Biology Chapter 2: Chemistry of Life PDF

CHAPTER 2 Chemistry of Life K E Y CO N C E P T S 2.1 Atoms, Ions, and Molecules All living things are based on atoms and their interactions. 2.2 Properties of Water...

CHAPTER 2 Chemistry of Life K E Y CO N C E P T S 2.1 Atoms, Ions, and Molecules All living things are based on atoms and their interactions. 2.2 Properties of Water Water’s unique properties allow life to exist on Earth. 2.3 Carbon-Based Molecules Carbon-based molecules are the foundation of life. 2.4 Chemical Reactions Life depends on chemical reactions. 2.5 Enzymes Enzymes are catalysts for chemical reactions in living things. BIO LO GY CL ASSZONE.COM RESOURCE CENTER BIOLOGY View animated chapter Keep current with biology news. Get more information on concepts. News feeds Elements of Life Hydrogen Bonding Careers Acids, Bases, and pH Energy and Chemical Polls Reactions Calorimetry Atoms and Bonding 34 Unit 1: Introducing Biology  How can this plant digest a frog? Connecting CO NCEP TS L ike other carnivores, the Venus flytrap eats animals to get nutrients that it needs to make molecules such as proteins Cell Function The Venus flytrap has specialized cells on the surfaces of its leaves. Some of these cells allow the plant to snap shut on its prey and nucleic acids. Other chemical com- within 0.5 seconds. Other cells, such as those that pounds made by the plant’s cells enable appear purple in this light the Venus flytrap to digest the animals micrograph, secrete digestive chemicals that allow the that it eats. These chemicals are similar to plant to consume its prey. (LM; magnification 500⫻) the chemicals that allow you to digest the food that you eat. Chapter 2: Chemistry of Life 35  2.1 Atoms, Ions, and Molecules KEY CONCEPT All living things are based on atoms and their interactions. MAIN IDEAS VOCABULARY Living things consist of atoms of atom, p. 36 ionic bond, p. 38 different elements. element, p. 36 covalent bond, p. 39 Ions form when atoms gain or lose molecule, p. 39 compound, p. 37 electrons. Atoms share pairs of electrons in ion, p. 38 Review covalent bonds. cell, organism Connect The Venus flytrap produces chemicals that allow it to consume and digest insects and other small animals, including an unlucky frog. Frogs also produce specialized chemicals that allow them to consume and digest their prey. Review Life Science In fact, all organisms depend on many chemicals and chemical reactions. For this 8.6.b Students know that living organisms are made of molecules reason, the study of living things also involves the study of chemistry. consisting largely of carbon, hydro- gen, nitrogen, oxygen, phosphorus, and sulfur. MAIN IDEA Living things consist of atoms of different elements. TAKING NOTES What do a frog, a skyscraper, a car, and your body all have in common? Every Use a main idea web to help you physical thing you can think of, living or not, is made of incredibly small make connections among ele- particles called atoms. An atom is the smallest basic unit of matter. Millions of ments, atoms, ions, compounds, and molecules. atoms could fit in a space the size of the period at the end of this sentence. And it would take you more than 1 trillion (1,000,000,000,000, or 1011) years atom:... to count the number of atoms in a single grain of sand. element Atoms and Elements ion:... Although there is a huge variety of matter on Earth, all atoms share the same basic structure. Atoms consist of three types of smaller particles: protons, neutrons, and electrons. Protons and neutrons form the dense center of an atom—the atomic nucleus. Electrons are much smaller particles outside of the nucleus. Protons have a positive electrical charge, and electrons have a negative electrical charge. Neutrons, as their name implies, are neutral—they have no charge. Because an atom has equal numbers of positively charged protons and negatively charged electrons, it is electrically neutral. An element is one particular type of atom, and it cannot be broken down into a simpler substance by ordinary chemical means. An element can also refer to a group of atoms of the same type. A few familiar elements include the gases hydrogen and oxygen and the metals aluminum and gold. Because all atoms are made of the same types of particles, what difference among atoms makes one element different from other elements? Atoms of different elements differ in the number of protons they have. All atoms of a given element have a specific number of protons that never varies. For example, all hydrogen atoms have one proton, and all oxygen atoms have eight protons. 36 Unit 1: Introducing Biology  The electrons in the atoms of each element determine the properties of FIGURE 2.1 The exact position that element. As FIGURE 2.1 shows, electrons are considered to be in a cloud of electrons cannot be known. They are somewhere in a around the nucleus. The simplified models of a hydrogen atom and an oxygen three-dimensional electron atom on the left side of FIGURE 2.2 illustrate how electrons move around the cloud around the nucleus. nucleus in regions called energy levels. Different energy levels can hold different numbers of electrons. For example, the first energy level can hold two electrons, and the second energy level can hold eight electrons. Atoms are most stable when they have a full outermost energy level. Of the 91 elements that naturally occur on Earth, only about 25 are found in organisms. Just 4 elements—carbon (C), oxygen (O), nitrogen (N), and hydrogen (H)—make up 96 percent of the human body’s mass. The other 4 percent consists of calcium (Ca), phosphorus (P), potassium (K), sulfur (S), sodium (Na), and several other trace elements. Trace elements are found in very small amounts in your body, but you need them to survive. For example, iron (Fe) is needed to transport oxygen in your blood. Chromium (Cr) is needed for your cells to break down sugars for usable energy. FIGURE 2.2 Representing Atoms BOHR’S ATOMIC MODEL SIMPLIFIED MODEL The model of the atom developed Hydrogen atom (H) Hydrogen atom (H) by Niels Bohr (left) shows that an nucleus: outermost energy atom’s electrons are located out- 1 proton (+) level: 1 electron (–) side the nucleus in regions called H energy levels. Different types of 0 neutrons atoms have different numbers of electrons and energy levels. Often, atoms are shown as Oxygen atom (O) Oxygen atom (O) simplified spheres (right). Different types of atoms are shown in nucleus: outermost energy different sizes and colors. 