Cellular Respiration Overview

Study Notes

Cellular respiration is the process by which cells convert glucose into ATP (adenosine triphosphate), the primary energy currency of the cell.
The process involves several stages, including glycolysis, the transition reaction, the Krebs cycle, and oxidative phosphorylation.

Glycolysis occurs in the cytoplasm and is the breakdown of glucose (a 6-carbon molecule) into two molecules of pyruvate (a 3-carbon molecule).
Two ATP molecules are used as initial investment, but four ATP molecules are produced, resulting in a net gain of 2 ATP molecules.
Two molecules of NAD+ are also reduced to NADH; carrying electrons to the electron transport chain.

The transition reaction links glycolysis to the Krebs cycle.
Pyruvate is transported into the mitochondrial matrix and converted to acetyl CoA (a 2-carbon molecule).
A carbon dioxide molecule is released, and one NAD+ is reduced to NADH.

The Krebs cycle takes place in the mitochondrial matrix.
Acetyl CoA combines with oxaloacetate to form citrate, starting the cycle.
The cycle releases two molecules of carbon dioxide.
One ATP, three NADH, and one FADH2 are produced per cycle, per acetyl CoA entering.
The cycle regenerates oxaloacetate, allowing the cycle to continue.

Oxidative phosphorylation occurs across the inner mitochondrial membrane.
NADH and FADH2 deliver electrons to the electron transport chain.
Electrons are passed through a series of protein complexes, releasing energy that is used to pump protons (H+) across the membrane, creating a proton gradient.
The flow of H+ back into the matrix through ATP synthase drives the synthesis of ATP, a process called chemiosmosis.
Oxygen is the final acceptor of electrons in the electron transport chain, forming water.

Pyruvate enters the mitochondria only when O2 is present.
The citric acid cycle has eight steps, each catalyzed by a specific enzyme.
The acetyl group of acetyl CoA joins the cycle by combining with oxaloacetate, forming citrate.
The reactions of the citric acid cycle follow a chemical logic.
The TCA cycle extracts the max potential energy from the pyruvate.
The cycle oxidizes organic fuel derived from pyruvate.