Week 1 Reading PDF

2 C H A P T E R The Chemical basis of life LEARN TO PREDICT Rafael is playing the soccer match of his life. However, hot weather conditions are making this match one of his hardest yet. As the game began, Rafael noticed himself sweating, which was helping to keep him cool. However, he had been ill t...

2 C H A P T E R The Chemical basis of life LEARN TO PREDICT Rafael is playing the soccer match of his life. However, hot weather conditions are making this match one of his hardest yet. As the game began, Rafael noticed himself sweating, which was helping to keep him cool. However, he had been ill the night before and had not been able to hydrate as was normal for him. Towards the end of the second half, he realized he was no longer sweating and he began to feel overheated and dizzy. After reading this chapter and learning how the properties of molecules contribute to their physiological functions, predict how Rafael’s inability to sweat affects his internal temperature. 2.1 BASIC CHEMISTRY learning Outcomes After reading this section, you should be able to A. Define chemistry and state its relevance to anatomy and physiology. B. Define matter, mass, and weight. C. Distinguish between an element and an atom. D. Define atomic number and mass number. E. Name the subatomic particles of an atom, and indicate their location. F. Compare and contrast ionic and covalent bonds. G. Explain what creates a hydrogen bond and relate its importance. H. Differentiate between a molecule and a compound. I. Describe the process of dissociation. Chemicals make up the body’s structures, and the interactions of chemicals with one another are responsible for the body’s functions. The processes of nerve impulse generation, digestion, muscle contraction, and metabolism can all be described in chemical terms. Many abnormal conditions and their treatments can also be explained in chemical terms, even though their symptoms appear as malfunctions in organ systems. For example, Parkinson disease, which causes uncontrolled shaking movements, results from a shortage of a chemical called dopamine in certain nerve cells in the brain. It can be treated by giving patients another chemical, which brain cells convert to dopamine. The sweat on Rafael's skin will evaporate, cooling his body. Module 2 Cells and Chemistry A basic knowledge of chemistry is essential for understanding anatomy and physiology. Chemistry is the scientific discipline concerned with the atomic composition and structure of substances and the reactions they undergo. This chapter outlines some basic chemical principles and emphasizes their relationship to humans. Matter, Mass, and Weight All living and nonliving things are composed of matter, which is anything that occupies space and has mass. Mass is the amount of matter in an object, and weight is the gravitational force acting on an object of a given mass. For example, the weight of an apple results from the force of gravity “pulling” on the apple’s mass. 21 Universal Free E-Book Store 22 Chapter 2 Predict 2 The difference between mass and weight can be illustrated by considering an astronaut. How would an astronaut’s mass and weight in outer space compare with that astronaut’s mass and weight on the earth’s surface? The international unit for mass is the kilogram (kg), which is the mass of a platinum-iridium cylinder kept at the International Bureau of Weights and Measurements in France. The mass of all other objects is compared with this cylinder. For example, a 2.2-lb lead weight and 1 liter (L) (1.06 qt) of water each have a mass of approximately 1 kg. An object with 1/1000 the mass of the standard kilogram cylinder is said to have a mass of 1 gram (g). some of which have an electrical charge. The three major types of subatomic particles are neutrons, protons, and electrons. Neutrons (noo′tronz) have no electrical charge, protons (prō′tonz) have positive charges, and electrons (e-lek′tronz) have negative charges. The positive charge of a proton is equal in magnitude to the negative charge of an electron. The number of protons and number of electrons in each atom are equal, and the individual charges cancel each other. Therefore, each atom is electrically neutral. Protons and neutrons form the nucleus at the center of the atom, and electrons move around the nucleus (figure 2.1). The nucleus accounts for 99.97% of an atom’s mass, but only 1-tentrillionth of its volume. Most of the volume of an atom is occupied Elements and Atoms An element is the simplest type of matter having unique chemical properties. A list of the elements commonly found in the human body appears in table 2.1. About 96% of the body’s weight results from the elements oxygen, carbon, hydrogen, and nitrogen. However, many other elements also play important roles in the human body. For example, calcium helps form bones, and sodium ions are essential for neuronal activity. Some of these elements are present in only trace amounts but are still essential for life. An atom (at′ ŏm; indivisible) is the smallest particle of an element that has the chemical characteristics of that element. An ­element is composed of atoms of only one kind. For example, the element carbon is composed of only carbon atoms, and the element oxygen is composed of only oxygen atoms. An element, or an atom of that element, is often represented by a symbol. Usually the symbol is the first letter or letters of the element’s name—for example, C for carbon, H for hydrogen, and Ca for calcium. Occasionally, the symbol is taken from the Latin, Greek, or Arabic name for the element; for example, the symbol for sodium is Na, from the Latin word natrium. The characteristics of matter result from the structure, organization, and behavior of atoms. Atoms are composed of subatomic particles, Element Nucleus Proton (positive charge) Neutron (no charge) Figure 2.1 Atomic Structure Table 2.1 Electron cloud Model of an Atom The tiny, dense nucleus consists of positively charged protons and uncharged neutrons. Most of the volume of an atom is occupied by rapidly moving, negatively charged electrons, which can be represented as an electron cloud. Common Elements in the Human Body Symbol Atomic Number Mass Number Percent in Human Body by Weight Percent in Human Body by Number of Atoms Hydrogen H 1 1 9.5 63.0 Carbon C 6 12 18.5 9.5 Nitrogen N 7 14 3.3 1.4 Oxygen O 8 16 65.0 25.5 Sodium Na 11 23 0.2 Phosphorus P 15 31 1.0 0.3 0.22 Sulfur S 16 32 0.3 0.05 Chlorine Cl 17 35 0.2 0.03 Potassium K 19 39 0.4 0.06 Calcium Ca 20 40 1.5 0.31 Iron Fe 26 56 Trace Trace Iodine I 53 127 Trace Trace Universal Free E-Book Store The Chemical Basis of Life 8e– 6e– 1e– 6p+ 6n0 1p+ Hydrogen atom 8p+ 8n0 Carbon atom 23 hand, have very little mass. The mass number of an element is the number of protons plus the number of neutrons in each atom. For example, the mass number for carbon is 12 because it has 6 protons and 6 neutrons. Predict 3 The atomic number of fluorine is 9, and the mass number is 19. What is the number of protons, neutrons, and electrons in an atom of fluorine? Oxygen atom Electrons and Chemical Bonding Figure 2.2 Hydrogen, Carbon, and Oxygen Atoms Within the nucleus of an atom are specific numbers of positively charged protons (p+) and uncharged neutrons (n0). The negatively charged electrons (e−) are around the nucleus. The atoms depicted here are electrically neutral because the number of protons and number of electrons within each atom are equal. by the electrons. Although it is impossible to know precisely where any given electron is located at any particular moment, the region where electrons are most likely to be found can be represented by an electron cloud (figure 2.1). Each element is uniquely defined by the number of protons in the atoms of that element. For example, only hydrogen atoms have one proton, only carbon atoms have six protons, and only oxygen atoms have eight protons (figure 2.2; see table 2.1). The number of protons in each atom is called the atomic number, and because the number of electrons and number of protons are equal, the atomic number is also the number of electrons. Protons and neutrons have about the same mass, and they are responsible for most of the mass of atoms. Electrons, on the other The chemical behavior of an atom is determined largely by its outermost electrons. Chemical bonding occurs when the outermost electrons are transferred or shared between atoms. Two major types of chemical bonding are ionic bonding and covalent bonding. Ionic Bonding An atom is electrically neutral because it has an equal number of protons and electrons. If an atom loses or gains electrons, the numbers of protons and electrons are no longer equal, and a charged particle called an ion (ı̄′ on) is formed. After an atom loses an electron, it has one more proton than it has electrons and is positively charged. For