Lesson 1 of 5 / Respiration and ATP production
Glycolysis starts in the cytosol
Why can some ATP be made without oxygen?
I can outline glycolysis location, inputs, outputs and substrate-level phosphorylation.
I can outline glycolysis: location, inputs and outputs.Syllabus
Syllabus 9477, Core 3(f). Outline glycolysis: location, inputs and outputs.
About 6 min
Make a guess. You are not marked.
Glycolysis makes 4 ATP per glucose. What is its net ATP yield?
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2, after 2 ATP are invested.
Two ATP phosphorylate early intermediates. Four are made later, giving a net gain of two.
The key idea
One glucose forms two pyruvate, with net two ATP and two reduced NAD in the cytosol.
Explore the idea
Distinguish ATP made from net ATP gained
Net ATP = 4 made - 2 invested = 2 gained. No CO2 is released in glycolysis. NADH means reduced NAD; it must be reoxidised for continued glycolysis.
This is a stage summary, not a complete balanced equation. Water, protons and individual intermediates are omitted. Direct phosphate transfer from an intermediate to ADP is substrate-level phosphorylation; oxygen is not a direct input to glycolysis.
Explanation
Glycolysis is a sequence of cytosolic reactions that converts six-carbon glucose into two three-carbon pyruvate molecules. It does not require a mitochondrion and does not directly consume oxygen. The pathway transfers some glucose energy into ATP and reduced NAD rather than releasing all of it at once.
An initial investment uses two ATP to phosphorylate intermediates. Subsequent reactions produce four ATP by substrate-level phosphorylation, giving a net gain of two per glucose. Substrate-level phosphorylation transfers phosphate from a phosphorylated intermediate to ADP directly.
Oxidation of intermediates reduces two NAD molecules to reduced NAD, often written NADH. NAD acts as an electron/hydrogen carrier; it is not permanently consumed if it is reoxidised later. Sustained glycolysis requires regeneration of oxidised NAD.
In aerobic conditions, pyruvate and reducing equivalents feed further energy-transfer pathways. Without sufficient oxygen, fermentation can regenerate NAD so glycolysis continues, but the low ATP yield remains. Do not add an extra ATP yield to fermentation itself in the standard yeast or muscle pathways.
Step by step
- 1
Balance carbon
Six carbons become two sets of three.
- 2
Separate gross and net ATP
Subtract the investment.
- 3
Track the carrier
Reduced NAD must later be reoxidised.
Worked example
Work through the evidence
Ten glucose molecules undergo glycolysis. State net ATP, pyruvate and reduced NAD outputs.
One way to explain it
Twenty net ATP, twenty pyruvate and twenty reduced NAD.
Why this answer works
- Scale the per-glucose outputs by ten.
- Net ATP differs from the forty ATP produced before subtracting investment.
Watch out for this
A student says: "Glycolysis stops instantly without oxygen because it directly uses O2." What is wrong with this?
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It does not directly use oxygen; its continuation depends on regenerating NAD, which fermentation can provide.
Check your understanding
What is the net glycolytic ATP yield per glucose?
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Two
Four produced minus two invested gives two net.
Quick recall
Why can some ATP be made without oxygen?