Cellular Respiration Overview

Overview

This section explains how pyruvate is oxidized to acetyl CoA and how the citric acid cycle (Krebs cycle) operates to harvest energy from glucose during cellular respiration.

Pyruvate Oxidation

• In eukaryotes, pyruvate from glycolysis enters the mitochondria for further oxidation.
• Pyruvate is converted to acetyl CoA in a three-step process: decarboxylation, oxidation, and transfer to CoA.
• One carbon is removed from pyruvate as CO₂, producing a two-carbon acetyl group.
• The acetyl group is oxidized, reducing NAD⁺ to NADH.
• The resulting acetyl group binds to coenzyme A, forming acetyl CoA.
• This process occurs twice per glucose molecule because glycolysis produces two pyruvate molecules.

The Citric Acid Cycle (Krebs Cycle)

• The citric acid cycle occurs in the mitochondrial matrix and is a circular (closed loop) pathway.
• Acetyl CoA donates its acetyl group to oxaloacetate, forming citrate.
• The cycle completes a series of redox, dehydration, hydration, and decarboxylation reactions.
• Each cycle turn releases two CO₂, produces three NADH, one FADH₂, and one GTP/ATP.
• Most ATP production is indirect, via NADH and FADH₂ for the electron transport chain.
• The cycle is aerobic because NADH and FADH₂ must transfer electrons to oxygen-dependent pathways.
• The last step regenerates oxaloacetate, allowing the cycle to continue.

Regulation and Functions

• The first step is highly exergonic and regulated by negative feedback from ATP.
• Intermediate steps are also regulated by ATP, NADH, ADP, and succinyl CoA.
• The cycle is amphibolic; some intermediates are used to make nonessential amino acids.

Key Terms & Definitions

• Pyruvate — 3-carbon molecule produced by glycolysis.
• Acetyl CoA — 2-carbon molecule attached to coenzyme A, enters the citric acid cycle.
• Coenzyme A (CoA) — Carrier molecule derived from vitamin B5 for acetyl groups.
• Citric acid cycle/Krebs cycle/TCA cycle — Pathway that oxidizes acetyl CoA, releasing CO₂ and generating NADH, FADH₂, and ATP/GTP.
• NADH/FADH₂ — Electron carriers that transport energy to the electron transport chain.
• Oxaloacetate — 4-carbon molecule that combines with acetyl CoA to start the cycle.
• Amphibolic — Pathway that is both catabolic and anabolic.

Action Items / Next Steps

• Review the steps and regulation of the citric acid cycle.
• Study how NADH and FADH₂ contribute to ATP production in the electron transport chain.
• Watch the recommended animation of the citric acid cycle.