Lactate and NAD: Higher Level only: This statement is Higher Level only. It explains why anaerobic respiration makes lactate (SL 3.2.5 says what is made; here you learn why).
Practise this as you read
- Outline how NAD is regenerated.
- State the purpose of converting pyruvate to lactate.
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The big idea: Glycolysis needs a supply of NAD. Without oxygen, reduced NAD cannot be oxidized by the electron transport chain, so muscle cells reduce pyruvate to lactate. This regenerates NAD, so glycolysis can go on making a little ATP.
The points to remember
- Glycolysis reduces NAD; without NAD it stops.
- With no oxygen, reduced NAD cannot be oxidized by the electron transport chain.
- Reduced NAD passes hydrogen to pyruvate, which is reduced to lactate.
- NAD is regenerated, so glycolysis continues: a net 2 ATP per glucose.
- No carbon dioxide is produced in humans' anaerobic respiration.
Remember it as: Lactate is the price of keeping NAD free.
Real example: the burning legs of a 400 m runner. In a 400 m race, muscles make ATP faster than oxygen arrives. Lactate builds up as pyruvate is reduced to keep glycolysis going, and the muscles tire.
Trap: The purpose of making lactate is not to make more ATP directly: it is to regenerate NAD so glycolysis can continue.
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How it is asked: Expect a question on the purpose of turning pyruvate into lactate, or an Outline of how NAD is regenerated.
IB-style questionOutline[2 marks]
Outline how NAD is regenerated in anaerobic respiration in human muscle.
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