AP Syllabus focus:
‘Glycolysis releases energy in glucose, forming ATP, NADH, and pyruvate in the cytosol.’
Glycolysis is a core energy-harvesting pathway that begins glucose breakdown without requiring oxygen.
Overview diagram of glycolysis showing glucose being converted into two pyruvate molecules. It highlights the two main phases (energy investment and energy payoff) and visually tracks ATP usage/production and NADH formation across the pathway. Source
It occurs in the cytosol, capturing some energy as ATP and NADH while producing pyruvate for further processing.
Core idea and location
Glycolysis: A cytosolic metabolic pathway that converts one glucose into two pyruvate while transferring energy to ATP and NADH.
Because glycolysis occurs in the cytosol, it can operate in cells lacking mitochondria and provides a rapid, enzyme-controlled way to begin extracting energy from glucose.
What glycolysis produces (and why it matters)
Glycolysis “pays” for energy capture by forming:
ATP via substrate-level phosphorylation (direct transfer of a phosphate group to ADP)
NADH (reduced electron carrier) by transferring electrons from intermediates to NAD
Pyruvate, a 3-carbon product that retains substantial potential energy
These outputs match the syllabus emphasis: ATP, NADH, and pyruvate are formed in the cytosol.
Full glycolysis pathway diagram with each intermediate, the enzyme catalyzing each step, and the paired energy/electron transfers. Co-reactants (ATP/ADP and ) are shown alongside the specific reactions where energy is invested and later harvested, ending in pyruvate. Source
Phases of glycolysis
1) Energy investment phase
This phase uses ATP to make glucose more reactive and symmetrical, preparing it for splitting.
Glucose is phosphorylated twice (consumes 2 ATP total).
The resulting 6-carbon sugar becomes primed for cleavage into two 3-carbon molecules.
2) Cleavage phase
The 6-carbon intermediate splits into two 3-carbon molecules.
These 3-carbon molecules are rearranged into forms that can be oxidised to capture energy.
3) Energy payoff phase
Energy is harvested as electrons and phosphate groups are transferred.
Oxidation of 3-carbon intermediates reduces NAD to NADH (forms 2 NADH per glucose).
Phosphates are transferred to ADP to form ATP (produces 4 ATP per glucose in this phase).
The final 3-carbon product is pyruvate (forms 2 pyruvate per glucose).
Net energy accounting
= net ATP gained per glucose (ATP molecules)
= total ATP generated in payoff phase (ATP molecules)
= ATP consumed in investment phase (ATP molecules)
For glycolysis, and , so the net yield is 2 ATP per glucose, alongside 2 NADH and 2 pyruvate.
Key controls and constraints within the pathway
Glycolysis proceeds through many enzyme-catalysed steps, but control is concentrated at large “commitment” steps.
The phosphorylation steps help trap glucose in the cell and control pathway flux.
Cells adjust glycolysis rate based on energy status:
High ATP tends to reduce glycolytic flux (cell already has energy).
High ADP/AMP tends to increase flux (cell needs ATP).
Why NADH formation is central
NADH stores high-energy electrons captured during oxidation steps.
Regenerating NAD is essential; without NAD, glycolysis cannot continue producing ATP.
This makes glycolysis tightly linked to cellular redox balance, even though the pathway itself occurs entirely in the cytosol.
FAQ
Phosphorylation raises the energy of intermediates and helps destabilise glucose for later splitting.
It also helps retain the sugar inside the cell by adding charged phosphate groups.
They rely on glycolysis in the cytosol for essentially all ATP production.
NAD$^+$ is regenerated by converting pyruvate to lactate to keep glycolysis running.
Two ATP are consumed to phosphorylate early intermediates.
Four ATP are later produced by substrate-level phosphorylation, giving $4-2=2$ net.
Several intermediates can be diverted as carbon skeletons for building molecules.
Examples include precursors for certain amino acids and lipid-related backbones.
It occurs in the cytosol and does not require oxygen or membrane-bound organelles.
Its broad distribution across diverse organisms suggests early origin and strong conservation.
Practice Questions
State two products of glycolysis and where glycolysis occurs in the cell. (2 marks)
Any two from: ATP, NADH, pyruvate (1 mark each)
Cytosol/cytoplasm (1 mark, if not already awarded full marks)
Explain how glycolysis captures energy from glucose, including ATP investment, ATP production, and NADH formation. (5 marks)
Uses ATP in an investment phase to phosphorylate/activate glucose (1)
Glucose (6C) is split into two 3C molecules that proceed to pyruvate (1)
ATP is produced by substrate-level phosphorylation (1)
More ATP is produced than invested; net gain of 2 ATP per glucose (1)
NAD is reduced to NADH during oxidation of intermediates (1)