example, a sodium atom (Na) can lose an electron to become a positively charged sodium ion (Na+) (figure 2.3a). After an atom gains an electron, it has one more electron than it has protons and is negatively charged. For example, a chlorine atom (Cl) can accept an electron to become a negatively charged chloride ion (Cl−). Because oppositely charged ions are attracted to each other, positively charged ions tend to remain close to negatively charged ions. Thus, an ionic (ı̄-on′ ik) bond occurs when electrons are transferred between atoms, creating oppositely charged ions. For example, Na+ and Cl− are held together by ionic bonding to form an array of ions called sodium chloride (NaCl), or table salt (figure 2.3b,c). Sodium atom (Na) 11e– 11p+ Sodium ion (Na+ ) Lo ses e 12n0 lectron 10e– 11p+ 12n0 Sodium chloride (NaCl) e– Na+ Cl– 17p+ 18n0 17p+ 18n0 ron Gains elect 17e– (a) Chlorine atom (Cl) 18e– (b) Chloride ion (Cl– ) Figure 2.3 (c) Ionic Bonding (a) A sodium atom loses an electron to become a smaller, positively charged ion, and a chlorine atom gains an electron to become a larger, negatively charged ion. The attraction between the oppositely charged ions results in ionic bonding and the formation of sodium chloride. (b) The Na+ and Cl− are organized to form a cube-shaped array. (c) A photomicrograph of salt crystals reflects the cubic arrangement of the ions. Universal Free E-Book Store 24 Chapter 2 Ions are denoted by using the symbol of the atom from which the ion was formed and adding a plus (+) or minus (−) superscript to indicate the ion’s charge. For example, a sodium ion is Na+, and a chloride ion is Cl−. If more than one electron has been lost or gained, a number is used with the plus or minus sign. Thus, Ca2+ is a calcium ion formed by the loss of two electrons. Table 2.2 lists some ions commonly found in the body. e– e– p–+ e p+ p+ e– e– p–+ e p+ p+ The two hydrogen atoms do not interact because they are too far apart. The two hydrogen atoms do not interact because they are too far apart. The two hydrogen atoms do not interact because they are too far apart. Predict 4 If an iron (Fe) atom loses three electrons, what is the charge of the resulting ion? Write the symbol for this ion. Covalent Bonding A covalent bond forms when atoms share one or more pairs of electrons. The resulting combination of atoms is called a molecule (mol′ ĕ-kūl). An example is the covalent bond between two hydrogen atoms to form a hydrogen molecule (figure 2.4). Each hydrogen atom has one electron. As the atoms get closer together, the positively charged nucleus of each atom begins to attract the electron of the other atom. At an optimal distance, the two nuclei mutually attract the two electrons, and each electron is shared by both nuclei. The two hydrogen atoms are now held together by a covalent bond. The sharing of one pair of electrons by two atoms results in a single covalent bond. A single line between the symbols of the atoms involved (for example, H H) represents a single covalent bond. A double covalent bond results when two atoms share two pairs of electrons. When a carbon atom combines with two oxygen atoms to form carbon dioxide, two double covalent bonds are formed. Double covalent bonds are indicated by a double line between the atoms (O C O). Important Ions in the Human Body Table 2.2 Ion Symbol Significance* Calcium Ca2+ Part of bones and teeth; functions in blood clotting, muscle contraction, release of neurotransmitters Sodium Na+ Membrane potentials, water balance Potassium Hydrogen Hydroxide K + H Membrane potentials + OH Acid-base balance − − Acid-base balance Chloride Cl Bicarbonate HCO3− Acid-base balance Ammonium NH4+ Acid-base balance Water balance 3− Phosphate PO4 Part of bones and teeth; functions in energy exchange, acid-base balance Iron Fe2+ Red blood cell function Magnesium Mg2 Necessary for enzymes Iodide I − Present in thyroid hormones *The ions are part of the structures or play important roles in the processes listed. p+ p+ p+ e– e– e– e– e– + p e– + p p+ The positively charged nucleus of each hydrogen atom begins to attract the of charged the other. Theelectron positively nucleus of each hydrogen atom begins to attract the electron of charged the other. The positively nucleus of each hydrogen atom begins to attract the electron of the other. p+ p+ p+ e– e– e– p+ p+ p+ e– e– e– A covalent bond forms when the electrons are shared between the nuclei because electrons are electrons equally attracted to each nucleus. A covalent bondthe forms when the are shared between the nuclei because electrons are electrons equally attracted to each nucleus. A covalent bondthe forms when the are shared between the nuclei because the electrons are equally attracted to each nucleus. Figure 2.4 Covalent Bonding Electrons can be shared unequally in covalent bonds. When there is an unequal, asymmetrical sharing of electrons, the bond is called a polar covalent bond because the unequal sharing of electrons results in one end (pole) of the molecule having a partial electrical charge opposite to that of the other end. For example, two hydrogen atoms can share their electrons with an oxygen atom to form a water molecule (H2O), as shown in figure 2.5. However, the hydrogen atoms do not share the electrons equally with the oxygen atom, and the electrons tend to spend more time around the oxygen atoms than around the hydrogen atoms. Molecules with this asymmetrical electrical charge are called polar molecules. When there is an equal sharing of electrons between atoms, the bond is called a nonpolar covalent bond. Molecules with a symmetrical electrical charge are called nonpolar molecules. Hydrogen Bonds A polar molecule has a positive “end” and a negative “end.” The positive end of one polar molecule can be weakly attracted to the negative end of another polar molecule. Although this attraction is called a hydrogen bond, it is not a chemical bond because electrons are not transferred or shared between the atoms of the different polar molecules. The attraction between molecules resulting from hydrogen bonds is much weaker than in ionic or covalent bonds. For example, the positively charged hydrogen of one water molecule is weakly attracted to a negatively charged oxygen of another water molecule (figure 2.6). Thus, the water molecules are held together by hydrogen bonds. Universal Free E-Book Store 25 The Chemical Basis of Life H Water molecule δ+ Hydrogen bond O δ– H (a) H δ+ Hydrogen Oxygen O Figure 2.6 Hydrogen Bonds H (b) δ– The positive hydrogen part of one water molecule (δ+) forms a hydrogen bond (red dotted line) with the negative oxygen part of another water molecule (δ−). As a result, hydrogen bonds hold separate water molecules together. Figure 2.5 Polar Covalent Bonds A molecule is formed when two or more atoms chemically combine to form a structure that behaves as an independent unit. Sometimes the atoms that combine are of the same type, as when two hydrogen atoms combine to form a hydrogen molecule. But more typically, a molecule consists of two or more different types of atoms, such as two hydrogen atoms and an oxygen atom combining to form water. Thus, a glass of water consists of a collection of individual water molecules positioned next to one another. A compound (kom′ pownd; to place together) is a substance resulting from the chemical combination of two or more different types of atoms. Water is an example of a substance that is a Bond Example Ionic Bond Na+Cl– Sodium chloride Polar Covalent Bond H   Hydrogen Bond The attraction of oppositely charged ends of one polar molecule to another polar molecule holds molecules or parts of molecules together. O  H Water Nonpolar Covalent Bond H O An equal sharing of electrons between two atoms results in an even charge distribution among the atoms of the molecule. O An unequal sharing of electrons between two atoms results in a slightly positive charge (δ+) on one side of the molecule and a slightly negative charge (δ–) on the other side of the molecule. O A complete transfer of electrons between two atoms results in separate positively charged and negatively charged ions. HOCOH H Methane H H... O HO HOO O O Molecules and Compounds Comparison of Bonds O Hydrogen bonds also play an important role in determining the shape of complex molecules. The bonds can occur between different polar parts of a single large molecule to hold the molecule in its normal three-dimensional shape (see “Proteins” and “Nucleic Acids: DNA and RNA” later in this chapter). Table 2.3 summarizes the important characteristics of chemical bonds (ionic and covalent) and forces between separate molecules (hydrogen bonds). Table 2.3 O O O O (a) A water molecule forms when two hydrogen atoms form covalent bonds with an oxygen atom. (b) The hydrogen atoms and oxygen atoms are sharing electron pairs (indicated by the black dots), but