8 protons (+) level: 6 electrons (–) 8 neutrons O inner energy level: 2 electrons (–) Apply How many electrons would need to be added to fill the outermost energy level of hydrogen? of oxygen? Compounds The atoms of elements found in organisms are often linked, or bonded, to other atoms. A compound is a substance made of atoms of different elements bonded together in a certain ratio. Common compounds in living things include water (H2O) and carbon dioxide (CO2). A compound’s properties are often different from the properties of the elements that make up the com- pound. At temperatures on Earth, for example, hydrogen and oxygen are both gases. Together, though, they can form water. Similarly, a diamond is pure carbon, but carbon atoms are also the basis of sugars, proteins, and millions of other compounds. Contrast How are elements different from compounds? Chapter 2: Chemistry of Life 37  MAIN IDEA Ions form when atoms gain or lose electrons. Connecting CONC EP TS An ion is an atom that has gained or lost one or more electrons. An ion forms Cell Structure and Function because an atom is more stable when its outermost energy level is full; the gain Several different ions are trans- or loss of electrons results in a full outermost energy level. An atom becomes ported across cell membranes during cell processes. You will an ion when its number of electrons changes and it gains an electrical charge. learn how this transport occurs This charge gives ions certain properties. For example, compounds consisting in Chapters 3 and 4. only of ions—ionic compounds—easily dissolve in water. Some ions are positively charged, and other ions are negatively charged. The type of ion that forms depends on the number of electrons in an atom’s outer energy level. An atom with few electrons in its outer energy level tends to lose those electrons. An atom that loses one or more electrons becomes a positively charged ion because it has more protons than electrons. In contrast, an atom with a nearly full outer energy level tends to gain electrons. An atom that gains one or more electrons becomes a negatively charged ion because it has more electrons than protons. Ions play large roles in organisms. For example, hydrogen ions (H+) are needed for the production of usable chemical energy in cells. Calcium ions (Ca2+) are necessary for every muscle movement in your body. And chloride ions (Cl–) are important for a certain type of chemical signal in the brain. Ions usually form when electrons are transferred from one atom to an- other. For example, FIGURE 2.3 shows the transfer of an electron from a sodium atom (Na) to a chlorine atom (Cl). When it loses its one outer electron, the sodium atom becomes a positively charged sodium ion (Na+). Its second energy level, which has eight electrons, is now a full outermost energy level. The transferred electron fills chlorine’s outermost energy level, forming a negatively charged chloride ion (Cl–). Positive ions, such as Na+, are attracted to negative ions, such as Cl–. An ionic bond forms through the electrical force between oppositely charged ions. Salt, or sodium chloride (NaCl), is an ionic compound of Na+ and Cl–. Sodium chloride is held together by ionic bonds. Apply What determines whether an atom becomes a positive ion or a negative ion? FIGURE 2.3 IONS AND IONIC BONDS 1 The sodium atom (Na) loses its one outer 2 The positive sodium ion (Na+) and negative chloride electron to the chlorine atom (Cl). ion (Cl–) attract each other and form an ionic bond. gained electron Na loses an ionic bond electron to Cl Sodium atom (Na) Chlorine atom (Cl) Sodium ion (Na+) Chloride ion (Cl–) 38 Unit 1: Introducing Biology  MAIN IDEA Atoms share pairs of electrons in covalent bonds. Not all atoms easily gain or lose electrons. Rather, the atoms of many elements VOCABULARY share pairs of electrons. The shared pairs of electrons fill the outermost energy The prefix co- means levels of the bonded atoms. A covalent bond forms when atoms share a pair “together,” and valent comes from a Latin word that means of electrons. Covalent bonds are generally very strong, and depending on how “power” or “strength.” many electrons an atom has, two atoms may form several covalent bonds to share several pairs of electrons. FIGURE 2.4 illustrates how atoms of carbon and oxygen share pairs of electrons in covalent bonds. All three atoms in a molecule of carbon dioxide (CO2) have full outer energy levels. FIGURE 2.4 COVALENT BONDS A carbon atom needs four electrons to fill its outer energy level. An oxygen atom needs two electrons to fill its outer energy level. In carbon dioxide, carbon makes a double bond, or shares two pairs of electrons, with each oxygen atom. covalent bonds Oxygen atom (O) Carbon atom (C) Oxygen atom (O) Carbon dioxide (CO2) A molecule is two or more atoms held together by covalent bonds. In the compound carbon dioxide, each oxygen atom shares two pairs of electrons (four electrons) with the carbon atom. Some elements occur naturally in the form of diatomic, or “two-atom,” molecules. For example, a molecule of oxygen (O2) consists of two oxygen atoms that share two pairs of electrons. Almost all of the substances that make up organisms, from lipids to nucleic acids to water, are molecules held together by covalent bonds. Summarize What happens to electrons in outer energy levels when two atoms form a covalent bond? ONLINE QUIZ 2.1 ASSESSMENT ClassZone.com REVIEWING MAIN IDEAS CRITICAL THINKING Connecting CONC EP TS 1. What distinguishes one 4. Compare and Contrast How does a 6. Chemistry A sodium atom has element from another? molecule differ from an atom? one outer electron, and a 2. Describe the formation of an 5. Apply Explain why a hydrogen carbon atom has four outer ionic compound. atom can become either an ion or electrons. How might this a part of a molecule. difference be related to the 3. What is the difference between an types of compounds formed by ionic bond and a covalent bond? atoms of these two elements? Chapter 2: Chemistry of Life 39  2.2 Properties of Water KEY CONCEPT Water’s unique properties allow life to exist on Earth. MAIN IDEAS VOCABULARY Life depends on hydrogen bonds hydrogen bond, p. 41 solute, p. 42 in water. cohesion, p. 41 acid, p. 42 Many compounds dissolve in water. base, p. 42 adhesion, p. 41 Some compounds form acids or bases. solution, p. 42 pH, p. 42 solvent, p. 42 Review ion, molecule Connect When you are thirsty, you need to drink something that is mostly water. Why is the water you drink absolutely necessary? Your cells, and the cells of every other living thing on Earth, are mostly water. Water gives cells structure Review Life Science and transports materials within organisms. All of the processes necessary for life 8.6.c Students know that living organisms have many different take place in that watery environment. Water’s unique properties, which are kinds of molecules, including small related to the structure of the water molecule, are important for living things. ones, such as water and salt, and very large ones, such as carbohy- drates, fats, proteins, and DNA. MAIN IDEA Life depends on hydrogen bonds in water. How do fish survive a cold winter if their pond freezes? Unlike most substances, water expands when it freezes. Water is less dense as a solid (ice) than as a liquid. In a pond, ice floats and covers the water’s surface. The ice acts as an insulator that allows the water underneath to remain a liquid. Ice’s low density is related to the structure of the water molecule. Water and Hydrogen Bonds Water is a polar molecule. You can think about polar molecules in the same way that you can think about a magnet’s poles. That is, polar molecules have a region with a slight positive O charge and a region with a slight negative charge. Polar molecules, such H H as the water molecule shown in FIGURE 2.5, form when atoms in a mol- ecule have unequal pulls on the elec- FIGURE 2.5 In water molecules, the trons they share. In a molecule of water, oxygen atom has a slightly negative charge, and the hydrogen atoms have the oxygen nucleus, with its eight slightly positive charges. protons, attracts the shared electrons more strongly than do the hydrogen nuclei, with only one proton each. The oxygen atom gains a small negative charge, and the hydrogen atoms gain small positive charges. Other molecules, called nonpolar molecules, do not have these charged regions. The atoms in nonpolar molecules share electrons more equally. 