《医学生理学 Medical Physiology》参考教材资料（PDF电子书籍）Mader Understanding Human Anatomy and Physiology 5th edition

Mader. Understanding T L Human Organization 2 Chemistry of Life T ② The McG ysiology, Fifth Edition 2.2 Water Acids and Bases Properties s of water Water is the most abundant molecule in living organisms, Polarity and hydrogen bonding cause water to have many usually making up about 60-70% of the total body weight. properties beneficial to life, including the three to be men- Even so, water is an inorganic molecule because it does not tioned here. contain carbon atoms. Carbon atoms are common to organic 1. Water is a solvent for polar(charged)molecules and thereby facilitates chemical reactions both outside and In water, the electrons spend more time circling the larger within our bodies oxygen(O)atom than the smaller hydrogen(H)atoms. This When ions and molecules disperse in water, they move oxygen and a slight positive charge (symbolized as &*)to the about and collide, allowing reactions to occur. Therefore, wa when a salt such as sodium chloride(Nacl) is put into water the negative ends of the water molecules are attracted to the sodium ions, and the positive ends of the water molecules are attracted to the chloride ions This causes the sodium ions and the chloride ions to separate and to dissolve in water: The diagram on the left shows the structural formula of water, and the one on the right is called a space-filling model Hydrogen Bonds The salt NaCl dissolves in water a hydrogen bond occurs whenever a covalently bonded hy drogen is positive and attracted to a negatively charged atom Ions and molecules that interact with water are said to be earby. A hydrogen bond is represented by a dotted line be- hydrophilic Nonionized and nonpolar molecules that do cause it is relatively weak and can be broken rather easily. In not interact with water are said to be hydrophobic. Figure 2.6, you can see that each hydrogen atom, being slightly positive, bonds to the slightly negative oxygen atom 2. Water molecules are cohesive, and therefore liquids fill vessels, such as blood vessels of another water molecule nearby Water molecules cling together because of hydrogen bonding, and yet water flows freely. This property allows dis- solved and suspended molecules to be evenly distributed Figure 2.6 Hydrogen bonding between water molecules.The throughout a system. Therefore, water is an excellent transport polarity of the water molecules causes hydrogen bonds( dotted medium. Within our bodies, the blood that fills our arteries lines) to form between the molecules. and veins is 92% water Blood transports oxygen and nutri- ents to the cells and removes wastes such as carbon dioxide 3. Water has a high heat of vaporization. Therefore, it absorbs much heat as it slowly rises, and gives off this heat It takes a large amount of heat to change water to steam. Converting one gram of the hottest water to steam require hydrogen heat of vaporization because hydrogen bonds must beb h an input of 540 calories of heat energy Water has a hi bond ken before boiling occurs and water molecules vaporia that is, evaporate into the environment. This property of wa- in a hot environment, we sweat; then the body cools as body heat is used to evaporate the sweat, which is mostly liquid 22 Part I Human Organization

Mader: Understanding Human Anatomy & Physiology, Fifth Edition I. Human Organization 2. Chemistry of Life © The McGraw−Hill Companies, 2004 2.2 Water, Acids, and Bases Water is the most abundant molecule in living organisms, usually making up about 60–70% of the total body weight. Even so, water is an inorganic molecule because it does not contain carbon atoms. Carbon atoms are common to organic molecules. In water, the electrons spend more time circling the larger oxygen (O) atom than the smaller hydrogen (H) atoms. This imparts a slight negative charge (symbolized as
) to the oxygen and a slight positive charge (symbolized as ) to the hydrogen atoms. Therefore, water is a polar molecule with negative and positive ends: The diagram on the left shows the structural formula of water, and the one on the right is called a space-filling model. Hydrogen Bonds A hydrogen bond occurs whenever a covalently bonded hy￾drogen is positive and attracted to a negatively charged atom nearby. A hydrogen bond is represented by a dotted line be￾cause it is relatively weak and can be broken rather easily. In Figure 2.6, you can see that each hydrogen atom, being slightly positive, bonds to the slightly negative oxygen atom of another water molecule nearby. Properties of Water Polarity and hydrogen bonding cause water to have many properties beneficial to life, including the three to be men￾tioned here. 1. Water is a solvent for polar (charged) molecules and thereby facilitates chemical reactions both outside and within our bodies. When ions and molecules disperse in water, they move about and collide, allowing reactions to occur. Therefore, wa￾ter is a solvent that facilitates chemical reactions. For example, when a salt such as sodium chloride (NaCl) is put into water, the negative ends of the water molecules are attracted to the sodium ions, and the positive ends of the water molecules are attracted to the chloride ions. This causes the sodium ions and the chloride ions to separate and to dissolve in water: Ions and molecules that interact with water are said to be hydrophilic. Nonionized and nonpolar molecules that do not interact with water are said to be hydrophobic. 