Energy and Electrons from Glucose
- Metabolic pathways occur in small steps, each catalyzed by a specific enzyme. They are often compartmentalized.
- When glucose burns, energy is released as heat and light. The same equation applies to the metabolism of glucose by cells but the reaction is accomplished in many separate steps so that the energy can be captured in ATP.
- Oxidation is the loss of electrons; reduction is the gain of electrons. As a material is oxidized, the electrons it loses are transferred to another material, which is thereby reduced. Such redox reactions transfer large amounts of energy.
- The coenzyme NAD is a key electron carrier in biological redox reactions. It exists in two forms, one oxidized (NAD+) and the other reduced (NADH + H+).
- Glycolysis operates in the presence or absence of O2. Under aerobic conditions cellular respiration continues the process of breaking down glucose. Under anaerobic conditions fermentation occurs.
- Cellular respiration consists of three pathways: pyruvate oxidation, the citric acid cycle and the respiratory chain.
- Pyruvate oxidation and the citric acid cycle produce CO2 and hydrogen atoms carried by NADH and FADH2. The respiratory chain combines these hydrogens with O2, releasing enough energy for the synthesis of ATP.
- In eukaryotes, glycolysis and fermentation take place in the cytoplasm outside of the mitochondria; pyruvate oxidation, the citric acid cycle, and the respiratory chain operate in association with mitochondria. In prokaryotes, glycolysis, fermentation, and the citric acid cycle take place in the cytoplasm and pyruvate oxidation and the respiratory chain operate in association with the plasma membrane.
Glycolysis: From Glucose to Pyruvate
- Glycolysis is a pathway of ten enzyme-catalyzed reactions located in the cytoplasm. Glycolysis provides starting materials for both cellular respiration and fermentation.
- The energy-investing reactions of glycolysis use two ATPs per glucose molecule and eventually yield two G3P molecules. In the energy-harvesting reactions, two NADH molecules are produced and four ATP molecules are generated by substrate-level phosphorylation. Two pyruvate molecules are produced for each glucose molecule.
- The pyruvate dehydrogenase complex catalyzes three reactions: (1) Pyruvate is oxidized to an acetyl group, releasing one CO2 molecule and considerable energy. (2) Some of this energy is captured when NAD+ is reduced to NADH + H+. (3) The remaining energy is captured when the acetyl group is combined with coenzyme A, yielding acetyl CoA.
The Citric Acid Cycle
- The energy in acetyl CoA drives the reaction of acetate with oxaloacetate to produce citrate. The citric acid cycle is a series of reactions in which citrate is oxidized and oxaloacetate regenerated (hence a “cycle”). It produces 2 CO2, 1 FADH2, 3 NADH, and 1 ATP for each acetyl CoA.
The Respiratory Chain: Electrons, Proton Pumping,and ATP Production
- NADH and FADH2 from glycolysis, pyruvate oxidation and the citric acid cycle are oxidized by the respiratory chain regenerating NAD+ and FAD. Most of the enzymes and other electron carriers of the chain are part of the inner mitochondrial membrane.
Oxygen (O2) is the final acceptor of electrons and protons forming water (H2O).
- The chemiosmotic mechanism couples proton transport to oxidative phosphorylation. As the electrons move along the respiratory chain, protons are pumped out of the mitochondrial matrix, establishing a gradient of both proton concentration and electric charge — the proton-motive force.
- The proton-motive force causes protons to diffuse back into the mitochondrial matrix through the membrane channel protein ATP synthase, which couples that diffusion to the production of ATP. Several key experiments demonstrate that chemiosmosis produces ATP.
Fermentation: ATP from Glucose, without O2
- Many organisms and some cells live without O2, deriving all their energy from glycolysis and fermentation. Together, these pathways partly oxidize glucose and generate energy-containing products such as lactic acid or ethanol.
Contrasting Energy Yields
- For each molecule of glucose used, fermentation yields 2 molecules of ATP. In contrast, glycolysis operating with pyruvate oxidation, the citric acid cycle, and the respiratory chain yields up to 36 molecules of ATP per molecule of glucose.
Relationships between Metabolic Pathways
- Catabolic pathways feed into the energy-harvesting metabolic pathways. Polysaccharides are broken down into glucose, which enters glycolysis. Glycerol from fats also enters glycolysis and acetyl CoA from fatty acid degradation enters the citric acid cycle. Proteins enter glycolysis and the citric acid cycle via amino acids.
- Anabolic pathways use intermediate components of energyharvesting pathways to synthesize fats, amino acids, and other essential building blocks.
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