40 Unit 1: Introducing Biology  BIOLOGY Opposite charges of polar molecules can interact See hydrogen to form hydrogen bonds. A hydrogen bond is an bonding in action at ClassZone.com. attraction between a slightly positive hydrogen atom and a slightly negative atom, often oxygen or nitrogen. Hydrogen bonding is shown among water molecules in FIGURE 2.6, but these hydrogen bonds are also found in many other molecules. bond For example, hydrogen bonds are part of the structures of proteins and of DNA, which is the genetic material for all organisms. FIGURE 2.6 Water’s surface ten- Properties Related to Hydrogen Bonds sion comes from hydrogen bonds Individual hydrogen bonds are about 20 times weaker than typical covalent (left) that cause water molecules bonds, but they are relatively strong among water molecules. As a result, a to stick together. large amount of energy is needed to overcome the attractions among water molecules. Without hydrogen bonds, water would boil at a much lower temperature than it does because less energy would be needed to change liquid water into water vapor. Water is a liquid at the temperatures that support most life on Earth only because of hydrogen bonds in water. Hydrogen bonds are responsible for three important properties of water. High specific heat Hydrogen bonds give water an abnormally high specific heat. This means that water resists changes in temperature. Compared to many other compounds, water must absorb more heat energy to increase in temperature. This property is very important in cells. The processes that produce usable chemical energy in cells release a great deal of heat. Water absorbs the heat, which helps to regulate cell temperatures. Cohesion The attraction among molecules of a substance is cohesion. Cohesion from hydrogen bonds makes water molecules stick to each other. You can see this when water forms beads, such as on a recently washed car. Cohesion also produces surface tension, which makes a kind of skin on water. Surface tension keeps the spider in FIGURE 2.6 from sinking. Adhesion The attraction among molecules of different substances is called FIGURE 2.7 The water’s surface adhesion. In other words, water molecules stick to other things. Adhesion (left, dyed red) is curved down is responsible for the upward curve on the surface of the water in FIGURE 2.7 because water has greater adhesion because water molecules are attracted to the glass of the test tube. Adhesion than cohesion. The surface of the mercury (right) is curved up helps plants transport water from their roots to their leaves because water because mercury has greater molecules stick to the sides of the vessels that carry water. cohesion than adhesion. Compare How are hydrogen bonds similar to ionic bonds? MAIN IDEA Many compounds dissolve in water. Molecules and ions cannot take part in chemical processes inside cells unless they dissolve in water. Important materials such as sugars and oxygen cannot be transported from one part of an organism to another unless they are dissolved in blood, plant sap, or other water-based fluids. Chapter 2: Chemistry of Life 41  Many substances dissolve in the VISUAL VOCAB water in your body. When one sub- The solvent is the substance that is stance dissolves in another, a solution present in the greatest amount, and is forms. A solution is a mixture of the substance that dissolves solutes. substances that is the same through- hdakZci out—it is a homogeneous mixture. A solution has two parts. The solvent is solution the substance that is present in the hdajiZ greater amount and that dissolves another substance. A solute is a A solute is the substance that dissolves. substance that dissolves in a solvent. The amount of solute dissolved in a certain amount of solvent is a solution’s concentration. One spoonful of a drink mix in water has little flavor because it has a low concentration. But a solution with four spoonfuls in the same amount of water tastes stronger because it has a higher concentration. The liquid part of your blood, called plasma, is about 95 percent water. Therefore, the solvent in plasma is water and all of the substances dissolved in it are solutes. Most of these solutes, such as sugars and proteins, dissolve in the water of blood plasma because they are polar. Polar molecules dissolve in water because the attraction between the water molecules and the solute molecules is greater than the attraction among the molecules of the solute. Similarly, ionic compounds, such as sodium chloride, dissolve in water because the charges of the water molecules attract the charges of the ions. The water molecules surround each ion and pull the compound apart. Nonpolar substances, such as fats and oils, rarely dissolve solution in in water. Nonpolar molecules do not have charged regions, so FIGURE 2.8 A mosquito they are not attracted to polar molecules. Polar molecules and injects a solution con- taining a protein solute nonpolar molecules tend to remain separate, which is why we that prevents blood from say, “Oil and water don’t mix.” But nonpolar molecules will clotting. The mosquito solution out dissolve in nonpolar solvents. For example, some vitamins, sucks in blood, which is a solution containing solutes such such as vitamin E, are nonpolar and dissolve in fat in your body. as ions, sugars, and proteins. Connect What are the solvent and solutes in a beverage you drink? MAIN IDEA Some compounds form acids or bases. Some compounds break up into ions when they dissolve in water. An acid is a compound that releases a proton—a hydrogen ion (H+)—when it dissolves in water. An acid increases the concentration of H+ ions in a solution. Bases are compounds that remove H+ ions from a solution. When a base dissolves in water, the solution has a low H+ concentration. A solution’s acidity, or H+ ion concentration, is measured by the pH scale. In FIGURE 2.9 you can see that pH is usually between 0 and 14. A solution with a pH of 0 is very acidic, with a high H+ concentration. A solution with a pH of 14 is very basic, with a low H+ concentration. Solutions with a pH of 7 are neutral—neither acidic nor basic. 42 Unit 1: Introducing Biology  FIGURE 2.9 Understanding pH The pH of a solution depends on the concentration of H+ ions. stomach acid pH between 1 and 3 blood pH 7.4 pure water pH 7 bile pH between 8 and 9 pH pH 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 more acidic neutral more basic The concentration of H+ ions varies depending on how acidic or basic a solution is. high H+ low H+ concentration concentration Summarize Describe the relationship between the H+ concentration and the pH value. Most organisms, including humans, need to keep their pH within a very narrow range around neutral (pH 7.0). However, some organisms need a very different pH range. The azalea plant thrives in acidic (pH 4.5) soil, and a microorganism called Picrophilus survives best at an extremely acidic pH of 0.7. For all of these different organisms, pH must be tightly controlled. One way pH is regulated in organisms is by substances called buffers. A Connecting CONC EP TS buffer is a compound that can bind to an H⫹ ion when the H+ concentration Human Biology In the human increases, and can release an H⫹ ion when the H⫹ concentration decreases. In body, both the respiratory other words, a buffer “locks up” H⫹ ions and helps to maintain homeostasis. system and the excretory system help regulate pH. You will learn For example, the normal pH of human blood is between 7.35 and 7.45, so it is about human systems and slightly basic. Just a small change in pH can disrupt processes in your cells, homeostasis in Chapter 28. and a blood pH greater than 7.8 or less than 6.8, for even a short time, is deadly. Buffers in your blood help prevent any large changes in blood pH. Apply Cells have higher H + concentrations than blood. Which has a higher pH? Why? ONLINE QUIZ 2.2 ASSESSMENT ClassZone.com