3.2.5Inquiry Process
Glycolysis — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 111 of 160
Curriculum Equation Key Balanced Cellular Respiration EquationFast Exam Solution
C₆H₁₂O₆ + 6O₂ ➔ 6CO₂ + 6H₂O + 36-38 ATP
Word Equation & Conditions: Glucose + Oxygen -> Carbon Dioxide + Water + Energy (36-38 ATP)
Part 1 of 5Where and Why
Glycolysis literally means the splitting of sugar, and it is the first stage of respiration.
Location
- Location Glycolysis takes place in the cytoplasm, outside the mitochondria, so it needs no organelles.
Substrate and product
- Substrate and product One six-carbon glucose molecule is converted into two three-carbon pyruvate molecules through a chain of enzyme-catalysed steps.
Oxygen not required
- Oxygen not required The pathway does not use oxygen, so it forms the first stage of both aerobic and anaerobic respiration.
Ancient pathway
- Ancient pathway Almost every living cell carries out glycolysis, which suggests that it evolved very early in the history of life.
Phosphorylated intermediates
- Phosphorylated intermediates Phosphate groups added to intermediates trap them inside the cell,
- because phosphorylated sugars cannot easily cross the plasma membrane.
Part 2 of 5Stages of Glycolysis
1
Phosphorylation: two ATP molecules each give a phosphate to glucose, producing hexose bisphosphate and making it more reactive.
2
Lysis: the hexose bisphosphate splits into two molecules of triose phosphate, each with three carbon atoms.
3
Oxidation: each triose phosphate loses hydrogen, which is picked up by NAD to form reduced NAD, so two NADH are made.
4
ATP formation: the remaining three-carbon compounds pass through steps in which four ATP are produced by substrate-level phosphorylation.
5
The end product is two molecules of pyruvate for each glucose molecule.
Part 3 of 5Energy Account
Energy Account
ATP used
- ATP used Two ATP are invested at the start of the pathway to phosphorylate glucose.
ATP made
- ATP made Four ATP are formed later, giving a net gain of two ATP per glucose molecule.
Reduced NAD
- Reduced NAD Two NADH are formed per glucose;
- in aerobic respiration they take hydrogen to the electron transport chain.
Substrate-level phosphorylation
- Substrate-level phosphorylation Direct transfer of a phosphate from an intermediate to ADP, without involving the electron transport chain.
| Item | Per glucose molecule |
|---|---|
| ATP used | 2 |
| ATP made | 4 |
| Net ATP | 2 |
| NADH made | 2 |
| Pyruvate made | 2 |
Part 4 of 5Worked Example
Question
- A yeast culture breaks down 5 molecules of glucose by glycolysis only.
- Calculate the total ATP used, ATP made, net ATP gain, and the number of reduced NAD and pyruvate molecules formed.
- ATP used = 5 x 2 = 10.
- ATP made = 5 x 4 = 20.
- Net ATP = 20 - 10 = 10, which is 5 x 2.
- Reduced NAD = 5 x 2 = 10 and pyruvate = 5 x 2 = 10.
Answer Net gain of 10 ATP, with 10 reduced NAD and 10 pyruvate molecules.
Part 5 of 5Exam Checklist & Tips
3.2.5 – Key Points for the Exam
- Glycolysis The breakdown of glucose to pyruvate in the cytoplasm.
- Pyruvate A three-carbon compound that is the product of glycolysis.
- Triose phosphate A three-carbon phosphorylated sugar formed when hexose bisphosphate splits.
- Substrate-level phosphorylation Formation of ATP by direct transfer of a phosphate group to ADP.
Common Mistakes
- Stating that glycolysis makes four ATP net; two are used so the net gain is two.
- Placing glycolysis in the mitochondria instead of the cytoplasm.
- Saying glycolysis needs oxygen, when it is anaerobic.
- Forgetting to double the products, because one glucose gives two triose phosphates and two pyruvates.
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