Hydration shells 8o a, 4% 9c ee Stable Unstable hydrogen bonds hydrogen bonds (a)Liquid water FIGURE 2.16 The role of hydrogen bonds in an ice crystal. (a) In liquid FIGURE 2.17 water, hydrogen bonds are not stable and constantly break and re- Why salt dissolves in water. When a crystal of table salt dissolves form. ()When water cools below 0C, the hydrogen bonds are in water, individual Na* and Cl- ions break away from the salt more stable, and a regular crystalline structure forms in which the lattice and become surrounded by water molecules. Water four partial charges of one water molecule interact with the molecules orient around Ch ions so that their partial positive poles opposite charges of other water molecules. Because water forms a face toward the negative CI- ion; water molecules surrounding Na' crystal latticework, ice is less dense than liquid water and floats. If ions orient in the opposite way, with their partial negative poles it did not, inland bodies of water far from the earths equator facing the positive Na* ion. Surrounded by hydration shells, Na* t never and Cl ions never reenter the salt lattice Water Is a Powerful Solvent in water. the water molecules act to exclude them. The Water is an effective solvent because of its ability to form nonpolar molecules are forced into association with one an hydrogen bonds. Water molecules gather closely around other, thus minimizing their disruption of the hydrogen onding of water. In effect, they shrink from contact with any substance that bears an electrical charge, whether that water and for this reason they are referred to as hydropho substance carries a full charge (ion)or a charge separation bic(Greek bydros, "water"and phobos, "fearing"). In con- (polar molecule). For example, sucrose(table sugar)is composed of molecules that contain slightly polar hydroxyl trast, polar molecules, which readily form hydrogen bonds (OH)groups. A sugar crystal dissolves rapidly in water be- with water, are said to be hydrophilic( "water-loving) cause water molecules can form hydrogen bonds with indi- The tendency of nonpolar molecules to aggregate in wa vidual hydroxyl groups of the sucrose molecules. There- er is known as hydrophobic exclusion. By forcing the hy drophobic portions of molecules together, water causes fore, sucrose is said to be soluble in water. Every time a these molecules to assume particular shapes. Different mo- sucrose molecule dissociates or breaks away from the crys- lecular shapes have evolved by alteration of the location tal, water molecules surround it in a cloud, forming a hy- and strength of nonpolar regions. As you will see, much of dration shell and preventing it from associating with oth- he evolution of life reflects changes in molecular shape er sucrose molecules. Hydration shells also form around that can be induced in just this way ions such as Na* and Cl-(figure 2. 17) Water molecules, which are very polar, cling to one Water Organizes Nonpolar Molecules nother, so that it takes considerable energy to separate Water molecules always tend to form the maximum possi them. Water also clings to other polar molecules, ble number of hydrogen bonds. When nonpolar molecules causing them to be soluble in water solution, but water such as oils, which do not form hydrogen bonds, are placed ends to exclude nonpolar molecules. Chapter2 The Nature of Molecules 31
Water Is a Powerful Solvent Water is an effective solvent because of its ability to form hydrogen bonds. Water molecules gather closely around any substance that bears an electrical charge, whether that substance carries a full charge (ion) or a charge separation (polar molecule). For example, sucrose (table sugar) is composed of molecules that contain slightly polar hydroxyl (OH) groups. A sugar crystal dissolves rapidly in water because water molecules can form hydrogen bonds with individual hydroxyl groups of the sucrose molecules. Therefore, sucrose is said to be soluble in water. Every time a sucrose molecule dissociates or breaks away from the crystal, water molecules surround it in a cloud, forming a hydration shell and preventing it from associating with other sucrose molecules. Hydration shells also form around ions such as Na+ and Cl– (figure 2.17). Water Organizes Nonpolar Molecules Water molecules always tend to form the maximum possible number of hydrogen bonds. When nonpolar molecules such as oils, which do not form hydrogen bonds, are placed in water, the water molecules act to exclude them. The nonpolar molecules are forced into association with one another, thus minimizing their disruption of the hydrogen bonding of water. In effect, they shrink from contact with water and for this reason they are referred to as hydrophobic (Greek hydros, “water” and phobos, “fearing”). In contrast, polar molecules, which