REVIEWING MAIN IDEAS CRITICAL THINKING Connecting CONC EP TS 1. How do polar molecules form 4. Compare and Contrast How do 6. Cellular Respiration When hydrogen bonds? polar molecules differ from non- sugars are broken down to 2. What determines whether a com- polar molecules? How does this produce usable energy for cells, pound will dissolve in water? difference affect their interactions? a large amount of heat is 5. Connect Describe an example of released. Explain how the water 3. Make a chart that compares acids cohesion or adhesion that you inside a cell helps to keep the and bases. might observe during your daily life. cell’s temperature constant. Chapter 2: Chemistry of Life 43  2.3 Carbon-Based Molecules KEY CONCEPT Carbon-based molecules are the foundation of life. MAIN IDEAS VOCABULARY Carbon atoms have unique monomer, p. 45 protein, p. 47 bonding properties. polymer, p. 45 amino acid, p. 47 Four main types of carbon-based nucleic acid, p. 48 carbohydrate, p. 45 molecules are found in living things. lipid, p. 46 Review fatty acid, p. 46 atom, molecule, covalent bond Connect Car manufacturers often build several types of cars from the same internal frame. The size and style of the cars might differ on the outside, but they have the same structure underneath. Carbon-based molecules are similar, but 1.h Students know most mac- they are much more varied. There are millions of different carbon-based mol- romolecules (polysaccharides, nucleic acids, proteins, lipids) in ecules, but they form around only a few simple frames composed of carbon atoms. cells and organisms are synthe- sized from a small collection of simple precursors. MAIN IDEA 4.e Students know proteins can differ from one another in Carbon atoms have unique bonding properties. the number and sequence of amino acids. Carbon is often called the building block of life because carbon atoms are the 4.f* Students know why pro- basis of most molecules that make up living things. These molecules form the teins having different amino acid structure of living things and carry out most of the processes that keep organ- sequences typically have different shapes and chemical properties. isms alive. Carbon is so important because its atomic structure gives it bond- 5.a Students know the general ing properties that are unique among elements. Each carbon atom has four structures and functions of DNA, unpaired electrons in its outer energy level. Therefore, carbon atoms can form RNA, and protein. covalent bonds with up to four other atoms, including other carbon atoms. Review Life Science 8.6.c Students know that living As FIGURE 2.10 shows, carbon-based molecules have three fundamental organisms have many different structures—straight chains, branched chains, and rings. All three types of kinds of molecules, including small molecules are the result of carbon’s ability to form four covalent bonds. ones, such as water and salt, and very large ones, such as carbohy- Carbon chains can bond with carbon rings to form very large, very complex drates, fats, proteins, and DNA. molecules. These large molecules can be made of many small molecules that are bonded together. In a sense, the way these molecules form is similar to the way in which individual links of metal come together to make a bicycle chain. FIGURE 2.10 CARBON CHAINS AND RINGS Straight chain Branched chain Ring O H H H H H CH H C C C C C CH3 C H H H H CH2 CH CH A simplified structure can also be shown as: CH3 CH CH2 CH3 CH C C O CH3 CH2 CH2 CH CH2 CH3 OH Pentene Hexane Vanillin 44 Unit 1: Introducing Biology  In many carbon-based molecules, VISUAL VOCAB small molecules are subunits of an Each smaller molecule is a subunit entire molecule, like links in a chain. called a monomer. mono- = one Each subunit in the complete molecule poly- = many is called a monomer. When monomers are linked, they form molecules called monomer polymers. A polymer is a large mol- ecule, or macromolecule, made of many monomers bonded together. All polymer of the monomers in a polymer may be A polymer is a molecule that contains the same, as they are in starches, or they many monomers bonded together. may be different, as they are in proteins. Synthesize Write your own analogy for the formation of a polymer from monomers. MAIN IDEA Four main types of carbon-based molecules are found in living things. TAKING NOTES Use a content frame to help you All organisms are made of four types of carbon-based molecules: carbohydrates, understand monomers and poly- lipids, proteins, and nucleic acids. These molecules have different structures and mers in carbon-based molecules. functions, but all are formed around carbon chains and rings. Monomer Polymer Example Function Carbohydrates Fruits and grains are in different food groups, but they both contain large amounts of carbohydrates. Carbohydrates are molecules composed of carbon, hydrogen, and oxygen, and they include sugars and starches. Carbohydrates can be broken down to provide a source of usable chemical energy for cells. Carbohydrates are also a major part of plant cell structure. The most basic carbohydrates are simple sugars, or monosaccharides (MAHN-uh-SAK-uh-RYDZ). Many simple sugars have either five or six carbon atoms. CH2OH Fruits contain a six-carbon sugar called C O fructose. Glucose, one of the sugars made by H H H plant cells during photosynthesis, is another C C OH H six-carbon sugar. Simple sugars can be HO OH C C bonded to make larger carbohydrates. For example, two sugars bonded together make H OH the disaccharide you know as table sugar, Glucose (C6H12O6) can be ring shaped and shown in FIGURE 2.11. Many glucose molecules is often shown as a simplified hexagon. can be linked to make polysaccharides (PAHL- ee-SAK-uh-RYDZ), which are polymers of monosaccharides. Starches, glycogen, and cellulose are polysaccharides. Starches and glycogen are similar, but they differ from cellulose because their glucose monomers are bonded together differently. Most starches are branched chains of glucose molecules. Starches are made and stored by plants, and they can be broken FIGURE 2.11 Household sugar (sucrose) is a disaccharide, or two- down as a source of energy by plant and animal cells. Glycogen, which is made sugar molecule, of glucose (inset) and stored in animals, is more highly branched than plant starches. and fructose. Chapter 2: Chemistry of Life 45  FIGURE 2.12 CARBOHYDRATE STRUCTURE Polymer (starch) Starch is a polymer of glucose monomers that often has a branched structure. Polymer (cellulose) monomer Cellulose is a polymer of glucose monomers that has a straight, rigid structure. Connecting CONC EP TS Cellulose is somewhat different from starch and glycogen. Its straight, rigid Cell Structure A cell wall made structure, shown in FIGURE 2.12, makes the cellulose molecule a major building of cellulose surrounds the block in plant cell structure. Cellulose makes up the cell wall that is the tough membrane of plant cells. You outer covering of plant cells. You have eaten cellulose in the stringy fibers of will learn more about cell walls in Chapter 3. vegetables such as celery, so you know that it is tough to chew and break up. Lipids Lipids are nonpolar molecules that include fats, oils, and cholesterol. Like carbohydrates, most lipids contain chains of carbon atoms bonded to oxygen and hydrogen atoms. Some lipids are broken down as a source of usable energy for cells. Other lipids are parts of a cell’s structure. Fats and oils are two familiar types of lipids. They store large amounts of chemical energy in organisms. Animal fats are found in foods such as meat and butter. You know plant