Respiration transfers energy in controlled steps
Glucose contains chemical potential energy, but releasing all of it in one uncontrolled event would waste much as heat. Cellular respiration uses enzyme-controlled reactions to transfer energy in smaller steps. Some is captured directly in ATP, while much is first transferred to reduced electron carriers.
ATP is the immediate energy currency for many cellular processes. It is useful because hydrolysis of its terminal phosphate is coupled to energy-requiring reactions, transport and movement. ATP is continually regenerated rather than stored in very large quantities.
Glycolysis splits glucose in the cytosol
Glycolysis occurs in the cytosol and does not require oxygen directly. Energy is first invested to phosphorylate and destabilise glucose. The six-carbon intermediate is split into two three-carbon molecules, which are oxidised to pyruvate.
Per glucose molecule, glycolysis produces a net gain of two ATP by substrate-level phosphorylation and reduces NAD to NADH. The two pyruvate molecules still contain much of the original energy and may enter mitochondria when aerobic conditions permit.
ATP activates a six-carbon sugar.
One six-carbon molecule becomes two three-carbon units.
Electrons reduce NAD to NADH.
ATP forms and two pyruvate remain.
The link reaction and Krebs cycle remove carbon
In the mitochondrial matrix, each pyruvate is decarboxylated and oxidised. Carbon dioxide is released, NAD is reduced, and the remaining two-carbon acetyl group joins coenzyme A. Acetyl-CoA then enters the Krebs cycle.
Acetyl combines with a four-carbon acceptor to form a six-carbon compound. A series of reactions releases two carbon dioxide molecules, regenerates the four-carbon acceptor, reduces NAD and FAD, and produces a small amount of ATP by substrate-level phosphorylation. The cycle's major energy output is reduced carriers, not ATP itself.
| Stage | Location | Main contribution |
|---|---|---|
| Glycolysis | Cytosol | Pyruvate, net ATP and NADH |
| Link reaction | Mitochondrial matrix | Acetyl-CoA, CO₂ and NADH |
| Krebs cycle | Mitochondrial matrix | CO₂, NADH, FADH₂ and a little ATP |
| Oxidative phosphorylation | Inner mitochondrial membrane | Most aerobic ATP |
Electron transport creates a proton gradient
NADH and FADH₂ donate high-energy electrons to protein complexes in the inner mitochondrial membrane. As electrons pass along the chain, released energy pumps protons from the matrix into the intermembrane space. The membrane stores potential energy as an electrochemical proton gradient.
Protons flow back through ATP synthase, whose activity couples that flow to ATP formation. This chemiosmotic mechanism is oxidative phosphorylation. Oxygen is the final electron acceptor; it combines with electrons and protons to form water. Without oxygen, electron flow stops and reduced carriers cannot be efficiently reoxidised.
NADH and FADH₂ feed the transport chain.
Electron energy builds a gradient.
Proton flow drives phosphorylation.
Oxygen accepts electrons and forms water.
Fermentation keeps glycolysis possible
When oxygen is unavailable, oxidative phosphorylation stops. Glycolysis can continue only if NADH transfers electrons to another acceptor and regenerates NAD. In animal cells, pyruvate is reduced to lactate; in yeast, pyruvate is converted to ethanol and carbon dioxide.
Fermentation adds no large ATP yield beyond glycolysis. Its crucial function is NAD regeneration. Because glucose is only partially oxidised, much chemical energy remains in lactate or ethanol.
The ideas to carry forward
- Glycolysis supplies pyruvate, a little ATP and reduced NAD.
- The Krebs cycle mainly loads electron carriers.
- Electron transport builds the proton gradient used by ATP synthase.
- Fermentation regenerates NAD so glycolysis can continue without oxygen.
Answer first. Then reveal the marking logic.
01Why does blocking the electron-transport chain eventually slow the Krebs cycle?2 marks · show the biological link
Answer: NADH and FADH₂ cannot be efficiently oxidised, so oxidised NAD and FAD become limiting for Krebs-cycle dehydrogenation reactions.
02What is the immediate purpose of fermentation?2 marks · show the biological link
Answer: To regenerate oxidised NAD, allowing glycolysis and its small ATP yield to continue.
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