readily form hydrogen bonds with water, are said to be hydrophilic (“water-loving”). The tendency of nonpolar molecules to aggregate in water is known as hydrophobic exclusion. By forcing the hydrophobic portions of molecules together, water causes these molecules to assume particular shapes. Different molecular shapes have evolved by alteration of the location and strength of nonpolar regions. As you will see, much of the evolution of life reflects changes in molecular shape that can be induced in just this way. Water molecules, which are very polar, cling to one another, so that it takes considerable energy to separate them. Water also clings to other polar molecules, causing them to be soluble in water solution, but water tends to exclude nonpolar molecules. Chapter 2 The Nature of Molecules 31 Water molecules Stable Unstable hydrogen bonds hydrogen bonds (a) Liquid water (b) Ice FIGURE 2.16 The role of hydrogen bonds in an ice crystal. (a) In liquid water, hydrogen bonds are not stable and constantly break and reform. (b) When water cools below 0°C, the hydrogen bonds are more stable, and a regular crystalline structure forms in which the four partial charges of one water molecule interact with the opposite charges of other water molecules. Because water forms a crystal latticework, ice is less dense than liquid water and floats. If it did not, inland bodies of water far from the earth’s equator might never fully thaw. Hydration shells Water molecules Salt crystal Na Cl Cl Cl Cl Na Na Na FIGURE 2.17 Why salt dissolves in water. When a crystal of table salt dissolves in water, individual Na+ and Cl– ions break away from the salt lattice and become surrounded by water molecules. Water molecules orient around Cl– ions so that their partial positive poles face toward the negative Cl– ion; water molecules surrounding Na+ ions orient in the opposite way, with their partial negative poles facing the positive Na+ ion. Surrounded by hydration shells, Na+ and Cl– ions never reenter the salt lattice.
Water onizes H+ lo Examples of Concentration The covalent bonds within a water molecule sometimes H Value break spontaneously. In pure water at 25oC, only 1 out of every 550 million water molecules undergoes this process When it happens, one of the protons(hydrogen atom nu- clei) dissociates from the molecule. Because the dissociated Lemon juice oroton lacks the negatively charged electron it was sharing Vinegar, cola, bee in the covalent bond with oxygen, its own positive charge is no longer counterbalanced, and it becomes a positively harged ion, H*. The rest of the dissociated water molecule, which has retained the shared electron from the covalent 10-5 Black coffee bond, is negatively charged and forms a hydroxide ion Normal rainwater (OH-). This process of spontaneous ion formation is called ionization: hydroxide ion hydrogenion(proton) At 25C, a liter of water contains 10000000(or 10-7)mole of H+ ions. (A mole is defined as the weight in Great Salt Lake grams that corresponds to the summed atomic masses of all of the atoms in a molecule. In the case ofH+. the atomic 11 Household ammonia mass is 1, and a mole of H+ ions would weigh 1 gram. One mole of any substance always contains 6.02 x 1023 mole Household bleach cules of the substance. ) Therefore the molar concentra- tion of hydrogen ions(represented as [H*D in pure water is Oven cleaner 10/ mole/liter. Actually, the hydrogen ion usually associ Sodium hydroxide ates with another water molecule to form a hydronium (H3O+)ion HI FIGURE 2.18 A more convenient way to express the hydrogen ion concen- The pH scale. The pH value of a solution indicates its This scale defines pH as the negative logarithm of the hy drogen ion concentration in the solution scale is logarithmic, so that a pH change of 1 means a tenfold change in the concentration of hydrogen ions. Thus, lemon juice is 100 times more acidic than tomato juice, and seawater is 10 tines nore Because the logarithm of the hydrogen ion concentration is basic than pure water, which has a pH of 7. simply the exponent of the molar concentration of H*, the oH equals the exponent times-l. Thus, pure water, with an tions have pH values below 7. The stronger an acid is, the H+l of 10-7 mole/liter, has a pH of 7. Recall that for every more H+ ions it produces and the lower its pH. For exam- H+ ion formed when water dissociates, an OH- ion is also ple, hydrochloric acid(HCi), which is abundant in your formed, meaning that the dissociation of water produces H+ stomach, ionizes completely in water. This means that 10 and OH- in equal amounts. Therefore, a pH value of 7 indi- mole per liter of HCl will dissociate to form 10- mole per cates neutrality-a balance between H++ and OH--on the liter of H* ions, giving the solution a pH of 1. The ph of pH scale. champagne, which