fats as oils, such as olive oil and peanut oil. The structures of fats and oils are similar. They both consist of a molecule called glycerol (GLIHS-uh-RAWL) bonded to molecules called fatty acids. Fatty acids are chains of carbon atoms bonded to hydrogen atoms. Two different types of fatty acids are shown in FIGURE 2.13. Many lipids, both fats and oils, contain three fatty acids bonded to glycerol. They are called triglycerides. Most animal fats are saturated fats, which means they have the maximum number of hydrogen atoms possible. That is, every place that a hydrogen atom can bond to a carbon atom is filled with a hydro- gen atom, and all carbon–carbon bonds are single bonds. You can think of the fatty acid as being “saturated” with hydrogen atoms. In contrast, fatty acids in FIGURE 2.13 Fatty acids can be oils have fewer hydrogen atoms because there is at least one double bond either saturated or unsaturated. between carbon atoms. These lipids are called unsaturated fats because the fatty acids are not saturated Saturated fatty acid with hydrogen atoms. Fats and Saturated fats contain O CH2 CH2 CH2 CH2 CH2 CH2 CH2 fatty acids in which all oils are very similar, but why C are animal fats solid and plant HO CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH3 carbon–carbon bonds are single bonds. oils liquid? The double bonds Unsaturated fatty acid in unsaturated fats make kinks Unsaturated fats have in the fatty acids. As a result, O CH CH CH CH CH2 CH2 CH2 fatty acids with at the molecules cannot pack C HO CH2 C CH2 C CH2 CH2 CH2 CH3 least one carbon– together tightly enough to carbon double bond. form a solid. 46 Unit 1: Introducing Biology  All cell membranes are made mostly of another type of lipid, called a phospholipid (FAHS-foh-LIHP-ihd). A phospholipid consists of glycerol, two fatty acids, and a phosphate group (PO4–) that is part of the polar “head” of the molecule. The fatty acids are the nonpolar “tails” of a phospholipid. Compare the structure of a phospholipid to the structure of a triglyceride in FIGURE 2.14. FIGURE 2.14 LIPID STRUCTURE Phospholipid A phospholipid Triglyceride A triglyceride has has nonpolar fatty three fatty acids PO4– acid “tails” and a and a molecule polar “head” that of glycerol, but contains a phos- no phosphate head tails phate group. group. Cholesterol (kuh-LEHS-tuh-RAWL) is a lipid that has a ring structure. You may hear about dangers of eating foods that contain a lot of cholesterol, such as eggs, but your body needs a certain amount of it to function. For example, cholesterol is a part of cell membranes, and your body uses it to make chemi- cals called steroid hormones. Cholesterol-based steroids have many functions. Some regulate your body’s response to stress. Others, such as testosterone and estrogen, control sexual development and the reproductive system. Proteins Proteins are the most varied of the carbon-based molecules in organisms. In FIGURE 2.15 Serine is one of 20 movement, eyesight, or digestion, proteins are at work. A protein is a polymer amino acids that make up proteins in organisms. made of monomers called amino acids. Amino acids are molecules that contain OH carbon, hydrogen, oxygen, nitrogen, and sometimes sulfur. Organisms use 20 H CH2 O different amino acids to build proteins. Your body can make 12 of the amino acids. The others come from foods you eat, such as meat, beans, and nuts. N C C H H OH Look at FIGURE 2.15 to see the amino acid serine. All amino acids have simi- lar structures. As FIGURE 2.16 shows, each amino acid monomer has a carbon atom that is bonded to four other parts. Three of these parts are the same in every amino acid: a hydrogen atom, an amino group (NH2), and a carboxyl group (COOH). Amino acids differ only in their side group, or the R-group. Amino acids form covalent bonds, called peptide bonds, with each other. The bonds form between the amino group of one amino acid and the car- boxyl group of another amino acid. Through peptide bonds, amino acids are linked into chains called polypeptides. A protein is one or more polypeptides. FIGURE 2.16 AMINO ACID AND PROTEIN STRUCTURE All amino acids have a carbon atom Monomer (amino acid) Polymer (protein) bonded to a hydrogen atom, an O R O amino group (NH2), and a carboxyl group (COOH). Different amino acids C N C C N have different side groups (R). H H H H R O peptide bonds peptide bonds N C C Peptide bonds form between the amino A polypeptide is a chain of precisely ordered H H OH group of one amino acid and the car- amino acids linked by peptide bonds. A pro- boxyl group of another amino acid. tein is made of one or more polypeptides. Chapter 2: Chemistry of Life 47  Proteins differ in the number and order of amino acids. The specific sequence of amino acids deter- mines a protein’s structure and function. Two types of interactions between the side groups of some amino acids are especially important in protein structure. First, some side groups contain sulfur atoms. The hydrogen bond sulfur atoms can form covalent bonds that force the protein to bend into a certain shape. Second, hydrogen bonds can form between the side groups of some amino acids. These hydrogen bonds cause the protein to fold into a specific shape. For example, FIGURE 2.17 shows the structure of one of the four polypeptides that makes up hemoglobin, the protein in your red blood cells that transports oxygen. Each of the four polypeptides contains an iron atom that bonds to an oxygen molecule. The four polypeptides are folded in a way that puts the four FIGURE 2.17 Hemoglobin in red oxygen-carrying sites together in a pocketlike structure inside the molecule. blood cells transports oxygen. The If a protein has incorrect amino acids, the structure may change in a way that structure of hemoglobin depends prevents the protein from working properly. Just one wrong amino acid of the on hydrogen bonds between spe- cific amino acids. Just one amino 574 amino acids in hemoglobin causes the disorder sickle cell anemia. acid change causes red blood cells to have the curved shape charac- Nucleic Acids teristic of sickle cell anemia. Detailed instructions to build proteins are stored in extremely long carbon- (colored SEM; magnification 3500⫻) based molecules called nucleic acids. Nucleic acids are polymers that are made up of monomers called nucleotides. A nucleotide is composed of a sugar, a phosphate group, and a nitrogen-containing molecule called a base. There are two general types of nucleic acids: DNA and RNA. Nucleic acids differ from the other types of carbon-based molecules. Carbohydrates, lipids, and proteins have a large number of structures and functions. Nucleic acids have just one function. They work together to make CHI6 proteins. DNA stores the information for putting amino acids together to hX^a^c`h#dg\ make proteins, and RNA helps to build proteins. DNA is the basis of genes and For more information on heredity, but cannot do anything by itself. Instead, the structure of DNA—the carbon-based molecules, visit scilinks.org. order of nucleotides—provides the code for the proper assembly of proteins. Keycode: MLB002 You will learn more about nucleic acids and how they build proteins in Unit 3. Apply What is the relationship between proteins and nucleic acids? ONLINE QUIZ 2.3 ASSESSMENT ClassZone.com REVIEWING MAIN IDEAS CRITICAL THINKING Connecting CONC EP TS 1. What is the relationship between a 3. Compare and Contrast How are 5. Biochemistry Why might polymer and a monomer? carbohydrates and lipids similar? fatty acids, amino acids, and 2. Explain how both nucleic acids and How are they different? nucleic acids increase the proteins are polymers. Be sure to 4. Infer Explain how the bonding hydrogen ion (H⫹) concentra- describe the monomers that make properties of carbon atoms result in tion of a solution? Explain your up the polymers. the large variety of carbon-based answer. molecules in living things. 