the sharing is unequal. The dashed outline shows the expected location of the electron cloud if the electrons are shared equally. But as the yellow area indicates, the actual electron cloud (yellow) is shifted toward the oxygen. Consequently, the oxygen side of the molecule has a slight negative charge (indicated by δ−), and the hydrogen side of the molecule has a slight positive charge (indicated by δ+). H H H H Water molecules molecules Water compound and a molecule. Not all molecules are compounds. For example, a hydrogen molecule is not a compound because it does not consist of different types of atoms. Some compounds are molecules and some are not. (Remember that, to be a molecule, a structure must be an independent unit.) Covalent compounds, in which different types of atoms are held Universal Free E-Book Store 26 Chapter 2 δ+ δ– Na+ Salt δ+ Na+ Cl– Water molecules δ– δ+ δ+ Cl– Salt crystal Figure 2.7 Dissociation Sodium chloride (table salt) dissociates in water. The positively charged Na+ are attracted to the negatively charged oxygen (red) end of the water molecule, and the negatively charged Cl− are attracted to the positively charged hydrogen (blue) end of the water molecule. together by covalent bonds, are molecules because the sharing of electrons results in distinct units. On the other hand, ionic compounds, in which ions are held together by the force of attraction between opposite charges, are not molecules because they do not consist of distinct units. Sodium chloride is an example of a substance that is a compound but not a molecule. A piece of sodium chloride does not consist of individual sodium chloride molecules positioned next to one another. Instead, it is an organized array of individual Na+ and individual Cl− in which each charged ion is surrounded by several ions of the opposite charge (see figure 2.3b). Molecules and compounds can be represented by the symbols of the atoms forming the molecule or compound plus subscripts denoting the quantity of each type of atom present. For example, glucose (a sugar) can be represented as C6H12O6, indicating that glucose is composed of 6 carbon, 12 hydrogen, and 6 oxygen atoms. Letter symbols represent most atoms and molecules. Throughout this chapter and the text, oxygen (O2), carbon dioxide (CO2), and other commonly discussed ions and molecules will be identified by their symbols when appropriate. Dissociation When ionic compounds dissolve in water, their ions dissociate (di-sō′ sē-āt′ ), or separate, from each other because the positively charged ions are attracted to the negative ends of the water molecules, and the negatively charged ions are attracted to the positive ends of the water molecules. For example, when sodium chloride dissociates in water, the Na+ and Cl− separate, and water molecules surround and isolate the ions, keeping them in solution (figure 2.7). These dissociated ions are sometimes called electrolytes (ē-lek′ trō-lı̄tz) because they have the capacity to conduct an electrical current, the flow of charged particles. For example, an electrocardiogram (ECG) is a recording of electrical currents produced by the heart. These currents can be detected by electrodes on the surface of the body because the ions in the body fluids conduct electrical currents. When molecules dissolve in water, the molecules usually remain intact even though they are surrounded by water molecules. Thus, in a glucose solution, glucose molecules are surrounded by water molecules. 2.2 Chemical Reactions Learning Outcomes After reading this section, you should be able to A. Summarize the characteristics of synthesis, decomposition, and exchange reactions. B. Explain how reversible reactions produce chemical equilibrium. C. Distinguish between chemical reactions that release energy and those that take in energy. D. Describe the factors that can affect the rate of chemical reactions. In a chemical reaction, atoms, ions, molecules, or compounds interact either to form or to break chemical bonds. The substances that enter into a chemical reaction are called the reactants, and the substances that result from the chemical reaction are called the products. Universal Free E-Book Store The Chemical Basis of Life Classification of Chemical Reactions For our purposes, chemical reactions can be classified as synthesis, decomposition, or exchange reactions. An example of an exchange reaction is the reaction of hydrochloric acid (HCl) with sodium hydroxide (NaOH) to form table salt (NaCl) and water (H2O): HCl + NaOH → NaCl + H2O Synthesis Reactions When two or more reactants combine to form a larger, more complex product, the process is called a synthesis reaction, represented symbolically as A + B → AB Examples of synthesis reactions include the synthesis of the complex molecules of the human body from the basic “building blocks” obtained in food and the synthesis of adenosine triphosphate (ATP) (ă-den′ ō-sēn trı̄-fos′ fāt) molecules. In ATP, A stands for adenosine, T stands for tri- (or three), and P stands for a phosphate group (PO43−). Thus, ATP consists of adenosine and three phosphate groups. ATP is synthesized when adenosine diphosphate (ADP), which has two (di-) phosphate groups, combines with a phosphate group to form the larger ATP molecule. The phosphate group that reacts with ADP is often denoted as Pi, where the i indicates that the phosphate group is associated with an inorganic substance (see “Inorganic Molecules ” later in this chapter). A-P-P    +    Pi    →    A-P-P-P (ADP) (Phosphate (ATP) group) All of the synthesis reactions that occur in the body are collectively referred to as anabolism (ă-nab′ -ō-lizm). Growth, maintenance, and repair of the body could not take place without anabolic reactions. Decomposition Reactions In a decomposition reaction, reactants are broken down into smaller, less complex products. A decomposition reaction is the reverse of a synthesis reaction and can be represented in this way: AB → A + B Examples of decomposition reactions include the breakdown of food molecules into basic building blocks and the breakdown of ATP to ADP and a phosphate group. A-P-P-P    →    A-P-P  +    Pi (ATP) (ADP) (Phosphate group) The decomposition reactions that occur in the body are collectively called catabolism (kă-tab′-ō-lizm). They include the digestion of food molecules in the intestine and within cells, the breakdown of fat stores, and the breakdown of foreign matter and microorganisms in certain blood cells that protect the body. All of the anabolic and catabolic reactions in the body are collectively defined as metabolism. Exchange Reactions An exchange reaction is a combination of a decomposition reaction and a synthesis reaction. In decomposition, the reactants are broken down. In synthesis, the products of the decomposition reaction are combined. The symbolic representation of an exchange reaction is AB + CD → AC + BD 27 Reversible Reactions A reversible reaction is a chemical reaction that can proceed from reactants to products and from products to reactants. When the rate of product formation is equal to the rate of reactant formation, the reaction is said to be at equilibrium. At equilibrium, the amount of the reactants relative to the amount of products remains constant. The following analogy may help clarify the concept of reversible reactions and equilibrium. Imagine a trough containing water. The trough is divided into two compartments by a partition, but the partition contains holes that allow the water to move freely between the compartments. Because the water can move in either direction, this is like a reversible reaction. The amount of water in the left compartment represents the amount of reactant, and the amount of water in the right compartment represents the amount of product. At equilibrium, the amount of reactant relative to the amount of product in each compartment is always the same because the partition allows water to pass between the two compartments until the level of water is the same in both compartments. If the amount of reactant is increased by adding water to the left compartment, water flows from the left compartment through the partition to the right compartment until the level of water is the same in both. Thus, the amounts of reactant and product are once again equal. Unlike this analogy, however, the amount of reactant relative to the amount of product in most reversible reactions is not one to one. Depending on the specific reversible reaction, there can be one part reactant to two parts product, two parts reactant to one part product, or many other possibilities. An important reversible reaction in the human body occurs when carbon dioxide (CO2) and water (H2O) form hydrogen ions (H+) and bicarbonate ions (HCO3−). The reversibility of the reaction is indicated by two arrows pointing in opposite directions: CO2 + H2O H+ + HCO3− If CO2 