Cellular respiration notes

Glycolysis

Glycolysis is the first step of cellular respiration, occurring in the cytosol. It begins with C6H{12}O_6} and splits it into 2 molecules of 3-carbon pyruvate. This step yields ATP and NADH and runs twice per glucose.

NAD^+ and FAD as electron carriers

NAD^+ and FAD are electron carriers that are reduced to NADH and FADH_2 during oxidation-reduction reactions. They carry 2 electrons each and then can act as reductants to deliver those electrons to the electron transport chain, becoming oxidized again.

The Citric Acid Cycle (Krebs)

Acetyl-CoA enters the cycle and binds to oxaloacetate to form a six-carbon molecule. Two turns of the cycle are required per glucose to fully oxidize the original molecule. The cycle releases carbon dioxide, and produces high-energy carriers (NADH and FADH2) and ATP, while regenerating oxaloacetate to start another turn.

Mitochondria and the inner membrane

A mitochondrion contains two spaces: the intermembrane space and the mitochondrial matrix. The inner membrane is studded with the electron transport chain complexes that drive ATP production by creating a proton gradient.

Electron transport chain and ATP synthesis (oxidative phosphorylation)

NADH and FADH2 donate electrons to the electron transport chain. Protons (H+) are pumped across the inner membrane, creating a gradient. ATP synthase uses the flow of H^+ to convert ADP and Pi into ATP: ADP + Pi \rightarrow ATP. The final electron acceptor is oxygen, and this step generates most of the cell’s ATP.

Fermentation under anaerobic conditions

When oxygen is unavailable, glycolysis can continue if NADH is recycled back to NAD^+ via fermentation. This process allows some ATP production in microbes (e.g., yeasts) and regenerates NAD^+. Yeast fermentation produces ethanol as a byproduct. Some parasites may rely on alternative strategies, but fermentation alone cannot support large multicellular organisms long-term.

Starting materials and metabolic flexibility

The starting molecule is glucose, but cells can synthesize glucose or convert other biomolecules into enter-point intermediates to fuel the citric acid cycle and electron transport chain. Carbohydrates are not strictly required in animal diets, as other biomolecules can feed into the pathway.

Heat production and brown fat in endotherms

Endotherms (mammals and birds) generate heat through cellular respiration in addition to meeting ATP needs. Some cells, particularly brown fat cells with many mitochondria and fat stores, can dissipate energy as heat rather than storing it all as ATP.

Redox concepts: OIL RIG

Cellular respiration involves many oxidation-reduction reactions. Oxidizers take electrons from reductants (Oxidation Is Loss, OIL) and reductants gain electrons (Reduction Is Gain, RIG). Oxygen-containing molecules often act as oxidizers.