Main Anatomy 101: From Muscles and Bones to Organs and Systems, Your Guide to How the Human Body Works
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tysm!!! i have always wanted to have this book <33
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ANATOMY 101 FROM MUSCLES AND BONES TO ORGANS AND SYSTEMS, YOUR GUIDE TO HOW THE HUMAN BODY WORKS KEVIN LANGFORD, PHD [image: ] Avon, Massachusetts CONTENTS INTRODUCTION: THE BUILDING BLOCKS OF ANATOMY AND PHYSIOLOGY THE CHEMISTRY OF CELLS CHEMICAL BONDS ORGANIC COMPOUNDS: CARBOHYDRATES AND PROTEINS ORGANIC COMPOUNDS: LIPIDS AND NUCLEIC ACIDS IMPORTANT INORGANIC COMPOUNDS COMPONENTS OF A CELL MOLECULAR TRAFFICKING CELL GROWTH AND REPLICATION DNA REPLICATION TRANSCRIPTION AND TRANSLATION ENZYMES AND CATALYSIS TISSUE ORIGINS AND DEVELOPMENT EPITHELIAL TISSUE CONNECTIVE TISSUE AND MUSCLE TISSUE NERVOUS TISSUE SKIN, HAIR, AND NAILS SKIN STRUCTURES AND FUNCTIONS SKIN DISEASES AND DISORDERS THE SKELETAL SYSTEM: BONE FUNCTIONS AXIAL SKELETON APPENDICULAR SKELETON AND JOINTS BONE GROWTH, REPAIR, AND DISEASES MAJOR SKELETAL MUSCLES NEUROMUSCULAR JUNCTION MUSCLE CONTRACTION MUSCLE DISEASES AND DISORDERS NERVOUS SYSTEM SIGNAL TRANSDUCTION AND NEUROTRANSMITTERS BRAIN AND SPINAL CORD PERIPHERAL NERVOUS SYSTEM AUTONOMIC NERVOUS SYSTEM NERVOUS SYSTEM DISEASES AND DISORDERS SENSORY SYSTEM RECEPTION AND PERCEPTION VISION HEARING, BALANCE, SMELL, AND TASTE SENSORY SYSTEM DISEASES AND DISORDERS CARDIOVASCULAR SYSTEM AND HEART STRUCTURE HEART REGULATION BLOOD VESSELS CIRCULATION RED BLOOD CELLS WHITE BLOOD CELLS PLASMA AND PLATELETS HEMOSTASIS BLOOD DISEASES AND DISORDERS LYMPH AND LYMPHATIC CIRCULATION PRIMARY AND SECONDARY LYMPHOID ORGANS INNATE AND NATURAL IMMUNITY ADAPTIVE IMMUNITY IMMUNE SYSTEM DISEASES AND DISORDERS DIGESTIVE SYSTEM THE UPPER GASTROINTESTINAL TRACT THE LOWER GASTROINTESTINAL TRACT NUTRITION DIGESTIVE SYSTEM DISEASES AND DISORDERS RESPIRATORY SYSTEM INHALATION, EXHALATION, AND GAS EXCHANGE RESPIRATORY DISEASES AND DISORDERS ENDOCRINE SYSTEM ENDOCRINE SYSTEM DISEASES AND DISORDERS URINARY SYSTEM STRUCTURE URINARY SYSTEM FUNCTIONS, DISEASES, AND DISORDERS MALE REPRODUCTIVE SYSTEM MALE REPRODUCTIVE SYSTEM DISEASES AND DISORDERS FEMALE REP; RODUCTIVE SYSTEM FEMALE REPRODUCTIVE SYSTEM DISEASES AND DISORDERS INTRODUCTION THE BUILDING BLOCKS OF ANATOMY AND PHYSIOLOGY The human body has always amazed mankind. Early scientific drawings and diagrams demonstrate the long-standing fascination with the body. Even cave drawings and later hieroglyphs illustrate that people were aware of the complex machinery of the human body. Our fascination continues to the present day, as we dig ever deeper into learning everything we can about the human body. Our understanding has advanced dramatically in just the last 20 years alone. The study of the human body is divided into two different but closely related disciplines. Human anatomy is the study of the structure of the human body while physiology is the study of its function. Together, they help us understand how the human body works. In this book, you won’t just learn the structure of the human body and the functions of its various parts, you’ll also discover why it does what it does. Cells, tissues, and organs are often intricately arranged to facilitate many functions simultaneously; complex biochemical processes take place that enable your body to perform those functions. In Anatomy 101, all of these processes and structures of the human body are explained. After reading this book you’ll know the human body inside and out. The amount of complexity can seem overwhelming when you’re studying anatomy and physiology, especially at first, and particularly if you don’t have a strong background in biology. Don’t be intimidated! This book is designed for the reader who doesn’t already have a PhD in biochemistry. Even if it’s been a few decades since high-school biology, with careful reading, you’ll be able to grasp the principles described in this book. By starting with a solid foundation, you will eventually master the intricacies of the human body. Don’t forget that you already have a head start: you own a human body. While it may seem obvious that the human body is made of organs and the structures that connect them to each other, this book doesn’t start there, at the macro, big-picture level. It starts at the micro level, inside your very cells, with a description of the processes at work that help your body’s cells know what to do, when, and how. We’ll look at the biochemical basis of human life—the organic and inorganic elements, compounds, and molecules that are necessary for the functioning of your body. We’ll look at how cells communicate and replicate. That will help create the solid foundation you’ll need to understand the rest of the material. Once those building blocks are in place, we’ll move on to a discussion of tissue, the foundation of all the organs in your body. Once this material is covered, we’ll move on to the major systems in your body, including the skeletal, nervous, cardiovascular, and respiratory systems (among others). For each system, common diseases and disorders are also described. Related material, such as how the senses integrate into the sensory system and the importance of nutrition to human health, are also covered. Consider this book your one-stop information source for understanding the human body from cranium (head) to phalange (toe). THE CHEMISTRY OF CELLS Nuclear Reactions and Why We Love Them Everything in the universe—from the largest of stars in the sky to the smallest grain of sand on the beach—is made up of matter. To be more precise, everything that takes up space and has mass is made up of matter. That small grain of sand may not seem like it takes up any space or has any real mass, but wait until it gets in your shoe. Then you’ll know it as a physical object. We might call matter “physical substance” (as opposed to that random thought you just had about what’s for lunch; random thoughts have no physical essence). The study of the structure of matter, its properties, and how different kinds of matter interact is called chemistry, and an understanding of basic chemistry is crucial to learning the principles of anatomy and physiology. The interaction of atoms—which you probably know as the building blocks of matter—has created the human body and the world it inhabits. Atoms together form elements, a type of matter that cannot be broken down by chemical means (that’s where the nuclear reaction comes in—elements can only be changed by nuclear means). Various elements combine together to create cells, which are the smallest structural units in the human body that perform a function. For example, your blood cells carry oxygen throughout your body. They have a distinct structure from cells that perform other functions, such as nerve cells or muscle cells. Chemistry rules not only how these cells are structured but also how they perform their functions. The Most Important Elements Just as the human body doesn’t have a single “most important” organ, several elements are essential for the creation of life. These are among the most important elements of all living things on earth: hydrogen (which is denoted by its chemical symbol, H) carbon (C) nitrogen (N) oxygen (O) Whether in the air we breathe, the food we eat, or the materials that make up the physical structures of the human body, without these elements humanity would not exist. What makes these elements so essential to the formation of life is their ability to interact with other elements and then organize them into important molecules (matter that is composed of more than one atom) or compounds (molecules composed of two or more different elements). They can do this because of their subatomic structure and particles. Anatomy of a Word molecule A molecule is a piece of matter consisting of more than one atom. A molecule can be made up of atoms that are all the same element (such as a molecule of oxygen) or it can be made up of different atoms, meaning that a molecule can be a compound (such as a molecule of water, which is a combination of hydrogen atoms and oxygen atoms). Subatomic Particles All atoms are made up of three basic subatomic (i.e., anything smaller than an atom) particles: protons (which have a positive electrical charge) neutrons (which have no charge) electrons (which carry a negative electrical charge) The number and organization of these particles dictates whether an atom will readily interact with any other atom—and also defines what type of atom it is. If an atom has only 1 proton, it must be a hydrogen atom. Positively charged protons are found in the nucleus of the atom. Where do atomic numbers come from? The number of protons present in an atom is the atomic number for that element. For example, carbon has an atomic number of 6 and oxygen has an atomic number of 8, which means carbon has 6 protons and oxygen has 8 protons in the nucleus. Another particle found in the nucleus of an atom is the neutron. While neutrons don’t contribute any charge to the atom, they do contribute to the mass of the atom. Therefore, the atomic mass of an atom is the number of protons and neutrons present in the atom. So while carbon has an atomic number of 6 (6 protons), it has an atomic mass of 12 (which means there are also 6 neutrons in the nucleus). However, while the nucleus is populated, there is an unequal charge for the atom. As with most things in the universe, atoms seek balance. To obtain this balance, atoms have negatively charged particles that orbit around the nucleus. These are called electrons. It is the electrostatic attraction between the electrons and the protons that keeps the electrons spinning in orbit around the nucleus, much like the moon is held close to the earth by gravity. In fact, to find a natural balance, atoms will have the same number of protons as electrons, leaving the atom with an overall net neutral charge. Electrons, however, are not restricted to a single location, such as the nucleus. They are found in orbitals (shells) around the nucleus. An atom can have many orbitals. In illustrations, these will often be drawn as concentric circles with the first being closest to the nucleus. The first orbital of any atom (that is, the orbital closest to the nucleus) can contain up to 2 electrons. After this orbital is filled, if the atom has more electrons, they will be packed into the next orbital, which can contain up to 8 electrons. Once the next orbital is filled (if there are more electrons), then they are packed into the next and so on. All orbitals after the first can contain up to 8 electrons. Orbitals by the numbers For carbon, with an atomic number of 6 (meaning 6 protons and thus 6 electrons), 2 of the electrons will be in the first orbital and the remaining 4 will be in the second (and outermost) orbital. With this basic understanding of atoms and subatomic particles, you can better understand how atoms will combine together to form molecules and compounds. The real building blocks of matter The fact that subatomic particles exist is why scientists cry out in despair when people say atoms are the building blocks of matter. Particles such as protons are smaller than atoms, and for many years scientists thought they were the building blocks of matter … until someone discovered quarks, which have itty bitty charges and combine to form protons and neutrons. No one has actually seen a quark, but experiments show they must exist. Thus it is quarks that are actually the building blocks of matter (and will continue to be until someone finds something smaller). Periodic Table of Elements In order to show the relationship of the various elements, scientists have arranged them into the periodic table of elements, which you probably remember from high-school chemistry class. The table of the elements begins with the element that has an atomic weight of 1 (hydrogen) and goes to—well, that depends on the table you’re looking at. There are 114 confirmed elements and several others are suspected to exist, such as 118 (ununoctium, a synthetic element no one knows much about, sort of like that weird neighbor down the street). Ninety-eight elements occur in nature; the others are only found in labs (where they are synthesized). Each entry in the table includes the element’s atomic number and its chemical symbol. Some tables may also show the atomic mass. Color coding is often used to indicate groups of elements that share similar qualities. CHEMICAL BONDS How Atoms Stick Together Atoms sometimes create connections (bonds) with other atoms, allowing them to form relationships we call molecules