2. Water molecules are cohesive, and therefore liquids fill vessels, such as blood vessels. Water molecules cling together because of hydrogen bonding, and yet water flows freely. This property allows dis￾solved and suspended molecules to be evenly distributed throughout a system. Therefore, water is an excellent transport medium. Within our bodies, the blood that fills our arteries and veins is 92% water. Blood transports oxygen and nutri￾ents to the cells and removes wastes such as carbon dioxide from the cells. 3. Water has a high heat of vaporization. Therefore, it absorbs much heat as it slowly rises, and gives off this heat as it slowly cools. It takes a large amount of heat to change water to steam. (Converting one gram of the hottest water to steam requires an input of 540 calories of heat energy.) Water has a high heat of vaporization because hydrogen bonds must be bro￾ken before boiling occurs and water molecules vaporize— that is, evaporate into the environment. This property of wa￾ter helps keep body temperature within normal limits. Also, in a hot environment, we sweat; then the body cools as body heat is used to evaporate the sweat, which is mostly liquid water. 22 Part I Human Organization H O H δ+ δ+ δ+ δ+ δ − δ − H H The salt NaCl dissolves in water. δ+ δ+ δ+ δ+ H H O O δ− δ− Cl− Na+ δ + δ + δ − hydrogen bond H H O Figure 2.6 Hydrogen bonding between water molecules. The polarity of the water molecules causes hydrogen bonds (dotted lines) to form between the molecules.

Mader. Understanding T L Human Organization 2 Chemistry of Life T ② The McG ysiology, Fifth Edition Acids and bases The ph of body fluids needs to be maintained within narrow range, or else health suffers. The pH of our blood when When water molecules dissociate(break up), they release an we are healthy is always about 7.4-that is, just slightly basic equal number of hydrogen ions(H")and hydroxide ions (alkaline). If the pH value drops below 7.35, the person is said to have acidosis if it rises above 7.45, the condition is called al kalosis. The pH stability is normally possible because the body has built-in mechanisms to prevent pH changes. Buffers are the H-。-H OH most important of these mechanisms. Buffers help keep the ph hydrogen within normal limits because they are chemicals or combina- tions of chemicals that take up excess hydro n Ions hydroxide ions(OH ). For example, the combination of car- Only a few water molecules at a time dissociate, and the ac- bonic acid(H2 3)and the bicarbonate ion [HCO3] helps tual number of H+ and oH is very small(1X10 keep the ph of the blood relatively constant because carbonic acid can dissociate to release hydrogen ions, while the bicar mo Acids are substances that dissociate in water, releasing bonate ion can take them up! hydrogen ions(H). For example, an important inorganic acid is hydrochloric acid(HCI), which dissociates in this Electrolytes As we have seen, salts, acids, and bases are molecules that dis. HCl→H++Cl sociate; that is, they ionize in water. For example, when a salt Dissociation is almost complete; therefore HCl is called a such as sodium chloride is put in water, the Naion separates strong acid. If hydrochloric acid is added to a beaker of water, from the cl ion Substances that release ions when put int to water are the number of hydrogen ions(H")increases greatly. Lemon called electrolytes, because the ions can conduct an electrical juice, vinegar, tomatoes, and coffee are all acidic solutions. Bases are substances that either take up hydroge It. The electrolyte balance in the blood and body tissues en Ion is important for good health because it affects the functi (H)or release hydroxide ions(OH"). For example, an im- of vital organs such as the heart and the brain portant inorganic base is sodium hydroxide(NaoH), which dissociates in this manner Figure 2.7 The pH scale. The proportionate amount of NaOH→Na++OH- ydrogen ions to hydroxide ions is indicated by the diagonal line Dissociation is almost complete; therefore, sodium hydroxide Any solution with a pH above 7 is basic, while any solution with a is called a strong base. If sodium hydroxide is added to a pH below 7 is acidic beaker of water, the number of hydroxide ions increases Milk hydrochloric acid(HCI) 0 of magnesia and ammonia are common basic solutions stomach acid 1 pH Scale lemon juice 2 The pH scale, which ranges from 0 to 14, is used to indicate Coca-Cola, beer, vinegar 3 the acidity and basicity(alkalinity )of a solution pH 7, which [H] is the ph of water, is neutral ph because water releases ar equal number of hydrogen ions(H*)and hydroxide ions black coffee 5 (OH ) Notice in Figure 2.7 that any pH above 7 is a base, ith more hydroxide ions than hydrogen ions. Any pH below neutral p 7 is an acid with more hydrogen ions than hydroxide ions. As pure water, tears 7 we move toward a higher pH, each unit has 10 times the ba- human blood sicity of the previous unit, and as we move toward a lower pH, ach unit has 10 times the acidity of the previous unit. This baking soda, stomach antacids 9 means that even a small change in pH represents a large ange in the proportional number of hydrogen and hydrox Great Salt Lake 10 milk of magnesia OHI de ions in the body. household ammonia 1 matter that contains as many objects (atoms, mole- cules, ions)as the number of atoms in exactly 12 grams ofC. pH is defined as the negative log of the hydrogen ion concentration H". A log is the power to which sodium hydroxide(NaOH) 14 Chapter 2 Chemistry of Life23

