Glycolysis and the Link Reaction
Glucose is split into two pyruvate molecules in the cytoplasm, with a small net ATP gain and no oxygen involved. Pyruvate then enters the mitochondrion and becomes acetyl coenzyme A.
What a learner can do afterwards
- Explains why glycolysis spends ATP before it yields any.
- States the net ATP and reduced NAD yield per glucose molecule.
- Says what is removed from pyruvate in the link reaction and where that happens.
1 · Read
Respiration releases energy by breaking down sugar, such as glucose built by photosynthesis. Glycolysis, its first stage, runs in the cytoplasm and needs no oxygen. Two ATP prime glucose by adding phosphate groups, so it can split evenly into two three-carbon pieces, which are oxidised into two pyruvates. The payoff banks four ATP and two reduced NAD. NAD is a carrier like NADP in photosynthesis, but a separate molecule. After the two-ATP entry fee, the net gain is two ATP plus two reduced NAD.
Pyruvate cannot enter the next stage as it is, so the link reaction changes it. Each pyruvate crosses into the mitochondrial matrix, loses carbon dioxide, and passes hydrogen to NAD. The leftover two-carbon acetyl group joins coenzyme A as acetyl CoA. Per glucose this runs twice, giving two carbon dioxides and two reduced NAD.
Keep a running tally per glucose. Glycolysis nets two ATP and two reduced NAD in the cytoplasm. The link step adds two carbon dioxides and two more reduced NAD in the matrix. These are small gains; the bigger payoffs come later. When oxygen runs short, fermentation recycles NAD in the cytoplasm so glycolysis keeps going.
A cell with no mitochondria still runs glycolysis, since the pathway lives in the cytoplasm. Fermenters recycle NAD in the cytoplasm to keep it spinning, while aerobes ship pyruvate toward the later stages.
Split glucose to two pyruvates for a net two ATP, then trim each pyruvate to acetyl CoA in the matrix.
The rewritten textbook readings were written for this dataset. Other reading material summarises a catalog listing for a Crash Course Biology episode by Complexly. Videos are linked from Khan Academy. License terms for these sources are not recorded.
2 · Watch
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