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C1.2.8 (HL)—Glycolysis

Glycolysis converts glucose to two pyruvate molecules in cytoplasm, producing net ATP and reduced NAD without oxygen, supporting usable cellular work and measurable biological activity.

Syllabus
First assessment 2025
Objective
C1.2.8
Level
HL

Exam analysis

Chance of appearing5%of analysed past papers
Latest appearanceMay 2025
Most common paperPaper1
Typical marks1

Common command terms

  • State
  • Identify
  • Outline

Recent exam appearances

May 2025Paper2 ["HL"] · TZ16(b)[ 1 ]C1.2.8 (HL)—Glycolysis
May 2025Paper2 ["HL"] · TZ16(a)(ii)[ 1 ]C1.2.8 (HL)—Glycolysis
May 2025Paper2 ["HL"] · TZ16(a)(i)[ 1 ]C1.2.8 (HL)—Glycolysis
November 2017Paper1 ["HL"] · TZ031[ 1 ]C1.2.8 (HL)—Glycolysis
May 2015Paper1 ["HL"] · TZ213[ 1 ]C1.2.8 (HL)—Glycolysis
Practice this objective

Coverage 2012–2025 · Updated 15 Jul 2026

Glycolysis Splits Glucose in the Cytoplasm

HL only

Glycolysis is a linear cytoplasmic pathway in which one glucose is converted stepwise into two pyruvate, with a net yield of ATP and reduced NAD.

Phosphorylation uses ATP to activate the six-carbon molecule; lysis splits it into two three-carbon compounds; oxidation transfers hydrogen to NAD; ATP formation then occurs by substrate-level phosphorylation.

Each step is catalysed by a different enzyme. Track the required net outputs rather than intermediate names: two pyruvate, two ATP net and reduced NAD per glucose.

The ATP investment precedes later ATP-producing steps, so gross production is larger than the net gain. NAD must be regenerated for the oxidation step to continue.

Glycolysis does not require oxygen or mitochondria and does not completely oxidize glucose. Do not count invested ATP as part of the net yield.

Glycolysis exam focus

HL only

Assessment in practice

1 marks
How it is assessed

This objective is assessed through structured response, commonly using State / Identify / Outline.

Command terms

State / Identify / Outline

What earns marks

Build the answer around this relationship: Glycolysis occurs in the cytoplasm.

Representative question

Question 1

[Maximum number: 6]

In anaerobic conditions, plants release energy by glycolysis. Outline the process of glycolysis.

Trace The Respiration Pathway

HL only

In HL respiration, carbon and hydrogen are followed separately. Glycolysis turns glucose into pyruvate, net ATP, and reduced NAD. Without oxygen, pyruvate becomes lactate in humans or ethanol and carbon dioxide in yeast to regenerate NAD. With oxygen, pyruvate enters the link reaction, forming acetyl-CoA, carbon dioxide, and reduced NAD. The Krebs cycle releases more carbon dioxide and reduced coenzymes. Electron transport uses reduced NAD/FAD to pump protons, chemiosmosis through ATP synthase makes ATP, and oxygen accepts electrons and protons to form water. Substrate comparisons depend on ATP yield, oxygen demand, water production, and speed. Regenerated NAD allows glycolysis to continue. Fermentation regenerates NAD for glycolysis and is used in baking and brewing. Electron transfers release energy while coenzymes are reoxidized.

  • Trace carbon: glucose -> pyruvate -> acetyl-CoA -> carbon dioxide, or anaerobic products.
  • Trace hydrogen/electrons: NAD/FAD become reduced and feed the electron transport chain.
  • Trace protons: electron transport builds the gradient; ATP synthase uses it for chemiosmosis.
  • Compare substrates by yield, oxygen demand, metabolic water, and speed.

Concept essentials

  • Glycolysis occurs in the cytoplasm.
  • One glucose molecule produces two pyruvate molecules.
  • Two ATP are used and four are produced, giving a net gain of two ATP.
  • Glycolysis reduces NAD and can proceed without oxygen.