or compounds. Sometimes these bonds are long-lasting, and other times they are shorter-lived than that thing you had with that drummer back in high school. Bonds between atoms are generally created by the attraction of opposite charges. For that reason, if an atom has an outer shell (orbital) that is already filled with electrons, it is unlikely to form molecules or compounds with other atoms/elements. However, if the atom has room in its life (outermost orbital), it is more receptive to a bond. A bond can be accomplished by the atom either giving or receiving electrons from other atoms, or by sharing electrons with other atoms. Ionic Bond An ionic bond is when 2 atoms form molecules by giving up or taking electrons from others to complete their outermost orbitals. The classic example is the compound salt (sodium chloride, NaCl). Na (sodium) has a single electron in the outermost (third) orbital. That is one lonely electron. To fill the outermost orbital, sodium could recruit 7 more electrons from other atoms, but that would be a lot of work and impractical and is totally against the law in several states. Therefore, Na gives up the single electron and leaves the complete second orbital filled with 8 electrons, a very stable arrangement. However, now this atom has 10 electrons and 11 protons. This imbalance between protons and electrons yields an ion. In this case, the sodium ion with 10 electrons has an overall positive charge. On the other hand, chlorine (Cl) has the dilemma of needing a single electron to complete its outermost shell. With an atomic number of 17, there are 7 electrons in the third orbital of chlorine and there is room for 8, making it a natural partner for sodium (and it didn’t even have to join an online dating service). Sodium gives up its electron to chlorine, which then uses the electron to complete its shell. Since it now has 1 more electron than proton, it has become a chloride ion with an overall negative charge. This is where the bond happens. The positive charge of the Na+ ion is attracted to the negative charge of the Cl- ion, and the two will form a moderately strong chemical bond to create NaCl, or salt. Anatomy of a Word ion An ion is a charged atom that has an unequal number of electrons and protons. An ion can be positively or negatively charged depending on whether it has fewer electrons than protons (positively charged) or more electrons than protons (negatively charged). Hydrogen Bond Hydrogen bonds are formed when atoms share electrons unequally in compounds. Water is the classical example of this type of bonding. Hydrogen has an atomic number of 1, so its shell is half full. Oxygen, with an atomic number of 8, lacks 2 electrons from filling its outermost shell. Thus, oxygen will share an electron with 2 hydrogen atoms, which will complete the outer shells of all three members of this compound, creating H2O, or water. (The subscript 2 on the chemical abbreviation for hydrogen indicates that there are 2 atoms of hydrogen in the compound.) However, with more protons in the nucleus of oxygen, the shared electrons will spend more time around that nucleus than around either hydrogen nucleus. This imbalance will create a slight negative charge on the oxygen side and slight positive charge on the hydrogen arms. This polarization of charge will cause water molecules to be attracted to each other. In this way, water will adhere to itself. This type of bond is the weakest of the three chemical bonds. It is also the type of bond that holds two strands of DNA (genetic code) together in chromosomes, which are the instruction books that tell an organism how to be what it is supposed to be. Covalent Bond The strongest of the chemical bonds, the covalent bond is when a molecule or compound shares electrons equally. Carbon, the foundation atom of organic molecules, is well adept at this type of bond formation since its atomic number of 6 means it needs 4 electrons to fill its outermost shell. Because of this, carbon can form 4 single covalent bonds with other atoms. What is an example of a compound with a covalent bond? A great example of a compound with a covalent bond is the basic structure of an amino acid. Amino acids are organic compounds that combine to form proteins (which are necessary to create tissue, organs, hair, skin—you name it, it needs an amino acid to help build it). The carbon is the central atom in an amino acid, onto which four components attach, each using one of the available bonds: a carbon group, a nitrogen group (called an amino group), a single hydrogen atom, and a fourth group, the structure of which changes from amino acid to amino acid. This variable side group (sometimes referred to as a side chain) is called an R group. pH: Ions, Acids, and Bases A pH measurement tells you whether a substance is an acid or base. An acid has a low pH and will release hydrogen ions (under certain circumstances) and a base has a high pH and will release hydroxide ions (under certain circumstances). Vinegar is an example of an acid. Baking soda is an example of a base. Acids and bases react if combined. If you mixed vinegar and baking soda together, you would produce a gas (which creates those bubbles and that hissing noise). A mixture’s pH is essentially a measure of its hydrogen ions. If a substance has molecules or compounds that will yield a large number of H+, the substance, based on a mathematical logarithm, will result in a lower pH number and be considered an acid or acidic solution (pH < 7.0). Conversely, with lower H+ concentrations, the pH will be above 7 and considered a base (also called a basic solution or alkaline). This standard is set internationally using known materials, such as pure water (pH of 7.0). Most living organisms can survive only within a small pH range. Change the pH range and you’ll create a reaction, and change (or kill!) the organism. Cells can change pH through the process of metabolism (think of your muscle cells creating lactic acid when you exercise hard). Your slightly alkaline bodily fluids The pH of plasma and body fluids is approximately 7.3–7.4. This is on the alkaline side of neutral (7.0). It is called the physiological neutral point. Your body will try to maintain this level of pH through various cellular processes. ORGANIC COMPOUNDS: CARBOHYDRATES AND PROTEINS It Does a Carbon-Based Life Form Good Most life on the planet is carbon-based. The chemistry associated with carbon-based living organisms is called organic chemistry and focuses on carbohydrates, proteins, lipids, and nucleic acids. These are called “organic” compounds and are used to build everything you need to run that marathon you’re entered in this weekend—from the lungs you use to breathe to the energy you use to power your stride. Carbohydrates Also known as “sugars,” carbohydrates (or saccharides) play a major role in energy conservation, transport, transfer, and storage. Plants capture energy from the sunlight and use it to assemble carbon molecules into carbohydrates. When you eat a plant, your body breaks down these complex molecules into individual CO2 molecules and recovers the energy generated from the breaking of the bonds to be used elsewhere in the body. In your body, energy can be stored either as fat or as long chains of carbohydrates (polysaccharides). A single carbohydrate molecule is often referred to as a monosaccharide with a typical chemical composition of (CH2O)n where n is at least 3. Thus, C3H6O3 is the simplest of monosaccharides (it is called glyceraldehyde). Glucose, one of the most important energy-bearing monosaccharides, is C6H12O6. Disaccharides are composed of 2 monosaccharides. Sucrose is a disaccharide composed of glucose and fructose, another important monosaccharide for metabolism. The common name for sucrose is table sugar. Beyond disaccharides Oligosaccharides are composed of between 3 and 9 monosaccharide units. Polysaccharides (a saccharide of more than 1 monosaccharide) may be much longer. Saccharides provide energy storage. Glucose is polymerized (the process of linking together molecules) into glycogen, which is stored intracellularly (inside the cell) in muscle and liver cells to be broken down in times of high energy needs and low blood glucose levels, such as when you oversleep and end up running out the door before you even have a chance to eat breakfast. Proteins Protein molecules serve as structural elements both inside and outside of the cell, as anchoring molecules to hold cells in place, as adhesive molecules to allow cells to move throughout the body, and as enzymes that facilitate much of the metabolic activity of the cell. Amino Acids Amino acids link together to form proteins, using a special linkage called a peptide bond. Because of this, proteins are often called polypeptides. There are twenty types of amino acids, each with a different structure. The final shape and function of a protein is determined by its amino acids. Since the only variable part of an amino acid is the R group, this is the portion of the amino acid that will confer different physical and functional properties to the protein. For instance, several amino acids, such as valine and isoleucine, have hydrocarbon (molecules consisting of only carbon and hydrogen) R groups. These groups will be neutrally charged and will not interact with charged (also called polar) molecules, such as water. Thus, these amino acids are said to be hydrophobic (“afraid of water”) and are often present in regions where a protein will span the plasma membrane (which is also a hydrophobic region of fatty acid hydrocarbon chains). Other amino acids are hydrophilic (attracting water). Some are acid; others are base. Amino acids and protein folding The type of amino acid in the protein will have an impact on the shape and folding of proteins into their final structure. A string of amino acids only becomes functional when it folds into a protein. For instance, glycine has the smallest of the R groups, with only hydrogen present. This will allow the protein to fold easily since there isn’t a large R group to physically get in the way. Protein Structure Methionine will always be the first amino acid in a protein since it is also the sequence (in the RNA) that signals the start of protein formation. While the protein sequence is usually written in a straight line and is considered the primary structure of the protein, proteins are flexible and typically fold back upon themselves into one of two patterns: in side-by-side runs of the protein, forming beta pleated sheets (sheetlike regions of the protein) twisted around neighboring regions of the protein, forming spiraling tubes called alpha helices Each of these folded patterns, which form the secondary structure of the protein, is held in place by hydrogen bonds between amino acids. As the protein folds, other amino acids may become closer to each other and form bonds. Cysteine, for example, is an amino acid with a sulfate group. When it is next to another cysteine it may form a disulfide bond. In this way, large loops of protein are held in place. These formations within the protein are called the tertiary structure of the protein. Lastly, separate protein units may be held together by bonds into large protein aggregates. This quaternary structure (meaning the combination of 2 or more chains that form a final structure) is illustrated in the hemoglobin molecule. Adult hemoglobin (the protein responsible for oxygen transport) is formed from 4 subunit proteins, bonded together in the large single molecule that moves oxygen throughout the blood stream. ORGANIC COMPOUNDS: LIPIDS AND NUCLEIC ACIDS Fatty Acids and the Code Talkers Carbohydrates and proteins may get all the press but two other organic compounds, lipids and nucleic acids, are fundamental to cellular biology—and therefore to human life. Lipids Lipids are hydrocarbon molecules used in plasma membranes and for energy storage. Because they are composed of neutrally charged hydrocarbon chains, they are hydrophobic. What keeps oil and water from mixing? Oil is a hydrocarbon (hydrophobic substance) and will not interact with charged (polar) molecules, such as water. Saturated versus Unsaturated Fatty Acids Fatty acid chains are polymers of hydrocarbons, which are attached to a carboxylic acid (a compound in which a carbon atom is bonded to an oxygen atom, such as COOH-, making it a weak acid). The carbons can be attached to each other via a single bond, with the remaining bonds completed with hydrogen molecules. This would generate a saturated fatty acid in which all of the bonds of carbon are occupied by additional atoms. A straight linear fatty