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6 Pathways that Harvest and Store Chemical Energy Chapter 6 Pathways that Harvest and Store Chemical Energy Key Concepts 6.1 ATP and Reduced Coenzymes Play Important Roles in Biological Energy Metabolism 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy 6.3 Carbohydrate Catabolism in the Absence of Oxygen Releases a Small Amount of Energy Chapter 6 Pathways that Harvest and Store Chemical Energy 6.4 Catabolic and Anabolic Pathways Are Integrated 6.5 During Photosynthesis, Light Energy Is Converted to Chemical Energy 6.6 Photosynthetic Organisms Use Chemical Energy to Convert CO2 to Carbohydrates Chapter 6 Opening Question Why does fresh air inhibit the formation of alcohol by yeast cells? Concept 6.1 ATP and Reduced Coenzymes Play Important Roles in Biological Energy Metabolism Energy is stored in chemical bonds and can be released and transformed by metabolic pathways. Chemical energy available to do work is termed free energy (G). Concept 6.1 ATP and Reduced Coenzymes Play Important Roles in Biological Energy Metabolism Five principles govern metabolic pathways: 1. Chemical transformations occur in a series of intermediate reactions that form a metabolic pathway. 2. Each reaction is catalyzed by a specific enzyme. 3. Most metabolic pathways are similar in all organisms. Concept 6.1 ATP and Reduced Coenzymes Play Important Roles in Biological Energy Metabolism 4. In eukaryotes, many metabolic pathways occur inside specific organelles. 5. Each metabolic pathway is controlled by enzymes that can be inhibited or activated. Concept 6.1 ATP and Reduced Coenzymes Play Important Roles in Biological Energy Metabolism In cells, energy-transforming reactions are often coupled: • An energy-releasing (exergonic) reaction is coupled to an energy-requiring (endergonic) reaction. • Two coupling molecules are the coenzymes ATP and NADH. Concept 6.1 ATP and Reduced Coenzymes Play Important Roles in Biological Energy Metabolism Adenosine triphosphate (ATP) is a kind of “energy currency” in cells. Energy released by exergonic reactions is stored in the bonds of ATP. When ATP is hydrolyzed, free energy is released to drive endergonic reactions. Figure 6.1 The Concept of Coupling Reactions Figure 6.2 ATP Concept 6.1 ATP and Reduced Coenzymes Play Important Roles in Biological Energy Metabolism Hydrolysis of ATP is exergonic: ATP + H2O → ADP + Pi + free energy ΔG is about –7.3 kcal/mol Concept 6.1 ATP and Reduced Coenzymes Play Important Roles in Biological Energy Metabolism The free energy of the bond between phosphate groups is much higher than the energy of the O—H bond that forms after hydrolysis. Concept 6.1 ATP and Reduced Coenzymes Play Important Roles in Biological Energy Metabolism Energy can also be transferred by the transfer of electrons in reduction–oxidation, or redox reactions. • Reduction is the gain of one or more electrons. • Oxidation is the loss of one or more electrons. Concept 6.1 ATP and Reduced Coenzymes Play Important Roles in Biological Energy Metabolism Oxidation and reduction always occur together. Concept 6.1 ATP and Reduced Coenzymes Play Important Roles in Biological Energy Metabolism It is also useful to think of oxidation and reduction in terms of gain or loss of hydrogen atoms: • Transfers of hydrogen atoms involve transfers of electrons (H = H+ + e–). • When a molecule loses a hydrogen atom, it becomes oxidized. Concept 6.1 ATP and Reduced Coenzymes Play Important Roles in Biological Energy Metabolism The more reduced a molecule is, the more energy is stored in its bonds. Energy is transferred in a redox reaction. Energy in the reducing agent is transferred to the reduced product. Figure 6.3 Oxidation, Reduction, and Energy Concept 6.1 ATP and Reduced Coenzymes Play Important Roles in Biological Energy Metabolism Coenzyme NAD is a key electron carrier in redox reactions. NAD+ (oxidized form) NADH (reduced form) Figure 6.4 NAD+/NADH Is an Electron Carrier in Redox Reactions (Part 1) Concept 6.1 ATP and Reduced Coenzymes Play Important Roles in Biological Energy Metabolism Reduction of NAD+ is highly endergonic: NAD+ + H+ + 2 e– → NADH Oxidation of NADH is highly exergonic: NADH + H+ + ½ O2 → NAD+ + H2O Figure 6.4 NAD+/NADH Is an Electron Carrier in Redox Reactions (Part 2) Concept 6.1 ATP and Reduced Coenzymes Play Important Roles in Biological Energy Metabolism Energy is released in catabolism by oxidation and trapped by reduction of coenzymes such as NADH. Energy for anabolic processes is supplied by ATP. Most energy-releasing reactions produce NADH, but most energy-consuming reactions require ATP. Oxidative phosphorylation transfers energy from NADH to ATP. Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy Cellular respiration: the set of metabolic reactions used by cells to harvest energy from food A lot of energy is released when reduced molecules with many C—C and C—H bonds are fully oxidized to CO2. The oxidation occurs in a series of small steps, allowing the cell to harvest about 34% of the energy released. Figure 6.5 Energy Metabolism Occurs in Small Steps Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy Catabolism of glucose under aerobic conditions (in the presence of O2), occurs in three linked biochemical pathways: • Glycolysis—glucose is converted to pyruvate. • Pyruvate oxidation—pyruvate is oxidized to acetyl CoA and CO2. • Citric acid cycle—acetyl CoA is oxidized to CO2. Figure 6.6 Energy-Releasing Metabolic Pathways Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy Glycolysis • Ten reactions • Takes place in the cytosol • Final products: § 2 molecules of pyruvate (pyruvic acid) § 2 molecules of ATP § 2 molecules of NADH Figure 6.7 Glycolysis Converts Glucose into Pyruvate (Part 1) Figure 6.7 Glycolysis Converts Glucose into Pyruvate (Part 2) Figure 6.7 Glycolysis Converts Glucose into Pyruvate (Part 3) Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy Steps 6 and 7 are examples of reactions that occur repeatedly in metabolic pathways: Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy Oxidation–reduction (step 6): exergonic; glyceraldehyde 3-phosphate is oxidized and energy is trapped via reduction of NAD+ to NADH. Substrate-level phosphorylation (step 7): also exergonic; energy released transfers a phosphate from 1,3-bisphosphoglycerate to ADP, forming ATP. Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy Pyruvate Oxidation • Occurs in mitochondria in eukaryotes. • Products: CO2 and acetate; acetate is then bound to coenzyme A (CoA) to form acetyl CoA. • NAD+ is reduced to NADH. Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy Citric Acid Cycle • Eight reactions • Occurs in mitochondria in eukaryotes • Operates twice for every glucose molecule that enters glycolysis • Starts with Acetyl CoA; acetyl group is oxidized to two CO2 • Oxaloacetate is regenerated in the last step Figure 6.8 The Citric Acid Cycle Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy Final reaction of citric acid cycle: Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy Cells transfer energy from NADH and FADH2 to ATP by oxidative phosphorylation: • NADH oxidation is used to actively transport protons (H+) across the inner mitochondrial membrane, resulting in a proton gradient. • Diffusion of protons back across the membrane then drives the synthesis of ATP. Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy When NADH is reoxidized to NAD+, O2 is reduced to H2O: NADH + H+ + ½ O2 → NAD+ + H2O This occurs in a series of redox electron carriers, called the respiratory chain, embedded in the inner membrane of the mitochondrion. Figure 6.9 Electron Transport and ATP Synthesis in Mitochondria Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy Electron transport: electrons from the oxidation of NADH and FADH2 pass from one carrier to the next in the chain. The oxidation reactions are exergonic, energy released is used to actively transport H+ ions across the membrane. Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy Oxidation is always coupled with reduction. When NADH is oxidized to NAD+, the reduction reaction is the formation of water from O2. 2 H+ + 2 e– + ½ O2 → H 2O The key role of O2 in cells is to act as an electron acceptor and become reduced. Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy ATP synthase uses the H+ gradient to drive synthesis of ATP by chemiosmosis: • Chemiosmosis: Movement of ions across a semipermeable barrier from a region of higher concentration to a region of lower concentration. ATP synthase converts the potential energy of the proton gradient into chemical energy in ATP. Figure 6.10 Chemiosmosis Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy ATP synthase is a molecular motor with two subunits: • F0 is a transmembrane domain that functions as the H+ channel. • F1 has six subunits. As protons pass through F0, it rotates, causing part of the F1 unit to rotate. ADP and Pi bind to active sites that become exposed on the F1 unit as it rotates, and ATP is made. Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy ATP synthase structure is similar in all organisms. • In prokaryotes, the proton gradient is set up across the cell membrane. • In eukaryotes, chemiosmosis occurs in mitochondria and chloroplasts. The mechanism of chemiosmosis is similar in almost all forms of life. Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy Chemiosmosis can be demonstrated experimentally. A proton gradient can be introduced artificially in chloroplasts or mitochondria in a test tube. ATP is synthesized if ATP synthase, ADP, and inorganic phosphate are present. Figure 6.11 An Experiment Demonstrates the Chemiosmotic Mechanism Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy About 32 molecules of ATP are produced for each fully oxidized glucose. The role of O2: most of the ATP is formed by oxidative phosphorylation, which is due to the reoxidation of NADH. Some bacteria and archaea use other electron acceptors. • Geobacter metallireducens can use iron (Fe3+) or uranium, making it potentially useful in environmental cleanup. Concept 6.3 Carbohydrate Catabolism in the Absence of Oxygen Releases a Small Amount of Energy Under anaerobic conditions, NADH is reoxidized by fermentation. There are many different types of fermentation, but all operate to regenerate NAD+. The overall yield of ATP is only two—the ATP made in glycolysis. Concept 6.3 Carbohydrate Catabolism in the Absence of Oxygen Releases a Small Amount of Energy Lactic acid fermentation: • End product is lactic acid (lactate). • NADH is used to reduce pyruvate to lactic acid, regenerating NAD+. Occurs in many microorganisms and complex organisms, including vertebrate muscle during exercise when O2 can not be delivered to the muscle fast enough. Figure 6.12 Fermentation (Part 1) Concept 6.3 Carbohydrate Catabolism in the Absence of Oxygen Releases a Small Amount of Energy Alcoholic fermentation: • End product is ethyl alcohol (ethanol). • Pyruvate is converted to acetaldehyde, and CO2 is released. NADH is used to reduce acetaldehyde to ethanol, regenerating NAD+. Occurs in certain yeasts and some plant cells under anaerobic conditions. Figure 6.12 Fermentation (Part 2) Concept 6.4 Catabolic and Anabolic Pathways Are Integrated Metabolic pathways are linked. There is an interchange of molecules into and out of the pathways for synthesis and breakdown. Carbon skeletons (molecules with covalently linked carbon atoms) can enter catabolic or anabolic pathways. These relationships comprise a metabolic system. Figure 6.13 Relationships among the Major Metabolic Pathways of the Cell Concept 6.4 Catabolic and Anabolic Pathways Are Integrated Catabolism: • Polysaccharides are hydrolyzed to glucose, which enters