Mader: Understanding Human Anatomy & Physiology, Fifth Edition I. Human Organization 2. Chemistry of Life © The McGraw−Hill Companies, 2004 Acids and Bases When water molecules dissociate (break up), they release an equal number of hydrogen ions (H) and hydroxide ions (OH
): Only a few water molecules at a time dissociate, and the ac￾tual number of H and OH
is very small (1  107 moles/liter).1 Acids are substances that dissociate in water, releasing hydrogen ions (H
). For example, an important inorganic acid is hydrochloric acid (HCl), which dissociates in this manner: HCl —→ H

Cl Dissociation is almost complete; therefore, HCl is called a strong acid. If hydrochloric acid is added to a beaker of water, the number of hydrogen ions (H
) increases greatly. Lemon juice, vinegar, tomatoes, and coffee are all acidic solutions. Bases are substances that either take up hydrogen ions (H
) or release hydroxide ions (OH). For example, an im￾portant inorganic base is sodium hydroxide (NaOH), which dissociates in this manner: NaOH —→ Na

OH Dissociation is almost complete; therefore, sodium hydroxide is called a strong base. If sodium hydroxide is added to a beaker of water, the number of hydroxide ions increases. Milk of magnesia and ammonia are common basic solutions. pH Scale The pH scale2 , which ranges from 0 to 14, is used to indicate the acidity and basicity (alkalinity) of a solution. pH 7, which is the pH of water, is neutral pH because water releases an equal number of hydrogen ions (H
) and hydroxide ions (OH). Notice in Figure 2.7 that any pH above 7 is a base, with more hydroxide ions than hydrogen ions. Any pH below 7 is an acid, with more hydrogen ions than hydroxide ions. As we move toward a higher pH, each unit has 10 times the ba￾sicity of the previous unit, and as we move toward a lower pH, each unit has 10 times the acidity of the previous unit. This means that even a small change in pH represents a large change in the proportional number of hydrogen and hydrox￾ide ions in the body. The pH of body fluids needs to be maintained within a narrow range, or else health suffers. The pH of our blood when we are healthy is always about 7.4—that is, just slightly basic (alkaline). If the pH value drops below 7.35, the person is said to have acidosis; if it rises above 7.45, the condition is called al￾kalosis. The pH stability is normally possible because the body has built-in mechanisms to prevent pH changes. Buffers are the most important of these mechanisms. Buffers help keep the pH within normal limits because they are chemicals or combina￾tions of chemicals that take up excess hydrogen ions (H
) or hydroxide ions (OH
). For example, the combination of car￾bonic acid (H2CO3) and the bicarbonate ion [HCO3 - ] helps keep the pH of the blood relatively constant because carbonic acid can dissociate to release hydrogen ions, while the bicar￾bonate ion can take them up! Electrolytes As we have seen, salts, acids, and bases are molecules that dis￾sociate; that is, they ionize in water. For example, when a salt such as sodium chloride is put in water, the Na+ ion separates from the Cl ion. Substances that release ions when put into water are called electrolytes, because the ions can conduct an electrical current. The electrolyte balance in the blood and body tissues is important for good health because it affects the functioning of vital organs such as the heart and the brain. Chapter 2 Chemistry of Life 23 H+ + OH– hydrogen ion hydroxide ion water H O H 1 In chemistry, a mole is defined as the amount of matter that contains as many objects (atoms, mole￾cules, ions) as the number of atoms in exactly 12 grams of 12C. 2 pH is defined as the negative log of the hydrogen ion concentration [H
]. A log is the power to which 10 must be raised to produce a given number. [H+] [OH–] 0 1 2 3 4 5 hydrochloric acid (HCl) stomach acid lemon juice , vinegar tomatoes black coffee normal rainwater urine saliva pure water, tears human blood seawater baking soda, stomach antacids Great Salt Lake milk of magnesia household ammonia bicarbonate of soda oven cleaner sodium hydroxide (NaOH) neutral pH Coca-Cola, beer 6 7 8 9 10 12 13 14 11 A C I D B A S E Figure 2.7 The pH scale. The proportionate amount of hydrogen ions to hydroxide ions is indicated by the diagonal line. Any solution with a pH above 7 is basic, while any solution with a pH below 7 is acidic.