acid chain is generated when the carbon bonds are saturated. However, a double bond may be present between carbons (and therefore one fewer hydrogen on each of the adjacent double-bonded carbons). This is an unsaturated fatty acid and will have a bend at each of the double-bonded regions. Any more than one double bond in a single fatty acid chain will result in a polyunsaturated fatty acid. Saturated and unsaturated fatty acids Eating foods that are high in saturated fats can increase your cholesterol (a waxy substance that can clog your arteries), possibly affecting your heart health. Most saturated fats come from animal sources (including meats and dairy). Unsaturated fats are considered healthier for your heart. Phospholipids Phospholipid is the main constituent in membranes within a cell, including the plasma, nuclear, mitochondrial (the mitochondria is the energy-producing structure in a cell), and vesicular (vesicles are structures used for storage and transport) membranes. Basically, 2 side-by-side fatty acid chains are attached at one end to a glycerol molecule. One end of the molecule is charged, making it hydrophilic, and the other end, composed of fatty acid chains, is not, making it hydrophobic. This duality is called amphipathic. Triglycerides Triglycerides are the storage form of energy in the body and typically are referred to as “fat.” This material is stored in fat cells called adipocytes and can be recruited when your body requires the use of a lot of energy. Releasing double the amount of energy as compared to glucose, triglycerides take longer to release energy than carbohydrates because it takes the cells longer to break down the fat and place it into the blood stream. As the name implies, these molecules are composed of 3 (tri-) fatty acid chains attached to a glycerol molecule. Sterols In humans, the principal sterol (a type of fat) is cholesterol. Despite all the bad press, your body cannot function without cholesterol. It plays a critical role in the proper spacing of the plasma membrane, which gives stability to the membrane. Additionally, hormones such as estrogen and testosterone are derived from cholesterol, and are crucial to proper body function. Nucleic Acids Nucleic acids are necessary for each and every cell of the body. Nucleic acids exist in two forms, deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). These linear molecules are the repository of genetic information (DNA) and copies of that information with which proteins are built (RNA). DNA Described as a double helical molecule, DNA has 2 chains held together by hydrogen bonds that can easily be separated for either DNA replication (during cell division) or RNA synthesis and transcription (the conversion of genetic information into proteins that carry out the directions). DNA strands are composed of a few basic units. First, a sugar molecule will form part of the backbone of the strand of nucleic acid (for DNA that sugar is deoxyribose, hence the “D” in DNA). The other portion of the backbone is a phosphate group, which will link the sugars together into a long strand. Also attached to the sugar is a nucleobase of either a purine or a pyrimidine. The purines of DNA are either adenine (A) or guanine (G) and the pyrimidines are thymine (T) and cytosine (C). The hydrogen bonds between the nucleobases are what hold the 2 DNA strands together into the double helix. The bases are always paired together: A-T and G-C. Because of their structure, the A-T pair is held together with 2 hydrogen bonds and the G-C pair with 3 hydrogen bonds. Thus, the G-C pair requires more energy to break than the A-T set. This becomes important for DNA replication and RNA synthesis (discussed in later sections). Purines and pyrimidines—what’s the difference? Both are nitrogenous bases. A purine has 2 carbon-nitrogen rings while a pyrimidine has a 1 carbon-nitrogen structure. They have similar functions. RNA Ribonucleic acid (RNA) is similar in its structure to DNA with some important differences. As the name implies, the first difference is the sugar used. For RNA, that sugar is ribose. Additionally, RNA will be synthesized as a single strand rather than a double strand. Lastly, while G, C, and A are found in RNA, T will not be present. Rather, uracil (U) is used instead. IMPORTANT INORGANIC COMPOUNDS The Night of the Living Dead While much of the focus in the study of anatomy is on the structure, function, and metabolism of organic molecules (molecules containing carbon), some inorganic compounds are essential for human existence and for life in general. One way to think of the difference between organic and inorganic molecules (other than organic molecules being carbon-based) is that organic molecules are generally synthesized by living organisms, whereas inorganic molecules are not (they are usually produced by other means, such as geologic processes). Water While most life on the planet is carbon-based, water, which is not carbon-based, is a compound without which life would not be possible. The human body, for instance, is considered to be composed of between 50 and 65 percent water. This water exists within your cells (about two-thirds of your water content) with the remainder outside of the cells in your tissues or blood stream. Your brain is about 85 percent water while your bones are about 10 percent water. Water is the universal solvent because it is composed of polar molecules—that is, molecules that contain a charge—that are capable of ionizing many molecules (e.g., NaCl). A solvent is used to form a solution when another substance is dissolved in it. A universal solvent is one that can dissolve a wide range of substances. Lemonade is an example of a solution. So is saline (which is water in which NaCl has been dissolved). Both of these solutions use water as the solvent. Water in the human body is used as a solvent for elements such as chloride. Proteins and other molecules also use water as a solvent. Anatomy of a Word ionizing Ionizing means any process that changes a neutral (noncharged) atom (or molecule) into one that carries a charge. Water is an essential substrate for many processes in the human body. A substrate is a molecule on which enzymes act to catalyze, or cause, biochemical and other essential reactions. So water is the basis upon which many biological processes take place. The connection between water pressure and blood pressure In the blood stream, the amount of water pressure has a large impact on blood pressure and heart activity. The kidneys respond to changes in the amount of water and water pressure in the body. For example, the kidneys will excrete more water and salt if the blood pressure increases to help reduce it. The body uses water for many functions, including: regulating body temperature lubricating joints and moistening tissues flushing waste (preventing constipation and reducing demand on the kidneys) aiding in the transport of materials and gases in the blood stream dissolving molecules (such as minerals) so the body can use them The Role of Salts Your body uses a number of inorganic compounds in the form of salts. (Calcium phosphate salts, described in the following section, are an example.) A salt is an ionic compound formed when a base reacts with an acid, which means it has net neutral charge despite being made up of charged parts (positively charged ions and negatively charged ions). Because of the properties of salts, in the body they can dissolve to become electrolytes (free ions that are conductive), making it possible for them to carry an electrical charge through a solution. Sodium chloride, for example, helps transmit neural impulses and is necessary for your muscles to contract. It is also used to aid digestion and to help regulate the amount of fluid in your body. Calcium Phosphates Calcium phosphates, a type of salt, make up much of the inorganic material in the bones and teeth. These body parts are essential for support, movement, and eating, but they also play the important bodily function of storing calcium as phosphates. Calcium is an essential ion for muscle contraction, nerve signaling, and protein activation, among other activities. If blood calcium levels decrease, calcium can be recruited from storage in the bones to maintain homeostasis of cellular activity. Endocrine glands secrete hormones that closely regulate blood calcium levels. Acids and Bases Your body uses acids and bases for a number of functions as well. Your body produces hydrochloric acid to digest food in the stomach. However, the hydrochloric acid must be neutralized once it is mixed with food and leaves your stomach or it would destroy other tissues, so your body produces a base, bicarbonate, to reduce the acidity. Your body also produces buffers that can make small changes in the base or acidity of a substance in your body to help keep your bodily fluids the proper pH. Minerals Your body needs other inorganic substances in the form of minerals to function. Minerals are naturally occurring solids that help your body with various processes. For example, iron helps bind oxygen to red blood cells and transport it throughout your body. Someone without enough iron in her body will be anemic, and suffer from fatigue and occasionally serious, life-threatening disorders. Other minerals are used to create hormones and to regulate your heartbeat. Minerals your body needs for proper functioning include: magnesium manganese iodine zinc potassium fluoride COMPONENTS OF A CELL The Secret Life of Cells Most cells in the human body consist of several organelles, units within the cell that have a specific function. Organelles include the membrane, cytoplasm, nucleus, endomembrane, and mitochondria. Membranes The cell membrane or plasma membrane forms the boundary between the inside and outside of a cell. It consists of both protein and lipid molecules in varying ratios depending on the cell type. Typically, there are 50 lipid molecules for each protein in the membrane. However, since proteins are much bigger than lipids, proteins make up 50 percent of the mass of the membrane. These molecules are arranged in two opposing sheets, creating a bilayer of lipid and protein. One layer faces the outside of the cell, or the extracellular surface, and the other faces the interior, the cytosolic surface. The main lipid type in the membrane is a phospholipid, a molecule made of a charged phosphate group attached to a base molecule (either a glycerol or sphingosine). Because of the negative charge of the phosphate, one end of the phospholipid layer can interact with water molecules, which are also charged, or polar. At the other end are fatty acid hydrocarbon chains that create a nonpolar (hydrophobic) area. Just as oil and water don’t mix, this part repels water or charged molecules and makes an effective filtration barrier, called a semipermeable membrane. One role of cholesterol in cell structure Cholesterol is abundant in the plasma membrane and serves to regulate membrane fluidity. Like phospholipids, a cholesterol molecule has a hydrophilic head and a hydrophobic tail, meaning the cholesterol molecule can align with the phospholipid and create a more rigid structure. Proteins embedded in the phospholipid bilayer may be associated with either or both of the surfaces of the membrane. This membrane-spanning arrangement enables the proteins to serve many cellular functions. They transport material into or out of the cell and provide membrane attachment for stationary cells or adhesion for migratory cells. Many proteins in the membrane are receptors that recognize chemical signals and relay those signals to the inside of the cell to alter cellular activity. Free-floating proteins Proteins in the membrane are not locked in place. They may float throughout the membrane, spin, or flip horizontally. Cytoplasm Effectively separated from the extracellular environment by the plasma membrane, the inside of the cell is the location of most metabolic activity. The cytoplasm houses the workshops of the cell. Here, incorporated material is broken down, new proteins are generated, and new phospholipids are produced. Nucleus The nucleus, which contains the DNA (genetic code for the cell), is positioned in the center of the cell. The nucleus turns copies of the DNA code into RNA, which is used in the cytoplasm to make proteins. The nuclear membrane consists of the same materials as the plasma membrane. But the nuclear membrane is composed of 4 phospholipid layers (2 bilayers) with a perinuclear space between. The most prominent structure within the nucleus is called the nucleolus and is made up of proteins and nucleic