Concept 6.1   ATP and Reduced Coenzymes Play Important Roles in Biological Energy Metabolism

• Metabolism is carried out in small steps and involves coenzymes as carriers of chemical energy.
• Adenosine triphosphate (ATP) serves as “energy currency” in the cell. Hydrolysis of ATP releases a large amount of free energy. Review Figure 6.1 and ACTIVITY 6.1
• In oxidation, a material loses electrons by transfer to another material, which thereby undergoes reduction. Such redox reactions transfer large amounts of energy.
• The coenzyme nicotinamide adenine dinucleotide (NAD) is a key electron carrier in biological redox reactions. It exists in two forms, one oxidized (NAD⁺) and the other reduced (NADH). Review Figure 6.4

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

• The sequential pathways of aerobic glucose catabolism are glycolysis, pyruvate oxidation, and the citric acid cycle. Review Figure 6.6
• In glycolysis, a series of ten enzyme-catalyzed reactions in the cell cytoplasm converts glucose to two molecules of pyruvate. Energy is released and captured as ATP and NADH. Review Figure 6.7
• The next pathway, pyruvate oxidation, links glycolysis to the citric acid cycle. Pyruvate oxidation converts pyruvate into the two-carbon molecule acetyl CoA.
• In the citric acid cycle, a series of eight enzyme-catalyzed reactions fully oxidizes acetyl CoA to CO₂. Much energy is released, and most is used to form NADH. Review Figure 6.8 and ACTIVITY 6.2
• The energy in NADH is used to make ATP via a series of electron transport carriers and chemiosmosis. Review Figure 6.9, ANIMATED TUTORIAL 6.1, and ACTIVITY 6.3
• In oxidative phosphorylation, ATP is formed with the energy derived from the reoxidation of reduced coenzymes. This depends on the process of chemiosmosis, in which a proton gradient across a membrane powers ATP formation. This occurs at the cell membrane in prokaryotes, and in the mitochondria and chloroplasts in eukaryotes. Review Figure 6.10, Figure 6.11 and ANIMATED TUTORIAL 6.2

Concept 6.3   Carbohydrate Catabolism in the Absence of Oxygen Releases a Small Amount of Energy

• In the absence of O₂, glycolysis is followed by fermentation. Together, these pathways partially oxidize pyruvate and generate the end products lactic acid or ethanol. In the process, NAD⁺ is regenerated from NADH so that glycolysis can continue, thus generating a small amount of ATP. Review Figure 6.12 and ACTIVITY 6.4
• For each molecule of glucose used, fermentation yields 2 molecules of ATP. In contrast, glycolysis, pyruvate oxidation, the citric acid cycle, and oxidative phosphorylation yield up to 32 molecules of ATP per molecule of glucose. Review ACTIVITY 6.5

Concept 6.4   Catabolic and Anabolic Pathways Are Integrated

• The catabolic pathways for the breakdown of carbohydrates, lipids, and proteins feed into the energy-harvesting metabolic pathways. Review Figure 6.13
• Anabolic pathways use intermediate components of the energy-harvesting pathways to synthesize fatty acids, amino acids, and other essential building blocks.
• The formation of glucose from intermediates of glycolysis and the citric acid cycle is called gluconeogenesis.
• The enzymes of glycolysis and the citric acid cycle are regulated by various mechanisms, including allosteric regulation. Excess acetyl CoA is diverted into fatty acid synthesis. Review ACTIVITY 6.6

Concept 6.5   During Photosynthesis, Light Energy Is Converted to Chemical Energy

• The light reactions of photosynthesis convert light energy into chemical energy. They produce ATP and reduce NADP⁺ to NADPH. Review Figure 6.15
• Light is a form of electromagnetic radiation. It is emitted in particle-like packets called photons but has wavelike properties. Molecules that absorb light in the visible spectrum are called pigments. Photosynthetic organisms have several pigments, most notably chlorophylls. Review Figure 6.16, Figure 6.17 and Figure 6.18
• The absorption of a photon puts a chlorophyll molecule into an excited state that has more energy than its ground state. This energy can be transferred via other chlorophylls to one in the reaction center of a photosystem. Review Figure 6.19
• An excited chlorophyll can act as a reducing agent, transferring excited electrons to other molecules. Oxidized chlorophyll regains electrons by the splitting of H₂O.
• In the thylakoid membrane of the chloroplast, photosystems I and II and a noncyclic electron transport system produce ATP via oxidative phosphorylation. NADPH and O₂ are also produced. Review Figure 6.20, ANIMATED TUTORIAL 6.3 and ANIMATED TUTORIAL 6.4
• Cyclic electron transport uses only photosystem I and produces only ATP. Review Figure 6.21

Concept 6.6   Photosynthetic Organisms Use Chemical Energy to Convert CO₂ to Carbohydrates

• The Calvin cycle makes carbohydrates from CO₂. The cycle consists of three processes: fixation of CO₂, reduction and sugar production, and regeneration of RuBP. Review Figure 6.22 and ACTIVITY 6.7
• RuBP is the initial CO₂ acceptor, and 3PG is the first stable product of CO₂ fixation. The enzyme rubisco catalyzes the reaction of CO₂ and RuBP to form 3PG. Review Figure 6.23 and ANIMATED TUTORIAL 6.5
• ATP and NADPH formed by the light reactions are used to fuel the reduction of 3PG to form glyceraldehyde 3-phosphate (G3P)—a starting material for the synthesis of glucose and other carbohydrates.