Mader. Understanding T L Human Organization 2 Chemistry of Life T ② The McG ysiology, Fifth Edition .3 Molecules of Life 2.4 Carbohydrates Four categories of molecules, called carbohydrates, lipids, Carbohydrates, like all organic molecules, always contain car- proteins, and nucleic acids, are unique to cells. They are called bon(C) and hydrogen(H)atoms. Carbohydrate molecules macromolecules because each is composed of many subur are characterized by the presence of the atomic grouping H C-OH, in which the ratio of hydrogen atoms(H)to oxygen Category Example Subunit(s) atoms(O) is approximately 2: 1. Because this ratio is the Polysaccharide same as the ratio in water, the name "hydrates of carbon seems appropriate. Carbohydrates first and foremost func- Lipids Glycerol and fatty acids tion for quick, short-term energy storage in all organisms, in- polypeptide Amino acid cluding humans. Figure 2.9 shows some foods that are rich in Nucleic acids Nucleotide carbohydrates. During synthesis of macromolecules, the cell uses a dehydra Simple Carbohydr tion reaction, so called because an -OH (hydroxyl group) If the number of carbon atoms in a carbohydrate is low(be and an -H(hydrogen atom)-the equivalent of a water tween three and seven), it is called a simple sugar, or monosac molecule-are removed as the molecule forms(Fig. 2.8a). charide. The designation pentose means a 5-carbon sugar, and The result is reminiscent of a train whose length is determined the designation hexose means a 6-carbon sugar Glucose, the by how boxcars are hitched together. To break up hexose our bodies use as an immediate source of energy, can be macromolecules, the cell uses a hydrolysis reaction, in which written in any one of these ways the components of water are added( Fig. 2.8b) igure 2.8 Synthesis and degradation of macromolecules. a In cells, synthesis often occurs when subunits bond following a dehydration reaction(removal of H2O). b Degradation occurs when the subunits in a macromolecule separate after a hydrolysis reaction(addition of H, o) H subunt H subunit C6H12O6 Figure 2.9 Common foods. Carbohydrates such as bread and pasta are digested to sugars; lipids such as oils are digested to glycerol and fatty acids; and proteins such as meat are digested to mino acids. Cells use these subunit molecules to build their own macromolecules }[m[咖m」 subunit subunit ous Potatoe }[[ M 24 Part I Human Organizatio

Mader: Understanding Human Anatomy & Physiology, Fifth Edition I. Human Organization 2. Chemistry of Life © The McGraw−Hill Companies, 2004 2.3 Molecules of Life Four categories of molecules, called carbohydrates, lipids, proteins, and nucleic acids, are unique to cells. They are called macromolecules because each is composed of many subunits: During synthesis of macromolecules, the cell uses a dehydra￾tion reaction, so called because an —OH (hydroxyl group) and an —H (hydrogen atom)—the equivalent of a water molecule—are removed as the molecule forms (Fig. 2.8a). The result is reminiscent of a train whose length is determined by how many boxcars are hitched together. To break up macromolecules, the cell uses a hydrolysis reaction, in which the components of water are added (Fig. 2.8b). 2.4 Carbohydrates Carbohydrates, like all organic molecules, always contain car￾bon (C) and hydrogen (H) atoms. Carbohydrate molecules are characterized by the presence of the atomic grouping H— C—OH, in which the ratio of hydrogen atoms (H) to oxygen atoms (O) is approximately 2:1. Because this ratio is the same as the ratio in water, the name “hydrates of carbon” seems appropriate. Carbohydrates first and foremost func￾tion for quick, short-term energy storage in all organisms, in￾cluding humans. Figure 2.9 shows some foods that are rich in carbohydrates. Simple Carbohydrates If the number of carbon atoms in a carbohydrate is low (be￾tween three and seven), it is called a simple sugar, or monosac￾charide. The designation pentose means a 5-carbon sugar, and the designation hexose means a 6-carbon sugar. Glucose, the hexose our bodies use as an immediate source of energy, can be written in any one of these ways: 24 Part I Human Organization Category Example Subunit(s) Carbohydrates Polysaccharide Monosaccharide Lipids Fat Glycerol and fatty acids Proteins Polypeptide Amino