acids. Here, rRNA (ribosomal RNA), required for protein production, is synthesized and prepared for transport outside the nucleus. Anatomy of a Word ribosome Ribosomes are structures found in all living cells, responsible for producing most of the proteins that are created in an organism. Protein complexes within the nuclear membrane regulate transport of materials into and out of the nucleus. While small water-soluble molecules may pass unimpeded, larger molecules must be assisted to get from place to place. The “helper” is a cotransport molecule, which must bind to the “cargo” molecule to allow the transport. The helper that moves molecules into the nucleus is called importin. Exportin moves molecules out of the nucleus. Endomembrane System Many of the membrane-bound organelles of a cell are either physically or functionally connected or both, and are thus grouped together as part of the endomembrane system. These include: nuclear envelope (the membrane surrounding the nucleus) endoplasmic reticulum Golgi apparatus vesicles plasma membrane Endoplasmic Reticulum The endoplasmic reticulum (ER), which plays a vital role in protein and lipid production, is made up of large folded sheets of membranes that occupy vast expanses of the cytoplasmic compartment. ER comes in two types: Rough ER (rER) is covered with ribosomes, the organelles for protein synthesis, which give the ER a rough appearance. Smooth ER (sER) does not contain ribosomes and is the site of lipid synthesis. Material of the ER is transported in membrane-bound spheres called vesicles that move toward and fuse with the membranes of the Golgi apparatus. Golgi Apparatus The Golgi apparatus, a cell structure made up of flat sacklike layers, is essential for sorting proteins and packaging them for specific targets. As the vesicles fuse together on the incoming side, they form a new layer termed the cis face of the Golgi. Like an assembly line, these layers are moved higher and higher in the stacks of the Golgi as new cis layers are added. At the opposite side of the stacks, the trans face, or last layer, breaks up into transport vesicles and shuttles material to its target. Some materials will be shipped to the plasma membrane, while others will be placed into vesicles going to other membrane-bound organelles, such as the mitochondria. Vesicles In addition to the transport vesicles of the endomembrane system, other vesicles are essential for proper cellular function. Lysosomes are spheres of enzymes that break down proteins, carbohydrates, or fats. Peroxisomes contain hydrogen peroxide and are prominent in liver and kidney cells where the hydrogen peroxide detoxifies ethanol and breaks down fatty acids. Mitochondria The mitochondria is a structure that creates the energy used by the cell. It consists of a double membrane system, much like the nuclear membrane. Like the nucleus, mitochondria also possess DNA. The mitochondrial genome encodes for over 30 genes whose products play essential roles in metabolism and energy production. Shaped like a capsule, the outer mitochondria membrane is flat over the surface of the organelle while the inner membrane is folded (to increase surface area) into sheets, which are called cristae. Proteins in the inner membrane create an electron transport system where protons or hydrogen ions (H+) are transported from the interior of the mitochondria, called the matrix, to the intermembrane space between the inner and outer membranes. The energy of the flowing H+ is used to produce ATP (adenosine triphosphate), the molecule that all cells of the human body use for energy. MOLECULAR TRAFFICKING Cellular Customs and Border Patrol Transport of material into and out of a cell can be passive, where no energy is used, or active, where the process must be helped through the expenditure of energy. In passive transport, molecules move from areas of high concentration to areas of low concentration. Diffusion and Osmosis Simple diffusion is a process by which molecules in areas of high concentration spread out into areas of low concentration. Spritz some air freshener in the trash can and pretty soon the whole kitchen will have a lovely lilac scent. This is an example of diffusion. You can think of the process of diffusion as a lot like riding a bicycle downhill. The only energy required was what you used to get up to the top of the hill. Afterward, it’s simply letting gravity coast you downhill. For cellular transport, the uphill push is the creation of a high concentration of molecules. This buildup of molecules might happen when, for example, food is digested into the nutrients your body needs to function. The nutrient molecules pile up. This process requires energy. But the distribution of the piled-up molecules—having them topple downhill (so to speak)—doesn’t (necessarily) require additional energy. Thus, the top of the hill is the area of high concentration and the bottom of the hill is low concentration. (If you have a stack of molecules at the bottom of the hill, the molecules at the top of the hill wouldn’t have any place to go, so diffusion could not occur.) Oxygen and carbon dioxide freely diffuse through the membrane during respiration. Anatomy of a Word respiration Respiration is the process of bringing oxygen into cells and getting rid of carbon dioxide (a waste product) from cells. As the cell does its work, it uses up its oxygen, converting it to carbon dioxide. The carbon dioxide piles up until the concentration is high enough that it can topple downhill (that is, through the cell membrane and out into the wild where it can be carried away). As the carbon dioxide builds up inside the cell, oxygen builds up outside the cell. Once the carbon dioxide moves out of the way, the high concentration of oxygen outside the cell can tumble inside the cell, evening out the distribution of oxygen molecules. And the cycle continues to infinity and beyond, or at least for a very long time. Water is also capable of freely diffusing through the plasma membrane; however, the diffusion of water is termed osmosis. Water molecules tend to dilute materials to an equal extent. If water on one side of a membrane has more solute added, such as sugar or salt, water will flow from the side of less solute (lesser concentration, or hypotonic) and into the side rich in solutes (hypertonic) in order to attempt to equalize the concentration of water and stuff on both sides of the membrane. Carrier-Mediated Transport Molecules that can’t diffuse through the plasma membrane (because they are too large or because they carry a charge) are transported through the membrane via protein channels. The molecule is still moving from an area of high concentration to an area of low concentration. Glucose and charged ions such as sodium are among the molecules and ions that must use a protein channel for diffusion into or out of a cell. No energy is used since this is still diffusion. The only difference is the specialized tunnel through which these molecules can diffuse. Active transport is a type of transport that is distinct from passive diffusion in that molecules are actively moved in and out of the cell, often with the help of transport proteins. The molecule in need of moving adheres to a transport protein, which brings it where it needs to go and releases it. This type of facilitated diffusion is still a matter of moving molecules from an area of high concentration to an area of low concentration. However, sometimes molecules need to be transported from an area of low concentration to an area of high concentration (against their concentration gradient). This is the opposite of diffusion, and it requires transmembrane proteins to carry the molecules plus a lot more energy to make the trip. (This is more like riding your bike up hill than it is coasting downhill.) Active transport protein channels bind the transport molecules and use energy from ATP to change their shape in such a way as to move the molecules across the membrane and against the gradient. The final step of the active transport process is to reset the channels so that the next active transport cycle may begin. Sometimes a cell uses membrane vesicles to transport molecules into and out of a cell. In this case, instead of using a transmembrane protein as a sled, the molecule is encased by a pocket in the cell membrane (creating a vesicle) and moved into or out of the cell. Endocytosis is the process of using vesicles to transport molecules into a cell. Exocytosis is the process of using vesicles to transport molecules out of a cell. CELL GROWTH AND REPLICATION Time for “The Talk” Cellular growth and division is a simple fact of life itself. Controlled at the molecular level and via secreted materials, growth and division are usually maintained with precision, unless a cell or group of cells begins to divide out of control, which leads to the formation of a tumor. Cell Cycle A typical cell will spend the majority of its cell cycle—the process of growth and replication—providing an essential function to the tissues and organs where it resides. This stage of the cell cycle is called interphase. In this beginning stage, the cell grows to its final size and may remain in this static, functional state until it prepares to divide again. At this stage, called mitosis, the cell divides its chromosomes and nucleus. The cell cycle finishes with cytokinesis, the division of the cytoplasm, resulting in 2 daughter cells identical to the single parent cell from which they were produced. The resulting cells begin the process of interphase and go through the cell cycle all over again. Interphase: G1 Once mitosis and cytokinesis are complete, each daughter cell enters the first phase of interphase: the gap 1 (G1) phase. Here, most cells increase in size, replicate essential organelles, and move the nucleus more toward the center of the cell. At the end of the G1 phase, the cell checks to be sure that the process of replication has begun without errors. If there is a problem, cell division will be halted and the cell will attempt to repair the problem. Interphase: S The synthesis (S) phase follows G1 and is the period when the chromosomes are duplicated so that each daughter cell can have a complete set of chromosomes. Interphase: G2 In the gap 2 (G2) phase, the cell begins to prepare for mitosis. During this time, the cell produces and organizes all of the structures and materials essential for mitosis. The most important point is the G2-M transition. Here, the cell size, DNA replication, and DNA damage are checked before the cell continues the process of replication. Mitosis: M During the M, or mitotic phase, a parent cell is cloned into 2 daughter cells. Each human parent cells possess 46 chromosomes in 23 pairs. Each resulting daughter cell will be a clone of the parent with exactly the same 46 chromosomes. Anatomy of a Word mitosis Mitosis is strictly defined as the process by which the chromosomes in a cell are duplicated and separated into their own nuclei. During prophase, the first phase of mitosis, the sister chromatids (a chromatid is one of a pair of duplicated chromosomes), which were formed during the S phase of interphase, are condensed and become more coiled in preparation for cell division. Additional changes occur in the cell during this phase, such as the nuclear membrane beginning to disintegrate. An intermediate period, prometaphase, is often considered late prophase. During this time, microtubules called spindle fibers, which stretch from each pole of the cell toward the opposite pole, are organized. The spindle fibers serve two critical functions during cell division. First, fibers from each pole connect to each side of the sister chromatids to move the chromatids to the middle of the cell. Other spindle fibers function to push the poles of the cell farther apart in preparation for the division of the cytoplasm in the last stage of mitosis. The most often illustrated phase of mitosis, metaphase, is the point when chromosomes become attached to the spindle fibers that were previously produced. It is recognizable because of the alignment of all 46 sister chromatids at the equator of the cell. The shortest of the phases of mitosis, anaphase is characterized by the pulling apart of the sister chromatids into 46 individual and identical chromosomes that are being moved in opposite directions. Anaphase continues until the chromosomes arrive at the poles, which signals the start of telophase. At this time, the chromosomes begin to rapidly relax and uncoil, the nuclear membrane begins to re-form, and many of the spindle fibers disappear. This concludes the division of the