is added to H2O, the amount of CO2 relative to the amount of H+ increases. However, the reaction of CO2 with H2O produces more H+, and the amount of CO2 relative to the amount of H+ returns to equilibrium. Conversely, adding H+ results in the formation of more CO2, and the equilibrium is restored. Maintaining a constant level of H+ in body fluids is necessary for the nervous system to function properly. This level can be maintained, in part, by controlling blood CO2 levels. For example, slowing the respiration rate causes blood CO2 levels to increase, which causes an increase in H+ concentration in the blood. Predict 5 If the respiration rate increases, CO2 is removed from the blood. What effect does this have on blood H+ levels? Energy and Chemical Reactions Energy is defined as the capacity to do work—that is, to move matter. Energy can be subdivided into potential energy and kinetic energy. Potential energy is stored energy that could do work but Universal Free E-Book Store 28 Chapter 2 CLINICAL IMPACT Protons, neutrons, and electrons are responsible for the chemical properties of atoms. They also have other properties that can be useful in a clinical setting. For example, some of these properties have enabled the development of methods for examining the inside of the body. Isotopes (ı̄ ′sō-tōpz; isos, equal + topos, part) are two or more forms of the same element that have the same number of protons and electrons but a different number of neutrons. Thus, isotopes have the same atomic number (i.e., number of protons) but different mass numbers (i.e., sum of the protons and neutrons). For example, hydrogen and its isotope deuterium each have an atomic number of 1 because they both have 1 proton. However, hydrogen has no neutrons, whereas deuterium has 1 neutron. Therefore, the mass number of hydrogen is 1, and that of deuterium is 2. Water made with deuterium is called heavy water because of the weight of the “extra” neutron. Because isotopes of the same atom have the same number of electrons, they are very similar in their chemical behavior. The nuclei of some isotopes are stable and do not change. Radioactive isotopes, however, have unstable nuclei that lose neutrons or protons. Several different kinds of radiation can be produced when neutrons and protons, or the products formed by their breakdown, are released from the nucleus of the isotope. The radiation given off by some radioactive isotopes can penetrate and destroy tissues. Rapidly dividing cells are more sensitive to radiation than are slowly dividing cells. Radiation is used to treat cancerous (malignant) tumors because cancer cells divide rapidly. If the treatment is effective, few healthy cells are destroyed, but the cancerous cells are killed. Clinical Uses of Atomic Particles Radioactive isotopes are also used in medical diagnosis. The radiation can be detected, and the movement of the radioactive isotopes throughout the body can be traced. For example, the thyroid gland normally takes up iodine and uses it in the formation of thyroid hormones. Radioactive iodine can be used to determine if iodine uptake is normal in the thyroid gland. Radiation can be produced in ways other than by changing the nucleus of atoms. X-rays are a type of radiation formed when electrons lose energy by moving from a higher energy state to a lower one. Health professionals use x-rays to examine bones to determine if they are broken and x-rays of teeth to see if they have caries (cavities). Mammograms, which are low-energy radiographs (x-ray films) of the breast, can reveal tumors because the tumors are slightly denser than normal tissue. Computers can be used to analyze a series of radiographs, each made at a slightly different body location. The computer assembles these radiographic “slices” through the body to form a three-dimensional image. A computed tomography (tō-mog′ră-fē) (CT) scan is an example of this technique (figure 2A). CT scans are used to detect tumors and other abnormalities in the body. Magnetic resonance imaging (MRI) is another method for looking into the body (figure 2B). The patient is placed into a very powerful magnetic field, which aligns the hydrogen nuclei. Radio waves given off by the hydrogen nuclei are monitored, and a computer uses these data to make an image of the body. Because MRI detects hydrogen, it is very effective for visualizing soft tissues that contain a lot of water. MRI technology can reveal tumors and other abnormalities. Figure 2A Figure 2B CT scan of the brain with iodine injection, showing one large brain tumor (green area) that has metastasized (spread) to the brain from cancer in the large intestine. Colorized MRI brain scan showing a stroke. The whitish area in the lower right part of the MRI is blood that has leaked into the surrounding tissue. is not doing so. For example, a coiled spring has potential energy. It could push against an object and move the object, but as long as the spring does not uncoil, no work is accomplished. Kinetic (ki-net′ ik; of motion) energy is energy caused by the movement of an object and is the form of energy that actually does work. An uncoiling spring pushing an object and causing it to move is an example. When potential energy is released, it becomes kinetic energy, thus doing work. Potential and kinetic energy exist in many different forms: chemical energy, mechanical energy, heat energy, electrical energy, and electromagnetic (radiant) energy. Here we examine how chemical energy and mechanical energy play important roles in the body. Universal Free E-Book Store The Chemical Basis of Life The chemical energy of a substance is a form of potential energy stored in chemical bonds. Consider two balls attached by a relaxed spring. In order to push the balls together and compress the spring, energy must be put into this system. As the spring is compressed, potential energy increases. When the compressed spring expands, potential energy decreases. In the same way, similarly charged particles, such as two negatively charged electrons or two positively charged nuclei, repel each other. As similarly charged particles move closer together, their potential energy increases, much like compressing a spring, and as they move farther apart, their potential energy decreases. Chemical bonding is a form of potential energy because of the charges and positions of the subatomic particles bound together. Chemical reactions are important because of the products they form and the energy changes that result as the relative positions of subatomic particles change. If the products of a chemical reaction contain less potential energy than the reactants, energy is released. For example, food molecules contain more potential energy than waste products. The difference in potential energy between food and waste products is used by the human body to drive activities such as growth, repair, movement, and heat ­production. An example of a reaction that releases energy is the breakdown of ATP to ADP and a phosphate group (figure 2.8a). The phosphate group is attached to the ADP molecule by a covalent bond, which has potential energy. After the breakdown of ATP, some of that energy is released as heat, and some is available to cells for activities such as synthesizing new molecules or contracting muscles: ATP → ADP + Pi + Energy (used by cells) REACTANT ADP More potential energy Less potential energy + Pi + Energy (a) REACTANTS PRODUCT ADP ATP + Pi + Energy According to the law of conservation of energy, the total energy of the universe is constant. Therefore, energy is neither created nor destroyed. However, one type of energy can be changed into another. Potential energy is converted into kinetic energy. Since the conversion between energy states is not 100% efficient, heat energy is released. For example, as a spring is released, its potential energy is converted to mechanical energy and heat energy. Mechanical energy is energy resulting from the position or movement of objects. Many of the activities of the human body, such as moving a limb, breathing, or circulating blood, involve mechanical energy. Predict 6 Why does body temperature increase during exercise? If the products of a chemical reaction contain more energy than the reactants (figure 2.8b), energy must be added from another source. The energy released during the breakdown of food molecules is the source of energy for this kind of reaction in the body. The energy from food molecules is used to synthesize molecules such as ATP, fats, and proteins: ADP + Pi + Energy (from food molecules) → ATP Rate of Chemical Reactions The rate at which a chemical reaction proceeds is influenced by several factors, including how easily the substances react with one another, their concentrations, the temperature, and the presence of a catalyst. Reactants Reactants differ from one another in their ability to