The Krebs Cycle and the Electron Transport System: Quick Revision

A brief note on The Krebs Cycle and the Electron Transport System. Read it before a test. Try each question at the end before you open its answer.

The mitochondrion

• The matrix holds the link reaction and the Krebs cycle. The inner membrane, folded into cristae, holds the electron transport system and ATP synthase.

The link reaction and the Krebs cycle

• Link reaction: pyruvic acid, CoA and NAD$^+$ give acetyl CoA, carbon dioxide and NADH. Enzyme: pyruvic dehydrogenase, with magnesium ions.
• Cycle: acetyl CoA (2C) and oxaloacetic acid (4C) give citric acid (6C). Enzyme: citrate synthase.
• Decarboxylations: alpha-ketoglutaric acid (5C), then succinyl CoA (4C).
• GTP is made at succinyl CoA to succinic acid, and it gives ATP.

The electron transport system

• Order for NADH: complex I, ubiquinone, complex III, cytochrome c, complex IV, oxygen. FADH$_2$ enters at complex II.
• I: NADH dehydrogenase. II: succinate dehydrogenase. III: cytochrome bc$_1$. IV: cytochrome c oxidase, with cytochromes a and a$_3$ and two copper centres.
• Oxygen is the final acceptor and makes water. One NADH gives 3 ATP. One FADH$_2$ gives 2 ATP.

Oxidative phosphorylation

• Electrons pump protons into the intermembrane space. The protons flow back through ATP synthase.
• F$_1$ is the head that makes ATP. F$_0$ is the channel. Two protons pass for each ATP.
• Without oxygen the chain stops, and the Krebs cycle stops for want of NAD$^+$ and FAD.

A worked example

Two glucose molecules are respired completely. How many NADH come from the Krebs cycle?

1. Two glucose give 4 pyruvic acid, so there are 4 turns.
2. Each turn forms 3 NADH, so the cycle forms $4 \times 3 = 12$ NADH.

Practice with new numbers

A program to try

The program works out what the cycle gives from some glucose.

Mistakes to avoid

• Putting the Krebs cycle on the inner membrane. It is in the matrix.
• Forgetting that one glucose gives two turns.
• Entering FADH$_2$ at complex I. It enters at complex II.
• Swapping F$_0$ and F$_1$. F$_0$ is the channel.

Check yourself

1. How many NADH does one turn of the cycle form?

2. Which complex gives electrons to oxygen?

3. How many protons pass through F$_0$ for each ATP?

4. Why does the Krebs cycle stop without oxygen?