Glycolysis and the link reaction – Cellular Respiration (Senior 6 Biology) | Ganzaa

Cellular Respiration

Lessons

Senior 6 · Biology

Cellular Respiration

5 lessons

Lesson 2 of 5

Glycolysis and the link reaction

20 min

The first ten steps

Every living cell on Earth, from a bacterium in the soil of Nyungwe to a neurone in your brain, begins to break down glucose in the same way: a pathway of ten enzyme-controlled steps called glycolysis. It needs no oxygen and no organelle, which suggests it evolved in the earliest cells, before the air contained oxygen. This lesson follows glycolysis and the link reaction that connects it to the mitochondrion, counting every carbon atom, ATP and reduced NAD.

Definition

Glycolysis: the splitting of one glucose molecule (6C) into two pyruvate molecules (3C) in the cytoplasm, with a net gain of 2 ATP and 2 reduced NAD. It does not need oxygen.

Phosphorylation: the addition of a phosphate group to a molecule. Lysis: splitting.

Oxidation: the loss of electrons, often as the removal of hydrogen (dehydrogenation). Reduction: the gain of electrons or hydrogen. When NAD accepts hydrogen, NAD is reduced and the substrate is oxidised.

Link reaction: the conversion of pyruvate to acetyl coenzyme A in the mitochondrial matrix, releasing CO₂ and forming reduced NAD.

Glycolysis in four stages

Figure 1: Glycolysis (in the cytoplasm) and the link reaction (in the mitochondrial matrix) for one glucose molecule. The link reaction is shown for one pyruvate; it happens twice per glucose.

Phosphorylation. Two phosphate groups from 2 ATP are added to glucose, forming hexose bisphosphate. This "investment" makes glucose more reactive and traps it in the cell, since the phosphorylated sugar cannot cross the membrane.
Lysis. Hexose bisphosphate splits into two molecules of triose phosphate (3C).
Oxidation. A dehydrogenase removes hydrogen from each triose phosphate; NAD accepts it, forming reduced NAD.
ATP formation. Each triose is converted to pyruvate, and its phosphate groups are transferred to ADP: 2 ATP per triose, 4 in all, by substrate-level phosphorylation.

Balance for one glucose: 2 ATP used, 4 ATP made, so a net gain of 2 ATP; 2 reduced NAD; 2 pyruvate.

The link reaction

If oxygen is available, pyruvate is actively transported into the mitochondrial matrix. There, for each pyruvate:

it is decarboxylated: one carbon is removed as CO₂;
it is dehydrogenated: hydrogen is removed and reduces NAD;
the remaining 2-carbon acetyl group joins coenzyme A, forming acetyl CoA.

\text{pyruvate} + \mathrm{NAD} + \mathrm{CoA} \rightarrow \text{acetyl CoA} + \mathrm{CO_2} + \text{reduced NAD}

Because each glucose gives two pyruvates, the link reaction happens twice per glucose. No ATP is made in the link reaction.

Worked example 1: the balance sheet so far

For one glucose molecule, total the ATP, reduced NAD and CO₂ produced by glycolysis and the link reaction, and check that the carbon atoms balance.

Stage	Net ATP	Reduced NAD	CO₂
Glycolysis	2	2	0
Link reaction ( $\times 2$ )	0	2	2
Total	2	4	2

Carbon check: glucose has 6 C. After the link reaction: $2 \times \text{acetyl (2C)} + 2 \times \mathrm{CO_2}\ (1\mathrm{C}) = 4 + 2 = 6$ carbons. ✓ The remaining 4 carbons, in the two acetyl groups, will leave as CO₂ in the Krebs cycle (Lesson 3).

Worked example 2: analysing an experiment with mitochondria

Mitochondria were separated from liver cells by centrifugation and kept in a buffer with oxygen. When glucose was added, they used no oxygen. When pyruvate was added, they used oxygen steadily. Explain these results.

