Respiration

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

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The idea

Cellular respiration releases the energy of glucose in small steps and stores it as ATP. It begins with glycolysis in the cytosol, which splits glucose into two molecules of pyruvic acid. Glycolysis needs no oxygen.

With oxygen, pyruvic acid enters the mitochondrion. It is oxidised to acetyl CoA, which enters the Krebs cycle in the matrix. The cycle releases CO₂ and loads hydrogen onto NAD⁺ and FAD. The respiratory chain on the inner membrane passes the electrons to oxygen, forming water, and most of the ATP is made here.

Without oxygen, pyruvic acid is reduced in the cytosol instead. This remakes NAD⁺ so that glycolysis can continue, but only 2 ATP are gained per glucose.

Rules to remember

Aerobic: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy.
Alcoholic fermentation (yeast): pyruvic acid → ethyl alcohol + CO₂. Lactic acid fermentation (muscle): pyruvic acid → lactic acid, no CO₂.
Glycolysis per glucose: net 2 ATP (4 made, 2 used), 2 NADH, 2 pyruvic acid.
Oxidation of each pyruvic acid: 1 acetyl CoA + 1 CO₂ + 1 NADH.
Each turn of the Krebs cycle: 2 CO₂, 3 NADH, 1 FADH₂, 1 ATP. Two turns per glucose.
In the respiratory chain, 1 NADH gives 3 ATP and 1 FADH₂ gives 2 ATP. Oxygen is the final electron acceptor.
Total per glucose with these values: 4 ATP made directly + 30 from 10 NADH + 4 from 2 FADH₂ = 38 ATP at most; 36 ATP net where 2 ATP are spent carrying the glycolysis NADH into the mitochondrion. Newer books give a lower yield, about 30 to 32.

Common mistake

Students forget to double. One glucose gives two pyruvic acid molecules, so pyruvate oxidation happens twice and the Krebs cycle turns twice. Multiply every per-turn figure by 2.

Worked example

Taking each NADH as 3 ATP and each FADH₂ as 2 ATP, how many ATP come from one turn of the Krebs cycle alone?

One turn gives 3 NADH, 1 FADH₂ and 1 ATP made directly.
From NADH: 3 × 3 = 9 ATP. From FADH₂: 1 × 2 = 2 ATP.
Total = 9 + 2 + 1 = 12 ATP.

Answer: 12 ATP per turn, so 24 ATP for the two turns from one glucose.

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Chapter 11: Bioenergetic

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