glycolysis. • Lipids break down to fatty acids and glycerol. Fatty acids can be converted to acetyl CoA. • Proteins are hydrolyzed to amino acids that can feed into glycolysis or the citric acid cycle. Concept 6.4 Catabolic and Anabolic Pathways Are Integrated Anabolism: • Many catabolic pathways can operate in reverse. • Gluconeogenesis—citric acid cycle and glycolysis intermediates can be reduced to form glucose. • Acetyl CoA can be used to form fatty acids. • Some citric acid cycle intermediates can form nucleic acids. Concept 6.4 Catabolic and Anabolic Pathways Are Integrated Amounts of different molecules are maintained at fairly constant levels in the metabolic pool. This is accomplished by regulation of enzymes— allosteric regulation and feedback inhibition. Enzymes can also be regulated by altering the transcription of genes that encode the enzymes. This is slower than feedback inhibition. Concept 6.4 Catabolic and Anabolic Pathways Are Integrated ATP and reduced coenzymes link catabolism, anabolism, and photosynthesis. Cellular respiration and photosynthesis are linked by their reactants and products and by the energy “currency” of ATP and reduced coenzymes. Concept 6.4 Catabolic and Anabolic Pathways Are Integrated In cellular respiration glucose is oxidized: glucose + 6 O2 → 6 CO2 + 6 H2O + chemical energy In photosynthesis, light energy is converted to chemical energy: CO2 + H2O + light energy → carbohydrates + O2 Figure 6.14 ATP, Reduced Coenzymes, and Metabolism Concept 6.5 During Photosynthesis, Light Energy Is Converted to Chemical Energy Photosynthesis (anabolic) involves two pathways: • Light reactions convert light energy into chemical energy (in ATP and the reduced electron carrier NADPH). • Carbon-fixation reactions use the ATP and NADPH to produce carbohydrates. Figure 6.15 An Overview of Photosynthesis Concept 6.5 During Photosynthesis, Light Energy Is Converted to Chemical Energy Light is a form of electromagnetic radiation; it is propagated as a wave but also behaves as particles (photons). The amount of energy in the radiation is inversely proportional to its wavelength. Figure 6.16 The Electromagnetic Spectrum Concept 6.5 During Photosynthesis, Light Energy Is Converted to Chemical Energy Photons can be absorbed by specific receptor molecules, which are raised to an excited state (higher energy). Concept 6.5 During Photosynthesis, Light Energy Is Converted to Chemical Energy Pigments: molecules that absorb wavelengths in the visible spectrum Chlorophyll absorbs blue and red light; the remaining light is mostly green. Absorption spectrum—plot of light energy absorbed against wavelength. Action spectrum—plot of the biological activity of an organism against wavelength. Figure 6.17 Absorption and Action Spectra Concept 6.5 During Photosynthesis, Light Energy Is Converted to Chemical Energy In plants, two chlorophylls absorb light energy— chlorophyll a and chlorophyll b. Accessory pigments absorb wavelengths between red and blue and transfer some of that energy to the chlorophylls. Figure 6.18 The Molecular Structure of Chlorophyll Concept 6.5 During Photosynthesis, Light Energy Is Converted to Chemical Energy The pigments are arranged into light-harvesting complexes, or antenna systems. A photosystem spans the thylakoid membrane in the chloroplast • It consists of multiple antenna systems surrounding a reaction center. Figure 6.19 Photosystem Organization Concept 6.5 During Photosynthesis, Light Energy Is Converted to Chemical Energy When chlorophyll (Chl) absorbs light, it enters an excited state (Chl*), then rapidly returns to ground state, releasing an excited electron. Chl* gives the excited electron to an acceptor and becomes oxidized to Chl+. The acceptor molecule is reduced. Chl* + acceptor → Chl+ + acceptor – The reaction center has converted light energy into chemical energy. Concept 6.5 During Photosynthesis, Light Energy Is Converted