Question

Summarize the two stages of cellular respiration. Be sure to explain where in the cell each stage occurs.

Answer 1

Cellular respiration can be described in two main stages:

1) Glycolysis (stage 1)
- Where: cytosol (cytoplasm) of the cell.
- What happens: one glucose (6C) is split into two molecules of pyruvate (3C each).
- Outputs (per glucose): net 2 ATP (by substrate-level phosphorylation) and 2 NADH; 2 pyruvate move on to the next stage.
- Note: if oxygen is absent, pyruvate is converted by fermentation in the cytosol instead of entering mitochondrial respiration.

2) Aerobic, mitochondrial stage (stage 2: pyruvate oxidation + citric acid cycle + oxidative phosphorylation)
- Where: mitochondrion — pyruvate is imported into the mitochondrial matrix; the citric acid (Krebs) cycle occurs in the matrix; the electron transport chain and ATP synthase are on the inner mitochondrial membrane.
- What happens:
- Pyruvate oxidation (link reaction) in the matrix: each pyruvate → acetyl‑CoA + CO2 + NADH.
- Citric acid cycle in the matrix: acetyl‑CoA is oxidized to CO2, producing ATP (or GTP), NADH and FADH2.
- Electron transport chain (inner membrane) uses electrons from NADH and FADH2 to pump protons and create a proton motive force; ATP synthase uses that gradient to make most of the ATP (oxidative phosphorylation). O2 is the final electron acceptor and water is formed.
- Outputs (per glucose, combined for the mitochondrial steps): additional NADH/FADH2 and CO2; the ETC/oxidative phosphorylation yields the bulk of ATP (overall total ATP per glucose is ~30–32, depending on shuttle systems and cell type).

Overall purpose: convert the energy in glucose into usable ATP, releasing CO2 and H2O as waste products.