nucleus, which coincides with the initiation of cytoplasmic division, or cytokinesis. Cytokinesis Near the end of telophase, proteins called actin begin to form a belt that extends around the equator of the cell. As cytokinesis continues, the actin ring becomes smaller and smaller, resulting in a narrowing at the waist of the cell called the cleavage furrow. This constriction continues until the 2 resulting daughter cells are pinched away from each other into independent yet identical cells. Meiosis Not all human cells divide by mitosis. During sexual reproduction, sperm and egg cells must be divided in half, so that each cell contains only 23 chromosomes rather than 46. This process of division is called meiosis. Thus, while mitosis is often referred to as cloning, meiosis is termed a reduction division. Why do sperm and egg cells contain only 23 chromosomes? When a sperm cell containing 23 chromosomes unites with an egg cell containing 23 chromosomes, they combine to form 46 chromosomes. The resulting new individual will have the final complete amount of genetic material. If each sperm and egg cell had the full complement of 46 chromosomes, the combined genetic material would result in an individual with 92 chromosomes. To complete this reduction of genomic material (“genome” meaning “the genetic material of a cell”) the cell undergoes 2 divisions, each given a name followed by the division number (e.g., prophase I, metaphase I). In meiosis I, the 46 chromosomes are divided so that each half now has 23 chromosomes. The second division is identical to mitosis in that the daughter cells from the first division will now have their sister chromatids pulled apart. The only difference between this division and mitosis is that the starting material for meiosis II are 23 sister chromatids (in mitosis there are 46). Thus after the completion of 2 divisions, 4 daughter cells will result. DNA REPLICATION Genes, Unzipped Before a cell can divide into two daughter cells, all of the DNA of the parent cells must be copied so that each daughter gets a copy. The DNA of a cell carries all of the organism’s genetic material—and instructions on how to use it. DNA Structure The structure of DNA is a double helix (spiral shape around a common axis) consisting of anti-parallel (running in opposite directions) strands of nucleic acid. It might be helpful to picture the DNA molecule as a ladder. Each leg of the ladder represents one strand of DNA and the rungs represent the nucleotide bases that hold the entire structure together. In DNA, the nucleotides on one strand are always paired with a partner in a very consistent manner. DNA nucleotides Nucleotides (the building blocks of DNA) are made up of a sugar molecule, a phosphate, and a nucleobase. DNA pairs contain only four different nucleotides, whose bases are either adenine (A), thymine (T), guanine (G), or cytosine (C). DNA strands are made up of a repeated pattern of a sugar (deoxyribose) and a phosphate. The sugar-phosphate molecule is called a ring compound because of its shape. The sugar is a form of ribose that has lost an oxygen atom (de-oxy meaning “minus oxygen”). If you think of the sugar molecule as being formed in the shape of a pentagon, then there are five angles where other molecules could attach. Imagine the base of the pentagon on the bottom and the point at the top. The first angle to the right of the top point is called 1', or 1 prime. Going in a clockwise direction, the second angle would be called 2', or 2 prime, and so on until all five angles are numbered. This helps scientists discuss where these molecules on a ring compound are attaching to each other. In DNA, in one nucleotide, the phosphate group attaches to the sugar at 5' and links to another nucleotide at the 3'. In this way, DNA strands exist as a one-way street that runs 5' to 3' in biochemical terms. But remember that DNA strands are connected to each other (the legs on the ladder). So, while one strand (the coding strand) is read left to right as 5' to 3', the other strand of the double helix (the complementary strand) is present 5' to 3' in the opposite direction. Preparing for Replication Before the DNA double helix can be replicated, it must be separated into single strands. This is like unzipping a pair of pants. On the DNA molecule, helicase, an enzyme, binds and separates the nucleotide pairs, freeing one strand from its partner. Although both strands are being replicated at the same time, for simplicity the strands and their replication will be considered separately. How does helicase work? Helicase breaks down the hydrogen bond at the center of a segment of DNA, where the bases connect to each other. This exposes the bases so that new bases can attach. Adenine always attaches to thymine and guanine always attaches to cytosine. Once the strands are unzipped, they no longer take a double helix shape. They are linear strands of nucleotides (denoted with the letters A, T, G, and C). Leading Strand Replication The so-called “complementary” strand of DNA is the “opposite” partner for the genetic code (coding strand). But it is used as a template to produce a new double helix for one daughter cell. By using each strand as a template, each daughter cell contains one new strand and one inherited strand from the parent and therefore forms the basis of semi-replicative division. With the DNA unzipped, an enzyme called DNA polymerase reads the template strand in a 3' to 5' direction, and assembles a new coding strand in a 5' to 3' direction. For example, if the sequence on the complementary DNA strand were 3'-A-T-C-G-G-T-T-A–5', then the new coding strand would be assembled in this order: 5'-T-A-G-C-C-A-A-T-3'. Since this is also the direction in which the DNA helicase enzyme is unzipping the double helix, DNA polymerase simply follows along until the entire strand has been copied into a long new coding strand, which is called the leading strand. Lagging Strand Replication Although the leading strand is continuously synthesized as one long new strand, the other replication, which uses the coding strand as a template, is more complex. Instead of a strand being continuously replicated, the lagging strand is produced in discontinuous pieces. Recall that as DNA helicase is unzipping the double helix, it moves in one direction. This works great for the leading strand replication because the new strand can be generated in the same direction that helicase is moving. However, for the other template, the new strand is generated in the opposite direction. This causes the new lagging strand to be generated in pieces. As soon as an area of single-stranded DNA is exposed, a new strand is being generated. But, in this case, the new strand is being formed in the opposite direction from the way the helicase is unzipping DNA. Thus, as helicase unzips away from the new strand segments, a gap of single-stranded DNA will be present between the new strand and the helicase. This is called an Okazaki fragment. The process of unzipping and replicating will continue producing distinct Okazaki fragments along the length of this template into a discontinuous lagging strand with gaps remaining between the fragments. These fragments have to be linked together to form a new, continuous strand of DNA. The gluing, or ligation, of these fragments into one long continuous strand is the function of first DNA polymerase and then DNA ligase, which bind to the gap site between adjacent Okazaki fragments and ligates them all together into a complete strand. TRANSCRIPTION AND TRANSLATION C’est la Vie The genetic code of a cell is housed in the nucleus as DNA. However, before this code can be read, interpreted, and used to create proteins, a copy must be made and shipped into the cytoplasm where protein synthesis occurs. The process of making this copy of the genetic code is called transcription and is very similar to DNA replication, with only a few differences. Transcription During transcription, nucleotides are assembled into an RNA molecule by the enzyme RNA polymerase. This enzyme, which is much larger than DNA polymerase, is capable of binding to specific sequences of DNA, unwinding the DNA, reading a single complementary strand of DNA as a template, and generating a single strand of RNA that will contain the genetic code for making protein. Once the RNA molecule is produced, it detaches from the RNA polymerase and the DNA strands bind back to one another to reform the double-stranded molecule. RNA nucleotides While the nucleotides in DNA are A:T and G:C pairs, the nucleotide uracil (U) replaces thymine (T) in RNA. Thus, the nucleotide pairs in RNA are A:U and G:C. The idea of a transcript in written language is to produce a word-for-word copy that uses the exact same language as is spoken. In the case of the genetic code and nucleotide alphabet, a strand of RNA is the transcript, consisting of the same code as found in the coding strand of DNA. Types of RNA Several types of RNA will be produced by transcription. Messenger RNA (mRNA) is the genetic code in RNA form. Ribosomal RNA (rRNA) will be combined with proteins to form ribosomes, which synthesize protein. Lastly, transfer RNA (tRNA) is molecules that transport amino acids in the correct order to the ribosome for assembly into protein. Translation When a translation of a spoken language occurs, the communication happens in a completely different language, often using a different alphabet. The same is true in cellular translation. Here, the machinery of the cell reads the genetic language of nucleotides and assembles proteins using an alphabet of amino acids. The essential components of translation are the 3 RNA molecules: mRNA, rRNA, and tRNA. Cellular translation can be considered the carrying out of the instructions in a genetic code. The code describes what to do and the translation does it. RNA Much like words are composed of letters, the genetic alphabet of nucleotides is arranged on mRNA into 3-letter words called codons, which are simply small units of genetic code—like syllables. With 4 different nucleotides arranged into 3-letter codons, there are 64 distinct codons that can be used by the translation machinery of the cell. While mRNA represents the code, the location where this code is read and interpreted and proteins are assembled is the ribosome. Some RNA molecules can bind to specific amino acids on one end while recognizing codons of the mRNA on the opposite end. Just as the coding strand of DNA binds to its complementary strand via nucleotide pairs (A:T, C:G), tRNA has 3 nucleotides (the anticodon) arranged that are complementary to the mRNA codons. This means that as the ribosome slides along the mRNA molecule, tRNA molecules bind to their respective codons and, in doing so, bring amino acids into the interior of the ribosome in the order directed by the mRNA code, and therefore into the correct sequence for the protein. The space inside the ribosome can only hold 2 tRNA molecules at a time. As the ribosome slides along the mRNA, the amino acids link together via peptide bonds, thus freeing 1 amino acid from its respective tRNA molecule and forming a growing chain of amino acids attached to the other tRNA. At this point, the empty tRNA molecule is ejected, the other tRNA and its growing chain slide into the space vacated by the ejected tRNA, and a new tRNA molecule with an amino acid enters. This assembly continues until the entire protein is finished. Codons and anticodons Given an mRNA codon with the nucleotides A-U-G, what would be the complementary anticodon of the tRNA molecule that would bind here? The anticodon that would recognize A-U-G would be U-A-C. Remember that there are no T nucleotides in RNA. Codons Although there are 64 possible codons, only 20 amino acids can occur in nature and be assembled into proteins. Does this mean many of the codons are irrelevant? The answer is no. Multiple codons can be recognized by the same tRNA. In addition, multiple codons encode for the same amino acid. For instance, the codons GGU, GGC, GGA, and GGG are all recognized by the tRNA for the amino acid glycine. In fact, many tRNA molecules can recognize 4 different codons. Specific codons called start codons and stop codons signal the beginning and end of protein synthesis. The codon AUG is the one and only start codon, and signals the assembly of the first amino acid in all proteins: methionine. The codons UAA, UAG, and UGA stop translation and signal for the ribosome complex to free the newly generated protein. ENZYMES AND CATALYSIS Making Things Happen Enzymes are a class of proteins made via translation. These molecules assist naturally occurring