Cellular Respiration On a warm summer day in 1974, 8-year-old Sarah suddenly felt pins and needles in the muscles of her legs as she walked. Within a year s time, she could no longer walk without experiencing muscle pain, shortness of breath, and a racing heart. By the age of 16, Sarah was attending school in a wheelchair. Tests showed that she had high levels of acidity in her blood. Her doctors were puzzled. After extensive study, a team of researchers traced Sarah s problem to a defect in her mitochondria. Her muscles could not make sufficient amounts of ATP and instead produced large amounts of a compound called lactic acid. In time, doctors discovered that a simple mixture of vitamin C and vitamin K reversed the condition. After taking the medication, Sarah began feeling better almost instantly. By the age of 20, she no longer used a wheelchair and was completely free of symptoms. The vitamin mixture did not cure Sarah s mitochondrial defect; it simply filled a gap in the chemical reactions that produce ATP. Without a team of investigators and laboratory technicians who understood the reactions that produce ATP (cellular respiration), Sarah would have been denied the opportunity to lead a normal life. Cellular Respiration Virtually all organisms (autotrophs and heterotrophs) transfer chemical energy from glucose to ATP by a process called cellular respiration. As you know, autotrophs, such as green plants, produce glucose by photosynthesis, and heterotrophs, such as humans, obtain glucose by eating other organisms. There are two forms of cellular respiration, aerobic cellular respiration and anaerobic cellular respiration. Aerobic cellular respiration uses oxygen, and produces large amounts of ATP. Anaerobic cellular respiration does not use oxygen, and produces small amounts of ATP. Large organisms like humans rely on aerobic cellular respiration to provide the large amounts of ATP they need to maintain their daily activities. Aerobic Cellular Respiration Aerobic cellular respiration occurs in two stages, glycolysis and oxidative respiration. Glycolysis is a series of 10 enzyme-catalyzed reactions, occurring in the cytoplasm, that essentially breaks one molecule of glucose (C 6 H 12 O 6 ) into two molecules of pyruvate (C 3 H 6 O 3 ). The reactions of glycolysis are exergonic reactions, meaning that they release energy. Some of the released energy is captured by producing two molecules of ATP. This is accomplished by attaching a phosphate group to each of two molecules of ADP. The formation of ATP absorbs the chemical energy released by glucose. A reaction that absorbs energy, like the formation of ATP, is called an endergonic reaction. The equation on the next page summarizes the process of glycolysis. 1.18 aerobic cellular respiration a form of cellular respiration that uses oxygen and produces large amounts of ATP anaerobic cellular respiration a form of cellular respiration that does not use oxygen and produces small amounts of ATP glycolysis a series of reactions occurring in the cytoplasm that breaks a glucose molecule into two pyruvate molecules and forms two molecules of ATP NEL Cellular Biology 77
glycolysis C 6 H 12 O 6 2C 3 H 6 O 3 glucose pyruvate 2 ADP 2P i 2ATP (Note: P i symbolizes a phosphate group.) oxidative respiration a series of reactions occurring in the mitochondrion that uses oxygen to convert pyruvate into carbon dioxide, water, and ATP DID YOU KNOW? Cyanide Cyanide blocks the reactions of oxidative respiration. This disruption severely reduces the amount of ATP that is produced in an animal s body, resulting in coma and death. This is why cyanide is a poison. The two molecules of pyruvate formed in glycolysis are transported into the matrix of a mitochondrion where, through a series of exergonic reactions involving oxygen, they are broken down into six molecules of carbon dioxide (CO 2 ) and six molecules of water. Because oxygen is used, this series of reactions is known as oxidative respiration. This process is also called the Krebs cycle in honour of Hans Krebs, the biochemist who discovered these reactions. Much of the large amount of energy released in this process is captured by the production of 34 molecules of ATP. The following equation summarizes the process of oxidative respiration. oxidative respiration 2C 3 H 6 O 3 6O 2 6CO 2 6H 2 O pyruvate oxygen carbon dioxide water 34 ADP 34 P i 34 ATP The oxygen used in oxidative respiration is obtained from air or water; that is why land animals have lungs and fish have gills. Carbon dioxide is released to the environment as a waste product. The overall process of aerobic respiration (glycolysis oxidative respiration) uses oxygen to convert glucose into carbon dioxide, water, and ATP. The following reaction summarizes the overall process of aerobic respiration: aerobic respiration C 6 H 12 O 6 6O 2 6CO 2 6H 2 O glucose oxygen carbon dioxide water 36 ADP 36 P i 36 ATP mitochondrial DNA the DNA in mitochondria The Mitochondrion and Aerobic Cellular Respiration The mitochondrion (Figure 1) is the site of oxidative respiration, and is therefore the location in a cell where most ATP is produced. That is why the mitochondrion is sometimes called the powerhouse of the cell. Some of the enzymes that catalyze the reactions of aerobic respiration are free floating in the matrix of the mitochondrion, while others are embedded in the inner mitochondrial membrane. Mitochondria are so important to the cells of larger organisms that they possess their own DNA molecules, called mitochondrial DNA, or mdna. Mitochondrial DNA allows mitochondria to reproduce. 78 Unit 1 NEL