undergo chemical reactions. For example, iron corrodes much more rapidly than does stainless steel. For this reason, during the refurbishment of the Statue of Liberty in the 1980s, the iron bars forming the statue’s skeleton were replaced with stainless steel bars. PRODUCTS ATP 29 Concentration Within limits, the greater the concentration of the reactants, the greater the rate at which a chemical reaction will occur because, as the concentration increases, the reacting molecules are more likely to come in contact with one another. For example, the normal concentration of oxygen inside cells enables it to come in contact with other molecules, producing the chemical reactions necessary for life. If the oxygen concentration decreases, the rate of chemical reactions decreases. A decrease in oxygen in cells can impair cell function and even result in cell death. Temperature Less potential energy More potential energy (b) Figure 2.8 Energy and Chemical Reactions In the two reactions shown here, the larger “sunburst” represents greater potential energy and the smaller “sunburst” represents less potential energy. (a) Energy is released as a result of the breakdown of ATP. (b) The input of energy is required for the synthesis of ATP. Because molecular motion changes as environmental temperature changes, the rate of chemical reactions is partially dependent on temperature. For example, reactions occur throughout the body at a faster rate when a person has a fever of only a few degrees. The result is increased activity in most organ systems, such as increased heart and respiratory rates. By contrast, the rate of reactions decreases when body temperature drops. The clumsy movement of very cold fingers results largely from the reduced rate of chemical reactions in cold muscle tissue. Universal Free E-Book Store Chapter 2 At normal body temperatures, most chemical reactions would take place too slowly to sustain life if not for substances called catalysts. A catalyst (kat′ ă-list) increases the rate of a chemical reaction, without itself being permanently changed or depleted. An enzyme is a protein molecule that acts as a catalyst. Many of the chemical reactions that occur in the body require enzymes. We consider them in greater detail later in this chapter, in the section titled “Enzymes.” 2.3 Acids and Bases Learning Outcomes Concentration in moles/liter [OH – ] [H +] — 10 0 — 0 Hydrochloric acid (HCl) 10 –13 — — 10 –1 — 1 Stomach acid 10 –12 — — 10 –2 — 2 Lemon juice — 10 –3 — 3 Vinegar, cola, beer — 10 –4 — 4 Tomatoes — 10 –5 — 5 Black coffee — 10 –6 — 6 Urine 10 –11 — 10 –10 — After reading this section, you should be able to A. Describe the pH scale and its relationship to acidic and basic solutions. B. Explain the importance of buffers in organisms. The body has many molecules and compounds, called acids and bases, that can alter body functions. An acid is a proton donor. Because a hydrogen atom without its electron is a proton, any substance that releases hydrogen ions (H+) in water is an acid. For example, hydrochloric acid (HCl) in the stomach forms H+ and chloride ions (Cl−): HCl → H+ + Cl− A base is a proton acceptor. For example, sodium hydroxide (NaOH) forms sodium ions (Na+) and hydroxide ions (OH−). It is a base because the OH− is a proton acceptor that binds with a H+ to form water. NaOH → Na+ + OH− H 2O pH Examples 10 –14 — 10 –9 — Increasing acidity Catalysts 10 –8 — Saliva (6.5) — 7 Distilled water 10 –7 — Neutral — 10 –7 10 –6 — — 10 –8 — 8 Seawater 10 –5 — — 10 –9 — 9 Baking soda — 10 –10 — 10 Great Salt Lake — 10 –11 — 11 Household ammonia — 10 –12 — 12 Soda ash — 10 –13 — 13 Oven cleaner — 10 –14 — 14 Sodium hydroxide (NaOH) 10 –4 — 10 –3 — 10 –2 — 10 –1 — 10 0 — Increasing alkalinity (basicity) 30 Blood (7.4) H+ The pH Scale The pH scale indicates the H+ concentration of a solution (figure 2.9). The scale ranges from 0 to 14. A neutral solution has an equal number of H+ and OH− and thus a pH of 7.0. An acidic solution has a greater concentration of H+ than of OH− and thus a pH less than 7.0. A basic, or alkaline (al′ kă-lı̄n), solution has fewer H+ than OH− and thus a pH greater than 7.0. Notice that the pH number and the actual H+ concentration are inversely related, meaning that the lower the pH number, the higher the H+ concentration. As the pH value becomes smaller, the solution becomes more acidic; as the pH value becomes larger, the solution becomes more basic. A change of one unit on the pH scale represents a 10-fold change in the H+ concentration. For example, a solution with a pH of 6.0 has 10 times more H+ than a solution with a pH of 7.0. Thus, small changes in pH represent large changes in H+ concentration. The normal pH range for human blood is 7.35 to 7.45. If blood pH drops below 7.35, a condition called acidosis (as-i-dō′ sis) results. The nervous system is depressed, and the individual becomes disoriented and possibly comatose. If blood pH rises above 7.45, alkalosis (al-kă-lō′ sis) results. The nervous system becomes overexcitable, and the individual can be extremely nervous or have convulsions. Both acidosis and alkalosis can result in death. Figure 2.9 pH Scale A pH of 7 is neutral. Values less than 7 are acidic (the lower the number, the more acidic). Values greater than 7 are basic (the higher the number, the more basic). This scale shows some representative fluids and their approximate pH values. Salts A salt is a compound consisting of a positive ion other than H+ and a negative ion other than OH−. Salts are formed by the reaction of an acid and a base. For example, hydrochloric acid (HCl) combines with sodium hydroxide (NaOH) to form the salt sodium chloride (NaCl): HCl    +    NaOH  →  NaCl    +    H2O (Acid) (Base) (Salt) (Water) Buffers The chemical behavior of many molecules changes as the pH of the solution in which they are dissolved changes. The survival of an organism depends on its ability to maintain homeostasis by keeping body fluid pH within a narrow range. One way normal body fluid pH is maintained is through the use of buffers. A buffer (bŭf′ er) is a chemical that resists changes in pH when either an Universal Free E-Book Store The Chemical Basis of Life Acidic solution Increased H+ Decreased pH H+ H+ Acidic solution Oxygen and Carbon Dioxide Buffer removes H+ Resists change in pH H+ B H+ B H+ H+ (a) H+ (b) 31 B H+ B H+ H+ B B H+ Oxygen (O2) is a small, nonpolar, inorganic molecule consisting of two oxygen atoms bound together by a double covalent bond. About 21% of the gas in the atmosphere is O2, and it is essential for most living organisms. Humans require O2 in the final step of a series of chemical reactions in which energy is extracted from food molecules (see chapter 17). Carbon dioxide (CO2) consists of one carbon atom bound to two oxygen atoms. Each oxygen atom is bound to the carbon atom by a double covalent bond. Carbon dioxide is produced when food molecules, such as glucose, are metabolized within the cells of the body. Once CO2 is produced, it is eliminated from the cell as a metabolic by-product, transferred to the lungs by the blood, and exhaled during respiration. If CO2 is allowed to accumulate within cells, it becomes toxic. Figure 2.10 Buffers Water (a) The addition of an acid to a nonbuffered solution results in an increase of H+ and a decrease in pH. (b) The addition of an acid to a buffered solution results in a much smaller change in pH. The added H+ bind to the buffer (symbolized by the letter B). Water (H2O) is an inorganic molecule that consists of one atom of oxygen joined by polar covalent bonds to two atoms of hydrogen. Water has many important roles in humans and all living organisms: acid or a base is added to a solution containing the buffer. When an acid is added to a buffered solution, the buffer binds to the H+, preventing these ions from causing a decrease in the pH of the solution (figure 2.10). Predict 7 If a base is added to a solution, will the pH of the solution increase or decrease? If the solution is buffered, what response from the buffer prevents the change in pH? 2.4 Inorganic Molecules Learning Outcomes After reading this section, you should be able to A. Distinguish between inorganic and organic molecules. B. Describe how the properties of O2, CO2, and water contribute to their physiological functions. Early scientists believed that inorganic substances came from nonliving sources and that organic substances were extracted from living organisms. As the science of chemistry developed, however, it became apparent that the body also contains inorganic substances and that organic substances can be manufactured in the laboratory. As currently defined, inorganic chemistry deals with those substances that do not contain carbon, whereas organic chemistry is the study of carbon-containing substances. These definitions have a few exceptions. For example, CO2 and carbon monoxide (CO) are classified as inorganic molecules, even though they contain carbon. Inorganic substances play many vital roles in human anatomy and physiology. Examples include the O2 and other gases we breathe, the calcium phosphate that makes up our bones, and the metals that are required for protein functions, such as iron in hemoglobin and zinc in alcohol dehydrogenase. In the next sections, we discuss the important roles of O2, CO2, and water—all inorganic molecules—in the body. 1. Stabilizing body temperature. Because heat energy causes not only movement of water molecules, but also disruption of hydrogen bonds, water can absorb large amounts of heat and remain at a stable temperature. Blood, which is mostly water, is warmed deep in the body and then flows to the surface, where the heat is released. In addition, water evaporation in the form of sweat results in significant heat loss from the body. 2. Providing protection. Water is an effective lubricant. For example, tears protect the surface of the eye from the rubbing of the eyelids. Water also forms a fluid cushion around organs, which helps protect them from damage. The fluid that surrounds the brain is an example. 