Cellular Respiration

Objective: Identify respiration inputs, outputs, and the role of glycolysis.

Cellular respiration transfers energy from organic fuel into ATP using a sequence of reactions. In aerobic respiration, a simplified net equation is C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + energy transferred to ATP and heat. Glycolysis occurs in the cytosol and splits one glucose into two pyruvate, with a net gain of two ATP and two NADH per glucose under the standard teaching model. Glycolysis itself does not directly require O₂, though sustained pathway flux depends on recycling electron carriers. In eukaryotes, subsequent aerobic stages occur mainly in mitochondria; prokaryotes use other cellular locations. Respiration also occurs in plants and other photosynthetic organisms.

Objective: Trace carbon release and reduced electron carriers before electron transport.

In eukaryotes during aerobic respiration, pyruvate is transported into mitochondria and converted to acetyl-CoA, releasing CO₂ and reducing NAD⁺ to NADH. Acetyl-CoA enters the citric acid cycle in the mitochondrial matrix. Across the cycle, acetyl carbon is oxidized, CO₂ is released, and electrons are transferred to NADH and FADH₂; a small amount of ATP or equivalent is formed by substrate-level phosphorylation. Carbon atoms from a newly entering acetyl group need not be the exact atoms released as CO₂ on that same turn. These stages capture much of the fuel’s energy in electron carriers rather than making most ATP directly.

Objective: Explain oxygen’s role and the proton gradient in aerobic ATP production.

In eukaryotic mitochondria, NADH and FADH₂ donate electrons to the electron transport chain in the inner membrane. Electron transfers help pump protons from the matrix into the intermembrane space, building an electrochemical gradient. Oxygen is the terminal electron acceptor in aerobic respiration; together with electrons and protons it forms water. Protons return through ATP synthase, which couples the gradient to ATP formation, called oxidative phosphorylation. ATP yield varies with organism and conditions, so a single exact number per glucose is not universal. The oxygen consumed here is not the oxygen gas released by photosynthetic water splitting in the same reaction.