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| Extended Lecture Outline |
Chapter 9: Cellular Respiration |
9.1 The biosphere operates on a carbon cycle.
a. Most organisms obtain energy from reduced compounds through the process of respiration.
b. Respiration means breathing–inhaling and exhaling–and it is closely related to getting energy (Figure 9.1).
c. Between 1755 and 1780, Joseph Black, Joseph Priestley, and Jan Ingenhousz experimented with animals and plants in an effort to better understand the process of respiration. They were working without the benefit of modern chemistry and therefore all of their findings are based on a nonexistent substance called "phlogiston" (Figure 9.2).
d. The observations of Priestley and Ingenhousz suggested a natural cycle of reactions in which plants and animals conduct opposite processes.
e. Antoine Lavoisier established that the gas he named oxygen is removed from the air during respiration or combustion and replaced by carbon dioxide.
f. De Saussure concluded that plants incorporate CO2 as they grow and they grow by consuming CO2 and producing oxygen at the same rate.
g. Lavoisier and Pierre de Laplace studied combustion and respiration quantitatively by using a chamber surrounded by ice as a calorimeter. The heat produced in the chamber by burning material or by the respiration of an animal is proportional to the amount of ice melted during the process (Figure 9.3).
h. The biosphere operates on a carbon cycle consisting largely of respiration and photosynthesis (Figure 9.4).
i. Autrotrophs, mostly photoautotrophs such as plants, synthesize organic compounds from CO2 as they grow.
j. Heterotrophs, mostly chemoheterotrophs such as animals, oxidize those compounds back to CO2.
k. Cellular respiration is a chemical process in which energy-rich materials are oxidized to release their energy.
l. In cellular respiration the hydrogen atoms of C6H12O6 are removed (and some oxygen is added), leaving CO2.
m. In respiration, sugar is oxidized to CO2 and water, whereas in photosynthesis essentially the opposite process occurs: CO2 and water are reduced to sugar.
9.2 Respiration produces NADH and ATP.
a. Metabolism, in heterotrophs, is divisible into five phases, and starts with a process of digestion, since food often consists of polymers that must be hydrolyzed into monomers for further metabolism (Figure 9.5).
1. The function of metabolism is to make stores of ATP (for energy) and NADH (for reducing power).
2. Energy is obtained from foodstuffs by oxidizing organic substrates while reducing NAD+ to NADH + H+.
3. A cell has a limited amount of NAD+ or NADH. Electrons from NADH are run through a respiratory electron transport system (ETS), which is a specialized set of membrane-bound proteins and coenzymes that conduct a flow of electrons and conserves some of their energy by synthesizing ATP.
4. Metabolism centers around pyruvate and a cycle of reactions called the Krebs cycle or the citric acid cycle, discovered by Sir Hans Krebs.
9.3 Overview: Respiration consists of two processes.
a. The first process in respiration is where the organic substrate (glucose) is oxidized completely to CO2, with the reduction of NAD+ to NADH + H+.
1. Metabolites in this pathway are mostly oxidized by dehydrogenation, usually by simply removing a pair of hydrogen atoms.
2. Some metabolites are oxidized by adding water and then removing two hydrogens.
b. In the second phase of respiration, the 24 hydrogens are used to reduce an electron transport system that generates ATP.
1. Oxygen is the terminal electron acceptor that removes the hydrogen atoms.
2. The sum of the two equations shows that sugar is completely oxidized to carbon dioxide and water.
9.4 Glucose is oxidized to pyruvate.
a. Glucose has three possible fates in a cell: to either be stored, oxidized, or converted into cell structure, depending on intracellular conditions.
1. A cell with overabundant glucose can store some as starch (in plants) or as glycogen (in animals).
2. Eventually, the glucose is oxidized, mostly through the pathway of glycolysis as shown in Figure 9.6.
3. The essence of glycolysis is splitting the 6-carbon (C6) sugar into two 3-carbon (C3) molecules and oxidizing them, while storing energy in NADH and ATP.
4. As a result, each glucose molecule becomes oxidized to two molecules of pyruvate (pyruvic acid).
5. The transfer of one phosphoryl group to ADP, making ATP, is called substrate-level phosphorylation.
9.5 Pyruvate is commonly oxidized to an acetyl group.
a. Pyruvate dehydrogenase, a complex enzyme, catalyzes reactions in which the carboxyl group of pyruvate, —COO-, is removed as CO2, leaving a C2 acetyl group.
