Trace carbon and electrons through glycolysis, the citric acid cycle, electron transport, and fermentation.
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Cellular respiration transfers energy from fuel into ATP. In aerobic respiration, glucose carbon ends up in carbon dioxide, and oxygen is ultimately reduced to water.
The overall equation summarizes many reactions rather than one explosive step: glucose + oxygen → carbon dioxide + water + energy transferred to ATP and heat.
Where does glycolysis occur, and what is its net ATP yield per glucose?
In the cytosol; net 2 ATP per glucose.
Glycolysis splits one six-carbon glucose into two three-carbon pyruvates. It makes four ATP but spends two, so its net yield is two. It also produces NADH.
NAD+ accepts high-energy electrons and hydrogen during fuel breakdown, becoming NADH. NADH can deliver electrons to the electron transport chain.
NADH is an electron carrier rather than the cell’s usual direct payment for mechanical work. ATP and electron carriers have related but different jobs.
Order these events: electron transport, glycolysis, citric acid cycle, pyruvate oxidation.
Glycolysis → pyruvate oxidation → citric acid cycle → electron transport.
Glycolysis makes pyruvate. Pyruvate oxidation makes acetyl-CoA, which enters the cycle. Reduced carriers from these stages feed electrons into electron transport.
In eukaryotes, pyruvate oxidation and the citric acid cycle occur mainly in the mitochondrial matrix. Carbon dioxide is released, and NADH and FADH₂ carry away much of the harvested energy.
The cycle regenerates its starting acceptor molecule. It makes a small amount of ATP directly, but most of its energy contribution reaches ATP production through electron carriers.
Electron transport uses released energy to pump H+ across the inner mitochondrial membrane. H+ then flows back through ATP synthase, helping drive ATP production.
The gradient stores potential energy. Electron transport and chemiosmosis together account for most ATP made in aerobic respiration.
What is oxygen’s direct role in aerobic cellular respiration?
It is the final electron acceptor in the electron transport chain, combining with electrons and H+ to form water.
Oxygen does not directly split glucose during glycolysis. Without oxygen, the aerobic chain cannot keep accepting electrons, and carriers cannot be regenerated by that chain.
When oxygen-dependent respiration cannot keep up, why does lactate fermentation help glycolysis continue?
It regenerates NAD+ from NADH, allowing glycolysis to keep running.
Fermentation itself does not add an extra ATP payoff beyond glycolysis. Its key service is recycling NAD+. Animal cells can make lactate; yeast can make ethanol and CO₂.
If H+ leaks freely across the inner mitochondrial membrane instead of passing through ATP synthase, what happens to ATP production?
ATP production by oxidative phosphorylation decreases.
The leak reduces the proton gradient. Electron flow can sometimes continue, but less of its released energy is captured as ATP and more can become heat.