Ev e ry living c e l l needs a source of

12 Photosynthesis and Cellular Respiration Ev e ry living c e l l needs a source of energy. Without energy, metabolism all of the chemical reactions that occur within cells will not occur. In this activity, you will learn how the complex chemical reactions of photosynthesis and cellular respiration help meet the energy needs of living things. You will examine the organelles, molecules, and chemical reactions involved in these two processes. You will also learn how a microbe or chemical that disrupts one or more of the steps of photosynthesis or cellular respiration causes disease. Challenge solar energy sunlight co 2 h 2 o photosynthesis in chloroplasts cellular respiration in mitochondria 00How do photosynthesis and cellular respiration meet the energy needs of all organisms? atp powers most cellular work heat carbon-containing molecules h 2 o Photosynthesis and cellular respiration meet the energy needs of organisms. Materials For each student Student Sheet 12.1, Anticipation Guide: Photosynthesis and Cellular Respiration Procedure 1. Fill in only the Before column of Student Sheet 12.1, Anticipation Guide: Photosynthesis and Cellular Respiration. 2. Visit the Science and Global Issues page of the SEPUP website at sepuplhs.org/sgi. With your partner, go to Photosynthesis and Cellular Respiration and follow the simulation. 3. Complete the Reading. 4. Fill in the After column on Student Sheet 12.1, Anticipation Guide: Photosynthesis and Cellular Respiration. 219

Science & Global Issues/Biology cell biology Reading Energy for Life e v e ry ce l l ne e d s a s o u r c e of en e rgy As you learned in the Ecology: Living on Earth unit of Science and Global Issues, all cells need energy if they are to function. Cellular respiration is the process by which cells break down complex molecules, such as sugars, to release energy. Some of the energy is released as heat while the rest is stored temporarily in other molecules, such as adenosine triphosphate (ATP). ATP is used when the cell needs energy. For example, whenever your muscles contract, ATP supplies the energy. As you also learned in the Ecology unit, photosynthesis is the process by which the cells of producers capture the sun s energy and store it in sugars. All producers and consumers ultimately depend on these sugars for their energy needs. The following equations summarize the two processes. Cellular respiration: sugars 1 oxygen & carbon dioxide 1 water 1 energy Photosynthesis: carbon dioxide 1 water 1 light energy & sugars 1 oxygen As you can see from the equations, the components of cellular respiration and photosynthesis are nearly identical. In fact, cellular respiration is often described as the opposite of photosynthesis. The two processes, however, are far more complex than the equations indicate. p h o t o s y n t h e s i s Photosynthesis only occurs in certain pigment-containing cells of producers. This differs from cellular respiration, which occurs in all types of cells in all organisms. A pigment is a molecule that absorbs light energy. All producers contain pigments, which are essential for photosynthesis. The most common pigment in producers is chlorophyll. Chlorophyll is what makes the stems and leaves of producers green. In most producers the chlorophyll is contained in chloroplasts. Light micrograph of Elodea cells 220

photosynthesis and cellular respiration Activity 12 Chloroplasts are relatively large organelles surrounded by two membranes. Chloroplasts absorb energy from sunlight, as shown in the diagram below. Inside the chloroplast is a fluid-filled space called the stroma and stacks of connected membrane sacs called thylakoids. The thylakoid membranes contain the chlorophyll, and the stacked structure greatly increases the membrane surface area that can absorb light. PlanT cell animal cell mitochondrion outer membrane inner membrane cristae matri chloroplast outer membrane inner membrane stroma thylakoids mitochondria chloroplast Mitochondria are present in both plants and animal cells, while chloroplasts are present only in plant cells. 221

Science & Global Issues/Biology cell biology t h e re a c t i o n s of ph o t o s y n t h e s i s The reactions occurring in photosynthesis are grouped into two stages, lightdependent and light-independent reactions. The light-dependent reactions rely on chlorophyll and other pigments in the thylakoid membranes to harness the energy of light. In a complex series of reactions, water breaks down into oxygen, hydrogen ions (protons), and electrons. Each reaction requires a unique enzyme. The oxygen is released to the atmosphere, some of the energy from the reactions goes into making ATP from ADP, and the protons and electrons combine with the carrier molecule, NADP +, to form NADPH. The NADPH and ATP are important to the light-independent Calvin cycle, which is a series of enzyme-catalyzed reactions that take place in the stroma of the chloroplast. During these reactions carbon dioxide combines with the hydrogen ions and electrons produced from water during the light-dependent reactions. This results in the production of the high-energy sugar glucose. The glucose is used in cellular respiration or is converted to another form, such as starch, and stored for later use. Starch is a type of carbo hydrate. A starch macromolecule is made of many glucose molecules linked together. 222