bubbles because of the carbonic acid Note that the ph scale is logarithmiC which means that a dissolved in it, is about 2 difference of I on the scale represents a tenfold change in Bases. A substance that combines with H+ ions when dis hydrogen ion concentration. This solved in water is called a base. By combining with H+ions, with a pH of 4 has 10 times the concentration of H+ than is a base lowers the H+ ion concentration in the solution present in one with a ph of 5 Basic(or alkaline)solutions, therefore, have pH values Acids. Any substance that dissociates in water to increase above 7. Very strong bases, such as sodium hydroxide the concentration of H+ ions is called an acid Acidic solu-(Naoh), have ph values of 12 or more 32 Part I The Origin of Living Things
Water Ionizes The covalent bonds within a water molecule sometimes break spontaneously. In pure water at 25°C, only 1 out of every 550 million water molecules undergoes this process. When it happens, one of the protons (hydrogen atom nuclei) dissociates from the molecule. Because the dissociated proton lacks the negatively charged electron it was sharing in the covalent bond with oxygen, its own positive charge is no longer counterbalanced, and it becomes a positively charged ion, H+. The rest of the dissociated water molecule, which has retained the shared electron from the covalent bond, is negatively charged and forms a hydroxide ion (OH–). This process of spontaneous ion formation is called ionization: H2O → OH– + H+ water hydroxide ion hydrogenion (proton) At 25°C, a liter of water contains 10,000 1 ,000 (or 10–7) mole of H+ ions. (A mole is defined as the weight in grams that corresponds to the summed atomic masses of all of the atoms in a molecule. In the case ofH+, the atomic mass is 1, and a mole of H+ ions would weigh 1 gram. One mole of any substance always contains 6.02 × 1023 molecules of the substance.) Therefore, the molar concentration of hydrogen ions (represented as [H+]) in pure water is 10–7 mole/liter. Actually, the hydrogen ion usually associates with another water molecule to form a hydronium (H3O+) ion. pH A more convenient way to express the hydrogen ion concentration of a solution is to use the pH scale (figure 2.18). This scale defines pH as the negative logarithm of the hydrogen ion concentration in the solution: pH = –log [H+] Because the logarithm of the hydrogen ion concentration is simply the exponent of the molar concentration of H+, the pH equals the exponent times –1. Thus, pure water, with an [H+] of 10–7 mole/liter, has a pH of 7. Recall that for every H+ ion formed when water dissociates, an OH– ion is also formed, meaning that the dissociation of water produces H+ and OH– in equal amounts. Therefore, a pH value of 7 indicates neutrality—a balance between H+ and OH–—on the pH scale. Note that the pH scale is logarithmic, which means that a difference of 1 on the scale represents a tenfold change in hydrogen ion concentration. This means that a solution with a pH of 4 has 10 times the concentration of H+ than is present in one with a pH of 5. Acids. Any substance that dissociates in water to increase the concentration of H+ ions is called an acid. Acidic solutions have pH values below 7. The stronger an acid is, the more H+ ions it produces and the lower its pH. For example, hydrochloric acid (HCl), which is abundant in your stomach, ionizes completely in water. This means that 10–1 mole per liter of HCl will dissociate to form 10–1 mole per liter of H+ ions, giving the solution a pH of 1. The pH of champagne, which bubbles because of the carbonic acid dissolved in it, is about 2. Bases. A substance that combines with H+ ions when dissolved in water is called a base. By combining with H+ ions, a base lowers the H+ ion concentration in the solution. Basic (or alkaline) solutions, therefore, have pH values above 7. Very strong bases, such as sodium hydroxide (NaOH), have pH values of 12 or more. 32 Part I The Origin of Living Things 10 1 H+ Ion Concentration Examples of Solutions Hydrochloric acid Stomach acid 1 pH Value 10 2 2 Lemon juice 10 3 3 Vinegar, cola, beer 10 4 4 Tomatoes 10 5 Black coffee Normal rainwater 5 10 6 Urine Saliva 6 10 7 Pure water Blood 7 10 8 8 Seawater 10 9 9 Baking soda 10 10 10 Great Salt Lake 10 11 11 Household ammonia 10 12 Household bleach 12 10 13 Oven cleaner 13 10 14 14 Sodium hydroxide FIGURE 2.18 The pH scale. The pH value of a solution indicates its concentration of hydrogen ions. Solutions with a pH less than 7 are acidic, while those with a pH greater than 7 are basic. The scale is logarithmic, so that a pH change of 1 means a tenfold change in the concentration of hydrogen ions. Thus, lemon juice is 100 times more acidic than tomato juice, and seawater is 10 times more basic than pure water, which has a pH of 7.
