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GlycolysisAQA A-Level Biology: Revision notes

Section 1

Respiration, ATP and respiratory substrates

Respiration produces ATP, the immediate energy currency of the cell. Various respiratory substrates can be used, mainly glucose, but also fatty acids and amino acids. The hydrolysis (breakdown) of respiratory substrates releases energy, which is used to make ATP from ADP and inorganic phosphate (Pi). Respiration happens in all living organisms.

Key termsrespirationrespiratory substrateATP

Section 2

Respiration and evolution

Glycolysis and respiration occur in nearly all organisms (bacteria, plants, animals, fungi) and use similar enzymes. This is indirect evidence for evolution: it suggests the pathways were inherited from a common ancestor. It is indirect because it infers relatedness rather than showing the actual order of events.

Key termscommon ancestorindirect evidence

Section 3

Glycolysis: where and what

Glycolysis is the first stage of both aerobic and anaerobic respiration. It occurs in the cytoplasm and is an anaerobic process (it does not need oxygen). It converts one glucose (6C) into two pyruvate (3C) molecules.

Key termsglycolysiscytoplasmanaerobic
Common mistake

Glycolysis is not aerobic: it happens in the cytoplasm and does not need oxygen, although it is the first stage of aerobic respiration too.

Section 4

Stage 1: phosphorylation

Glucose is phosphorylated using ATP (hydrolysed to ADP) to give glucose phosphate. A second phosphorylation, again using ATP, gives hexose bisphosphate. Phosphorylation makes the sugar more reactive, so it is easier to split, and keeps it in the cell. Two ATP are used per glucose.

Key termsphosphorylationhexose bisphosphate

Section 5

Stage 2: splitting to triose phosphate

Hexose bisphosphate is split into two molecules of triose phosphate (3C each). Each is then taken through stage 3, so everything from here is counted twice per glucose.

Key termstriose phosphate

Section 6

Stage 3: oxidation to pyruvate

Each triose phosphate is oxidised (hydrogen removed) to pyruvate. NAD accepts the hydrogen and is reduced to reduced NAD. ATP is made by substrate-level phosphorylation: four ATP are made per glucose (two per triose phosphate). With two used earlier, the net gain is 2 ATP, 2 reduced NAD and 2 pyruvate per glucose.

Key termsoxidationreduced NADsubstrate-level phosphorylationnet gain
Exam tip

Write 4 ATP made, 2 ATP used, net 2. Examiners credit the arithmetic.

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Carry on to the next subtopic.

Exam questions on Glycolysis

  1. Glycolysis was investigated in a cell-free extract of yeast, supplied with glucose, ATP, ADP, NAD and inorganic phosphate, in a sealed tube containing no oxygen.
    Explain why glycolysis begins with the phosphorylation of glucose using ATP.2 marks
  2. A biochemist measures the ATP and reduced NAD produced when glucose is converted to pyruvate in a cell-free extract, with all the enzymes and cofactors in excess.
    Explain how glycolysis gives a net gain of two ATP per glucose molecule.2 marks
  3. The metabolic poison iodoacetate inhibits the enzyme that oxidises triose phosphate during glycolysis. Cultured human cells in which glucose is the only respiratory substrate are treated with iodoacetate and supplied with glucose and oxygen.
    Predict and explain the effect of iodoacetate on the concentration of triose phosphate and on the production of ATP and reduced NAD in the cells.3 marks
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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).