photosynthesis and cellular respiration Activity 12 The reactions in the Calvin cycle occur in the presence or absence of light and are referred to as light-independent reactions. During the Calvin cycle ATP and NADPH are converted back into ADP and NADP+. These will be used again in the light-dependent reactions in the thylakoid membranes. light-independent reactions chloroplast outer membrane solar energy sunlight h 2 o co 2 lightdependent reactions carrier molecule adp P atp calvin cycle inner membrane stroma thylakoids o 2 carrier molecule with hydrogen glucose The reactions of the Calvin cycle continue as long as carbon dioxide, ATP, NADPH, and enzymes are present. Certain plant diseases, such as the plum pox virus that attacks several kinds of fruit trees, inhibit enzymes involved in the Calvin cycle. This prevents the plant from making the sugar it needs to support cellular respiration. Trees infected with plum pox may eventually stop producing fruit. Organisms that feed off the fruit may need to find another food source, and communities that rely on fruit sales may suffer economically. c e l l u l a r re s p i r at i o n Each reaction in cellular respiration also requires a unique enzyme, and, as in photosynthesis, diseases or poisons may reduce the activity of these enzymes. This can have serious consequences for an organism, since cellular respiration is the process by which energy is released for cells to use. It is important not to confuse cellular respiration with breathing, which is often called respiration. Breathing gets oxygen into your lungs, but the oxygen has no purpose until it enters your cells and plays its part in cellular respiration. 223

Science & Global Issues/Biology cell biology In prokaryotes, such as bacteria, the reactions of cellular respiration that require oxygen occur on the cell membrane. In eukaryotes, these reactions take place in the cells mitochondria. Mitochondria are organelles that have two membranes, an outer membrane and an inner membrane that is folded within the outer, which play central roles in the reactions of cellular respiration. The outer membrane contains specialized proteins that make the membrane very permeable to ions and small molecules. The inner membrane is less permeable, but it contains transport proteins to allow for the passage of some molecules. The inner membrane also contains enzymes needed for cellular respiration. The inner membrane is folded to form many cristae, finger-like projections that provide a large surface area for the reactions of cellular respiration. Inside a mitochondrion is a space called the matrix. t h e re a c t i o n s of ce l l u l a r re s p i r at i o n Cellular respiration begins with the process of glycolysis. Glycolysis means the breaking of sugar. This first step takes place in the cytoplasm. In glycolysis, the energy from ATP breaks a glucose molecule, releasing hydrogen ions and creating two smaller molecules of pyruvic acid. The carrier molecule, NAD +, combines with these hydrogen ions and with electrons to form NADH. Glycolysis also produces new ATP molecules, resulting in a net gain of 2 ATP molecules for each molecule of glucose that is broken down. If oxygen is present, the pyruvic acid molecules produced during glycolysis are then further broken down, releasing large amounts of energy in a process called aerobic respiration. In eukaryotic cells, the pyruvic acid molecules first enter the mitochondrion where they are stripped of a carbon atom and are also temporarily 224