chemical reactions by making it easier to cut, modify, process, or further manipulate material into a final product. In other words, enzymes cause things to happen. Activation Energy Activation energy can be considered the threshold a chemical reaction must overcome in order to form a product. An enzyme lowers this threshold. Therefore less energy is required to create the product, the rate of the reaction increases, and the entire process becomes more efficient. Enzymes save energy and make the metabolic job of a cell much easier and faster. The limits of enzymes No enzyme can catalyze a chemical reaction that would not occur in nature. Enzymes only enable the reaction to occur faster. Induced Fit Model The induced fit model explains how only specific substrates can bind and be modified by specific enzymes. The model suggests that the connection between an enzyme and a substrate changes as they interact. When a substrate molecule binds to the enzyme, the enzyme changes shape, and since the substrate molecule is bound by the enzyme, its shape is altered, too. This moves the substrate molecule into a better position to transform into the final product, thereby reducing the energy and time required for the chemical reaction to occur. You could compare this to cutting snowflakes out of a sheet of paper. If someone had to cut every side of the snowflake by hand, it would take much longer, require more energy, and likely not look very symmetrical in its final form. This would be like a chemical reaction occurring naturally. However, if the paper is first folded and then cut, fewer cuts are required, the snowflake is produced faster, and all sides will be the same. Enzymes fold and process substrates in much the same fashion. Glycolysis Enzymes are essential for processing organic molecules and releasing the energy stored within chemical bonds so cells can use it. Anatomy of a Word catabolism Catabolism, or the catabolic process, is a process by which complex molecules are broken down into simpler ones, releasing energy as a result. Although many organic molecules can release energy to cells, the principal molecule used to release energy is glucose. The catabolic process of breaking down glucose is termed glycolysis. Anatomy of a Word fermentation Fermentation is how cells produce energy when oxygen is in short supply. In yeast, a by-product of this process is CO2 and ethanol. In animal cells, the by-product is lactic acid. During glycolysis, glucose is processed through 10 enzymatic steps from a 6-carbon molecule into 2 separate, 3-carbon molecules called pyruvate. These much smaller molecules can be transported into mitochondria where they support its functions. Energy is also released during glycolysis. The reaction also causes electrons to be released. Nicotinamide adenine dinucleotide (NAD+), a coenzyme, captures and transports electrons from one chemical reaction to another. When stored energy in the form of ATP (adenosine triphosphate) is used by a cell, one of the phosphate groups is broken off the molecule, turning the ATP into ADP (adenosine diphosphate). During the process of glycolysis, as chemical bonds are broken and rearranged, some of the bonding energy is released and is used to convert ADP back into ATP. Net energy gain during glycolysis The products of glycolysis are 2 pyruvate molecules, 2 NADH (reduced NAD+) coenzymes, and 4 molecules of ATP. In the early stages of glycolysis, 2 ATP molecules are used to prepare intermediate molecules for the next steps of glycolysis. While 4 ATP molecules are produced during glycolysis, the net production of ATP is only 2. TCA Cycle The next stage of energy release, also known as the citric acid or the Krebs cycle, consists of 9 enzymatic steps that break the 2-pyruvate molecules down into 6 CO2 molecules and release the remaining energy in chemical bonds. Each pyruvate (3-carbon molecule) must first be converted into acetyl-CoA (a 2-carbon molecule). This also results in the formation and release of a CO2 molecule as well as the formation of another NADH coenzyme. The further breakdown of acetyl-CoA continues as a cycle of carbon intermediates is introduced. Intermediates are highly reactive molecules with short lives and lots of energy, sort of like your first crush. The 2-carbon acetyl-CoA is combined with a 4-carbon molecule to generate the first intermediate of the TCA cycle, the 6-carbon molecule citrate. Through 8 more enzymatic reactions, 2 more carbons will be released as CO2, electrons will be captured in the form of 3 NADH (and 1 FADH2), and an additional molecule of ATP will be generated for each acetyl-CoA molecule that enters the cycle. Thus, by the end of the TCA cycle, all carbons from glucose have been released as 6 CO2 molecules and the energy of those bonds captured within 10 NADH, 2 FADH2, and 6 ATP molecules (net ATP production remains 4 ATP). Clearly, the bulk of the energy from glucose is contained within the coenzymes and has not yet been converted into ATP for the cell. That process of ATP formation also happens in the mitochondria as the electron transport system. Electron Transport System The electron transport chain recovers energy contained within the coenzymes NADH and FADH2 (an energy-carrying molecule that is the reduced form of flavin adenine dinucleotide). This system is similar to a hydroelectric reservoir, where the dam is the inner membrane of the mitochondria and the water is the electrons. Just as water builds up in the lake and, because of gravity, has a large potential energy to flow through the turbines of the dam to create electricity, protein complexes in the mitochondrial membrane use the energy of the electrons from the coenzymes to move hydrogen ions into the intermembrane space. This reservoir of ions then flows back into the matrix of the mitochondria through the last protein complex called ATP synthase. Just as the flow of water powers the turbine in the dam, the flow of hydrogen ions enables ATP synthase to generate ATP molecules. Although only a net of 4 ATP molecules were produced during the stages leading up to the electron transport chain, the energy bound in the coenzymes as electrons from glucose are used to generate an additional 32 molecules of ATP. At the end of this process, 36 net molecules of ATP and 6 molecules of CO2 are produced for each molecule of glucose. TISSUE ORIGINS AND DEVELOPMENT Making a Tissue of It Tissues are collections of similar cells that define a specific layer of and relate an essential function to an organ. While the adult human body consists of over 200 different cell types, each human began his or her life as a single fertilized egg cell, which divided and gave rise to all the rest of their cells. The Creation of Tissue To understand how tissue fits in the organization of the human body, think of it as the level between cells and organs. Tissue is made up of cells and in turn it is used to make organs. Stem cells and tissue Stem cells are unspecialized cells that can, under certain conditions, take on specific cell, organ, and/or tissue functions. For example, under the right circumstances, a stem cell might be persuaded to become a neuron, helping to conduct electrical impulses throughout the body. In some tissues, stem cells work to repair damage, sort of like a corps of engineers. Early in embryonic development, the newly (and rapidly!) dividing cells produce three distinct layers of cells from which all the cells of the body are derived. This process, called gastrulation, begins when a cluster of cells (called a blastula) organizes into layers. At first there is only an inner and an outer layer. These layers work together to produce a third middle layer. Organisms, such as humans, that have these three layers are called triploblastic. These layers are called germ layers. Here the use of the word “germ” has nothing to do with disease-causing pathogens, but is related to the word “germination.” Germ layers form during the embryonic (“germination”) stage of human development, and they are the origin of all tissues and organs in the body (thus they “germinate” these structures). The layers are called: the ectoderm the mesoderm the endoderm The ectoderm (ecto meaning “outside”) refers to the tissue that makes up your skin, which is the outer covering of the body. The ectoderm also creates the neural tube that eventually becomes home to your central nervous system. The human body is often considered a tube within a tube. In this sense, the ectoderm produces the outside tube of skin and the endoderm (endo meaning “internal”) produces the tube on the inside. That tube is the digestive tract. This leaves a lot of tissue and organs in between, which make up the mesoderm (meso meaning “middle”). Muscle, bone, blood, and connective tissue are all derived from this middle layer that is produced early in cell development. Anatomy of a Word organogenesis Organogenesis is the name for the process by which the germ layers form all of the organs in the human body. Your body has four main types of tissue: epithelial, which generally works to protect your body connective, which joins structures together muscle, which contracts nervous, which coordinates your body’s movements How does the anatomy of a tissue relate to its physiology? Think of anatomy as “form” or “structure” and physiology as “function.” They are linked together and can’t really be separated. For example, muscle tissue has the property of being able to contract (anatomy), so it is used to move the body around (physiology). Histology is the study of tissues and how they work to allow the human body to function as a whole. Scientists often refer to the parenchyma of an organ, which is the tissue of the organ that performs its essential function, and the stroma, which is other tissue in the organ, such as a vein or a nerve, which does not perform the essential function. A true understanding of how an organ works (or why it may not be functioning correctly) requires attention to both the parenchyma and the stroma. EPITHELIAL TISSUE I Feel Your Pain Epithelial tissue, one of the four basic types of human tissue, covers a surface or forms the lining of a hollow organ. The surface of your skin is made of epithelial tissue, as is the inside lining of your stomach and intestines. In fact, the inside of your body cavity is covered with a thin layer of epithelial cells. The function of these covering cells is to protect and to provide a watertight barrier to keep material out (skin preventing pathogens from entering) or keeping material in (lining of the stomach keeping hydrochloric acid from damaging other areas of the body). Types of Epithelial Cells Epithelial cells are classified, in part, based on their shape. Cells can be very flat, much like a fried egg, with the nucleus of the cells bulging upward like the yolk of the egg. These flat cells are called squamous epithelium. Epithelial cells can also be cube-shaped, where the width of the side is the same as the height of the cell. These are known as cuboidal epithelium. Lastly, cells that are taller than they are wide, thus looking like columns, are classified as columnar epithelium. Number of Layers Epithelial tissue composed of a single layer of cells is called a simple epithelium. In simple epithelium, all of the cells in the tissue are in contact with the structure that underlies the tissue (called a basement membrane). When epithelial tissue has multiple layers of epithelial cells, it is referred to as stratified. In this stratified layer, only the cells at the bottom of the layer are in contact with the underlying tissue. In some areas of the body, epithelial tissue appears to be made up of multiple layers of cells, but closer observation shows that all of the cells are in contact with the basement membrane and it just happens that the cells are tall or have a variety of heights. This type of epithelium is classified as pseudostratified, and is easy to confuse with the truly stratified layers. When describing the epithelium, both their shape and the number of layers are considered. A single layer of flattened cells would be called a simple squamous epithelium. Likewise, an epithelium consisting of multiple layers and having a surface layer of cube-shaped cells would be called a stratified cuboidal epithelium. How is epithelial tissue classified when multiple layers are present? When multiple layers are present, the shape of the cells at the surface of the epithelial tissue will be used in the classification regardless of the underlying cells. Transitional epithelial tissue is found along the urinary tract and in the lining of the bladder. While