Metabolism, Cell Respiration and Photosynthesis (AHL) - Cell Respiration

Cell respiration is the controlled release of energy from organic compounds to produce $ATP$. In AHL, this focus shifts to the specific redox reactions: Oxidation (loss of electrons/$H$) and Reduction (gain of electrons/$H$).

Phosphorylation is the addition of a phosphate group ($PO_4^{3-}$) to an organic molecule, which makes the molecule less stable and more likely to react. This occurs during the initial stages of Glycolysis ($Glucose \rightarrow Glucose-6-phosphate$).

Glycolysis occurs in the cytosol. One molecule of glucose ($C_6H_{12}O_6$) is broken down into two molecules of pyruvate ($C_3H_4O_3$), with a net yield of $2$ $ATP$ and $2$ $NADH + H^+$.

The Link Reaction occurs in the mitochondrial matrix. Pyruvate is decarboxylated (removes $CO_2$) and oxidized (forms $NADH + H^+$) to produce an acetyl group, which attaches to Coenzyme A to form $Acetyl-CoA$.

The Krebs Cycle involves the oxidation of acetyl groups. For every turn of the cycle, $2 \times CO_2$ are released, $3 \times NADH + H^+$ are produced, $1 \times FADH_2$ is produced, and $1 \times ATP$ is generated via substrate-level phosphorylation.

The Electron Transport Chain (ETC) is located on the inner mitochondrial membrane (cristae). Electrons from $NADH$ and $FADH_2$ are passed through carriers, releasing energy used to pump protons ($H^+$) into the intermembrane space.

Chemiosmosis is the process where the $H^+$ gradient drives $ATP$ synthesis as protons flow back into the matrix through $ATP$ synthase. Oxygen ($O_2$) acts as the final electron acceptor, forming $H_2O$.

Mitochondrial structure is adapted to function: the small intermembrane space allows for rapid build-up of $H^+$ concentration, and the folded cristae increase surface area for the ETC.