acid Nucleic acids DNA, RNA Nucleotide subunit subunit OH H subunit subunit H2O dehydration reaction subunit subunit OH H H2O subunit subunit hydrolysis reaction a. b. Figure 2.8 Synthesis and degradation of macromolecules. a. In cells, synthesis often occurs when subunits bond following a dehydration reaction (removal of H2O). b. Degradation occurs when the subunits in a macromolecule separate after a hydrolysis reaction (addition of H2O). Figure 2.9 Common foods. Carbohydrates such as bread and pasta are digested to sugars; lipids such as oils are digested to glycerol and fatty acids; and proteins such as meat are digested to amino acids. Cells use these subunit molecules to build their own macromolecules. H H OH HO OH H H CH2OH OH H C 1 H OH HO OH H H CH2OH C6H12O6 OH H O O O 3 4 5 6 2 C C

Mader. Understanding T L Human Organization 2 Chemistry of Life T ② The McG ysiology, Fifth Edition Other common hexoses are fructose, found in fruits, and galac- in starch are long chains of up to 4, 000 glucose units. Starch tose, a constituent of milk. A disaccharide(di, two; saccharide, has fewer side branches, or chains of glucose that branch off sugar) is made by joining only two monosaccharides together from the main chain, than does glycogen, as shown in Fi by a dehydration reaction(see Fig. 2. 8a). Maltose is a disaccha- ures 2.10 and 2. 11. Flour, usually acquired by grinding ride that contains two glucose molecules wheat and used for baking, is high in starch, and so are After we eat starchy foods such as potatoes, bread, and cake, glucose enters the bloodstream, and the liver stores glucose as glycogen. In between eating, the liver releases glucose so that the blood glucose concentration is always about 0. 1%. If blood contains more glucose, it spills over into the urine, signaling that the condition diabetes mell When glucose and fructose join, the disaccharide sucrose forms. Sucrose, which is ordinarily derived from sugarcan Cellulose and sugar beets, is commonly known as table sugar. The polysaccharide cellulose is found in plant cell walls. In Complex Carbohydrates(Polysaccharides) cellulose, the glucose units are joined by a slightly different type of linkage from that in starch or glycogen. Although Macromolecules such as starch, glycogen, and cellulose are this might seem to be a technicality, actually it is important polysaccharides that contain many glucose units. Although because humans are unable to digest foods containing this polysaccharides can contain other sugars, we will study the type of linkage; therefore, cellulose largely passes through ones that use glucose. our digestive tract as fiber, or roughage. It is believed that fiber in the diet is necessary to good health, and some re- Starch and Glycogen searchers have suggested it may even help prevent colon Starch and glycogen are ready storage forms of glucose in cancer plants and animals, respectively. Some of the macromolecule Figure 2.11 Glycogen structure and function. Glycogen is more branched than starch. The electron micrograph shows Figure 2.10 Starch structure and function. Starch has stra glycogen granules in liver cells. Glycogen is the storage form of chains of glucose molecules. Some chains are also branched, glucose in humans indicated. The electron micrograph shows starch granules in potato cells. Starch is the storage form of glucose in plants. ③。0○ granules Chapter 2 Chemistry of Life

Mader: Understanding Human Anatomy & Physiology, Fifth Edition I. Human Organization 2. Chemistry of Life © The McGraw−Hill Companies, 2004 Other common hexoses are fructose, found in fruits, and galac￾tose, a constituent of milk. A disaccharide (di, two; saccharide, sugar) is made by joining only two monosaccharides together by a dehydration reaction (see Fig. 2.8a). Maltose is a disaccha￾ride that contains two glucose molecules: When glucose and fructose join, the disaccharide sucrose forms. Sucrose, which is ordinarily derived from sugarcane and sugar beets, is commonly known as table sugar. Complex Carbohydrates (Polysaccharides) Macromolecules such as starch, glycogen, and cellulose are polysaccharides that contain many glucose units. Although polysaccharides can contain other sugars, we will study the ones that use glucose. Starch and Glycogen Starch and