3. Facilitating chemical reactions. Most of the chemical reactions necessary for life do not take place unless the reacting molecules are dissolved in water. For example, NaCl must dissociate in water into Na+ and Cl− before those ions can react with other ions. Water also directly participates in many chemical reactions. For example, during the digestion of food, large molecules and water react to form smaller molecules. 4. Transporting substances. Many substances dissolve in water and can be moved from place to place as the water moves. For example, blood transports nutrients, gases, and waste products within the body. 2.5 Organic molecules Learning Outcomes After reading this section, you should be able to A. Describe the structural organization and major functions of carbohydrates, lipids, proteins, and nucleic acids. B. Explain how enzymes work. Carbon’s ability to form covalent bonds with other atoms makes possible the formation of the large, diverse, complicated molecules necessary for life. Carbon atoms bound together by covalent bonds Universal Free E-Book Store 32 Chapter 2 constitute the “framework” of many large molecules. Two mechanisms that allow the formation of a wide variety of molecules are variation in the length of the carbon chains and the combination of the atoms bound to the carbon framework. For example, proteins have thousands of carbon atoms bound by covalent bonds to one another and to other atoms, such as nitrogen, sulfur, hydrogen, and oxygen. The four major groups of organic molecules essential to living organisms are carbohydrates, lipids, proteins, and nucleic acids. Each of these groups has specific structural and functional characteristics (table 2.4). Carbohydrates Carbohydrates are composed of carbon, hydrogen, and oxygen atoms. In most carbohydrates, for each carbon atom there are two hydrogen atoms and one oxygen atom. Note that this two-to-one ratio is the same as in water (H2O). The molecules are called carbohydrates because each carbon (carbo-) is combined with the same atoms that form water (hydrated). For example, the chemical formula for glucose is C6H12O6. The smallest carbohydrates are monosaccharides (mon-ōsak′ ă-rı̄dz; one sugar), or simple sugars. Glucose (blood sugar) and fructose (fruit sugar) are important monosaccharide energy sources for many of the body’s cells. Larger carbohydrates are formed by chemically binding monosaccharides together. For this reason, monosaccharides are considered the building blocks Table 2.4 of ­carbohydrates. Disaccharides (d ı̄-sak′ ă-r ı̄dz; two sugars) are formed when two monosaccharides are joined by a covalent bond. For example, glucose and fructose combine to form the disaccharide sucrose (table sugar) (figure 2.11a). Polysaccharides (pol-ē-sak′ ă-r ı̄dz; many sugars) consist of many monosaccharides bound in long chains. Glycogen, or animal starch, is a polysaccharide of glucose (figure 2.11b). When cells containing glycogen need energy, the glycogen is broken down into individual glucose molecules, which can be used as energy sources. Plant starch, also a polysaccharide of glucose, can be ingested and broken down into glucose. Cellulose, another polysaccharide of glucose, is an important structural component of plant cell walls. Humans cannot digest cellulose, however, and it is eliminated in the feces, where the ­cellulose fibers provide bulk. Lipids Lipids are substances that dissolve in nonpolar solvents, such as alcohol or acetone, but not in polar solvents, such as water. Lipids are composed mainly of carbon, hydrogen, and oxygen, but other elements, such as phosphorus and nitrogen, are minor components of some lipids. Lipids contain a lower proportion of oxygen to carbon than do carbohydrates. Fats, phospholipids, eicosanoids, and steroids are examples of lipids. Fats are important energy-storage molecules; they also pad and insulate the body. The building blocks of fats are glycerol (glis′ er-ol) and fatty acids (figure 2.12). Glycerol is a 3-carbon Important Organic Molecules and Their Functions in the Body Molecule Elements Building Blocks Function Examples Carbohydrate C, H, O Monosaccharides Energy Monosaccharides can be used as energy sources. Glycogen (a polysaccharide) is an energy-storage molecule. Lipid C, H, O (P, N in some) Glycerol and fatty    acids (for fats) Energy Fats can be stored and broken down later for energy; per unit of weight, fats yield twice as much energy as carbohydrates. Structure Phospholipids and cholesterol are important components of cell membranes. Regulation Steroid hormones regulate many physiological processes (e.g., estrogen and testosterone are responsible for many of the differences between males and females). Regulation Enzymes control the rate of chemical reactions. Hormones regulate many physiological processes (e.g., insulin affects glucose transport into cells). Structure Collagen fibers form a structural framework in many parts of the body. Energy Proteins can be broken down for energy; per unit of weight, they yield the same energy as carbohydrates. Contraction Actin and myosin in muscle are responsible for muscle contraction. Protein Nucleic acid C, H, O, N (S in most) C, H, O, N, P Amino acids Nucleotides Transport Hemoglobin transports O2 in the blood. Protection Antibodies and complement protect against microorganisms and other foreign substances. Regulation DNA directs the activities of the cell. Heredity Genes are pieces of DNA that can be passed from one generation to the next. Gene expression RNA is involved in gene expression. Universal Free E-Book Store The Chemical Basis of Life CH2OH O O + OH HO OH HO HO CH2OH H2O OH OH Glucose (a) CH2OH O CH2OH HO 33 CH2OH O OH HO O OH OH Fructose CH2OH Sucrose O Glycogen granules CH2OH O OH OH Branch O OH CH2OH O OH Nucleus TEM 32000x (c) O CH2OH O O OH O OH O OH (b) CH2OH O CH2 O OH CH2OH O O OH OH OH CH2OH O O OH OH O OH Glycogen main chain Figure 2.11 Carbohydrates (a) Glucose and fructose are monosaccharides that combine to form the disaccharide sucrose. (b) Glycogen is a polysaccharide formed by combining many glucose molecules. (c) Transmission electron micrograph of stored glycogen in a human cell. Glycogen clusters together into particles called granules. molecule with a hydroxyl (hı̄-drok′ sil) group (— OH) attached to each carbon atom, and fatty acids consist of a carbon chain with a carboxyl (kar-bok′ sil) group attached at one end. A carboxyl group consists of both an oxygen atom and a hydroxyl group attached to a carbon atom (—COOH): O O — C— OH    or    HO — C — The carboxyl group is responsible for the acidic nature of the molecule because it releases H+ into solution. Tri­glycerides (tr ı̄-glis′ er-ı̄dz) are the most common type of fat molecules. Triglycerides have three fatty acids bound to a glycerol molecule. Fatty acids differ from one another according to the length and degree of saturation of their carbon chains. Most naturally occurring fatty acids contain 14 to 18 carbon atoms. A fatty acid is saturated if it contains only single covalent bonds between the carbon atoms (figure 2.13a). Sources of saturated fats include beef, pork, whole milk, cheese, butter, eggs, coconut oil, and palm oil. The carbon chain is unsaturated if it has one or more double covalent bonds (figure 2.13b). Because the double covalent bonds can occur anywhere along the carbon chain, many types of unsaturated fatty acids with an equal degree of unsaturation are possible. Monounsaturated fats, such as olive and peanut oils, have one double covalent bond between carbon atoms. Polyunsaturated fats, such as safflower, sunflower, corn, and fish oils, have two or more double covalent bonds