Buffers The pH inside almost all living cells, and in the fluid sur- rounding cells in multicellular organisms, is fairly close to 7. Most of the biological catalysts(enzymes)in living systems are extremely sensitive to pH; often even a small change in H will alter their shape, thereby disrupting their activities and rendering them useless. For this reason it is important that a cell maintain a constant pHlevel. range Yet the chemical reactions of life constantly produce acids and bases within cells. Furthermore, many animals eat sub- stances that are acidic or basic; cola, for example, is a stror (although dilute)acidic solution. Despite such variations in the concentrations of H+ and OH-, the ph of an organism is kept at a relatively constant level by buffers(figure 2. 19) a buffer is a substance that acts as a reservoir for hy rogen ions donating them to the solution when their con- dr centration falls and taking them from the solution when Amount of base added their concentration rises. What sort of substance will act in FIGURE 2. 19 this way? Within organisms, most buffers consist of pairs of Buffers minimize changes in pH. Adding a base to a solution substances, one an acid and the other a base the key buffer neutralizes some of the acid present, and so raises the pH. Thus, in human blood is an acid-base pair consisting of carbonic as the curve moves to the right, reflecting more and more base, it acid(acid) and bicarbonate(base). These two substances in- also rises to higher ph values. What a buffer does is to make the teract in a pair of reversible reactions. first, carbon dioxide curve rise or fall very slowly over a portion of the pH scale, called (CO2)and HO join to form carbonic acid(H]CO3), which the "buffering range"of that buffer in a second reaction dissociates to yield bicarbonate ion (HCO3)and H+(figure 2. 20). If some acid or other sub- stance adds H+ ions to the blood, the Hco3- ions act as a In a condition called blood acidosis human blood which base and remove the excess H+ ions by forming H2 CO3. normally has a pH of about 7.4, drops 0.2 to 0. 4 points on the Similarly, if a basic substance removes H+ ions from the pH scale. This condition is fatal if not treated immediately blood, H2CO3 dissociates, releasing more H+ ions into the The reverse condition, blood alkalosis, involves an increase in blood. The forward and reverse reactions that interconvert blood pH of a similar magnitude and is just as serious H2 CO3 and HCO3- thus stabilize the bloods pH The reaction of carbon dioxide and water to form car- The pH of a solution is the negative logarithm of the H bonic acid is important because it permits carbon, essential n concentration in the solution. Thus, low pH values to life. to enter water from the air. As we will discuss in indicate high H* concentrations(acidic solutions), and chapter 4, biologists believe that life first evolved in the high pH values indicate low H*concentrations(basic early oceans. These oceans were rich in carbon because of solutions). Even small changes in pH can be harmful to the reaction of carbon dioxide with water life ③+○③ HO H Water Carbonic acid Bicarbonate FIGURE 2.20 freeing H+ ions. This reaction makes carbonated beverages acidic, and produced the carbon-rich early oceans that craded lif water Buffer formation. Carbon dioxide and water combine chemically to form carbonic acid (H2CO 3 ) The acid then dissociates in Chapter2 The Nature of Molecules 33
Buffers The pH inside almost all living cells, and in the fluid surrounding cells in multicellular organisms, is fairly close to 7. Most of the biological catalysts (enzymes) in living systems are extremely sensitive to pH; often even a small change in pH will alter their shape, thereby disrupting their activities and rendering them useless. For this reason it is important that a cell maintain a constant pH level. Yet the chemical reactions of life constantly produce acids and bases within cells. Furthermore, many animals eat substances that are acidic or basic; cola, for example, is a strong (although dilute) acidic solution. Despite such variations in the concentrations of H+ and OH–, the pH of an organism is kept at a relatively constant level by buffers (figure 2.19). A buffer is a substance that acts as a reservoir for hydrogen ions, donating them to the solution when their concentration falls and taking them from the solution when their concentration rises. What sort of substance will act in this way? Within organisms, most buffers consist of pairs of substances, one an acid and the other a base. The key buffer in human blood is an acid-base pair consisting of