The respiratory balance sheet

Adding it up

We have followed one glucose molecule through four stages. Now we can add up the ATP. A table of this kind is called a balance sheet.

The carriers give ATP by the electron transport system: 3 ATP for each NADH and 2 ATP for each FADH$_2$. ATP made without carriers, by substrate level phosphorylation, is counted as it is.

The NCERT count: 36

NCERT states that the net gain is 36 ATP for one glucose in aerobic respiration. Here is how the figure is reached.

• Made directly: $2 + 2 = 4$ ATP.
• NADH of the link reaction and the Krebs cycle: $8 \times 3 = 24$ ATP.
• FADH$_2$ of the Krebs cycle: $2 \times 2 = 4$ ATP.
• NADH of glycolysis, made in the cytoplasm: $2 \times 2 = 4$ ATP.

Total: $4 + 24 + 4 + 4 = 36$ ATP.

The NADH of glycolysis is worth only 2 ATP each, because its electrons must be carried into the mitochondrion, and this costs some of their energy. If a book gives this NADH 3 ATP, as for any other NADH, it reaches 38 ATP. Newer measurements give about 2.5 ATP for a NADH and 1.5 ATP for a FADH$_2$, and a total of about 30 to 32 ATP.

So three numbers are in use: 36, 38 and about 30. In an exam, follow what the question gives you. If it names no source, 36 is the NCERT answer.

Why the figure is only theoretical

The count rests on four assumptions.

• The pathway runs in order, with glycolysis, the Krebs cycle and the electron transport system following one after another.
• The NADH of glycolysis is moved into the mitochondrion and undergoes oxidative phosphorylation.
• None of the intermediates is used to make other compounds.
• Only glucose is respired. No other substrate enters.

None of these is true in a living cell.

• All the pathways work at the same time, and not one after another.
• Substrates enter the pathway and are withdrawn from it whenever they are needed.
• ATP is used as soon as it is made.
• The rates of the enzymes are controlled in many ways.

So the real gain is not exactly 36. The figure is a good guide, and it shows how much more aerobic respiration gives than fermentation.

Fermentation and aerobic respiration

The 2 NADH of glycolysis are turned back into NAD$^+$ in fermentation. In aerobic respiration, the electron transport system does it.

You should find that the NCERT setting gives 36 ATP, that giving the cytoplasmic NADH 3 ATP gives 38, and that the newer values, 2.5, 1.5 and 1.5, give 30. The share of the glucose energy that is kept in ATP changes with the count, and it stays close to two fifths.

Try it in Python

The program counts the ATP of one glucose in three ways.

Practise with new numbers

Common mistakes

• Giving the glycolysis NADH 3 ATP in a count that says 36. In the NCERT count it is worth 2.
• Counting the NADH of glycolysis twice, or leaving out the NADH of the link reaction.
• Calling 36 an exact figure. It is a theoretical gain, and the real gain in a cell is different.
• Forgetting the ATP made directly. There are 4: 2 in glycolysis and 2 in the Krebs cycle.
• Saying that fermentation gives the same ATP. It gives only 2.

Remember these

• One NADH gives 3 ATP, and one FADH$_2$ gives 2 ATP, in the usual count.
• For one glucose: 4 ATP made directly, 10 NADH and 2 FADH$_2$.
• NCERT: net gain of 36 ATP in aerobic respiration. Other counts: 38, and about 30 to 32.
• The count assumes an orderly pathway, no intermediates withdrawn and only glucose. A living cell does not work in this way.
• Fermentation gives 2 ATP. Aerobic respiration gives many times more.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. How many ATP does NCERT give as the net gain for one glucose in aerobic respiration?

2. How many ATP are made directly in the whole of aerobic respiration, without carriers?

3. Why is the count of ATP only a theoretical figure?

4. Why do some books give 38 ATP?

5. Compare the net ATP of fermentation and of aerobic respiration.