Glycolysis happens in the cytoplasm, and its enzymes were left behind when the mitochondria were separated. So the mitochondria could not convert glucose to pyruvate.
Pyruvate can be taken into the matrix, where the link reaction and Krebs cycle oxidise it, producing reduced NAD and reduced FAD.
These pass hydrogen to the electron transport chain on the inner membrane, where oxygen is the final acceptor: so oxygen is used only when pyruvate is supplied.

Control: a tube of mitochondria with buffer only, and no substrate, shows the background oxygen use.

Warning

Common mistakes

"Glycolysis makes 4 ATP." It makes 4 but uses 2: the net gain is 2.
"Glycolysis happens in the mitochondrion." It happens in the cytoplasm; only pyruvate enters the mitochondrion.
Forgetting to double. One glucose gives two pyruvates, so every quantity from the link reaction onwards is multiplied by 2.

Amoeba Sisters: "Cellular Respiration (UPDATED)". Watch for where glycolysis, the link reaction ("intermediate step"), the Krebs cycle and the electron transport chain happen, and how many ATP each gives.

Tip

Exam technique. For each stage, learn four facts: where it happens, what goes in, what comes out, and the ATP, reduced NAD, reduced FAD and CO₂ per glucose. A table like the one in Worked example 1, extended through Lesson 3, is the fastest way to answer counting questions.

✦

Practice: glycolysis and the link reaction

State where glycolysis happens and why it does not need oxygen.
Explain why ATP is used at the start of glycolysis.
Name the 3-carbon products of (a) lysis, (b) glycolysis as a whole.
Explain what is meant by "NAD is reduced" in glycolysis.
Write a word equation for the link reaction, and state how many times it happens per glucose.
Calculate the number of ATP and reduced NAD molecules formed from 5 molecules of glucose by glycolysis and the link reaction together.
A poison blocks the transport of pyruvate into mitochondria. Predict its effect on (a) glycolysis, (b) the link reaction, and explain.

Show answersHide answers

In the cytoplasm. Its steps are oxidations in which hydrogen is taken up by NAD, not by oxygen, and its ATP is made by substrate-level phosphorylation.
Phosphorylation makes glucose more reactive (lowers the activation energy for splitting it) and traps it inside the cell.
(a) Triose phosphate. (b) Pyruvate.
NAD accepts hydrogen (electrons) removed from triose phosphate by a dehydrogenase, forming reduced NAD; the triose phosphate is oxidised.
Pyruvate + NAD + coenzyme A → acetyl CoA + CO₂ + reduced NAD. Twice per glucose.
Per glucose: net 2 ATP and 4 reduced NAD. For 5 glucose: 10 ATP and 20 reduced NAD.
(a) Glycolysis continues at first (it happens in the cytoplasm), though pyruvate and reduced NAD build up unless they are removed another way, such as by fermentation. (b) The link reaction stops, because its enzymes are in the matrix and pyruvate cannot reach them.

Key points

Glycolysis (cytoplasm): glucose → 2 pyruvate; phosphorylation, lysis, oxidation, ATP formation; net 2 ATP and 2 reduced NAD.
Link reaction (matrix, twice per glucose): pyruvate → acetyl CoA + CO₂ + reduced NAD; no ATP.
After these stages: 2 ATP, 4 reduced NAD, 2 CO₂ per glucose; 4 carbons remain in acetyl groups.

Sources and credits

2.4.3: Glycolysis, from Biology 2e by OpenStax (Mary Ann Clark, Matthew Douglas, Jung Choi), via LibreTexts Biology, licensed under CC BY 4.0. Used for the stages of glycolysis and its yield. Adapted by Ganzaa.
2.4.4: Oxidation of Pyruvate and the Citric Acid Cycle, from Biology 2e by OpenStax (Mary Ann Clark, Matthew Douglas, Jung Choi), via LibreTexts Biology, licensed under CC BY 4.0. Used for the link reaction. Adapted by Ganzaa.
Video: Cellular Respiration (UPDATED), Amoeba Sisters, embedded from YouTube.

Figure 1, the tables, the worked examples and the practice questions are written by Ganzaa.

1The first ten steps

2Glycolysis in four stages

3The link reaction

The first ten steps

Glycolysis in four stages

The link reaction