ZIMSEC A Level · Biology · Form 6
Respiration releases energy as ATP.
ATP
Adenosine triphosphate is the universal energy currency; hydrolysis to ADP and phosphate releases energy.
Stages of aerobic respiration
- Glycolysis (cytoplasm): glucose is phosphorylated and split into two pyruvate molecules; net gain of 2 ATP and 2 reduced NAD.
- Link reaction (mitochondrial matrix): pyruvate is decarboxylated and dehydrogenated to form acetyl coenzyme A; CO₂ and reduced NAD are produced.
- Krebs cycle (matrix): acetyl CoA combines with oxaloacetate (4C) to form citrate (6C), which is regenerated to oxaloacetate, releasing CO₂, reduced NAD, reduced FAD and ATP.
- Oxidative phosphorylation (inner membrane/cristae): electrons pass along the electron transport chain; energy pumps protons into the intermembrane space; protons flow back through ATP synthase (chemiosmosis) making ATP. Oxygen is the final electron acceptor, forming water.
Anaerobic respiration
- Mammals: pyruvate is reduced to lactate, regenerating NAD so glycolysis continues.
- Yeast: pyruvate → ethanal → ethanol + CO₂ (alcoholic fermentation).
Respiratory quotient
RQ = CO₂ produced ÷ O₂ used: carbohydrate about 1.0, lipid about 0.7, protein about 0.9.
Exam tip: Track where each stage occurs and what it produces – a summary table is essential revision.
Check yourself
- Where does the Krebs cycle take place?
- What is the role of oxygen in aerobic respiration?
- Why is lactate produced in muscles during anaerobic respiration?
Show answers
- In the mitochondrial matrix.
- It is the final electron acceptor, combining with electrons and protons to form water.
- To regenerate NAD so that glycolysis can continue producing ATP.
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