glycogen are ready storage forms of glucose in plants and animals, respectively. Some of the macromolecules in starch are long chains of up to 4,000 glucose units. Starch has fewer side branches, or chains of glucose that branch off from the main chain, than does glycogen, as shown in Fig￾ures 2.10 and 2.11. Flour, usually acquired by grinding wheat and used for baking, is high in starch, and so are potatoes. After we eat starchy foods such as potatoes, bread, and cake, glucose enters the bloodstream, and the liver stores glucose as glycogen. In between eating, the liver releases glucose so that the blood glucose concentration is always about 0.1%. If blood contains more glucose, it spills over into the urine, signaling that the condition diabetes melli￾tus exists. Cellulose The polysaccharide cellulose is found in plant cell walls. In cellulose, the glucose units are joined by a slightly different type of linkage from that in starch or glycogen. Although this might seem to be a technicality, actually it is important because humans are unable to digest foods containing this type of linkage; therefore, cellulose largely passes through our digestive tract as fiber, or roughage. It is believed that fiber in the diet is necessary to good health, and some re￾searchers have suggested it may even help prevent colon cancer. Chapter 2 Chemistry of Life 25 O O O O O O O O O CH2 O O O O O O O O O O starch granule cell wall potato cells Figure 2.10 Starch structure and function. Starch has straight chains of glucose molecules. Some chains are also branched, as indicated. The electron micrograph shows starch granules in potato cells. Starch is the storage form of glucose in plants. O O O O O O O O O O O O O O O O O O O O O CH2 O CH2 O CH2 O O O O O O O O CH2 O O liver cells glycogen granules Figure 2.11 Glycogen structure and function. Glycogen is more branched than starch. The electron micrograph shows glycogen granules in liver cells. Glycogen is the storage form of glucose in humans. maltose O O O

Mader. Understanding T L Human Organization 2 Chemistry of Life T ② The McG ysiology, Fifth Edition 2.5 Lipids nonpolar ends project. Now the droplet disperses in water, which means that emulsification has occurred Lipids contain more energy per gram than other biol molecules, and some function as long- term energy molecules in organisms. Others form a membrane that rates a cell from its environment and has inner compartments as well. Steroids are a large class of lipids that includes, among other molecules, the sex hormones Lipids are diverse in structure and function, but they have nonpolar a common characteristic: They do not dissolve in water. Their low solubility in water is due to an absence of polar groups emulsifier fat They contain little oxygen and consist mostly of carbon and hydrogen atoms Fats and oils Emulsification takes place when dirty clothes are washed with soaps or detergents. Also, prior to the digestion of fatty The most familiar lipids are those found in fats and oils foods, fats are emulsified by bile. The gallbladder stores bile Fats, which are usually of animal origin(e.g, lard and but- for emulsifying fats prior to the digestive process ter), are solid at room temperature. Oils, which are usually of plant origin(e-g corn oil and soybean oil), are liquid at Saturated and Unsaturated Fattv Acids It is used for long-term energy storage, it insulates against A fatty acid is a carbon-hydrogen chain that end heat loss, and it forms a protective cushion around major acidic group -COOH(Fig. 2. 12). Most of the fatty acids in cells contain 16 or 18 carbon atoms per molecule, although Fats and oils form when one glycerol molecule reacts smaller ones with fewer carbons are also known with three fatty acid molecules(Fig. 2.12). A fat is sometimes Fatty acids are either saturated or unsaturated. Satu called a triglyceride, because of its three-part structure, or a fatty acids have only single covalent bonds because the carbon neutral fat, because the molecule is nonpolar and carries no chain is saturated, so to speak, with all the hydrogens it can hold Saturated fatty acids account for the solid nature at room temperature of fats such as lard and butter. Unsaturated fatty acids have double bonds between carbon atoms wherever fewer