12.2.312 · Energy and Respiration

Aerobic Respiration- Glycolysis — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

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📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision

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12.2.3 Aerobic Respiration: Glycolysis — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Describe the steps of glycolysis, its inputs and outputs, and how it is controlled.Meaning — glycolysis (glucose splitting) is the first stage of respiration.Location — the cytopl…

Conversion
Phosphorylation
ATP formation
Why use ATP
Net products per glucose
With oxygen

Part 1 of 7What glycolysis is

Meaning

Meaning glycolysis is the first stage of respiration.
- (glucose splitting)

Location

Location the cytoplasm of all cells.

Oxygen

Oxygen it is not needed, so it is common to aerobic and anaerobic respiration.

Conversion

Conversion one glucose (6C) becomes two pyruvate (3C) molecules.

Origin

Origin it evolved very early, which fits its presence in nearly all organisms.

Purpose

Purpose it traps glucose in the cell by phosphorylation and splits the molecule in two.

Part 2 of 7Steps of glycolysis

Phosphorylation

Phosphorylation glucose is phosphorylated by ATP to glucose phosphate, which also traps it in the cell.

Glucose phosphate is converted to fructose phosphate, and a second ATP adds another phosphate to make fructose (hexose) bisphosphate.

Lysis

Lysis the hexose bisphosphate (6C) splits into two triose phosphate (3C) molecules.

Oxidation

Oxidation hydrogen is removed from each triose phosphate by a dehydrogenase and accepted by NAD, forming 2 reduced NAD.

Dephosphorylation

Dephosphorylation the energy released adds a further phosphate, and phosphates are transferred to ADP.

ATP formation

4 ATP are made by substrate-level phosphorylation.
- (2 from each triose phosphate)

Pyruvate

2 triose phosphate become 2 pyruvate, the end product.

Why use ATP

Why use ATP the investment of 2 ATP makes the stable glucose reactive and lowers the activation energy of later steps.

Part 3 of 7ATP balance

Stage	ATP used	ATP made	Net
Per glucose	2	4	2

Net products per glucose

2 pyruvate, 2 ATP and 2 reduced NAD.

Count

Count two ATP in, four ATP out, two reduced NAD out, two pyruvate out.

Part 4 of 7What happens next

1With oxygen

With oxygen pyruvate is actively transported into the mitochondrion for the link reaction,
- and reduced NAD delivers electrons to the electron transport chain.

2Without oxygen

Without oxygen pyruvate becomes lactate (animals) or ethanol and CO₂ , which regenerates NAD so glycolysis continues.
- (yeast and plants)

3Nature of the pathway

Nature of the pathway it is quick but low-yield, and the only ATP source for red blood cells.

Part 5 of 7Control of glycolysis

Phosphofructokinase

Phosphofructokinase it catalyses fructose phosphate to fructose bisphosphate.
High ATP inhibits it.
ADP and AMP stimulate it.

Allosteric effect

ATP binds to a site other than the active site.

Negative feedback

Negative feedback when ATP is plentiful glycolysis slows, and when ATP is used rapidly, as in contracting muscle, it speeds up.

Part 6 of 7Links to other pathways

1Cells without mitochondria

Cells without mitochondria glycolysis can occur in mature mammalian red cells and in prokaryotes.

2Other substrates

Other substrates glycerol is converted to triose phosphate, and fructose and galactose are converted to glycolysis intermediates.

3Products diverted

Products diverted triose phosphate can be converted to glycerol for lipid synthesis, and pyruvate to alanine.

Data skills.

ATP made in an oxygen-free tube must come from glycolysis, not oxidative phosphorylation.
Radioactive carbon from glucose appears in pyruvate first, and in CO₂ only later after the link and Krebs reactions.

5Metabolic chain

Metabolic chain each step has its own enzyme, small energy changes occur at each step, and intermediates can be diverted into other pathways.

Part 7 of 7More from the specification

Four Stages of Aerobic Respiration

Aerobic respiration proceeds through four sequential stages: glycolysis in the cytoplasm, the link reaction and Krebs cycle in the mitochondrial matrix,
and oxidative phosphorylation at the inner mitochondrial membrane.

Mitochondrial Structure and Function

The folded cristae of the inner membrane provide an extensive surface area for electron transport chains and ATP synthase enzymes,
while the aqueous matrix contains metabolic enzymes and dissolved substrates at optimal concentration and temperature.

Terminal Oxidation and Chemiosmosis

As electrons flow through the inner membrane complexes, protons are pumped into the intermembrane space;
- their return through ATP synthase powers ATP synthesis,
while oxygen acts as terminal electron acceptor to form metabolic water.

Matrix

The solution inside the inner membrane containing Krebs cycle enzymes.

Fermentation

Anaerobic respiration by yeast producing ethanol and carbon dioxide.

Krebs cycle

Cycle in the mitochondrial matrix producing NADH, FADH2, ATP and CO₂.

Substrate concentration

The amount of substrate per unit volume.

Intermembrane space

The gap between the two membranes where protons accumulate.

Mitochondrial matrix

Fluid centre of the mitochondrion containing enzymes for the link reaction and Krebs cycle.

Ethanol fermentation

Decarboxylation of pyruvate to ethanal, then reduction to ethanol.

Key terms

Term	Meaning
Glycolysis	Breakdown of glucose to pyruvate in the cytoplasm.
Phosphorylation	Addition of a phosphate group to a molecule.
Triose phosphate	A 3C sugar phosphate formed when hexose bisphosphate splits.
Pyruvate	The 3C end product of glycolysis.
Substrate-level phosphorylation	Direct transfer of phosphate from a substrate to ADP.
Phosphofructokinase	An enzyme of glycolysis inhibited by ATP.

Avoid these mistakes

Saying glycolysis needs oxygen — it does not.

Saying glycolysis takes place in the mitochondrion — it takes place in the cytoplasm.

Saying 4 ATP is the net yield — 2 are used, so the net is 2.

Saying glycolysis makes ATP by chemiosmosis — it uses substrate-level phosphorylation.

Last updated: 18 October 2026