Pathways that Harvest and Store Chemical Energy 6 Pathways that Harvest and Store Chemical Energy

Memorization of the steps in glycolysis and the Krebs cycle, or of the structures of the molecules and the names of the enzymes involved, are beyond the scope of the course and the AP Exam.

Figure 6.1 The Concept of Coupling Reactions Figure 6.1 The Concept of Coupling Reactions Some exergonic cellular reactions are coupled with the formation of ATP from ADP and Pi (an endergonic reaction). The cell can later couple the (exergonic) hydrolysis of ATP with endergonic cellular processes.

Reduction is the gain of one or more electrons. 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.

Oxidation and reduction always occur together. Concept 6.1 ATP and Reduced Coenzymes Play Important Roles in Biological Energy Metabolism Oxidation and reduction always occur together.

Reduction is the gain of one or more electrons or gain of H 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–). Reduction is the gain of one or more electrons or gain of H Oxidation is the loss of one or more electrons or loss of H

Energy is transferred in a redox reaction. 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.

Coenzyme NAD is a key electron carrier in redox reactions. 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)

highly exergonic highly endergonic Figure 6.4 NAD+/NADH Is an Electron Carrier in Redox Reactions (Part 2) highly exergonic highly endergonic Figure 6.4 NAD+/NADH Is an Electron Carrier in Redox Reactions (A) NAD+ is an important electron acceptor in redox reactions, and its reduced form, NADH, is an important energy intermediary in cells. The unshaded portion of the molecule (left) remains unchanged by the redox reaction. (B) Coupling of redox reactions using NAD+/NADH.

Figure 6.6 Energy-Releasing Metabolic Pathways Figure 6.6 Energy-Releasing Metabolic Pathways The catabolism of glucose under aerobic conditions occurs in three sequential metabolic pathways: glycolysis, pyruvate oxidation, and the citric acid cycle. The reduced coenzymes are then oxidized by the respiratory chain, and ATP is made.

Mitochondria intermembrane space inner membrane outer matrix cristae

Takes place in the cytosol Final products: 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) Endergonic: Requires input of energy from ATP hydrolysis Figure 6.7 Glycolysis Converts Glucose into Pyruvate Glucose is converted to pyruvate in ten enzyme-catalyzed steps. Along the way, energy is released to form ATP and NADH.

Figure 6.7 Glycolysis Converts Glucose into Pyruvate (Part 2) A 6-carbon sugar is cleaved into 2 3-carbon sugars Figure 6.7 Glycolysis Converts Glucose into Pyruvate Glucose is converted to pyruvate in ten enzyme-catalyzed steps. Along the way, energy is released to form ATP and NADH.

Figure 6.7 Glycolysis Converts Glucose into Pyruvate (Part 3) Exergonic: Releases energy, forming ATP and NADH Glycolysis rearranges the bonds in glucose, resulting in pyruvate Figure 6.7 Glycolysis Converts Glucose into Pyruvate Glucose is converted to pyruvate in ten enzyme-catalyzed steps. Along the way, energy is released to form ATP and NADH.

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. 16

Occurs in mitochondria in eukaryotes. 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.  Pyruvate is transported from the cytoplasm to the mitochondrion, where further oxidation occurs.

Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy 18

Occurs in mitochondria in eukaryotes 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 NADH is formed Figure 6.8 The Citric Acid Cycle Also called the Krebs cycle for its discoverer, Hans Krebs, the citric acid cycle involves eight steps and fully oxidizes acetyl CoA to CO2. FADH2 is formed 2 CO2 are released GTP can transfer Pi to ADP

carbon dioxide is released from organic intermediates Summary of Krebs carbon dioxide is released from organic intermediates ATP is synthesized from ADP and inorganic phosphate via substrate level phosphorylation electrons are captured by the coenzymes NADH and FADH2 and carried to the electron transport chain (ETC)

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.

Figure 6.9 Electron Transport and ATP Synthesis in Mitochondria e- transport proteins pass e- from NADH to O2, releasing energy that pumps H+ out of mitochondrial matrix Figure 6.9 Electron Transport and ATP Synthesis in Mitochondria As electrons pass through the protein complexes of the respiratory chain, protons are pumped from the mitochondrial matrix into the intermembrane space. As the protons return to the matrix through ATP synthase, ATP is formed.

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. 24

Oxidation is always coupled with reduction. 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 H2O The key role of O2 in cells is to act as an electron acceptor and become reduced. Oxygen is the terminal electron acceptor 25

Electron transport chain reactions also occur in chloroplasts (photosynthesis) prokaryotic plasma membranes

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 Cellular metabolism transfers H+ to one side of the membrane, creating an H+ gradient Chemiosmosis results in H+ returning, providing ATP synthase energy to make ATP Figure 6.10 Chemiosmosis (A) If a cell can generate a proton (H+) gradient across a membrane, the potential energy resulting from the concentration gradient can be used by a membrane-spanning enzyme to make ATP. (B) ATP synthase has a membrane-embedded channel for H+ diffusion and a motor that turns, releasing some energy to produce ATP.

Figure 6.10 Chemiosmosis ATP synthase has a membrane-embedded channel for H+ diffusion and a motor that turns, releasing some energy to produce ATP. Figure 6.10 Chemiosmosis (A) If a cell can generate a proton (H+) gradient across a membrane, the potential energy resulting from the concentration gradient can be used by a membrane-spanning enzyme to make ATP. (B) ATP synthase has a membrane-embedded channel for H+ diffusion and a motor that turns, releasing some energy to produce ATP.

ATP synthase structure is similar in all organisms. 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.

Some bacteria and archaea use other electron acceptors. 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.

Figure 6.12 Fermentation (Part 1) 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 (A) In lactic acid fermentation, NADH is used to reduce pyruvate to lactic acid, thus regenerating NAD+ to keep glycolysis operating. (B) In alcoholic fermentation, pyruvate is converted to acetaldehyde, and CO2 is released. NADH is used to reduce acetaldehyde to ethanol, again regenerating NAD+ for glycolysis.

Figure 6.12 Fermentation (Part 2) Occurs in certain yeasts and some plant cells under anaerobic conditions Figure 6.12 Fermentation (A) In lactic acid fermentation, NADH is used to reduce pyruvate to lactic acid, thus regenerating NAD+ to keep glycolysis operating. (B) In alcoholic fermentation, pyruvate is converted to acetaldehyde, and CO2 is released. NADH is used to reduce acetaldehyde to ethanol, again regenerating NAD+ for glycolysis.