than two hydrogens are bonded to a carbon atom. Un Emulsifiers can cause fats to mix with water. They contain saturated fatty acids account for the liquid nature of vegetable cules position themselves about an oil droplet so that their cos ert the to margarine and products such as crisco, Figure 2.12 Synthesis and degradation of a fat molecule Fatty acids can be saturated (no double bonds between carbon atoms)or insaturated(have double bonds, colored yellow, between carbon atoms). When a fat molecule forms, three fatty acids combine with glycerol, and three water molecules are produced. H-C-OH H-C-OH C-C-C-C-C-C-C-H reaction H-C-O-C-C-C-C-C C-H 十3 HHHHH H-C-OH C-C-C=C-C-C-H H-C-0-C-C-C=C-C-C-H H 26PartI Human Organization

Mader: Understanding Human Anatomy & Physiology, Fifth Edition I. Human Organization 2. Chemistry of Life © The McGraw−Hill Companies, 2004 2.5 Lipids Lipids contain more energy per gram than other biological molecules, and some function as long-term energy storage molecules in organisms. Others form a membrane that sepa￾rates a cell from its environment and has inner compartments as well. Steroids are a large class of lipids that includes, among other molecules, the sex hormones. Lipids are diverse in structure and function, but they have a common characteristic: They do not dissolve in water. Their low solubility in water is due to an absence of polar groups. They contain little oxygen and consist mostly of carbon and hydrogen atoms. Fats and Oils The most familiar lipids are those found in fats and oils. Fats, which are usually of animal origin (e.g., lard and but￾ter), are solid at room temperature. Oils, which are usually of plant origin (e.g., corn oil and soybean oil), are liquid at room temperature. Fat has several functions in the body: It is used for long-term energy storage, it insulates against heat loss, and it forms a protective cushion around major organs. Fats and oils form when one glycerol molecule reacts with three fatty acid molecules (Fig. 2.12). A fat is sometimes called a triglyceride, because of its three-part structure, or a neutral fat, because the molecule is nonpolar and carries no charge. Emulsification Emulsifiers can cause fats to mix with water. They contain molecules with a nonpolar end and a polar end. The mole￾cules position themselves about an oil droplet so that their nonpolar ends project. Now the droplet disperses in water, which means that emulsification has occurred. Emulsification takes place when dirty clothes are washed with soaps or detergents. Also, prior to the digestion of fatty foods, fats are emulsified by bile. The gallbladder stores bile for emulsifying fats prior to the digestive process. Saturated and Unsaturated Fatty Acids A fatty acid is a carbon–hydrogen chain that ends with the acidic group —COOH (Fig. 2.12). Most of the fatty acids in cells contain 16 or 18 carbon atoms per molecule, although smaller ones with fewer carbons are also known. Fatty acids are either saturated or unsaturated. Saturated fatty acids have only single covalent bonds because the carbon chain is saturated, so to speak, with all the hydrogens it can hold. Saturated fatty acids account for the solid nature at room temperature of fats such as lard and butter. Unsaturated fatty acids have double bonds between carbon atoms wherever fewer than two hydrogens are bonded to a carbon atom. Un￾saturated fatty acids account for the liquid nature of vegetable oils at room temperature. Hydrogenation of vegetable oils can convert them to margarine and products such as Crisco. 26 Part I Human Organization + 3 H2O H C OH H H C OH C C H OH H H H H C O H H C O H C O H C H C H C H H C O C H H H C O H H C H C H C H H C C C H H H O C H H H H C H C H C H H C O HO C H H C H H H C H H H H C H C H C H H C O C H H C H H H C H H H H C H C H C H H C O H C H H H H C H C H C H H H glycerol 3 fatty acids fat 3 waters HO HO dehydration reaction hydrolysis reaction + Figure 2.12 Synthesis and degradation of a fat molecule. Fatty acids can be saturated (no double bonds between carbon atoms) or unsaturated (have double bonds, colored yellow, between carbon atoms). When a fat molecule forms, three fatty acids combine with glycerol, and three water molecules are produced. + polar end nonpolar end emulsifier emulsion fat