carbonic acid (acid) and bicarbonate (base). These two substances interact in a pair of reversible reactions. First, carbon dioxide (CO2) and H2O join to form carbonic acid (H2CO3), which in a second reaction dissociates to yield bicarbonate ion (HCO3 –) and H+ (figure 2.20). If some acid or other substance adds H+ ions to the blood, the HCO3 – ions act as a base and remove the excess H+ ions by forming H2CO3. Similarly, if a basic substance removes H+ ions from the blood, H2CO3 dissociates, releasing more H+ ions into the blood. The forward and reverse reactions that interconvert H2CO3 and HCO3 – thus stabilize the blood’s pH. The reaction of carbon dioxide and water to form carbonic acid is important because it permits carbon, essential to life, to enter water from the air. As we will discuss in chapter 4, biologists believe that life first evolved in the early oceans. These oceans were rich in carbon because of the reaction of carbon dioxide with water. In a condition called blood acidosis, human blood, which normally has a pH of about 7.4, drops 0.2 to 0.4 points on the pH scale. This condition is fatal if not treated immediately. The reverse condition, blood alkalosis, involves an increase in blood pH of a similar magnitude and is just as serious. The pH of a solution is the negative logarithm of the H+ ion concentration in the solution. Thus, low pH values indicate high H+ concentrations (acidic solutions), and high pH values indicate low H+ concentrations (basic solutions). Even small changes in pH can be harmful to life. Chapter 2 The Nature of Molecules 33 0 1 0 1 2 3 4 5 6 7 8 9 3 Amount of base added Buffering range pH 2 4 5 FIGURE 2.19 Buffers minimize changes in pH. Adding a base to a solution neutralizes some of the acid present, and so raises the pH. Thus, as the curve moves to the right, reflecting more and more base, it also rises to higher pH values. What a buffer does is to make the curve rise or fall very slowly over a portion of the pH scale, called the “buffering range” of that buffer. FIGURE 2.20 Buffer formation. Carbon dioxide and water combine chemically to form carbonic acid (H2CO3). The acid then dissociates in water, freeing H+ ions. This reaction makes carbonated beverages acidic, and produced the carbon-rich early oceans that cradled life. H2O Water CO2 Carbon dioxide H2CO3 Carbonic acid HCO 3 Bicarbonate ion H Hydrogen ion – + + +
Chapter 2 http://www.mhhe.com/raven6ehttp://www.biocourse.com Questions Media resource 2.1 Atoms are nature' s building material The smallest stable particles of matter are protons, neutrons, 1. An atom of nitrogen has 7 Atomic Structure and electrons which associate to form atoms protons and 7 neutrons. What its atomic number? what is The core, or nucleus, of an atom consists of protons and aBe sons; the electrons orbit around the nucleus in a cloud. trons does it have. any elec- arther an electron is from the nucleus. the faster it 2. How do the isotopes of a sin- Basic Chemistry ves and the more energy it possesses gle element differ from each The chemical behavior of an atom is largely determined by other? the distribution of its electrons and in particular by the num- 3. The half-life of radium-226 is ber of electrons in its outermost(highest)energy level 620 years. If a sample of mate- There is a strong tendency for atoms to have a completely rial contains 16 milligrams of ra- filled outer level; electrons are lost, gained, or shared until diurm-226, how much will it con tain in 1620 years? How much will it contain in 3240 years? How long will it take for the sample to contain 1 milligram of radium-2262 2.2 The atoms of living things are among the smallest. More than 95% of the weight of an organism consists 4. What is the octet rule, and of oxygen, hydrogen, carbon, and nitrogen, all of how does it affect the chemical which form strong covalent bonds with one another behavior of atom 2.3 Chemical bonds hold molecules together. lonic bonds form when electrons transfer from one 5. what is the difference be atom to another, and the resulting oppositely charged tween an ionic bond and a cov · Ionic bonds ons attract one another lent bond: Give an example of Covalent bonds form when two atoms share elec- trons. They are responsible for the formation of most Bonds biologically important molecules 2.4 Water is the cradle of life The chemistry of life is the chemistry of water(HO). 6. What types of atoms partici The central oxygen atom in water attracts the elec ate in the formation of hydro- trons it shares with the two hydrogen atoms. This harge separation makes water a polar molecule nds contribute to water's high A hydrogen bond is formed between the partial pe specific heat? tive charge of a hydrogen atom in one molecule 7. What types of molecules are rophobic? Wha ently in water? Water is cohesive and adhesive, has a great 8. What is the pH of a solution molecules,and tends to exclude nonpolar molecules. tration of 10-3 moleliter ncen- for storing heat, is a good solvent for other p that has a hydrogen ion co The H+ concentration in a solution is expressed by Would such a solution be acidic he ph scale, in which pH equals the negative loga rithm of the h+ concentration 34 Part I The Origin of Living Things