1. If this reaction occurred outside a cell, the remaining 2-carbon molecule would be acetate (CH3COO-), but in metabolism the acetate (minus one oxygen atom) is attached as an acetyl group to a coenzyme called coenzyme A (CoA) to make acetyl-CoA.
2. The main role of coenzyme A is to carry C2 groups.
3. Pyruvate dehydrogenase required four coenzymes, which animals obtain as vitamins.
9.6 Mitochondria are the principal sites of respiration in eucaryotes.
a. Glycolysis occurs in the cytosol, in both procaryotic and eucaryotic cells.
b. In eucaryotic cells, pyruvate oxidation, the Krebs cycle, and electron transport are all confined to the mitochondria (Figure 9.7).
1. Mitochondria are about 0.5 µm wide and a few micrometers long and are made of two membranes.
2. The outer membrane is simple and permeable, but the inner one restricts the passage of materials and has extensive folds known as cristae.
3. The composition of the matrix space inside the inner membrane is very different from the cytosol, since the inner membrane contains several transport proteins that regulate the passage of molecules such as ATP, ADP, and intermediary metabolites of the Krebs cycle.
9.7 The Krebs cycle is the core of metabolism.
a. Cycles of reactions are critical to metabolism.
b. Figure 9.8 shows the Krebs cycle and some other closely related reactions.
c. The Krebs cycle oxidizes acetyl groups to CO2, while generating the reduced compounds NADH and FADH2 that can be used for ATP synthesis.
9.8 The electron transport system synthesizes ATP.
a. As a cell carries out oxidative metabolism, it accumulates NADH and FADH2, whose energy can be used to convert ADP into ARP in the process of oxidative phosphorylation.
b. The ETS, which carries out the final stage of respiration, is located in the inner membrane of mitochondria in eucaryotes and in the plasma membrane in procaryotes.
1. The ETS creates a proton gradient, an electrochemical gradient that has enough energy to drive ATP synthesis.
2. At the end, protons and electrons come back together and combine with oxygen to form water (Figure 9.9).
c. The components of an electron transport system form four protein complexes that are embedded in the inner mitochondrial membrane like a mosaic, but are free to move laterally because membranes are fluid (Figure 9.10).
d. Electrons pass from one complex to another via mobile carriers and finally to Complex III, which becomes oxidized again by reducing oxygen to water.
e. Lack of oxygen causes the ETS to stay reduced, which stops the Krebs cycle and ATP synthesis. Lack of oxygen can result in a lack of energy and even death in a short time.
9.9 A proton gradient across a membrane can be used to synthesize ATP.
a. The step-by-step transfer of electrons through an ETS leads to the synthesis of ATP through a remarkable mechanism that conserves some energy from the oxidation of organic molecules.
b. The inner mitochondrial membrane has distinctive little knobs along its inside face; these are the proteins that form an ATP synthase, which combines ADP with inorganic phosphate to make ATP (Figure 9.11).
c. Peter Mitchell developed the theory of chemiosmotic coupling, in which ATP is synthesized by a membrane system that creates a proton gradient.
1. Mitchell identified this gradient as a proton-motive force which is defined as a combination of a chemical potential (due to the gradient of protons) plus an electrical potential (since each proton carries a positive charge).
2. Andre Jagendorf confirmed Mitchell's hypothesis with an experiment where he soaked chloroplasts in a solution at pH 4 for several hours then moved them to a solution of pH 8 and observed a burst of ATP synthesis as protons moved down their concentration gradient (Figure 9.12).
d. An ATP synthase can generate one ATP for every three protons that pass through it.
9.10 A proton gradient itself can do work.
a. While the energy of the proton gradient can be stored in the form of ATP, it can also be used directly to drive a symport mechanism just as a sodium-ion gradient does.
1. The galactoside permease system of some bacteria is a proton-lactose symporter which uses the energy of the proton gradient for active transport of the sugar lactose (Figure 9.13).
2. The same proton gradient also supplies the motive force for bacterial flagella, which are protein rods that rotate within an anchoring ring in the membrane.
