Respiration

Biology A2 · Energy and respiration. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

Test this topic free All Biology A2 topics

The idea

Aerobic respiration has four stages. Glycolysis takes place in the cytoplasm. The link reaction and the Krebs cycle take place in the mitochondrial matrix. Oxidative phosphorylation takes place on the inner mitochondrial membrane.

In the first three stages hydrogen is removed from the substrate and carried by NAD and FAD. On the inner membrane the hydrogen splits into protons and electrons. Energy from the electrons is used to move protons into the intermembrane space. The protons return through ATP synthase, and this makes most of the ATP. Oxygen is the final electron acceptor and forms water.

Without oxygen only glycolysis continues, so the yield of ATP is very small.

Rules to remember

Glycolysis: glucose (6C) is phosphorylated to fructose 1,6-bisphosphate, which splits into 2 triose phosphate (3C), then oxidised to 2 pyruvate (3C); net gain 2 ATP and 2 reduced NAD
Link reaction: pyruvate (3C) loses carbon dioxide and hydrogen to form an acetyl group (2C); coenzyme A carries the acetyl group into the Krebs cycle
Krebs cycle: acetyl (2C) + oxaloacetate (4C) → citrate (6C); each turn gives 2 CO₂, 3 reduced NAD, 1 reduced FAD and 1 ATP
Decarboxylation removes carbon dioxide; dehydrogenation removes hydrogen, which reduces NAD or FAD
Lactate fermentation (mammals): pyruvate + reduced NAD → lactate + NAD. Ethanol fermentation (yeast): pyruvate → ethanal + CO₂, then ethanal + reduced NAD → ethanol + NAD
Rice in flooded fields: aerenchyma in the roots carries oxygen down, root cells use ethanol fermentation, and stems grow faster to keep leaves above the water
DCPIP and methylene blue turn from blue to colourless when reduced; rate of respiration is proportional to 1 ÷ time taken

Common mistake

Do not say that fermentation makes extra ATP. Turning pyruvate into lactate or ethanol only regenerates NAD, so that glycolysis can continue to make its 2 ATP per glucose.

Worked example

How many molecules of reduced NAD are formed in total from one molecule of glucose in glycolysis, the link reaction and the Krebs cycle?

Glycolysis gives 2 reduced NAD and 2 pyruvate per glucose.
The link reaction happens twice (once per pyruvate): 2 × 1 = 2 reduced NAD.
The Krebs cycle turns twice (once per acetyl group): 2 × 3 = 6 reduced NAD.
Total = 2 + 2 + 6 = 10.

Answer: 10 molecules of reduced NAD

Ask about this

Type what you are stuck on. The helper finds the explanation, worked examples and the notes to read, free and instant.

Read the full notes

Full-course notes

Keep going

← Energy Photosynthesis as an energy transfer process →Take a Biology A2 test