Citric acid cycle

Where does the citric acid cycle occur in eukaryotic cells?

The mitochondrial matrix.

Where does the citric acid cycle occur in prokaryotic cells?

The cytoplasm.

What molecule enters the citric acid cycle?

Acetyl-CoA, produced from pyruvate oxidation.

What are the products of pyruvate oxidation?

Acetyl-CoA, NADH, and CO2.

What is substrate-level phosphorylation?

Direct transfer of a phosphate group from a phosphorylated substrate to ADP, producing ATP.

What is an isomer?

A molecule with the same molecular formula as another but a different arrangement of atoms.

What is ADP?

A two-phosphate molecule that can be converted into ATP.

What is GTP?

An energy-storing molecule similar to ATP, used in specific pathways such as protein synthesis.

What happens in Step 1 of the citric acid cycle?

Acetyl-CoA joins oxaloacetate to form citrate. CoA is released.

What happens in Step 2 of the citric acid cycle?

Citrate is rearranged into its isomer, isocitrate. No ATP or electron carriers are produced.

What happens in Step 3 of the citric acid cycle?

Isocitrate is oxidized into alpha-ketoglutarate. NADH and CO2 are produced.

What enzyme catalyzes Step 3?

Isocitrate dehydrogenase.

What happens in Step 4 of the citric acid cycle?

Alpha-ketoglutarate is oxidized into succinyl-CoA. NADH and CO2 are produced.

What enzyme catalyzes Step 4?

Alpha-ketoglutarate dehydrogenase.

What happens in Step 5 of the citric acid cycle?

Succinyl-CoA becomes succinate. Substrate-level phosphorylation produces ATP or GTP.

What happens in Step 6 of the citric acid cycle?

Succinate is oxidized into fumarate, producing FADH2.

What happens in Step 7 of the citric acid cycle?

Water is added to fumarate to form malate.

What happens in Step 8 of the citric acid cycle?

Malate is oxidized into oxaloacetate, producing NADH.

What molecule is regenerated at the end of each cycle?

Oxaloacetate.

What is the mnemonic for remembering the 8 intermediates?

Can I Keep Selling Seashells for Money, Officer.

What are the 8 intermediates in order?

Citrate, Isocitrate, Alpha-ketoglutarate, Succinyl-CoA, Succinate, Fumarate, Malate, Oxaloacetate.

What are the products of one turn of the citric acid cycle?

3 NADH, 1 FADH2, 1 ATP or GTP, and 2 CO2.

What are the products of two turns of the citric acid cycle?

6 NADH, 2 FADH2, 2 ATP or GTP, and 4 CO2.

Why does the citric acid cycle produce NADH and FADH2?

They carry high-energy electrons to the electron transport chain for oxidative phosphorylation.

How does NADH contribute to ATP production?

It delivers electrons to Complex I of the ETC, helping drive proton pumping and ATP synthesis.

How does the proton gradient produce ATP?

H+ flows through ATP synthase, driving ATP production by chemiosmosis.

Does the citric acid cycle directly use oxygen?

No. Oxygen is the final electron acceptor in the ETC.

Why does the citric acid cycle depend indirectly on oxygen?

Oxygen allows NADH and FADH2 to be oxidized back into NAD+ and FAD for reuse.

What happens to oxygen at the end of the ETC?

It accepts electrons and combines with H+ to form water.

How many enzyme-catalyzed reactions occur in the citric acid cycle?

Eight reactions, each catalyzed by an enzyme.

What is the main purpose of the citric acid cycle?

To harvest energy from acetyl-CoA by producing NADH and FADH2 for oxidative phosphorylation.