9.11 Many organisms obtain energy through fermentation.
a. If an organism metabolizes glucose completely to CO and water, it is able to store 50-60 percent of the energy of the sugar in the form of ATP; to do this it must be living in an aerobic environment (an environment with oxygen).
b. Many organisms are adapted to anaerobic environments (without oxygen), and without oxygen are unable to carry out complete respiration as described earlier.
c. Fermentation is an alternative to respiration that uses glycolysis to produce ATP and NADH; in fermentation, the final oxidizing agent is an organic compound (Figure 9.14), as NADH + H+ is oxidized to NAD+ by reducing pyruvate to some end product.
d. Some yeasts convert the pyruvate to CO and acetaldehyde, then reduce the acetaldehyde to ethanol (ethyl alcohol).The process is responsible for the making of bread, beer, wine, and other alcoholic beverages.
e. Other fermentations produce various by-products that we find either delectable or detestable, depending on our tastes and cultures.
1. Propionic acid provides the distinctive taste of Swiss cheese and the CO2 produced in the fermentation makes the big holes in the product.
2. Lactate provides the sharp flavors of sauerkraut, sour cream, and yogurt.
3. Many important products are synthesized by bacteria and fungi as fermenters (Figure 9.15).
f. Animals depend on the lactate fermentation as a backup to respiration.
1. Animal cells use glucose as their main energy source, and when they are breathing normally and not working too hard, they oxidize glucose completely.
2. Glucose cannot be oxidized completely if the lungs and circulatory system can't supply oxygen quickly enough.
3. Glycolysis can supply needed ATP quickly enough to sustain exercise for some time, even without enough oxygen to supply the muscles completely.
9.12 Excess sugar can be made into fatty acids.
a. Sometimes animal cells receive more carbohydrate than they can use immediately, usually as a result of consuming too much food and not exerting enough energy through work or exercise.
b. Glucose can be stored as glycogen, but an animal's capacity for storage of this type is limited.
c. Fat is an efficient storage material that yields about 9 kcal when oxidized, in contrast to about 4 kcal/gram for carbohydrate.
d. Sugars can be oxidized, primarily in liver and adipose cells, to pyruvate then to acetyl-CoA and then the acetyl groups are put together end-to-end to make fatty acids as shown in Figure 9.16.
e. Cells retrieve the energy stored in fat through essentially the reverse of the synthetic reactions.
9.13 Some organisms use other types of respiration and inorganic energy sources.
a. There are several types of respiration using different electron donors and acceptors.
1. Even though oxygen is the most common terminal electron acceptor, many kinds of bacteria oxidize nitrite to nitrate.
2. A few bacteria employ sulfate respiration in which they reduce sulfate to sulfide.
3. None of the organisms in this category are prominent in ecosystems, because they don't get as much energy from their food as do organisms that use oxygen.
b. Bacteria known as chemoautotrophs take advantage of a variety of inorganic chemical reactions that yield energy, instead of using organic compounds or sunlight as energy sources.
1. Chemoautotrophs can oxidize inorganic compounds to get ATP and NADH, which are then used to reduce CO2 to organic compounds, as in photosynthetic organisms.
2. Chemoautotrophs grow slowly because these reactions don't yield a lot of energy. To increase their energy production, these bacteria develop huge internal membrane structures (Figure 9.17) to provide large amounts of the enzyme systems for generating ATP.
3. Chemoautotrophs that transform nitrogen and sulfur compounds play important parts in the cycling of materials in ecosystems.
9.14 Many compounds are catabolized into the central pathways.
a. An animal's food supplies carbohydrates and fats, which are usually oxidized for their energy, but most of the bulk (aside from water) consists of proteins and nucleic acids.
1. Humans require eight of the twenty amino acids ready-made in food since they can't make them.
2. Much of the protein and other compounds can be catabolized for their energy, and many chemotrophs depend on these organic constituents as energy sources (Figure 9.18).
3. When amino acids are catabolized, all their amino groups generally cannot be used so they are removed and eliminated as ammonia, urea, or uric acid.
9.15 Summary: Heterotrophic metabolism consists of five phases.
a. Figure 9.19 summarizes the pathways involved in heterotrophic metabolism.
b. Metabolism is complicated because a cell is simultaneously carrying out respiration and supplying molecules for biosynthesis.
1. Many of the molecules that move through the pathways of respiration are not oxidized completely but are saved to make the structure of the organism (Figure 9.20).
2. Several of the metabolites in the central pathways also serve as precursors of new monomers.
3. For this reason, the Krebs cycle and some pathways that serve it are called amphibolic pathways, because they have both catabolic and anabolic functions.
4. All chemical activity in organisms is eventually directed toward growth or maintenance of the existing structure and metabolism simultaneously provides energy and raw materials for growth.
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