7.2.3Topic 7 · Run for your Life
Glycolysis — Edexcel A Level Biology A (SNAB) 9BI0 Revision Notes & Topical Questions
5 min read 8 parts Lesson 9 of 25 1 quiz
7.2.3 Glycolysis — Edexcel A Level Biology A (SNAB) 9BI0 notes: Describe the stages of glycolysis, its net products, and what happens to pyruvate with and without oxygen.Glycolysis ("splitting of sugar") is the first stage of both aerobic and anaerobic respiration.
- Phosphorylation
- hexose bisphosphate
- triose phosphate
- ATP formation
- net gain of 2 ATP
- Animals and some bacteria
Section 1 of 8What and where
Glycolysis ("splitting of sugar") is the first stage of both aerobic and anaerobic respiration. It takes place in the cytoplasm and does not need oxygen. It converts one six-carbon glucose into two three-carbon pyruvate molecules. Because it is found in almost all organisms, including archaea and bacteria, it is thought to be one of the oldest metabolic pathways.
Section 2 of 8Stages of glycolysis
- Two ATP molecules are used to phosphorylate glucose, first to glucose phosphate and then to hexose bisphosphate .
- (fructose 1,6-bisphosphate)
- The phosphate groups make the sugar more reactive and prevent it leaving the cell, and the energy comes from the hydrolysis of ATP.
- Each triose phosphate is oxidised by dehydrogenase enzymes: hydrogen is removed and accepted by NAD, forming reduced NAD.
- Each triose phosphate loses hydrogen, so 2 reduced NAD are formed per glucose.
- The remaining phosphates are transferred to ADP by substrate-level phosphorylation.
- Four ATP are made in total, giving a net gain of 2 ATP .
- (4 made − 2 used)
- The end product of each triose phosphate is pyruvate.
Each step is catalysed by a specific enzyme in the cytoplasm, so the pathway is sensitive to temperature and pH like any enzyme-controlled sequence. The oxidation step is the energy-yielding one: the hydrogen removed from triose phosphate is the "fuel" that will later drive proton pumping, which is why reduced NAD from glycolysis is worth roughly 1.5 to 2.5 ATP each if oxygen is available.
Section 3 of 8Fate of pyruvate
If oxygen is available, pyruvate is actively transported into the mitochondrial matrix for the link reaction. The reduced NAD delivers its hydrogen to the electron transport chain, so NAD is regenerated.
Without oxygen, reduced NAD cannot be re-oxidised by the electron transport chain. To prevent glycolysis stopping, cells use alternative pathways to regenerate NAD:
- This is reversible and the lactate can be oxidised back to pyruvate later.
- Ethanal is then reduced by reduced NAD to ethanol, a process that is irreversible.
Anaerobic respiration in this way yields only 2 ATP per glucose, so organisms that rely on it need to consume much more glucose. This is the principle behind brewing and baking, where yeast fermentation produces ethanol and CO2 respectively used and released.
Section 4 of 8Why glycolysis matters
Glycolysis is rapid and needs no oxygen or mitochondria, so it can supply ATP during intense exercise, in red blood cells (which have no mitochondria) and in fast twitch muscle fibres. The pathway is controlled by enzymes; high ATP levels inhibit key enzymes such as phosphofructokinase, which is an example of negative feedback that prevents waste of glucose when the cell has plenty of ATP.
Section 5 of 8Exam approach and data
If a question asks you to account for a change in ATP concentration or in the concentration of intermediates, use the pathway order: glucose, hexose bisphosphate, triose phosphate, pyruvate.
Be ready to compare yields.
- and why a sprinter produces a large amount of lactate: fast twitch fibres can make ATP by glycolysis about 100 times faster than oxidative phosphorylation, though for a much shorter time.
Section 6 of 8Exam-style questions
Explain why two molecules of ATP are needed at the start of glycolysis. [2]2 marks
The ATP provides phosphate groups that phosphorylate glucose to hexose bisphosphate. This makes the sugar more reactive (lowers activation energy for later steps) and traps it inside the cell.
State the net products of glycolysis for one molecule of glucose. [3]3 marks
Two molecules of pyruvate, two molecules of reduced NAD and a net gain of two ATP (four made minus two used).
Explain the importance of the conversion of pyruvate to lactate in anaerobic respiration. [3]3 marks
Lactate formation oxidises reduced NAD to NAD. This regenerates the NAD required for glycolysis, so glycolysis and the net production of 2 ATP can continue without oxygen.
Section 7 of 8Key terms
| Term | Meaning |
|---|---|
| Glycolysis | Splitting of glucose into two pyruvate in the cytoplasm |
| Pyruvate | Three-carbon end product of glycolysis |
| Hexose bisphosphate | Six-carbon sugar with two phosphates, formed after ATP phosphorylates glucose |
| Triose phosphate | Three-carbon intermediate formed by lysis of hexose bisphosphate |
| Phosphorylation | Addition of a phosphate group to a molecule |
| Fermentation | Anaerobic pathway that regenerates NAD, such as ethanol production in yeast |
| Lactate dehydrogenase | Enzyme that converts pyruvate to lactate |
Section 8 of 8Exam technique
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