BIOL 112 - LECTURE 11: Citric Acid Cycle, Electron Transport Chain & Fermentation

What is the main purpose of the citric acid cycle?

The citric acid cycle completes the oxidation of glucose by converting acetyl-CoA into 2 molecules of CO2, while producing reduced electron carriers (NADH and FADH2) and GTP.

Why is FAD used in the citric acid cycle instead of NAD?

FAD is used because the oxidation of succinate does not release enough energy to reduce NAD+, but it is sufficient for reducing FAD to FADH2.

What is the function of the electron transport chain (ETC)?

The ETC is a series of redox reactions that transport electrons from NADH and FADH2 across the inner mitochondrial membrane to produce ATP through oxidative phosphorylation.

Why doesn't NADH directly donate electrons to O2?

NADH donates electrons to the first enzyme complex of the ETC, ensuring gradual energy release and proton transport across the membrane, rather than a direct electron transfer to oxygen.

What are the roles of ubiquinone and cytochrome c in the ETC?

Ubiquinone and cytochrome c are hydrophobic electron carriers that shuttle electrons between the enzyme complexes within the inner mitochondrial membrane.

What is the proton-motive force in the ETC?

The proton-motive force is generated by the electrochemical gradient of protons (H+) across the inner mitochondrial membrane, which is used to drive ATP synthesis through ATP synthase.

What is oxidative phosphorylation?

Oxidative phosphorylation is the process where ATP is synthesized from ADP and inorganic phosphate by ATP synthase, driven by the proton gradient established by the electron transport chain.

How does ATP synthase function in ATP production?

ATP synthase uses the flow of protons back across the membrane to drive the phosphorylation of ADP to ATP.

What happens in brown fat in newborns and hibernating animals?

In brown fat, the ETC is uncoupled from ATP synthesis, and instead, the energy is used to generate heat through proton flow, helping to maintain body temperature.

How is the citric acid cycle regulated?

The citric acid cycle is regulated by the concentrations of ATP (which inhibits glycolysis), NADH (which inhibits the cycle), and the proton gradient (which inhibits the ETC).

What happens to the electron transport chain in the absence of oxygen?

Without oxygen, the ETC shuts down because oxygen is the final electron acceptor. Without NAD+ regeneration, pyruvate oxidation and the citric acid cycle also stop.

What is fermentation and when does it occur?

Fermentation occurs under anoxic conditions, where pyruvate acts as an electron acceptor to regenerate NAD+, allowing glycolysis to continue and produce ATP without oxygen.

What is the main product of fermentation in muscles?

In muscles, fermentation produces lactic acid as a byproduct when oxygen is insufficient during strenuous exercise.

What is the role of fermentation in food production?

Fermentation is used in food production (e.g., yogurt, cheese, salami) because the acid and absence of oxygen inhibit bacterial growth, preserving the food.

Why is it said that fermentation is a "step backward" in energy production?

Fermentation is a less efficient process compared to respiration, as it does not generate much ATP and instead just recycles NAD+ without generating additional energy.

How do the three major food ingredients (carbohydrates, fats, proteins) get metabolized?

Carbohydrates, fats, and proteins are all broken down into acetyl-CoA, which enters the citric acid cycle. Excess acetyl-CoA can lead to fat synthesis or be used in metabolism depending on the body’s needs.

Why does fat provide more energy than carbohydrates or proteins?

Fats have more C-H bonds, which take longer to oxidize, releasing more energy and producing more NADH compared to carbohydrates or proteins.

Why is it a good idea to reduce sugar consumption?

Excessive sugar consumption can disrupt the body’s regulatory system, contributing to conditions like diabetes, as sugar was not historically a major part of the diet.

How does regular snacking affect metabolism?

Regular snacking can reduce the catabolism of stored fat, as the body metabolizes carbohydrates first, followed by fat and protein during periods of fasting.

Why is it important to understand carbohydrate metabolism for physical or mental exercise?

Carbohydrates provide the necessary energy for physical and mental exertion, as they are metabolized first to fuel immediate energy needs.