Respiration Pathways and ATP
How glycolysis, the link reaction, the Krebs cycle, and oxidative phosphorylation transfer energy from fuels into ATP.
Cellular respiration transfers energy from organic molecules into ATP. Rather than releasing all of the energy in one step, cells use enzyme-controlled pathways and electron carriers to capture it in manageable stages.
C6H12O6 + 6 O2 -> 6 CO2 + 6 H2OGlycolysis
Glycolysis occurs in the cytosol. One glucose molecule is phosphorylated, split, and oxidized to form two molecules of pyruvate. The pathway makes a small net yield of ATP by substrate-level phosphorylation and reduces NAD to NADH.
Link reaction and Krebs cycle
When oxygen is available, pyruvate enters the mitochondrial matrix. The link reaction removes carbon dioxide and transfers hydrogen/electrons to NAD, producing acetyl-CoA. Acetyl-CoA then enters the Krebs cycle, where its carbon atoms are released as CO2 while NAD and FAD are reduced and a small amount of ATP is formed.
| Stage | Location | Key outputs |
|---|---|---|
| Glycolysis | Cytosol | Pyruvate, NADH, small ATP yield |
| Link reaction | Mitochondrial matrix | Acetyl-CoA, CO2, NADH |
| Krebs cycle | Mitochondrial matrix | CO2, NADH, FADH2, small ATP yield |
| Oxidative phosphorylation | Inner mitochondrial membrane | Most ATP; water formed when oxygen accepts electrons |
Electron transport and chemiosmosis
NADH and FADH2 donate high-energy electrons to carriers in the inner mitochondrial membrane. As electrons move through the chain, released energy is used to pump protons from the matrix into the intermembrane space.
The proton electrochemical gradient stores potential energy. Protons flow back into the matrix through ATP synthase, and that flow drives phosphorylation of ADP to ATP. Oxygen is the terminal electron acceptor and combines with electrons and protons to form water.
When oxygen is limited
Without oxygen, the electron transport chain cannot oxidize NADH at its normal rate. Fermentation pathways regenerate NAD+ so glycolysis can continue, but they do not add the large ATP yield of oxidative phosphorylation.
| Cells | Pyruvate pathway | Purpose |
|---|---|---|
| Many animal cells under short-term oxygen shortage | Lactate formation | Regenerates NAD+ for glycolysis |
| Yeast and some microorganisms | Ethanol + CO2 formation | Regenerates NAD+ for glycolysis |
Reason through a respirometer
- Keep temperature constant because gas volume changes with temperature.
- Use a CO2 absorbent when the aim is to infer oxygen uptake from a fall in gas volume.
- Include a control so non-biological pressure or temperature changes can be distinguished from respiration.
- Compare rate using movement per unit time and normalize for organism mass when appropriate.