Glycolysis

CIE A-Level Biology· 35 min read

1. Location and Overview★★☆☆☆⏱ 10 min

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📘 Definition

Glycolysis

An anaerobic sequence of reactions that converts one 6-carbon glucose molecule into two 3-carbon pyruvate molecules, producing a net gain of ATP. It occurs in the cytosol of all living cells.

📐 Worked Example

State where glycolysis occurs in a eukaryotic cell and explain why it can proceed without oxygen.

1
Glycolysis is an evolutionarily ancient pathway that occurs in the soluble cytosol (cytoplasm), not inside mitochondria.
2
No reaction in glycolysis uses oxygen as a terminal electron acceptor. All oxidation reactions are coupled to the reduction of the coenzyme $N A D^{+}$ .
3
Final answer: Glycolysis occurs in the cytosol, and no oxygen is required for any of its steps.

2. Key Steps of Glycolysis★★★☆☆⏱ 20 min

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📘 Definition

Substrate-level Phosphorylation

The formation of ATP by direct transfer of a phosphate group from a high-energy intermediate substrate to ADP, rather than via the proton gradient used in oxidative phosphorylation.

Example:

This is the only method of ATP production used in glycolysis.

Phosphorylation: Glucose (6C) is phosphorylated by 2 ATP to form hexose bisphosphate, activating glucose for downstream reactions and trapping it in the cell.
Lysis: Hexose bisphosphate splits into two identical molecules of triose phosphate (TP, 3C each).
Oxidation: Each TP is oxidized, losing hydrogen and electrons to reduce $N A D^{+}$ to NADH. 2 ATP are produced via substrate-level phosphorylation here.
Conversion: Each 3-carbon intermediate is converted to pyruvate, producing an additional 2 ATP.

📐 Worked Example

Describe the role of ATP in the first stage of glycolysis.

1
Free glucose is a stable molecule that does not spontaneously break down to release energy for the cell.
2
Two ATP molecules are hydrolyzed to transfer phosphate groups to glucose, forming first glucose phosphate then hexose bisphosphate.
3
This phosphorylation lowers the activation energy for subsequent reactions, activates glucose, and traps it in the cell (phosphorylated glucose cannot cross the cell membrane).
4
This initial ATP investment is recovered later in glycolysis to produce a net gain of ATP.

3. Inputs and Outputs★★☆☆☆⏱ 15 min

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Molecule	Number per Glucose	Role
Glucose	1	Starting respiratory substrate
ATP	2	Invested to phosphorylate glucose
NAD+	2	Reduced to NADH during oxidation
ADP	4	Converted to ATP via substrate-level phosphorylation
Pyruvate	2	End product of glycolysis
NADH	2	Electron carrier for ETC
ATP (net)	2	Net ATP for immediate cell use

📐 Worked Example

Calculate the net ATP yield of glycolysis from one glucose molecule, and explain why it differs from the gross yield.

1
Total gross ATP produced during glycolysis: 2 ATP from each of the two 3-carbon intermediates, so 4 ATP total gross.
2
Two ATP molecules were invested in the initial phosphorylation step to activate glucose, so we subtract this investment from the gross yield.
3
Calculation: $4 - 2 = 2$
4
Net yield accounts for the initial energy investment, while gross yield only counts total ATP produced regardless of investment. The net yield is 2 ATP per glucose.

4. Role in Aerobic and Anaerobic Respiration★★★☆☆⏱ 15 min

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After glycolysis, the fate of pyruvate and NADH depends on the presence of oxygen. In aerobic respiration, pyruvate is transported into the mitochondrial matrix to enter the link reaction, and NADH carries electrons to the electron transport chain. In anaerobic conditions, NAD+ must be regenerated to allow glycolysis to continue: mammalian muscle uses lactate fermentation, while yeast and plants use ethanol fermentation, both oxidizing NADH back to NAD+.

📐 Worked Example

Explain why glycolysis cannot continue indefinitely in anaerobic mammalian muscle without lactate fermentation.

1
The oxidation of triose phosphate in glycolysis requires $N A D^{+}$ as a coenzyme to accept hydrogen and electrons, producing NADH.
2
Without oxygen, NADH cannot release electrons to the electron transport chain, so it cannot be reoxidized back to $N A D^{+}$ .
3
Lactate fermentation reduces pyruvate to lactate, which oxidizes NADH back to $N A D^{+}$ .
4
This recycled $N A D^{+}$ is reused for the oxidation step of glycolysis, allowing ATP production to continue.

5. Common Pitfalls

Wrong move:

Stating that glycolysis occurs in the mitochondria

Why:

All steps of glycolysis occur before any mitochondrial respiratory stages

Correct move:

Glycolysis occurs in the cytosol (cytoplasm) of all cells

Wrong move:

Claiming the net ATP yield of glycolysis is 4 ATP

Why:

4 ATP is the gross yield; 2 ATP are used in the initial phosphorylation step

Correct move:

Net yield of ATP from glycolysis is 2 ATP per glucose molecule

Wrong move:

Saying glycolysis requires oxygen to proceed

Why:

No step of glycolysis uses oxygen, so it works in both aerobic and anaerobic conditions

Correct move:

Glycolysis does not require oxygen and is the first stage of both respiration types

Wrong move:

Calling ATP production in glycolysis oxidative phosphorylation

Why:

Oxidative phosphorylation uses the ETC and proton gradient; glycolysis uses direct phosphate transfer

Correct move:

ATP production in glycolysis is substrate-level phosphorylation

Wrong move:

Stating one glucose produces one pyruvate in glycolysis

Why:

Glucose is a 6-carbon molecule that splits into two 3-carbon products

Correct move:

One glucose molecule produces two pyruvate molecules at the end of glycolysis

6. Quick Reference Cheatsheet

Feature	Glycolysis Fact
Location	Cytosol
Oxygen required?	No
Starting substrate	1 × 6C glucose
End product	2 × 3C pyruvate
Net ATP yield	2 ATP
Gross ATP yield	4 ATP
ATP production method	Substrate-level phosphorylation
NADH produced	2 molecules
Aerobic pyruvate fate	Mitochondrial link reaction
Anaerobic (muscle) fate	Converted to lactate
Anaerobic (yeast/plants) fate	Converted to ethanol + CO₂

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2022 · 1
Question on glycolysis location
2021 · 2
Question on net ATP yield
2023 · 1
Question on glycolysis inputs

Going deeper

syllabusCIE 9700 Official SyllabusUnit 12: Energy and Respiration

What's Next

Glycolysis is the foundation for all subsequent stages of cellular respiration, and understanding its steps is critical for tracking carbon and energy flow through the entire respiratory pathway, a common long answer question in CIE AS exams. After mastering glycolysis, you will move on to the aerobic respiratory stages that occur inside mitochondria, which produce the majority of ATP for the cell. Glycolysis also connects directly to anaerobic fermentation pathways, which are frequently tested in multiple choice and structured questions for their role in sustaining ATP production when oxygen is limited.