GlycolysisEdexcel A-Level Biology B: Revision notes
Section 1
Where glycolysis happens
Glycolysis is the first stage of respiration. It converts a hexose sugar such as glucose (6 carbons) into two molecules of pyruvate (3 carbons). It takes place in the cytoplasm of the cell, not in mitochondria, and it does not need oxygen. It is found in almost all living cells.
The pathway is a series of enzyme-controlled reactions. You do not need to learn the names of the intermediate compounds, only the key stages below.
Name the location (cytoplasm) in every answer on glycolysis.
Section 2
Phosphorylation of hexose
Two molecules of ATP are used to phosphorylate the hexose, adding two phosphate groups to form hexose bisphosphate. The phosphate groups come from ATP, which is converted to ADP. Phosphorylation makes the hexose more reactive, so it can be split in the next stage. This is an investment of ATP: the cell spends 2 ATP first, then gains more later.
Section 3
Splitting, oxidation and ATP production
Hexose bisphosphate is split into two 3-carbon molecules (triose phosphate). These are oxidised: hydrogen is removed from them and picked up by the coenzyme NAD, forming reduced NAD (NADH). They are converted through glycerate 3-phosphate (GP) to pyruvate, and ATP is made in these steps.
Four ATP are made per glucose. The ATP is made by transferring phosphate groups directly to ADP, not using a membrane.
Do not say glycolysis produces 4 ATP overall. It makes 4 but uses 2, so the net gain is 2.
Section 4
Yield and worked example
Per molecule of glucose:
- ATP used: 2
- ATP made: 4
- Net gain: 2 ATP
- NADH formed: 2
- Pyruvate formed: 2
Worked example. 0.50 mmol of glucose passes through glycolysis. ATP used = 2 × 0.50 = 1.0 mmol. ATP made = 4 × 0.50 = 2.0 mmol. Net gain = 1.0 mmol. NADH = 1.0 mmol and pyruvate = 1.0 mmol.
Section 5
Why glycolysis matters
The pyruvate and NADH from glycolysis pass on to the later stages of aerobic respiration. In the mitochondria the pyruvate is oxidised further, and the NADH is used in oxidative phosphorylation to make much more ATP. Because glycolysis takes place in the cytoplasm and does not need oxygen, it can supply ATP in cells without mitochondria, such as mature red blood cells.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Glycolysis
- A biochemist adds glucose to an extract of yeast cytoplasm that contains all the enzymes of glycolysis, together with NAD, ADP and phosphate, but no ATP. No reaction takes place. When a small amount of ATP is added, glycolysis begins and pyruvate is soon detected.Explain why glycolysis does not begin in this extract unless ATP is added.2 marks
- A student supplies a cytoplasm extract with 2.0 mmol of glucose, with excess ATP, ADP, NAD and phosphate. After incubation she finds that all of the glucose has been converted to pyruvate.Calculate the net amount of ATP gained by the extract. Show your working.2 marks
- A researcher adds an inhibitor to a yeast cytoplasm extract. The inhibitor blocks the enzyme that converts glycerate 3-phosphate (GP) to pyruvate. She supplies glucose and a limited amount of ATP, and measures the concentrations of ATP, GP and pyruvate every minute for 10 minutes.Explain why the concentration of ATP in the inhibited extract falls and stays low.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).