this is a stratified epithelium, the surface cells are large and either dome-shaped (when the bladder is empty) or flattened (when the bladder is full). Often, the cells of this tissue will contain 2 nuclei, making for easy identification. Apical Modification The top of the epithelial cells that are adjacent to the lumen, or the hollow space of an organ, is referred to as the apical surface; these cells have membrane specializations that affect the physiological function of the tissue. One such modification on the epithelial cells lining the respiratory tract is cilia. These hairlike structures extend upward from the apical cell surface and can bend back and forth to move materials. On cells in the intestines, the apical modifications are called microvilli. These fingerlike projections increase the surface area of the cell for greater absorption of nutrients and water. Basement Membrane Beneath every epithelial layer is a zone of molecules that aids in anchoring the cells to the underlying tissues in much the same way as a foundation or a basement secures a home to the ground. The basement membrane (BM) is also a transition zone where cells anchor to molecules such as laminin (a cell adhesion molecule) and other BM molecules interconnect with the underlying connective tissue, firmly anchoring the epithelial layers to it. The BM consists of three zones, each referred to as a lamina, which is another term for layer: The lamina lucida is a clear layer directly beneath and in contact with the bottommost epithelial cell, containing cell adhesion molecules. The next layer is called lamina densa, so named because it’s dark, a result of its highly compact network of type IV collagen fibers that resemble a net. This provides another anchorage point for the cells. The deepest layer is the lamina reticularis. In this layer, fibers from the underlying connective tissue extend upward and interconnect with the molecules of the lamina densa. Anatomy of a Word basal lamina Together the lamina lucida and lamina densa compose the basal lamina. Some people confuse this with the basement membrane, but they are not interchangeable terms. CONNECTIVE TISSUE AND MUSCLE TISSUE Tissues of the World, Unite! Two basic types of human tissue are connective tissue and muscle tissue. They have distinct forms and functions, but along with epithelial tissue and nervous tissue, they work together to make the human body function. Connective Tissue As the name implies, connective tissue joins other tissues together. It is composed of cells and molecules that function together for this adhesive process. Connective Tissue Cells Fibroblast is the principal cell of connective tissue. This cell deposits fibers that are found in all connective tissues: collagen, a protein resistant to stretching, thus giving the tissue tensile strength. These cells also produce elastic fibers that allow the tissues to rebound after being stretched. Macrophages are also found in connective tissues; they function as the vacuum cleaners of the body by removing pathogens and debris. Fat cells, or adipocytes, may also be present in connective tissue. These cells are mostly droplets of fats (lipids, cholesterol, or fatty acids) that are stored and released depending on the body’s available energy. When fuel for the body is abundant, materials are stored as fat. In times when fuel is scarce in the blood, fat is converted into a useable form of energy. Connective Tissue Classification Connective tissue is classified based on the percentage of cells to fibers and how tightly packed the fibers are within the connective tissue. Loose connective tissue consists of widely spaced fibers and many cells migrating within the open spaces. Areas where you find loose connective tissue include the tissue around large blood vessels, beneath the epithelium of the skin, and in the digestive and respiratory tracts. Dense irregular connective tissue has many more fibers and fewer cells than loose connective tissue and is classified based on the orientation of the fibers. In the dermis of the skin, the collagen fibers have a swirling and disorganized pattern, so the connective tissue is called dense irregular. Dense regular connective tissue is made up almost entirely of collagen or elastic fibers and contains few cells. Since the fibers are tightly packed and arranged parallel to one another, this tissue is classified as dense regular connective tissue. Ligaments and tendons that connect bone to bone or muscle to bone, respectively, are resistant to stretching and are made of dense regular connective tissue. Muscle Tissue Muscle not only moves the body, it moves materials through the body. All muscles have one job: to contract. Muscle contraction can only happen with the sliding action of two proteins, actin and myosin. The overlapping actin and myosin molecules slide toward each other, pulling each end of the cell and shortening the muscle. Skeletal Muscle Skeletal muscles are muscles attached to bones. This arrangement allows the muscles, which are the engines, to move the bones, which are the levers upon which actions can occur and work can be done. During embryonic development, individual muscle cells fuse together to create long tubes of muscle cells, which contain many nuclei. These are called skeletal muscle fibers. All of the nuclei are pressed to the periphery of the cell membrane because the middle of each muscle fiber is filled with long columns of overlapping actin and myosin molecules. The repeating and overlapping nature of the actin and myosin give skeletal muscle cells a striated appearance. Anatomy of a Word sarcomere The repeating units of actin and myosin, which are arranged in series like the links in a chain, are called sarcomeres. To contract, each sarcomere shortens a small amount. When added together, all sarcomeres shortening at the same time results in the entire muscle organ shortening by as much as a few centimeters. Skeletal muscle is the only type of muscle in the human body under voluntary control. Picking up a glass or running a race could not occur without your conscious control. Cardiac Muscle Like skeletal muscle cells, cardiac muscle also contains overlapping actin and myosin arranged into sarcomeres, yielding a striated appearance. However, cardiac muscle is under subconscious control and is considered involuntary. While you can increase or decrease your heart rate by your level of activity (running can increase the rate, while lying down will decrease the rate), heart rate cannot be voluntarily controlled by thought itself. Cardiac cells are branched at many different points, unlike linear tubes of skeletal muscle. These branch points link cardiac muscle cells together in interwoven layers (laminae). Laminae allow the 3-dimensional contraction of the heart (rather than the linear contraction of skeletal muscle). Cardiac muscle features attachment points between cells, called intercalated disks. Intercalated disks allow the muscle cells to hold on to each other tightly while contracting. These disks also contain membrane tunnels called gap junctions, which allow the cytoplasm of one cell to flow unimpeded into the adjacent cell. Thus, muscle cells joined via gap junctions contract at the same time. The heart functions as a single unit although composed of many parts. Smooth Muscle Smooth muscle lacks the striated pattern of skeletal and cardiac muscle. This does not mean smooth muscle lacks actin and myosin. Instead, it lacks the sarcomeric arrangement of the contractile proteins. In smooth muscle, the overlapping actin and myosin are attached to points on the plasma membrane called dense bodies, which are scattered all over the surface of the cell. This 3-dimensional pattern causes the cell to collapse upon itself when contracted. Like cardiac muscle, smooth muscle is involuntary and may be joined together with gap junctions to form belts or bands of smooth muscle tissue. These are found in surrounding hollow organs, such as the digestive tract, urinary tract, and blood vessels, and assist in the movement of materials such as urine. For example, the pyloric sphincter is a belt of muscle between the stomach and small intestine that regulates when and if material passes from the stomach to the intestines. Contraction of this smooth muscle belt causes a narrowing and even closing of the passageway and a relaxation leads to an open passage. NERVOUS TISSUE That Takes a Lot of Nerve [image: ] NERVOUS TISSUE Nervous tissue sends electrical signals from one place to another in the body. These signals can either bring information to the central nervous system from the body for processing or send information out to the body. You would trigger nervous tissue reaction if you touched a hot stove, for example. Your neurons would tell your brain that you are touching something hot (bringing information to the central nervous system). Your brain would tell your body to move your hand off the stove (sending information out to the body). Neurons Neurons are the signaling cells of the nervous system and come in a myriad of shapes and sizes. Generally, neurons have structures called neurites extending from the cell body, which make the cell look somewhat spiky and create the characteristic appearance of a neuron. What these neurites are named is based on the direction the signals travel. For instance, if the electrical signal travels toward the cell body, the neurite is called a dendrite. If the signal moves away from the cell body, it is an axon. Typically neurons will have many smaller-diameter dendrites and one longer, thicker axon. Neuroglia While neurons are the signaling cells of the nervous system, they only make up 20 percent of the nervous system. The bulk is made up of the supporting cells of the nervous system that are collectively referred to as neuroglia. Why are neuroglia sometimes called “nerve glue”? The word “neuroglia” comes from two Greek words that together mean “nerve glue.” Neuroglial cells don’t actually glue anything to anything. When they were first discovered, scientists thought they served a binding purpose, thus their name. In much the same way as the movie stars on the screen are only a small fraction of the people involved in making a movie, the behind-the-scenes supporting members for the nervous system are the neuroglial cells (they are also called glial cells or glia). Glial cells are smaller than neurons and do not have neurites. They are not involved in actively sending or receiving signals. Rather, they support neurons by maintaining their surroundings and modulating the uptake of neurotransmitters. Glial cells also help neurons recover from injury and perform other supporting functions. Anatomy of a Word neurotransmitters Neurotransmitters are the chemicals that transmit signals between neurons. Neurons have small spaces between them, called synapses, and neurotransmitters help signals pass through synapses by converting the electrical impulse of the neuron into a chemical messenger. Myelinating Cells One group of neuroglia insulates the axons of neurons. These cells wrap their plasma membranes around the axon 40 or 50 times, much like an electrician wrapping electrical tape around a bare wire. The term myelin refers to the area of the membrane that is tightly wrapped around the axon (to myelinate something is to wrap around it). Myelin insulates the axon, and in doing so creates a much more rapid conduction velocity of the electrical signal. For the neurons in areas of the body such as in your arms and legs, these neurons will be myelinated by Schwann cells, named after Theodor Schwann, the physiologist who discovered them. They are specific types of glial cells that exist in the peripheral nervous system and are responsible for wrapping peripheral nerves. In the central nervous system, however, oligodendrocytes myelinate axons. Much like an octopus with many arms, these cells extend several cellular structures to axons and thus one oligodendrocyte can myelinate several different axons. Astrocytes Astrocytes, so-called because of their star shape, are another type of neuroglial cell. They protect the nervous system from infection by covering blood vessels with their structures and regulating the movement of materials out of the blood stream and into the nervous system. As part of the blood-brain barrier, they screen for pathogens and assist in the transportation of nutrients as well as the processing of waste. Microglia Microglia cells are the vacuum cleaners of the central nervous system. Known as phagocytic cells, they patrol the nervous system looking for pathogens and debris and removing them through a process