• The smallest stable particles of matter are protons, neutrons, and electrons, which associate to form atoms. • The core, or nucleus, of an atom consists of protons and neutrons; the electrons orbit around the nucleus in a cloud. The farther an electron is from the nucleus, the faster it moves and the more energy it possesses. • The chemical behavior of an atom is largely determined by the distribution of its electrons and in particular by the number of electrons in its outermost (highest) energy level. There is a strong tendency for atoms to have a completely filled outer level; electrons are lost, gained, or shared until this condition is reached. 2.2 The atoms of living things are among the smallest. 34 Part I The Origin of Living Things Chapter 2 Summary Questions Media Resources 2.1 Atoms are nature’s building material. 1. An atom of nitrogen has 7 protons and 7 neutrons. What is its atomic number? What is its atomic mass? How many electrons does it have? 2. How do the isotopes of a single element differ from each other? 3. The half-life of radium-226 is 1620 years. If a sample of material contains 16 milligrams of radium-226, how much will it contain in 1620 years? How much will it contain in 3240 years? How long will it take for the sample to contain 1 milligram of radium-226? • More than 95% of the weight of an organism consists of oxygen, hydrogen, carbon, and nitrogen, all of which form strong covalent bonds with one another. • Ionic bonds form when electrons transfer from one atom to another, and the resulting oppositely charged ions attract one another. • Covalent bonds form when two atoms share electrons. They are responsible for the formation of most biologically important molecules. 4. What is the octet rule, and how does it affect the chemical behavior of atoms? 5. What is the difference between an ionic bond and a covalent bond? Give an example of each. 2.3 Chemical bonds hold molecules together. 2.4 Water is the cradle of life. • The chemistry of life is the chemistry of water (H2O). The central oxygen atom in water attracts the electrons it shares with the two hydrogen atoms. This charge separation makes water a polar molecule. • A hydrogen bond is formed between the partial positive charge of a hydrogen atom in one molecule and the partial negative charge of another atom, either in another molecule or in a different portion of the same molecule. • Water is cohesive and adhesive, has a great capacity for storing heat, is a good solvent for other polar molecules, and tends to exclude nonpolar molecules. • The H+ concentration in a solution is expressed by the pH scale, in which pH equals the negative logarithm of the H+ concentration. 6. What types of atoms participate in the formation of hydrogen bonds? How do hydrogen bonds contribute to water’s high specific heat? 7. What types of molecules are hydrophobic? What types are hydrophilic? Why do these two types of molecules behave differently in water? 8. What is the pH of a solution that has a hydrogen ion concentration of 10–3 mole/liter? Would such a solution be acidic or basic? • Atomic Structure • Basic Chemistry • Atoms • Bonds • Ionic Bonds • Bonds • Water • ph Scale http://www.mhhe.com/raven6e http://www.biocourse.com
The Chemical building Blocks of life Concept Outline 3.1 Molecules are the building blocks of life. The Chemistry of Carbon. Because individual carbon atoms can form multiple covalent bonds, organic molecules can be quite complex. 