between carbon atoms. Unsaturated fats are the best type of fats in the diet because, unlike saturated fats, they do not contribute to the development of cardiovascular disease. Trans fats are unsaturated fats that have been chemically altered by the addition of H atoms. The process makes the fats more saturated and hence more solid and stable (longer shelf-life). However, the change in structure of these chemicals makes the consumption of trans fats an even greater factor than saturated fats in the risk for cardiovascular disease. Universal Free E-Book Store 34 Chapter 2 H O H–C–OH H H H H H H H–C–O HO – C – C – C – C – C – C – H O H–C–OH H H H H H H H H H H O H–C–OH H H H H H H H H H H H H H H H H H H H H H H H H H H C – C – C –C – C – C– H 3 H2O O H–C–O H H H H H H H H H H C – C – C –C – C – C– H H H H C – C – C –C – C – C– H H–C–O HO – C – C – C – C – C – C – H H H O Enzymes HO – C – C – C – C – C – C – H H O H H H H H Fatty acids Triglyceride molecule Glycerol Figure 2.12 Triglyceride One glycerol molecule and three fatty acids are combined to produce a triglyceride. H H — H — H — — H — H — H — H — H — H — H — H — H — H — H — — — O — — — — — — — H — — H — — H — — H — — HO— C — C — C — C — C — C — C — C — C — C — C — C — C — C — C — C —H H H H H H H H H H H H Palmitic acid (saturated) — — — — — — — — H H H H — — H H H Double bond (b) —C — C— C H Double bond H H —C —C — H — — H — — H — —C H — —C H H C— H — — — —C HO— C — C — C — C — C — C — C — C — C — H H — — H — H — H — H — H — H — H — H — H — O — — (a) H Double bond Linolenic acid (unsaturated) Figure 2.13 Fatty Acids (a) Palmitic acid is a saturated fatty acid; it contains no double bonds between the carbons. (b) Linolenic acid is an unsaturated fatty acid; note the three double bonds between the carbons, which cause the molecule to have a bent shape. Phospholipids are similar to triglycerides, except that one of the fatty acids bound to the glycerol is replaced by a molecule containing phosphorus (figure 2.14). A phospholipid is polar at the end of the molecule to which the phosphate is bound and nonpolar at the other end. The polar end of the molecule is attracted to water and is said to be hydrophilic (water-loving). The non­polar end is repelled by water and is said to be hydrophobic (water-fearing). Phospholipids are important structural components of cell membranes (see chapter 3). The eicosanoids ( ı̄′ kō-să-noydz) are a group of important chemicals derived from fatty acids. Eicosanoids are made in most cells and are important regulatory molecules. Among their numerous effects is their role in the response of tissues to injuries. One example of eicosanoids is prostaglandins (pros′ tă-glan′ dinz), Universal Free E-Book Store The Chemical Basis of Life which have been implicated in regulating the secretion of some hormones, blood clotting, some reproductive functions, and many other processes. Many of the therapeutic effects of aspirin and other anti-inflammatory drugs result from their ability to inhibit prostaglandin synthesis. Steroids are composed of carbon atoms bound together into four ringlike structures. Cholesterol is an important steroid because Nitrogen 35 other steroid molecules are synthesized from it. For example, bile salts, which increase fat absorption in the intestines, are derived from cholesterol, as are the reproductive hormones estrogen, progesterone, and testosterone (figure 2.15). In addition, cholesterol is an important component of cell membranes. Although high levels of cholesterol in the blood increase the risk of cardiovascular disease, a certain amount of cholesterol is vital for normal function. Polar (hydrophilic) region (phosphatecontaining region) Phosphorus Oxygen Carbon Hydrogen Nonpolar (hydrophobic) region (fatty acids) (a) (b) Figure 2.14 Phospholipids (a) Molecular model of a phospholipid. Note that one of the fatty acid tails is bent, indicating that it is unsaturated. (b) A simplified depiction of a phospholipid. CH3 CH CH3 CH3 CH2CH2CH2CH OH CH3 CH3 CH3 Cholesterol HO HO Estrogen (estradiol) CH3 OH CH CH3 O CH2CH2 C O NH CH2 C OH CH3 O– CH3 HO CH3 OH Bile salt (glycocholate) O Testosterone Figure 2.15 Steroids Steroids are four-ringed molecules that differ from one another according to the groups attached to the rings. Cholesterol, the most common steroid, can be modified to produce other steroids. Universal Free E-Book Store 36 Chapter 2 Proteins All proteins contain carbon, hydrogen, oxygen, and nitrogen, and most have some sulfur. The building blocks of proteins are amino (ă-mē′ nō) acids, which are organic acids containing an amine (ă-mēn′ ) group (–NH2) and a carboxyl group (figure 2.16a). There are 20 basic types of amino acids. Humans can synthesize 12 of them from simple organic molecules, but the remaining 8 so-called essential amino acids must be obtained in the diet. A protein consists of many amino acids joined together to form a chain (figure 2.16b,c). Although there are only 20 amino acids, they can combine to form numerous types of proteins with unique structures and functions. Different proteins have different kinds and numbers of amino acids. Hydrogen bonds between amino acids in the chain cause the chain to fold or coil into a specific three-dimensional shape (figure 2.16d,e). The ability of proteins to perform their functions depends on their shape. If the hydrogen bonds that maintain the shape of the protein are broken, the protein becomes nonfunctional. This change in shape is called denaturation, and it can be caused by abnormally high temperatures or changes in pH. Proteins perform many important functions. For example, enzymes are proteins that regulate the rate of chemical reactions, structural proteins provide the framework for many of the body’s tissues, and muscles contain proteins that are responsible for muscle contraction. Enzymes An enzyme (en′ zı̄m) is a protein catalyst that increases the rate at which a chemical reaction proceeds without the enzyme being permanently changed. Enzymes increase the rate of chemical ­reactions by lowering the activation energy, which is the energy necessary to start a chemical reaction. For example, heat in the form of a spark is required to start the reaction between O2 and gasoline. Most of the chemical reactions that occur in the body have high activation energies, which are decreased by enzymes (figure 2.17). The lowered activation energies enable reactions to proceed at rates that sustain life. Consider an analogy in which paper clips represent amino acids and your hands represent enzymes. Paper clips in a box only occasionally join together. Using your hands, however, you can rapidly make a chain of paper clips. In a similar fashion, enzymes can quickly join amino acids into a chain, forming a protein. An enzyme allows the rate of a chemical reaction to take place more than a million times faster than it would without the enzyme. The three-dimensional shape of enzymes is critical for their normal function. According to the lock-and-key model of enzyme action, the shape of an enzyme and those of the reactants allow the enzyme to bind easily to the reactants. Bringing the reactants very close to one another reduces the activation energy for the reaction. Because the enzyme and the reactants must fit together, enzymes are very specific for the reactions they control, and each enzyme controls only one type of chemical reaction. After the reaction takes place, the enzyme is released and can be used again (figure 2.18). The body’s chemical events are regulated primarily by mechanisms that control either the concentration or the activity of enzymes. Either (1) the rate at which enzymes are produced in cells or (2) whether the enzymes are in an active or inactive form determines the rate of each chemical reaction. Nucleic Acids: DNA and RNA Deoxyribonucleic (dē̄-oks′ ē-r ı̄′ bō-noo-klē′ ik) acid (DNA) is the genetic material of cells, and copies of DNA are transferred from one generation of cells to the next. DNA contains the information that determines the structure of proteins. Ribonucleic (r ı̄′ bō-nooklē′ ik) acid (RNA) is structurally related to DNA, and three types of RNA also play important roles in gene expression or protein synthesis. In chapter 3, we explore the means by which DNA and RNA direct the functions of the cell. The nucleic (noo-klē′ ik, noo-klā′ ik) acids are large mol­ ecules composed of carbon, hydrogen, oxygen, nitrogen, and phosphorus. Both DNA and RNA consist of basic building blocks called nucleotides (noo′ klē-ō-t ı̄dz). Each nucleotide is composed of a sugar (monosaccharide) to which a nitrogenous organic base and a phosphate group are attached (figure 2.19). The sugar is deoxyribose for DNA, ribose for RNA. The organic bases are thymine (thı̄′ mēn, thı̄′ min), cytosine (sı̄′ tō-sēn), and uracil (ūr′ ă-sil), which are single-ringed molecules, and adenine (ad′ ĕ-nēn) and guanine (gwahn′ ēn), which are double-ringed molecules. DNA has two strands of nucleotides joined together to form a twisted, ladderlike structure called a double helix. The sides of the ladder are formed by covalent bonds between the sugar molecules and phosphate groups of adjacent nucleotides. The rungs of the ladder are formed by the bases of the nucleotides of one side connected to the bases of the other side by hydrogen bonds. Each nucleotide of DNA contains one of the organic bases: adenine, thymine, cytosine, or guanine. Adenine binds only to thymine because the structure of these organic bases allows two hydrogen bonds to form between them. Cytosine binds only to guanine because the structure of these organic bases allows three hydrogen bonds to form between them. The sequence of organic bases in DNA molecules stores genetic information. Each DNA molecule consists of millions of organic bases, and their sequence ultimately determines the type and sequence of amino acids found in protein molecules. Because enzymes are proteins, DNA structure determines the rate and type of chemical reactions that occur in cells by controlling enzyme structure. Therefore, the information contained in DNA ultimately defines all cellular activities. Other proteins, such as collagen, that are coded by DNA determine many of the structural features of humans. RNA has a structure similar to a single strand of DNA. Like DNA, four different nucleotides make up the RNA molecule, and the organic bases are the same, except that thymine is replaced with uracil. Uracil can bind only to adenine. Adenosine Triphosphate Adenosine triphosphate (ă-den′ ō-sēn tr ı̄-fos′ fāt) (ATP) is an especially important organic molecule found in all living organisms. It consists of adenosine (the sugar ribose with the organic base adenine) and three phosphate groups (figure 2.20). Adenosine diphosphate (ADP) has only two phosphate groups. The potential energy stored in the covalent bond between the second and third phosphate groups is important to living organisms because it ­provides the energy used in nearly all of the chemical reactions within cells. ATP is often called the energy currency of cells because it is capable of both storing and providing energy. The concentration of ATP is maintained within a narrow range of values, and essentially all energy-requiring chemical reactions stop when the quantity of ATP becomes inadequate. Universal Free E-Book Store 37 The Chemical Basis of Life (a) Two examples of amino acids. Each amino.acid has an amine group (—NH2) and a.carboxyl group (—COOH). Amino acid H (alanine) H CH3 N C C H O H OH H H N C C H O OH Amino acid (glycine) H2O (b) The individual amino acids are joined. H H H CH3 H H N C C N C C H O H O OH H N HO (c) A protein consists of a chain of different.amino acids (represented by different.colored spheres). O C C C H N C C O H C (d) A three-dimensional representation of the.amino acid chain shows the hydrogen bonds.(dotted red lines) between different amino.acids. The hydrogen bonds cause the amino.acid chain to become folded or coiled. N O H O N H C N O C H N H C O C H O C O H N C C N N H C O C C Coiled N C C N C O O H C N C HO H C C C O N N H O C C C Folded N N C O C HO H C C C H C C C H O N C C HO N C N C O H C N O C (e) An entire protein has a complex.three-dimensional shape. Figure 2.16 Protein Structure Universal Free E-Book Store 38 Chapter 2 Molecule A Free energy Progress of reaction without enzyme EA without enzyme Molecule B EA with enzyme is lower Enzyme Reactants Progress of reaction with enzyme Products Progress of the reaction New molecule AB Figure 2.17 Activation Energy and Enzymes Activation energy is required to initiate chemical reactions. Without an enzyme, a chemical reaction can proceed, but it needs more energy input. Enzymes lower the activation energy, making it easier for the reaction to proceed. Figure 2.18 Enzyme Action The enzyme brings two reacting molecules together. This is possible because the reacting molecules “fit” the shape of the enzyme (lock-and-key model). After the reaction, the unaltered enzyme can be used again. 1 The building blocks of nucleic acids are nucleotides, which consist of a phosphate group, a sugar, and a nitrogen base. 2 The phosphate groups connect the sugars to form two strands of nucleotides (purple columns). P Nitrogen base (thymine) O O CH3 C N O C H O N CH2 O H H O G N H H H C N N H C H H O C C N H CH2 N C O H C N C H C O N C C H H P H 3 H O N P O– H – C N C H H H O C A 2 N H C H H O H N O H T N N O H H H O A C C H O O C H H H CH2 C 1 H O Nucleotide H – 4 G Sugar (deoxyribose) O 4 The two nucleotide strands coil to form a double-stranded helix. T Phosphate group 2 O 3 Hydrogen bonds (dotted red lines) between the nucleotides join the two nucleotide strands together. Adenine binds to thymine, and cytosine binds to guanine. O CH2 Adenine (A) O Guanine (G) Thymine (T) O Cytosine (C) P O– O Figure 2.19 Structure of DNA Universal Free E-Book Store The Chemical Basis of Life NH2 N H C C C A CASE IN POINT N Adenine N C N C H CH2 O H O Ribose H H OH OH Cyanide Poisoning P O O– H O O O P O O– P O– O– Phosphate groups Adenosine Adenosine diphosphate (ADP) Adenosine triphosphate (ATP) Figure 2.20 Structure of ATP ATP releases energy when the bond between phosphate groups is broken. Notice that the bond between the oxygen atoms and phosphorus atoms of adjacent phosphate groups is drawn as a squiggly line. This symbolizes a less stable bond that is partly due to the negative charge on each phosphate and that, when broken, releases a great amount of energy. Consequently, this squiggly line is often referred to as a "high-energy" bond. ANSWER TO 39 Learn to Predict In this question, you learn that Rafael had not been able to properly hydrate the night before the soccer match and the weather is hot. The question also tells you that Rafael was cool when he was sweating and became overheated when he stopped sweating. Sweat consists mainly of water, and as you learned in section 2.1, water molecules are polar, allowing them to hydrogen Although Justin Hale was rescued from his burning home, he suffered from cyanide poisoning. Inhalation of smoke released by the burning of rubber and plastic during household fires is the most common cause of cyanide poisoning. Cyanide compounds can be lethal to humans because they interfere with the production of ATP in mitochondria (see chapter 3). Without adequate ATP, cells malfunction and can die. The heart and brain are especially susceptible to cyanide poisoning. Cyanide poisoning by inhalation or absorption through the skin can also occur in certain manufacturing industries, and cyanide gas was used during the Holocaust to kill people. Deliberate suicide by ingesting cyanide is rare but has been made famous by the “suicide capsules” in spy movies. In 1982, seven people in the Chicago area died after taking Tylenol that someone had laced with cyanide. Subsequent copycat tamperings occurred and led to the widespread use of tamper-proof capsules and packaging. bond together. Later, in section 2.4, you learned that hydrogen bonds stabilize water, allowing it to absorb large amounts of heat. Thus, sweating effectively creates a cooler layer of air next to your skin because the water molecules in sweat can absorb heat. Because Rafael was dehydrated, he could not continue sweating, became overheated, and suffered a mild heat stroke. Answers to the rest of this chapter’s Predict questions are in Appendix E. SUMMARY Chemistry is the study of the composition and structure of substances and the reactions they undergo. 2.1 Basic Chemistry   (p. 21) Matter, Mass, and Weight 1. Matter is anything that occupies space and has mass. 2. Mass is the amount of matter in an object, and weight results from the gravitational attraction between the earth and an object. Elements and Atoms 1. An element is the simplest type of matter having unique chemical and physical properties. 2. An atom is the smallest particle of an element that has the chemical characteristics of that element. An element is composed of only one kind of atom. Atomic Structure 1. Atoms consist of neutrons, positively charged protons, and negatively charged electrons. 2. An atom is electrically neutral because the number of protons equals the number of electrons. 3. Protons and neutrons are in the nucleus, and electrons can be represented by an electron cloud around the nucleus. 4. The atomic number is the unique number of protons in each atom of an element. The mass number is the number of protons and neutrons. Electrons and Chemical Bonding 1. An ionic bond results when an electron is transferred from one atom to another. 2. A covalent bond results when a pair of electrons is shared between atoms. A polar covalent bond is an unequal sharing of electron pairs. Universal Free E-Book Store