called phagocytosis, or engulfing the foreign object into their cell bodies, effectively eliminating these detrimental materials before they can do any damage. Ependymal Cells The last type of neuroglial cell present in the central nervous system is the ependymal cell. These cells produce cerebrospinal fluid, which flows around the spinal cord and the brain and functions as a shock absorber against blows to the head and spine. Additionally, as cerebrospinal fluid is produced and as it is eliminated it creates a current that assists the blood-brain barrier; pathogens have to swim across this river of flowing cerebrospinal fluid before gaining access to the central nervous system. Together these various cells form the tissues and organs of the central and peripheral nervous systems in the human body. SKIN, HAIR, AND NAILS Beauty Is Only Skin-Deep but Skin Is Pretty Deep [image: ] SKIN ANATOMY Skin is the largest organ in the human body. Composed of several layers, it’s the external covering of the body and it serves to protect against infection and dehydration. Also present in skin is hair, which can detect touch and helps keep the body warm. To cool the body, glands in the skin produce sweat that cools the body as it evaporates from the surface. Epidermis While skin covers the entire body surface, all skin isn’t created equal. The distinction between the skin on your arm and the skin on the soles of your feet is significant and largely based on the thickness of the epidermal (top) part of the skin. Up to five individual layers of cells (strata) make up the top layer, or the epidermis, of the skin: stratum basale stratum spinosum stratum granulosum stratum lucidum stratum corneum Let’s look at each in turn. Starting at the bottom, adjacent to the basement membrane, is the stratum basale where cells divide and produce a continuous supply of new cells as the old ones are shed from the surface. The next layer as you move up is stratum spinosum, which gets its name because its cells have a spiny shape. Anatomy of a Word melanocyte A melanocyte is one of the pigmented cells that give skin cells their individual color. They occur throughout stratum spinosum layer of the epidermis. They cover over and protect the dividing cells at the bottom from damage from ultraviolet radiation, kind of like an umbrella shading you on a beach. As skin cells (keratinocytes) move higher and higher (through the process of replacing dead cells with living ones), they become cells of the stratum granulosum. These cells are filled with granules of keratohyalin, which help give skin its structure and give these cells a grainy appearance. This is the last layer of living cells. The stratum lucidum is next. It’s a thin translucent layer of the skin and is composed of dead skin cells. On top of this layer is the final layer, which is the most variable in its thickness. This stratum corneum is composed of multiple dead cell layers and is the true first-line defense barrier against infection for the body. Anatomy of a Word desquamation Desquamation is the process of shedding dead skin cells. Cells are continually shed from the epidermal surface and new cells are continually made. It takes a new skin cell about 14 days to move from the bottom layer to the surface of the skin. Thick skin is found on the palms of the hands and soles of the feet. Of the five previously mentioned layers, the stratum corneum is by far the thickest layer. It provides great protection against the friction that occurs when walking or grasping objects; however, unlike the rest of the skin that covers your body, you will not find hair follicles or sebaceous glands in thick skin. Furthermore, you will find fewer sweat glands in thick skin than in thin skin. While your feet and palms may get moist, the volume of sweat produced is much lower than what is produced in other areas of the body. The majority of the body is covered with thin skin. In fact, the epidermis as a whole is thinner in this type of skin since it lacks two of the five layers that are found in thick skin (stratum granulosum and stratum lucidum are missing). Thin skin possesses many hair follicles and sebaceous glands that support the growing hair. Additionally, while both skin types contain sweat glands, thin skin has a much higher density of these glands to aid in cooling the body. Dermis The dermis is the layer beneath the epidermis that forms a transition zone between the underlying connective tissue and the epidermal layer above. It consists of dense irregular connective tissue made up largely of collagen fibers, elastic fibers, and fat tissue. Within the dermis are blood vessels that support the skin, as well as many nerve endings and receptors that detect pressure, pain, and temperature. Hair and Nails Human skin may also be modified into the structures known as hair and nails, which are composed of the same material that makes up the surface stratum corneum of the skin. The major difference is how compact these layers are and how they are arranged with the other dead cells in these layers. Hair Imagine taking the dead cells of the stratum corneum layer and rolling them into a tightly wound tube of dead cells: that is a hair. The hair follicle is simply a deep pit on the epidermis that projects downward into the deeper dermis of the skin. The other layers of the epidermis surround and support the shaft of the hair as it grows. At the deepest part of the follicle is the hair bulb. This is where the cells divide. This is also where they are nourished by blood vessels that enter into the follicle and support the growth of the hair. New layers are added to the hair and continually extrude the hair out of the follicle and onto the surface of the skin. Nails The surface of the skin is relatively soft, as is most of the hair on the body. However, the nails are drastically harder and consist of tightly packed layers of dead cells. What are commonly known as nails are in fact nail plates. This hardened plate of dead cells remains attached to the underlying epidermis via the nail bed, which tightly holds on to the plate to prevent the nail from falling off. At the base of the nail, you’ll find the cuticle (eponychium). The cuticle is a portion of the skin’s epidermis that overlaps the newly forming nail plate as it moves forward. Beneath the cuticle new nail plate material is continuously being produced and pushing the nail forward. At the end of the nail, the plate extends beyond the tip of the finger forming a crevasse, which is great at collecting dirt, and is technically called the hyponychium. This is the part of the nail that you are most familiar with and must trim on a regular basis to prevent the nails from growing too long. SKIN STRUCTURES AND FUNCTIONS You Positively Glow! Your skin uses a number of different structures to perform its functions—and it does more than just keep germs from getting in. Sweat and Sebaceous Glands The skin is an intact (contiguous) sheet of cells that protects the human body. Accessory structures enable the skin to cool the body and condition the skin itself. These important glands are the sweat glands and the sebaceous glands. Sweat Glands In ancient times, Egyptians hung wet linen cloth in doorways and windows to cool the inside of their living areas. As the water evaporated, it cut down on the heat in the air and effectively cooled the rooms. The human body uses this same approach to cool down. This is called sweating. When the surface of the skin is wetted, evaporation occurs, and as it does, heat dissipates and your body temperature is lowered. Why some sweat smells Eccrine sweat glands can be found over the majority of the body and produce the fluid that cools the human body known as sweat. Apocrine sweat glands can be found in greater density in the armpits and produce a different type of sweat that when metabolized by bacteria produces a distinctive and often unpleasant odor. The secretory portion of the sweat gland resides in the deeper parts of the dermis and produces the salty protein-and-lipid-rich fluid known as sweat. This is passed through long, coiled ducts upward and through the epidermis to be released onto the surface of the skin. Sebaceous Glands Sebaceous glands can be found in the dermal layer of the skin and are attached to the sides of hair follicles. Sebum, a waxy secretion produced by these glands, is injected into the hair follicle, coats the shaft of the hair, and is eliminated onto the surface of the skin. This material aids in the waterproofing and lubricating of the skin and hair in mammals. Wound Healing An intact layer of skin is critical in preventing infectious agents from gaining access to deeper regions of the body including the blood stream. Therefore, when an injury to the skin occurs, rapid and complete repair or closure of the wounded area must be accomplished. All layers of skin are involved in the repair process. Inflammatory Phase The body’s initial response to a wound is to minimize blood loss. Damaged blood vessels reflexively constrict to slow the flow of blood to the damaged area. At the same time, platelets, blood cells that perform a clotting function, activate and begin the process of forming a platelet plug to prevent the further loss of blood cells and to decrease the loss of blood plasma at the injury site. This plug forms the initial foundation of the full blood clot. At the same time, the body floods the injured site with body fluid and immune cells to flush the area of pathogens and bring in white blood cells to destroy any pathogens that remain. This inflammatory process is a nonspecific means by which the body fights off any potential infection. Proliferative Phase As the wound site becomes filled with the blood clot and infiltrating white blood cells, connective tissue cells, or fibroblasts, migrate into the area and begin to deposit a temporary scaffolding of connective tissue molecules to fill in the open gap in the skin. This material is the start of granulation tissue; it helps close the wound and provides a foundation for the normal tissue constituents to regrow and re-form the original state of the skin. Since the surface of your skin is epithelial and not connective tissue, those epithelial cells on the edges of the wound start to multiply and spread. They overgrow the granulation tissue. To provide nutrients to the newly growing tissue, blood vessels grow into the granulation tissue and form new circulation. Tissue reconstruction continues as the wound is repaired and as the body attempts to close it. The blood clot starts contracting and pulls the edges of the wound closer together, resulting in a smaller area to form new tissue. How do scars occur? Scars occur when there is an overproduction of connective tissue proteins such as collagen, which prevents the formation of normal epithelial tissue over the wound site. If the wound is too wide or the granulation tissue grows too extensively, then re-epithelialization cannot occur. This results in a scar. Maturation Phase In the final weeks of wound repair, the last portions of granulation tissue will be removed, all clotting material will be eliminated from the site, and the resident constituents of the skin will be properly formed in the exact proportions and in the correct area. Following this period, most repairs of small wounds will result in skin that shows little to no sign of any injury or defect at all. Temperature Regulation Most people know that the skin protects your body from disease and from dehydration. However, few are aware of the role the skin plays in temperature regulation of the body. Sweat As discussed earlier, when you’re hot, your body will sweat. As the liquid on the surface of your skin evaporates, heat is liberated from the skin and dispersed into the air, cooling the skin. In this way, the skin and the associated blood vessels that supply the skin can be thought of as a radiator or a heat exchanger between the human body and the environment. Conserving Heat The skin and its vascular supply also play a critical role in conserving body heat when the core body temperature begins to decrease, such as on very cold days. When exposed to the cold, the skin of the hands and face will initially flush and turn red, which indicates the dilation of the blood vessels near the skin surface in an attempt to bring more of the body’s core temperature to the skin and keep the tissue warm. This attempt to warm the body will only continue up to a point. When the body’s core temperature falls enough, the blood supply to the skin is restricted and the blood is redirected to deeper (core) areas of the body. This is the body’s wa