Citric Acid Cycle

02/10/2025

What is the main purpose of the citric acid cycle?

Completes oxidation of glucose by converting Acetyl-CoA (2 carbon compound) into 2 CO2.

What types of reactions dominate the citric acid cycle?

Redox reactions catalyzed by dehydrogenases.

Why is FAD used during the oxidation of succinate?

Because the oxidation of succinate does not release enough energy to reduce NAD. FAD’s redox potential makes it a better electron carrier.

Why is the electron transport chain required after the citric acid cycle?

ETC reoxidizes NADH to NAD+, which is necessary to keep the citric acid cycle and other pathways running. It harnesses the released redox energy to produce ATP.

How does NADH deliver its electrons to the ETC?

NADH binds to the first enzyme complex of the ETC, ensuring electrons are released gradually. This allows proton pumping to occur.

How does FADH2 differ from NADH in the ETC?

FADH2 donates its electrons to ubiquinone via complex II because its oxidation releases less energy. It bypasses the first complex used by NADH.

What role does ubiquinone play in the ETC?

• it is a mobile, hydrophobic electron carrier.

• Shuttles electrons between complexes.

• Reduced on one side of the membrane and oxidized on the other, allowing it to transport protons across.

How is oxygen utilized in the ETC?

Oxygen is held by a heme group in complex IV until it accepts four electrons.

Why is the controlled use of oxygen important in the ETC?

Controlled binding prevents the formation of dangerous intermediates (like cyanide).

Why is cyanide dangerous?

Cyanide blocks the binding of oxygen to the heme site on complex IV, halting the ETC.

What is the electrochemical proton gradient? How does it form?

the ETC pumps protons into the intermembrane space, creating an electrochemical gradient that stores energy for ATP synthesis.

How is ATP produced via oxidative phosphorylation?

As protons flow back into the mitochondrial matrix through ATP synthase, about three protons are needed to synthesize one ATP molecule.

Can ATP synthase operate independently of the electron transport chain?

Yes, experiments with vesicles show that a proton gradient alone can drive ATP synthase to produce ATP, even when ETC is not present. ATP synthase can also run in reverse to pump protons.

What is uncoupling in energy metabolism?

Uncoupling happens when the ETC is connected to a proton channel instead of ATP synthase. Produces heat instead of ATP. (ex. hibernating animals).

How are glycolysis, the citric acid cycle and the ETC regulated?

• High ATP levels shut down glycolysis

• High NADH levels inhibit citric acid cycle

• HIgh proton gradient slows the ETC.

What happens to cellular respiration when no oxygen is present?

Without oxygen as the final electron acceptor, the ETC stops which halts NADH oxidation. This causes pyruvate oxidation and citric acid cycle to stop. ONLY FERMENTATION CAN OCCUR.

What is fermentation? Why is it less efficient?

Fermentation is an anaerobic process that regenerates NAD+ from pyruvate. It only allows glycolysis to continue, meaning it produces much less ATP.

What is fermentation’s role in food production?

Fermentation produces acids or alcohol that inhibit harmful bacteria growth. It preserves foods like yogurt, cheese and salami.

How are carbohydrates, fats and proteins interconnected in metabolism?

They are all converted to acetyl-CoA (except some amino acids that enter directly into the citric acid cycle).

What diet info can be learned from looking at cell metabolism?

Since the brain relies exclusively on glucose, overconsumption of sugar can disrupt metabolic regulation, whereas slow releasing carbs (starch) are more manageable. Late night snacking may reduce fat catabolism.

What electron carrier goes into complex IV?

Cytochrome C.

What is the reaction that occurs in complex IV of the citric acid cycle?

2e- + 2H+ + 1/2O2- → H2O + H+

What goes into complex 1?

NADH

What goes into complex 2?

FADH2