photosynthesis and cellular respiration Activity 12 combined with the enzyme helper, coenzyme A. This combination, called acetyl CoA, becomes part of the next stage of cellular respiration, the Krebs cycle. The Krebs cycle takes place in the matrix of the mitochondrion. mitochondrion outer membrane inner membrane aerobic respiration pyruvate oygen glucose glycolysis Krebs cycle electron transport chain cristae cytoplasm matri cytoplasm 2 atp co 2 h 2 atp 2 o 32 atp electrons transported on carrier molecules In the Krebs cycle the acetyl CoA breaks down and releases coenzyme A and pyruvic acid. The carbon atoms of the pyruvic acid end up in carbon dioxide, which is released as waste. Two molecules of ATP are produced and the carrier molecules NAD + and FAD + pick up electrons and hydrogen ions to form NADH and FADH, respectively. These are transferred by the carriers to the cristae of the mitochondria s inner membranes to take part in the final stage of cellular respiration, the electron transport chain. During this stage, electrons and hydrogen ions are released when NADH and FADH, are converted back to NAD + and FAD +. In a series of enzyme-catalyzed reactions, electrons combine with hydrogen ions and oxygen to make water, which then leaves the mitochondrion. The movement of electrons through the electron transport chain releases energy, which converts 32 more molecules of ADP into ATP. Aerobic respiration, therefore, produces a total of 36 ATP molecules for each glucose molecule. Since the reactions of the electron transport chain need oxygen, they will stop if oxygen is not available, which prevents the Krebs cycle from operating. Some organisms do not need oxygen for cellular respiration and instead use a process called anaerobic respiration. This process also occurs for short periods of time in muscle cells when insufficient oxygen is present to conduct aerobic respiration. Anaerobic respiration, also known as fermentation, takes place in the 225

Science & Global Issues/Biology cell biology cytoplasm of a cell. There are two types of fermentation: lactate fermentation and alcoholic fermentation. In lactate fermentation the pyruvic acid that is produced by glycolysis is changed into a form of lactic acid. This is the type of anaerobic respiration that occurs in muscle cells. However, anaerobic respiration does not sustain muscle cells for very long: as lactic acid builds up, fatigue and muscle cramps result. This same process in bacteria causes food to spoil, and with certain bacteria such fermentation allows us to make cheese and yogurt. In alcoholic fermentation, shown in the figure below, pyruvic acid reacts with water to form acetaldehyde and release carbon dioxide. (When yeast is added to bread dough, alcoholic fermentation releases carbon dioxide, which helps the bread to rise.) In the presence of electrons and hydrogen ions that have been transported by NADH the acetaldehyde is then converted into ethanol. This is the same process that occurs, for example, when energy companies convert corn into the biofuel ethanol and when people make wine from grapes. During fermentation there is a net gain of only two molecules of ATP. When compared to the net gain of 32 ATP molecules produced during the Krebs cycle and the electron transport chain stage, it is clear that aerobic respiration provides much more energy to cells than does anaerobic respiration. 226

photosynthesis and cellular respiration Activity 12 Some diseases and toxins disrupt cellular respiration in plants and animals. For example, powdery mildew fungus and cyanide both inhibit the electron transport chain. Powdery mildew causes premature death of infected plants, and cyanide is highly toxic to organisms that respire aerobically. Some genetic disorders involve mutations in certain enzymes of cellular respiration. In lactate dehydrogenase deficiency, for example, the enzyme that converts pyruvic acid to lactic acid is affected, which causes fatigue, and muscle damage if the afflicted person exercises intensely. Diabetes can also disrupt respiration. If insulin levels become too low, cells don t get enough glucose to meet their cellular respiration needs, and the body switches to fat as an energy source. As fat is broken down to be used for cellular respiration, a by-product of the process builds up and makes the blood acidic. This condition can lead to coma and death. Analysis 1. Describe the roles of enzymes in photosynthesis and respiration. 2. a. What is chlorophyll? b. What is chlorophyll s role in photosynthesis? 3. Describe how a plant cell is specialized for photosynthesis at: a. the level of organelles and other cellular structures. b. the molecular level. 4. Compare aerobic and anaerobic respiration. What are the similarities? What are the differences? 5. In a chart like the one below, write the differences between photosynthesis and cellular respiration. Photosynthesis and Cellular Respiration Comparison In what cells and cell structures does it happen? Reactants Products How does it contribute to the organism s energy needs? Photosynthesis Cellular respiration 227

Science & Global Issues/Biology cell biology 6. What are some of the similarities of photosynthesis and cellular respiration? 7. If you mixed carbon dioxide and water in a test tube, and placed the test tube in the sunlight, would photosynthesis take place? Explain. 8. How might reduced enzyme function lead to disease? Give at least two specific examples. Key vocabulary adenosine triphosphate (ATP) aerobic respiration anaerobic respiration Calvin cycle cellular respiration chloroplast cristae electron transport chain glycolysis Krebs cycle light-dependent reactions light-independent reactions matrix mitochondria, mitrochondrion organelle photosynthesis pigment stroma thylakoids 228