48 Unit 1: Introducing Biology D ATA A N A LY S I S I D E N T I F Y I N G VA R I A B L E S DATA ANALYSIS ClassZone.com Independent and Dependent Variables In an experiment, a scientist determines the effect one variable has on another. A scientist changes, or manipulates, the independent variable and measures or observes the dependent variables. Therefore, data from an experiment are measurements of dependent variables. Changes in dependent variables “depend upon” the independent variable. EXAMPLE A scientist studied the effect of jogging on the GRAPH 1. JOGGING AND CALORIES number of Calories used. (The Calories in food are kilocalories, or 1000 calories.) People jogged (%% for three different lengths of time—10 minutes, '*% 20 minutes, and 30 minutes. The number of Calories used was measured, recorded, and '%% 8Vadg^ZhjhZY plotted on a graph like the one shown on the dependent right. What are the independent and dependent &*% variable variables? &%% The independent variable is the length of time spent jogging (10 minutes, 20 minutes, *% or 30 minutes). % The dependent variable is the number of &% '% (% Calories used while jogging—the number of independent I^bZ_d\\^c\b^cjiZh Calories “depends on” time. variable IDENTIFY VARIABLES A company that makes nutritional products is developing a new type of protein drink for athletes. A scientist at the company is studying the pH at which a digestive enzyme best breaks down the different proteins in the drink. The scientist uses the following experimental procedure: Five test tubes each contain 2 mL of the protein drink. Five different solutions contain the digestive enzyme, but each solution has a different pH—1.5, 2.0, 2.5, 3.0, and 3.5. One enzyme solution is added to each test tube of protein drink. Protein levels are measured in each of the five test tubes. 1. Identify What are the independent and dependent variables in the experiment? Explain your answers. 2. Apply Time is often used as a dependent variable in experiments. Describe how time could be used as a dependent variable in this experiment. Chapter 2: Chemistry of Life 49  2.4 Chemical Reactions KEY CONCEPT Life depends on chemical reactions. MAIN IDEAS VOCABULARY Bonds break and form during chemical reaction, p. 50 activation energy, p. 53 chemical reactions. reactant, p. 50 exothermic, p. 53 Chemical reactions release or endothermic, p. 53 product, p. 50 absorb energy. bond energy, p. 51 Review equilibrium, p. 51 atom, molecule Connect When you hear the term chemical reaction, what comes to mind? Maybe you think of liquids bubbling in beakers. You probably do not think of the air in your breath, but most of the carbon dioxide and water vapor that you 1 The fundamental life breathe out are made by chemical reactions in your cells. processes of plants and animals depend on a variety of chemical reactions that occur in specialized areas of the organism’s cells. MAIN IDEA Bonds break and form during chemical reactions. Plant cells make cellulose by linking simple sugars together. Plant and animal cells break down sugars to get usable energy. And all cells build protein mol- ecules by bonding amino acids together. These are just a few of the chemical reactions in living things. Chemical reactions change substances into different substances by breaking and forming chemical bonds. Reactants, Products, and Bond Energy Your cells need the oxygen molecules that you breathe in. Oxygen (O2) plays a part in a series of chemical reactions that provides usable energy for your cells. These reactions, which are described in detail in Chapter 4, break down the simple sugar glucose (C6H12O6). The process uses oxygen and glucose and results in carbon dioxide (CO2), water (H2O), and usable energy. Oxygen and glucose are the reactants. Reactants are the substances changed during a chemical reaction. Carbon dioxide and water are the products. Products are the substances made by a chemical reaction. Chemical equations are used to show what happens during a reaction. The overall equation for the process that changes oxygen and glucose into carbon dioxide and water is 6O2 + C6H12O6 6CO2 + 6H2O Reactants Direction Products The reactants are on the left side of the equation, and the products are on the right side. The arrow shows the direction of the reaction. This process, which is called cellular respiration, makes the carbon dioxide and water vapor that you breathe out. But for carbon dioxide and water to be made, bonds FIGURE 2.18 The breakdown must be broken in the reactants, and bonds must form in the products. What of glucose provides chemical energy for all activities, causes bonds in oxygen and glucose molecules to break? And what happens including speed skating. when new bonds form in carbon dioxide and water? 50 Unit 1: Introducing Biology  QUICK LAB MODELING Chemical Bonding You use energy to put things together, but chemical bonding is dif- MATERIALS ferent. Energy is added to break bonds, and energy is released when 2 flat magnets bonds form. PROBLEM How is chemical bonding similar to the interaction between two magnets? PROCEDURE 1. Bring the magnets close to each other until they snap together. 2. Pull the magnets away from each other. ANALYZE AND CONCLUDE 1. Infer How is bond formation represented by the snapping sound? 2. Apply How is bond energy related to your separation of the magnets? First, energy is added to break bonds in molecules of oxygen and glucose. Bond energy is the amount of energy that will break a bond between two atoms. Bonds between different types of atoms have different bond energies. A certain amount of energy is needed to break bonds in an oxygen molecule. A different amount of energy is needed to break bonds in a glucose molecule. Energy is released when bonds form, such as when molecules of water and carbon dioxide are made. When a bond forms, the amount of energy released is equal to the amount of energy that breaks the same bond. For example, energy is released when hydrogen and oxygen atoms bond to form a water molecule. The same amount of energy is needed to break apart a water molecule. Chemical Equilibrium Some reactions go from reactants to products until the reactants are used up. However, many reactions in living things are reversible. They move in both directions at the same time. These reactions tend to go in one direction or the other depending on the concentrations of the reactants and products. One such reaction lets blood, shown in FIGURE 2.19, carry carbon dioxide. Carbon dioxide reacts with water in blood to form a compound called carbonic acid (H2CO3). Your body needs this reaction to get rid of carbon dioxide waste from your cells. CO2 + H2O H2CO3 The arrows in the equation above show that the reaction goes in both directions. When the carbon dioxide concentration is high, as it is around your cells, the reaction moves toward the right and carbonic acid forms. In your lungs, the carbon dioxide concentration is low. The reaction goes in the other FIGURE 2.19 Blood cells and direction, and carbonic acid breaks down. plasma transport materials When a reaction takes place at an equal rate in both directions, the reactant throughout the body. Carbonic and product concentrations stay the same. This state is called equilibrium. acid dissolves in the blood so that carbon dioxide can be trans- Equilibrium (EE-kwuh-LIHB-ree-uhm) is reached when both the reactants and ported to the lungs. (composite products are made at the same rate. colored SEM; magnification 1000⫻) Apply Explain why concentration is important in a chemical reaction. Chapter 2: Chemistry of Life 51  FIGURE 2.20 Energy and Chemical Reactions BIOLOGY Energy is required to break bonds in reactants, and energy is released Watch exothermic when bonds form in products. Overall, a chemical reaction either absorbs and endothermic or releases energy. reactions at ClassZone.com. ACTIVATION ENERGY When enough activation energy is added to the reactants, bonds in the reactants break and the reaction begins. VXi^kVi^dc ZcZg\n activation energy energy in Energy reactants Reaction progress EXOTHERMIC REACTION Energy Released ENDOTHERMIC REACTION Energy Absorbed total energy total energy added added total energy total energy released released Energy Energy reactants products reactants difference in energy products difference in energy (released) (absorbed) Reaction progress Reaction progress The products in an exothermic reaction have a lower bond The products in an endothermic reaction have a higher bond energy than the reactants, and the difference in bond energy energy than the reactants, and the difference in bond energy is released to the surroundings. is absorbed from the surroundings. CRITICAL Is the amount of activation energy related to whether a reaction VIEWING is exothermic or endothermic? Why or why not? 52 Unit 1: Introducing Biology  MAIN IDEA Chemical reactions release or absorb energy. All chemical reactions involve changes in energy. Energy that is added to the VOCABULARY reactants breaks their chemical bonds. When new bonds form in the products, The prefix exo- means “out,” energy is released. This means that energy is both absorbed and released and the prefix endo- means “in.” Energy “moves out of” during a chemical reaction. Some chemical reactions release more energy than an exothermic reaction, and they absorb. Other chemical reactions absorb more energy than they release. energy “moves into” an Whether a reaction releases or absorbs energy depends on bond energy. endothermic reaction. Some energy must be absorbed by the reactants in any chemical reaction. Activation energy is the amount of energy that needs to be absorbed for a chemical reaction to start. Activation energy is like the energy you would need to push a rock up a hill. Once the rock is at the top of the hill, it rolls down the other side by itself. A graph of the activation energy that is added to start a chemical reaction is shown at the top of FIGURE 2.20. An exothermic chemical reaction releases more energy than it absorbs. If the products have a lower bond energy than the reactants, the reaction is exothermic. The excess energy—the difference in bond energy between the reactants and products—is often given off as heat or light. Some animals, such as squids and fireflies, give off light that comes from exothermic reactions, as shown in FIGURE 2.21. Cellular respiration, the process that uses glucose and oxygen to provide usable energy for cells, is also exothermic. Cellular respiration releases not only usable energy for your cells but also heat that keeps your body warm. An endothermic chemical reaction absorbs more energy than it releases. If products have a higher bond energy than reactants, the reaction is endo- thermic. Energy must be absorbed to make up the difference. One of the FIGURE 2.21 The glow of most important processes for life on Earth, photosynthesis, is endothermic. the bugeye squid comes from During photosynthesis, plants absorb energy from sunlight and use that an exothermic reaction that energy to make simple sugars and complex carbohydrates. releases light. Analyze How is activation energy related to bond energy? ONLINE QUIZ 2.4 ASSESSMENT ClassZone.com REVIEWING MAIN IDEAS CRITICAL THINKING Connecting CONC EP TS 1. Hydrogen peroxide (H2O2) 3. Infer The process below is exothermic. 5. Biochemistry A chemical breaks down into water (H2O) What must be true about the bond reaction can start when and oxygen (O2). Explain why energies of the reactants and the enough activation energy is this is a chemical reaction. products? Explain. added to the reactants. Do you What are the reactants and 6O2 + C6H12O6 6CO2 + 6H2O think the activation energy for the products in the reaction? chemical reactions in living 4. Evaluate Why might it not always be 2. How do endothermic and things is high or low? Explain possible to determine the reactants and exothermic reactions differ? your answer. the products in a reaction? Explain your answer in terms of chemical equilibrium. Chapter 2: Chemistry of Life 53  2.5 Enzymes KEY CONCEPT Enzymes are catalysts for chemical reactions in living things. MAIN IDEAS VOCABULARY A catalyst lowers activation energy. catalyst, p. 54 Review Enzymes allow chemical reactions to occur enzyme, p. 55 chemical reaction, under tightly controlled conditions. activation energy, protein, substrate, p. 56 hydrogen bond Connect Just how can a Venus flytrap digest a frog? It happens through the action of proteins called enzymes. Enzymes help to start and run chemical reactions in living things. For example, enzymes are needed to break down food 1.b Students know enzymes into smaller molecules that cells can use. Without enzymes, a Venus flytrap are proteins that catalyze biochemical reactions without couldn’t break down its food, and neither could you. altering the reaction equilibrium and the activities of enzymes depend on the temperature, MAIN IDEA ionic conditions, and the pH of the surroundings. A catalyst lowers activation energy. Remember what you learned about activation energy in Section 2.4. Activa- tion energy for a chemical reaction is like the energy that is needed to push a rock up a hill. When enough energy is added to get the rock to the top of a hill, the rock can roll down the other side by itself. Activation energy gives a similar push to a chemical reaction. Once a chemical reaction starts, it can continue by itself, and it will go at a certain rate. Often, the activation energy for a chemical reaction comes from an increase in temperature. But even after a chemical reaction starts, it may happen very slowly. The reactants may not interact enough, or they may not be at a high enough concentration, to quickly form the products of the reaction. However, both the activation energy and rate of a chemical reaction can be changed by a chemical catalyst, as shown in FIGURE 2.22. A catalyst (KAT-l-ihst) is a substance that decreases the activation energy needed to start a chemical reaction and, as a result, also increases the rate of the chemical reaction. FIGURE 2.22 CATALYSTS AND ACTIVATION ENERGY Under normal conditions, a certain amount of activation Normal reaction energy is needed to start a chemical reaction. A catalyst activation energy (uncatalyzed) Catalyzed reaction decreases the activation energy needed. products Energy reactants activation energy (catalyzed) Reaction progress 54 Unit 1: Introducing Biology  Compare the activation energies and the reaction rates in the graph in FIGURE 2.22. Under normal conditions, the reaction requires a certain amount of activation energy, and it occurs at a certain rate. When a catalyst is present, less energy is needed and the products form faster. Although catalysts take part in chemical reactions, catalysts are not considered to be either reactants or products because catalysts are not changed or used up during a reaction. Summarize Describe two functions of catalysts in chemical reactions. MAIN IDEA Enzymes allow