to Chemical Energy Concept 6.5 During Photosynthesis, Light Energy Is Converted to Chemical Energy The electron acceptor is the first carrier in an electron transport system in the thylakoid membrane. The final acceptor is NADP+, which gets reduced: NADP+ + H+ + 2 e– → NADPH ATP is produced chemiosmotically during electron transport (photophosphorylation). Figure 6.20 Noncyclic Electron Transport Uses Two Photosystems Concept 6.5 During Photosynthesis, Light Energy Is Converted to Chemical Energy Two photosystems: • Photosystem I absorbs light energy at 700 nm and passes an excited electron to NADP+, reducing it to NADPH. • Photosystem II absorbs light energy at 680 nm, oxidizes water, and initiates ATP production. Concept 6.5 During Photosynthesis, Light Energy Is Converted to Chemical Energy Photosystem II When Chl* gives up an electron, it is unstable and grabs an electron from H2O, which splits the H—O—H bonds. 2 Chl* + H2O → 2 Chl + 2 H+ + ½ O2 Concept 6.5 During Photosynthesis, Light Energy Is Converted to Chemical Energy The excited (energetic) electron is passed through a series of thylakoid membrane-bound carriers to a final acceptor at a lower energy level. A proton gradient is generated and used by ATP synthase to make ATP. Concept 6.5 During Photosynthesis, Light Energy Is Converted to Chemical Energy Photosystem I When Chl* gives up an electron, it grabs another electron from the last carrier in the transport system of Photosystem II. This electron ends up reducing NADP+ to NADPH. Concept 6.5 During Photosynthesis, Light Energy Is Converted to Chemical Energy ATP is needed for carbon-fixation pathways. The noncyclic light reactions would not provide enough ATP. Cyclic electron transport uses only photosystem I and produces only ATP. An electron is passed from an excited chlorophyll, through the electron transport chain, and recycles back to the same chlorophyll. Figure 6.21 Cyclic Electron Transport Traps Light Energy as ATP Concept 6.6 Photosynthetic Organisms Use Chemical Energy to Convert CO2 to Carbohydrates Calvin cycle: the energy in ATP and NADPH is used to “fix” CO2 in reduced form in carbohydrates • Occurs in the stroma of the chloroplast. • Each reaction is catalyzed by a specific enzyme. The cycle is composed of three distinct processes. Figure 6.22 The Calvin Cycle Concept 6.6 Photosynthetic Organisms Use Chemical Energy to Convert CO2 to Carbohydrates 1. Fixation of CO2: • CO2 is added to ribulose 1,5-bisphosphate (RuBP). • Ribulose bisphosphate carboxylase/ oxygenase (rubisco) catalyzes the reaction. • A 6-carbon molecule results, which quickly splits into two 3-carbon molecules: 3-phosphoglycerate (3PG) Figure 6.23 RuBP Is the Carbon Dioxide Acceptor Concept 6.6 Photosynthetic Organisms Use Chemical Energy to Convert CO2 to Carbohydrates 2. 3PG is reduced to form glyceraldehyde 3phosphate (G3P). Concept 6.6 Photosynthetic Organisms Use Chemical Energy to Convert CO2 to Carbohydrates 3. The CO2 acceptor RuBP is regenerated from G3P. • Some of the extra G3P is exported to the cytosol and is converted to hexoses (glucose and fructose). • When glucose accumulates, it is linked to form starch, a storage carbohydrate. Concept 6.6 Photosynthetic Organisms Use Chemical Energy to Convert CO2 to Carbohydrates The C—H bonds generated by the Calvin cycle provide almost all the energy for life on Earth. Photosynthetic organisms (autotrophs) use most of this energy to support their own growth and reproduction. Heterotrophs cannot photosynthesize and depend on autotrophs for chemical energy. Answer to Opening Question Pasteur’s findings: Catabolism of the beet sugar is a cellular process, so living yeast cells must be present. In air (O2), yeasts use aerobic metabolism to fully oxidize glucose to CO2. Without air, yeasts use alcoholic fermentation, producing ethanol, less CO2, and less energy (slower growth). Figure 6.24 Products of Glucose Metabolism