Glycolysis

Where it happens

Glycolysis takes place in the cytoplasm of the cell, not inside the mitochondrion. It is the first stage of respiration and happens in every living cell, whether or not oxygen is available.

Because it needs no membrane-bound organelle, glycolysis runs in cells that have no mitochondria at all, such as mature red blood cells and bacteria, and in cells whose oxygen supply is temporarily cut off, such as muscle fibres during a sprint. The enzymes that catalyse it are dissolved in the cytoplasm, so the reactions happen throughout the cell rather than at one fixed site.

Inputs and outputs

• Input: one molecule of glucose (a six-carbon sugar), taken up from the blood or released from stored glycogen or starch.
• Input: two molecules of ATP, used at the start to activate the glucose molecule.
• Input: the cytoplasmic enzymes that catalyse each step, and the hydrogen carrier NAD.
• Output: two molecules of pyruvate (each with three carbon atoms).
• Output: four molecules of ATP produced, giving a net gain of two ATP per glucose after the two used at the start.
• Output: two molecules of reduced NAD, carrying hydrogen removed from the glucose for use in the later stages of aerobic respiration or in the regeneration step of anaerobic respiration.

The steps

1. A glucose molecule, with six carbon atoms, enters the cytoplasm.
2. Two molecules of ATP are used to add phosphate groups to the glucose, making it more reactive; this is the activation step.
3. Enzymes break the activated glucose molecule down in a series of steps.
4. The six-carbon molecule is split into two three-carbon molecules.
5. Hydrogen is removed from each three-carbon molecule and picked up by the hydrogen carrier NAD, forming reduced NAD.
6. The three-carbon molecules are converted into two molecules of pyruvate, and four molecules of ATP are made in the process.
7. The net result is two pyruvate, two ATP and two reduced NAD for every glucose molecule broken down.

Why it matters and how it is controlled

Glycolysis is the starting point shared by both aerobic and anaerobic respiration, so it happens no matter whether oxygen is present. What happens to the pyruvate next depends on oxygen: with oxygen, it moves into the mitochondrion for aerobic respiration; without oxygen, it is converted into lactic acid in muscle cells or into ethanol and carbon dioxide in yeast.

The rate of glycolysis is controlled by the cell's demand for ATP. When ATP is plentiful, it slows one of the early enzymes of the pathway, so glucose is not wasted.

When ATP is used up quickly, as in a contracting muscle, the enzymes speed up and glucose is broken down faster. This is a form of feedback control, and it is the same principle as the control of enzyme activity by product concentration that you meet in the enzymes chapter.

The pathway also explains why anaerobic respiration is so much less efficient. Without oxygen, the reduced NAD made in glycolysis cannot pass its hydrogen to the mitochondrion, so the cell converts pyruvate into lactic acid or ethanol simply to free the NAD and let glycolysis continue.

The only ATP the cell gains is the two molecules from glycolysis itself.

How it is examined

Questions may ask you to state where glycolysis occurs, to explain why it does not need oxygen, to identify glycolysis as the common first stage of aerobic and anaerobic respiration, or to compare the small ATP yield of glycolysis with the much larger yield of the later aerobic stages.

Flow-diagram questions give a partly completed scheme of respiration and ask you to fill in glucose, pyruvate, the site of each stage and the products under each condition. Comparison questions ask for a table of aerobic and anaerobic respiration; glycolysis is the row that is identical in both columns.

Essay questions on respiration expect you to name glycolysis as the first stage, state that it happens in the cytoplasm, and give its products before moving on to the mitochondrial stages.

Common misconceptions

Worked exam-style question

Question. A sample of yeast cells was placed in a glucose solution in a sealed flask from which the air had been removed. A second sample was placed in a glucose solution in an open flask with air bubbled through it.

After thirty minutes, the ATP content of the cells in each flask was measured. The cells in the open flask contained far more ATP.

(a) Name the stage of respiration that took place in the cells of both flasks. (b) State where in the cell this stage occurs and name its products.

(c) Explain why the cells in the open flask contained more ATP.

Model answer. (a) Glycolysis. (b) It occurs in the cytoplasm.

One glucose is broken down into two pyruvate molecules, with a net gain of two ATP and reduced NAD. (c) In the open flask, oxygen is available, so pyruvate enters the mitochondria and is broken down completely by aerobic respiration to carbon dioxide and water, releasing a large amount of ATP.

In the sealed flask, no oxygen is present, so pyruvate is converted to ethanol and carbon dioxide by anaerobic respiration, which produces no further ATP. The only ATP gained is the two from glycolysis, so the ATP content is much lower.

Source:SRC-DSKP-EN

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