Alberts - Essential Cell Biology (4th ed.)

How Cells Obtain Energy From Food

Aerobic Metabolism and Acetyl CoA Production

1. The pyruvate produced by glycolysis is transported into the mitochondrial matrix, where crucial energy-producing processes occur.

2. Pyruvate Dehydrogenase Complex: A large multienzyme complex that decarboxylates pyruvate, which involves the removal of a carbon atom as carbon dioxide (CO2), while forming NADH (an electron carrier) and acetyl CoA, an essential molecule for subsequent metabolic pathways.

   • This step is vital for linking glycolysis and the citric acid cycle (CAC).

3. Sources of Acetyl CoA:

   • Sugars: The breakdown of glucose through glycolysis produces pyruvate, which is then converted into acetyl CoA.

   • Fats: Fatty acids undergo a process called beta-oxidation, where they are converted into acetyl CoA after being activated by covalent attachment to CoA. This metabolic pathway involves a cycle that breaks down two-carbon units from the fatty acids, generating acetyl CoA, NADH, and FADH2, thus providing a significant energy source.

   • Proteins: Certain amino acids, through deamination and subsequent conversion, can be transformed into acetyl CoA or enter the citric acid cycle at different points, thus contributing to energy production.

4. In aerobic bacteria, metabolism primarily occurs in the cytosol due to the absence of mitochondria, utilizing similar biochemical pathways to generate energy.

Citric Acid Cycle (CAC)

1. The CAC serves as a central hub for aerobic metabolism, accounting for significant carbon oxidation and the generation of CO2, NADH, and FADH2, which are crucial for ATP production.

2. Major steps include:

   • The reaction of acetyl CoA with oxaloacetate to form citric acid (6C), initiating a cyclic pathway of oxidation and reduction.

   • Progressive oxidation of citric acid leads to the generation of NADH and FADH2, along with the release of CO2, contributing to the cellular pool of electron carriers necessary for ATP synthesis.

3. Electrons from NADH and FADH2 are transferred through the electron transport chain (ETC), which ultimately results in ATP production.

   • While the CAC does not directly utilize oxygen (O2), it is profoundly dependent on the electron transport chain, which consumes oxygen and produces water as a byproduct.

Electron Transport Chain (ETC)

1. The ETC is embedded in the inner mitochondrial membrane and performs a critical role in cellular respiration.

2. NADH and FADH2 donate their electrons to the chain, which passes them through a series of proteins known as complexes (I, II, III, and IV). This transfer creates a proton gradient across the inner mitochondrial membrane.

3. Final Electron Acceptors: The electrons combine with molecular oxygen to form water (H2O), a crucial reaction for sustaining aerobic respiration.

4. This process culminates in the synthesis of ATP through oxidative phosphorylation, utilizing the proton gradient established by the movement of protons (H+) back into the mitochondrial matrix through ATP synthase.

Summary of ATP Production

1. The complete oxidation of one glucose molecule can yield approximately 30-32 ATP molecules, a stark contrast to glycolysis alone, which only yields 2 ATP.

   • This enhanced efficiency of ATP production underscores the importance of aerobic metabolism for sustaining life processes.

Regulation of Metabolism

1. Cells require versatile mechanisms of regulation to maintain energy balance and adapt to changing environmental conditions.

2. Feedback Regulation enables cells to switch between anabolic (building up) and catabolic (breaking down) pathways based on energy availability and demand.

3. Gluconeogenesis: The synthesis of glucose from pyruvate or other precursors is tightly regulated to ensure a continuous supply of glucose during periods of fasting or intense physical activity, critical for maintaining blood sugar levels.

Food Storage in Cells

1. Cells store glucose in the form of glycogen in animals and starch in plants to provide readily available energy reserves.

   • Glycogen, with its branched structure, can be rapidly mobilized to meet sudden energy needs, making it an efficient storage form.

2. Fat Storage: Fatty acids are stored in adipocytes as triacylglycerols, providing significantly more energy per gram than glycogen due to their higher energy density, making fats an efficient long-term energy store.

3. Glycogen Metabolism: Glycogen can be hydrolyzed quickly through well-regulated pathways to release glucose when energy is demanded, minimizing energy wastage.

Diverse Energy Sources

1. Cells can utilize a variety of organic molecules (sugars, fats, proteins) as energy sources, each being metabolized through interconnected pathways to yield ATP efficiently.

2. The interplay between catabolism (energy production) and anabolism (the synthesis of necessary cellular components) exemplifies the integrated nature of cellular metabolism.

Conclusion

1. The pathways of glycolysis, the citric acid cycle, and oxidative phosphorylation exemplify the intricate biochemical processes that enable cells to harness energy from food efficiently and sustain vital cellular functions.

2. Understanding these pathways provides valuable insights into cellular metabolism and its regulation, essential for the maintenance of life and understanding metabolic disorders.