3.2 Proteins perform the chemistry of the cell The Many Functions of Proteins. Proteins can be cata- ysts, transporters, supporters, and reg Amino acids are the building blocks of proteins proteins are long chains of various combinations of amino acids. A Protein,'s Function Depends on the Shape of the Molecule. A protein s shape is determined by its amino acid sequence How Proteins Fold Into Their Functional Shape. The distribution of nonpolar amino acids along a protein chain largely determines how the protein folds. How Proteins Unfold. When conditions such as pH or temperature fluctuate, proteins may denature or unfold FIGURE 3.1 3.3 Nucleic acids store and transfer genetic information Computer-generated model of a macromolecule. Pictured is Information Molecules. Nucleic acids store information in an enzyme responsible for releasing energy from sugar. Thi Ils RNA is a single-chain polymer of nucleotides, while complex molecule consists of hundreds of different amino acids DNA possesses two chains twisted around each other linked into chains that form the characteristic coils and folds seen 3.4 Lipids make membranes and store energy. Phospholipids Form Membranes. The spontaneous ag gregation of phospholipids in water is responsible for the olecules are extremely small compared with the fa formation of biological membranes miliar world we see about us. Imagine: there are Fats and Other Kinds of Lipids. Organisms utilize a wide more water molecules in a cup than there are stars in the variety of water-insoluble molecules ky. Many other molecules are gigantic, compared with wa Fats as Food. Fats are very efficient energy storage ter, consisting of thousands of atoms. These atoms are or- molecules because of their high proportion of C-H bonds. anized into hundreds of smaller molecules that are linked ogether into long chains(figure 3. 1). These enormous 3.5 Carbohydrates store energy and provide building molecules, almost always synthesized by living things,are called macromolecules. As we shall see, there are four gen Simple Carbohydrates. Sugars are simple carbohydrates, eral types of macromolecules, the basic chemical building consisting of six-carbon rin blocks from which all organisms are assembled Linking Sugars Together. Sugars can be linked together to form long polymers, or polysaccharides Structural Carbohydrates. Structural carbohydrates like cellulose are chains of sugars linked in a way that enzymes cannot easily attack
35 3 The Chemical Building Blocks of Life Concept Outline 3.1 Molecules are the building blocks of life. The Chemistry of Carbon. Because individual carbon atoms can form multiple covalent bonds, organic molecules can be quite complex. 3.2 Proteins perform the chemistry of the cell. The Many Functions of Proteins. Proteins can be catalysts, transporters, supporters, and regulators. Amino Acids Are the Building Blocks of Proteins. Proteins are long chains of various combinations of amino acids. A Protein’s Function Depends on the Shape of the Molecule. A protein’s shape is determined by its amino acid sequence. How Proteins Fold Into Their Functional Shape. The distribution of nonpolar amino acids along a protein chain largely determines how the protein folds. How Proteins Unfold. When conditions such as pH or temperature fluctuate, proteins may denature or unfold. 3.3 Nucleic acids store and transfer genetic information. Information Molecules. Nucleic acids store information in cells. RNA is a single-chain polymer of nucleotides, while DNA possesses two chains twisted around each other. 3.4 Lipids make membranes and store energy. Phospholipids Form Membranes. The spontaneous aggregation of phospholipids in water is responsible for the formation of biological membranes. Fats and Other Kinds of Lipids. Organisms utilize a wide variety of water-insoluble molecules. Fats as Food. Fats are very efficient energy storage molecules because of their high proportion of C—H bonds. 3.5 Carbohydrates store energy and provide building materials. Simple Carbohydrates. Sugars are simple carbohydrates, often consisting of six-carbon rings. Linking Sugars Together. Sugars can be linked together to form long polymers, or polysaccharides. Structural Carbohydrates. Structural carbohydrates like cellulose are chains of sugars linked in a way that enzymes cannot easily attack. Molecules are extremely small compared with the familiar world we see about us. Imagine: there are more water molecules in a cup than there are stars in the sky. Many other molecules are gigantic, compared with water, consisting of thousands of atoms. These atoms are organized into hundreds of smaller molecules that are linked together into long chains (figure 3.1). These enormous molecules, almost always synthesized by living things, are called macromolecules. As we shall see, there are four general types of macromolecules, the basic chemical building blocks from which all organisms are assembled. FIGURE 3.1 Computer-generated model of a macromolecule. Pictured is an enzyme responsible for releasing energy from sugar. This complex molecule consists of hundreds of different amino acids linked into chains that form the characteristic coils and folds seen here