Glycolysis

Glycolysis is the first stage of both aerobic and anaerobic respiration. In A-level Biology you need to know that it happens in the cytoplasm, doesn't need oxygen, and turns one glucose molecule into two molecules of pyruvate in three stages: phosphorylation of glucose using ATP, splitting into triose phosphate, and oxidation of triose phosphate to pyruvate. There is a net gain of 2 ATP and 2 reduced NAD per glucose.

Where glycolysis happens

Glycolysis takes place in the cytoplasm of the cell. It is an anaerobic process: none of its reactions use oxygen. That is why every cell, with or without oxygen, can carry it out. What happens to the pyruvate afterwards depends on whether oxygen is available.

The stages of glycolysis

1. Phosphorylation of glucose. Glucose (6C) is phosphorylated using two ATP molecules, which give their phosphate groups to it. This makes the glucose more reactive and gives glucose phosphate (hexose bisphosphate). Two ATP are used up.
2. Splitting into triose phosphate. The phosphorylated six-carbon sugar splits into two molecules of triose phosphate, each with three carbon atoms.
3. Oxidation of triose phosphate to pyruvate. Each triose phosphate is oxidised: hydrogen is removed and accepted by NAD, making reduced NAD. Each one is converted to pyruvate (3C), and in the process two ATP are made. With two triose phosphates, that is 4 ATP and 2 reduced NAD.

The ATP in stage 3 is made by substrate-level phosphorylation: a phosphate group is passed directly from an intermediate to ADP, without the electron transfer chain.

Products of glycolysis

AQA A-level Biology (3.5.2) summarises glycolysis as phosphorylation of glucose to glucose phosphate using ATP, production of triose phosphate, and oxidation of triose phosphate to pyruvate with a net gain of ATP and reduced NAD.

Check your understanding.

What happens to pyruvate next

With oxygen: aerobic respiration

Pyruvate is actively transported into the mitochondrial matrix, where it enters the link reaction and then the Krebs cycle. The reduced NAD made in glycolysis is later oxidised by the electron transfer chain, which uses it to make more ATP by oxidative phosphorylation.

Without oxygen: anaerobic respiration

Without oxygen, the electron transfer chain stops, so reduced NAD can't be oxidised back to NAD there. Glycolysis would soon stop for lack of NAD. Instead, pyruvate is converted into another product using the reduced NAD, which turns it back into NAD so glycolysis can carry on:

Anaerobic respiration only gives the 2 ATP from glycolysis for each glucose, far less than aerobic respiration, because the rest of the energy stays locked up in lactate or ethanol.

Worked examples

Example 1: counting carbon atoms

Glucose has 6 carbons. Glycolysis splits it into two triose phosphates of 3 carbons each, which become two pyruvates of 3 carbons each. No carbon is lost as carbon dioxide in glycolysis: 6 carbons in, 6 carbons out.

Example 2: ATP from three glucose molecules

Three glucose molecules go through glycolysis. How many ATP are made, and what is the net gain? Each makes 4 ATP and uses 2, so 3 × 4 = 12 ATP made, 3 × 2 = 6 used, and the net gain is 6 ATP.

Example 3: why NAD matters

Explain why a muscle cell short of oxygen converts pyruvate to lactate. The conversion uses reduced NAD and regenerates NAD. Without a supply of NAD, the oxidation of triose phosphate in stage 3 cannot happen, so making NAD again keeps glycolysis, and its small ATP yield, going.

Common mistakes

• Saying glycolysis happens in the mitochondria. It is in the cytoplasm.
• Giving 4 ATP as the gain. 4 are made, but 2 are used: the net gain is 2.
• Saying glycolysis needs oxygen. It is anaerobic and happens in both types of respiration.
• Forgetting the two pyruvates. One glucose gives two of everything after the split.
• Saying CO₂ is made in glycolysis. It isn't; CO₂ comes from the link reaction, the Krebs cycle, or the ethanol pathway.
• Missing the role of NAD. Reduced NAD is formed when triose phosphate is oxidised.

Practice questions

1. Where in the cell does glycolysis happen?
2. Name the three-carbon molecule formed when phosphorylated glucose splits.
3. How many molecules of reduced NAD are made when 5 glucose molecules go through glycolysis?
4. In yeast without oxygen, what is pyruvate converted into?

Answers: 1) The cytoplasm. 2) Triose phosphate. 3) 2 per glucose, so 10. 4) Ethanol and carbon dioxide.

Practise with a short timed paper

Respiration in detail is in the second year of A-level Biology. The daily A-level Biology: Year 2 paper is a short timed paper of 8 questions, with a new one every day. For the whole of respiration, see the cellular respiration guide.

Read next

Biology · Respiration and photosynthesisCalvin cycle A-level Biology: the 3 stages explainedThe Calvin cycle for A-level Biology: fixation by rubisco, reduction of GP to triose phosphate, regeneration of RuBP, the ATP and reduced NADP it uses.