Section 1.18 mitochondrial matrix cristae outer mitochondrial membrane inner mitochondrial membrane Figure 1 Oxidative ATP synthesis occurs in the matrix and the cristae of a mitochondrion. Anaerobic Respiration When oxygen is not available, organisms may still produce ATP from glucose through the process of anaerobic cellular respiration. Some cells accomplish this through a series of reactions called fermentation. There are two forms of fermentation, ethanol fermentation and lactate (lactic acid) fermentation. Ethanol fermentation occurs in yeast cells (single-celled fungi), and lactate fermentation occurs in human muscle cells during periods of strenuous exercise. Ethanol Fermentation In ethanol fermentation, glucose is first broken down by the reactions of glycolysis (Figure 2). This produces two molecules of pyruvate and two molecules of ATP. An enzyme in the cytoplasm called pyruvate decarboxylase removes a carbon dioxide molecule from pyruvate, converting it into ethanol, the alcohol found in alcoholic beverages. The reactions of ethanol fermentation all occur in the cytoplasm of the cells. The two ATP molecules produced satisfy the organism s energy needs, and the ethanol and carbon dioxide are released as waste products. Humans have learned ways of making use of these cellular wastes. Ethanol fermentation carried out by yeast cells is of great historical, economic, and cultural importance. Breads and pastries, wine, beer, liquor, and soy sauce are all products of fermentation (Figure 3, on the next page). ethanol fermentation a form of anaerobic respiration in which glucose is converted into ethanol, ATP, and carbon dioxide gas glucose 2 ADP GLYCOLYSIS 2 ATP 2 pyruvate CO 2 2 ethanol Figure 2 Ethanol fermentation creates ethanol, carbon dioxide, and ATP from glucose. NEL Cellular Biology 79
Figure 3 Ethanol fermentation is used in the production of baked goods, wine, and beer. lactate (lactic acid) fermentation a form of anaerobic respiration in which glucose is converted into lactate and ATP oxygen debt the extra oxygen required to clear the body of lactate that accumulates in muscle cells during vigorous exercise Bread may be leavened (assisted to rise) by mixing live yeast cells with starches (in flour) and water. The yeast cells ferment the glucose (from the starch in flour) and release carbon dioxide and ethanol. Small bubbles of carbon dioxide gas cause the bread to rise, and the ethanol evaporates away when the bread is baked. In beer making and winemaking, yeast cells ferment the sugars found in sugar-rich fruit juices such as grape juice. The mixture bubbles as the yeast cells release carbon dioxide gas and ethanol during fermentation. In winemaking, fermentation ends when the concentration of ethanol reaches approximately 12%. At this point, the yeast cells die as a result of ethanol accumulation and the product is used as a beverage. Plants with flooded roots also undergo ethanol fermentation (in the roots) and may die if the roots are not exposed to oxygen. Lactate (Lactic Acid) Fermentation When oxygen is plentiful, animals such as humans respire glucose by aerobic respiration. However, during strenuous exercise, muscle cells break down glucose faster than oxygen can be supplied. Under such low-oxygen conditions, oxidative respiration slows down and lactate fermentation begins. Like ethanol fermentation in yeast cells, lactate fermentation begins with glucose being broken down in the reactions of glycolysis (Figure 4). As usual, this produces two molecules of pyruvate and two molecules of ATP. An enzyme in the cytoplasm called lactate dehydrogenase then converts pyruvate into lactate. glucose GLYCOLYSIS 2 pyruvate 2 lactate Figure 5 Marathon runners are fatigued after a race because of the accumulation of lactate in their muscles. Panting provides the oxygen needed to respire the excess lactate. 2 ADP Figure 4 Lactate fermentation 2 ATP The accumulation of lactate molecules in muscle tissue causes stiffness, soreness, and fatigue. When vigorous exercise ends, lactate is converted back to pyruvate, which then goes into mitochondria and proceeds through the reactions of oxidative respiration. As usual, this produces lots of ATP, but requires oxygen. The extra oxygen needed to clear the body of lactate that accumulates during vigorous exercise is referred to as the oxygen debt. Panting after bouts of strenuous exercise is the body s way of paying the oxygen debt (Figure 5). 80 Unit 1 NEL