chemical reactions to occur under tightly controlled conditions. Chemical reactions in organisms have to take place at an organism’s body temperature. Often, reactants are found in low concentrations. Because the reactions must take place very quickly, they usually need a catalyst. Enzymes are catalysts for chemical reactions in living things. Enzymes, like other catalysts, lower the activation energy and increase the rate of chemical reac- tions. In reactions that are reversible, such as the carbon dioxide and carbonic acid reaction described in Section 2.4, enzymes do not affect chemical equilib- rium. This means that enzymes do not change the direction of a reaction— they just change the amount of time needed for equilibrium to be reached. Enzymes are involved in almost every process in organisms. From breaking down food to building proteins, enzymes are needed. For example, amylase is an enzyme in saliva that breaks down starch into simpler sugars. This reaction occurs up to a million times faster with amylase than without it. Enzymes are also an important part of your immune system, as shown in FIGURE 2.23. Almost all enzymes are proteins. These enzymes, like other proteins, are long chains of amino acids. Each enzyme also depends on its structure to function properly. Conditions such as temperature and pH can affect the shape and func- tion, or activity, of an enzyme. Enzymes work best in a small temperature range around the organism’s normal body temperature. At only slightly higher temperatures, the hydrogen bonds in an enzyme may begin to break apart. The enzyme’s structure changes and it loses its ability to function. This is one reason why a very high fever is so dangerous to a person. A change in pH can also affect the hydrogen bonds in enzymes. Many FIGURE 2.23 The inset micrograph (top) shows a white blood cell engulfing enzymes in humans work best at the an invading pathogen. The larger micrograph shows a pathogen after it has nearly neutral pH that is maintained been captured. Once inside a white blood cell, enzymes are used to destroy the pathogen. (inset image: colored SEM; magnification about 3000⫻; large image: within cells of the human body. colored TEM; magnification 11,000⫻) Chapter 2: Chemistry of Life 55  Connecting CONC EP TS Enzyme structure is important because each enzyme’s shape allows only Biochemistry The order of certain reactants to bind to the enzyme. The specific reactants that an enzyme amino acids determines the acts on are called substrates. For example, amylase only breaks down starch. structure and function of an Therefore, starch is the substrate for amylase. Substrates temporarily bind to enzyme. An enzyme’s structure often depends on hydrogen enzymes at specific places called active sites. Like a key fits into a lock, sub- bonds between amino acids. strates exactly fit the active sites of enzymes. This is why if an enzyme’s struc- ture changes, it may not work at all. This idea of enzyme function, which is called the lock-and-key model, is shown below. hjWhigViZh egdYjXi gZVXiVcih ZconbZ 1 2 3 Substrates bind to an The enzyme brings The catalyzed reaction forms enzyme at certain places substrates together and a product that is released called active sites. weakens their bonds. from the enzyme. The lock-and-key model helps explain how enzymes work. First, enzymes bring substrate molecules close together. Because of the low concentrations of reactants in cells, many reactions would be unlikely to take place without enzymes bringing substrates together. Second, enzymes decrease activation energy. When substrates bind to the enzyme at the enzyme’s active site, the bonds inside these molecules become strained. If bonds are strained, or stretched slightly out of their normal positions, they become weaker. Less activation energy is needed for these slightly weakened bonds to be broken. The lock-and-key model is a good starting point for understanding enzyme function. However, scientists have recently found that the structures of enzymes are not fixed in place. Instead, enzymes actually bend slightly when they are bound to their substrates. In terms of a lock and key, it is as if the lock bends around the key to make the key fit better. The bending of the enzyme is one way in which bonds in the substrates are weakened. Apply How does the structure of an enzyme affect its function? ONLINE QUIZ 2.5 ASSESSMENT ClassZone.com REVIEWING MAIN IDEAS CRITICAL THINKING Connecting CONC EP TS 1. How does a catalyst affect the 3. Infer Some organisms live in very 5. Homeostasis Organisms need activation energy of a chemical hot or very acidic environments. to maintain homeostasis, or reaction? Would their enzymes function in a stable internal conditions. Why 2. Describe how the interaction person’s cells? Why or why not? is homeostasis important for between an enzyme and its 4. Predict Suppose that the amino the function of enzymes? substrates changes a chemical acids that make up an enzyme’s reaction. active site are changed. How might this change affect the enzyme? 56 Unit 1: Introducing Biology CHAPTER 2 I N V E S T I G AT I O N MATERIALS 5 test tubes Enzymatic Activity test tube rack Enzymes are necessary for many processes, including digestion and fighting disease. marker Conditions such as temperature and pH must be tightly controlled so that enzymes 7 10-mL graduated cylinders can function properly. In this lab, you will study an enzyme called catalase that 4 mL each of solutions helps break down hydrogen peroxide into water and oxygen. of pH 3, 5, 7, 9, and 11 2 mL 60% catalase PROBLEM How does pH affect enzymatic activity? solution 1 mL 3% hydrogen PROCEDURE peroxide solution 1. Label 5 test tubes pH 3, pH 5, pH 7, pH 9, pH 11. Place them in the test tube rack. metric ruler 2. Add 4 mL of the appropriate pH solution to each test tube. Be sure to use a different graduated cylinder for each of the solutions. 3. Add 2 mL of the catalase enzyme solution to each of the test tubes. Gently swirl the test tubes to mix the solutions. Allow the test tubes to sit for 5 minutes. PROCESS SKILLS 4. Design a data table that has rows labeled with the independent variable and Identifying Variables columns labeled with the dependent variable. Read step 6 to determine whether Observing the foam height is the independent variable or the dependent variable. Measuring 5. Add 1 mL of the hydrogen peroxide solution to each test tube. Allow 5 minutes Collecting Data for the solutions to react. Foam should appear on the solution tops. Interpreting Data Caution: Avoid skin contact with hydrogen peroxide. 6. Measure the distance in millimeters from the bottom of each test tube to the top of the foam in the test tube. Record your measurements in your data table. 86A>;DGC>6 HI6C96G9H ANALYZE AND CONCLUDE 1.b Students know 1. Identify Variables What are the independent and dependent variables? What is enzymes are proteins that the operational definition of the dependent variable? catalyze biochemical reac- 2. Analyze Choose a type of graph to appropriately display your data. Construct tions without altering the reaction equilibrium and your graph. the activities of enzymes 3. Analyze How is the enzymatic activity of catalase related to pH? What does this depend on the tempera- tell you about the pH of your cells? ture, ionic conditions, and 4. Infer The activity of an enzyme depends upon its structure. What do your results the pH of the surroundings. suggest about the effect of pH on the structure of catalase? Explain. 1E.1.a Select and use appropriate tools and tech- 5. Experimental Design Why is it important that each test tu