Section 1.18 TRY THIS activity Clothespins and Muscle Fatigue In order to contract, your muscles require energy in the form of ATP. Muscles can produce ATP by using oxygen (aerobic respiration) or not (anaerobic respiration). Anaerobic respiration in muscle cells produces lactic acid. When muscles do a lot of workquickly, the buildup of lactic acid reduces their ability to contract, until eventually exhaustion sets in and contraction stops altogether. This is called muscle fatigue. Materials: clothespin, timer 1. Hold a clothespin between the thumb and index finger of your dominant hand (the one you use the most). 2. Count the number of times you can open and close the clothespin in a 20-s period while holding the other fingers of the hand straight out. Make sure to squeeze quickly and completely to get the maximum number of squeezes for each trial. 3. Repeat the process for nine more 20-s periods, recording the result for each trial in a suitable table. Do not rest between trials. 4. Repeat steps 1, 2, and 3 for the nondominant hand. (a) What happened to your strength as you progressed through each trial? (b) Describe how your hand and fingers felt near the end of your trials. (c) Were your results different for the dominant and the nondominant hand? If yes, explain why. (d) Your muscles will likely recover after about 10 min of rest. Explain why. Photosynthesis and Cellular Respiration Photosynthesis and cellular respiration are closely related. Plants carry out photosynthesis and cellular respiration because they contain chloroplasts and mitochondria. Animals can carry out cellular respiration but not photosynthesis because they possess mitochondria but not chloroplasts. Nevertheless, both types of organisms rely on each other for the raw materials of both processes. Plants use carbon dioxide for photosynthesis, and animals, plants, fungi, and bacteria give off carbon dioxide as a waste product of cellular respiration. Plants give off oxygen as a byproduct of photosynthesis, and all aerobic organisms use oxygen in cellular respiration. Thus, the products of one process are the raw materials of the other (Figure 6). carbohydrate and oxygen high-energy compound light energy mitochondria plants and animals plants chloroplast ATP usable energy low-energy compounds CO 2 and H 2 O Figure 6 Photosynthesis uses the products of cellular respiration, and cellular respiration uses the products of photosynthesis. NEL Cellular Biology 81
DID YOU KNOW? Rigor Mortis When an animal dies, all cells of the body do not die at the same time. Immediately after death, the temperature of the body begins to drop and muscles become stiff. This is called rigor mortis. However, rigor mortis is not caused by the drop in body temperature but by the fermentation of glucose in muscle cells, leading to high levels of lactic acid. The lactic acid causes muscle tissue to become rigid. Rigor mortis sets in much sooner if death occurs immediately following strenuous activity such as running. The relationship that exists between photosynthesis and cellular respiration reveals a dependency between autotrophs and heterotrophs. Autotrophs produce almost all of the oxygen in the environment, and heterotrophs produce almost all of the carbon dioxide in the environment. Section 1.18 Questions Understanding Concepts 1. Why is it possible for organisms to carry out photosynthesis and cellular respiration but not photosynthesis only? 2. (a) What is the key difference between aerobic respiration and anaerobic respiration? (b) Which form of respiration is more suited to large organisms such as humans, cows, and fish? Which form is adequate for single-celled organisms such as yeast cells and bacteria? Explain. 3. Describe three cell processes that require the use of ATP. 4. (a) What is meant by the term exergonic reaction? (b) Is the formation of ATP during the reactions of glycolysis an endergonic reaction or an exergonic reaction? Explain. 5. (a) Write an overall chemical equation for glycolysis. (b) Where in a cell do the reactions of glycolysis occur? (c) In which organelle do the reactions of oxidative respiration occur? 6. (a) How many ATP molecules are created in glycolysis from one glucose molecule? (b) How many ATP molecules are created in the overall process of aerobic respiration from one glucose molecule? 7. (a) What is the cause of the pain and stiffness felt after strenuous exercise? (b) Why does the pain eventually subside? 8. How many ATP molecules does a yeast cell obtain from every glucose molecule that undergoes ethanol fermentation? Applying Inquiry Skills 9. A sealed, flexible container of apple juice is left on a counter at room temperature. After one week, the container looks inflated. (a) State a hypothesis that may explain this observation. (b) Describe an experiment you could perform to test your hypothesis. Making Connections 10. Describe two ways in which humans make use of ethanol fermentation. 82 Unit 1 NEL