Balance Sheet, Amphibolic Pathway and Respiratory Quotient: Quick Revision

A brief note on Balance Sheet, Amphibolic Pathway and Respiratory Quotient. Read it before a test. Try each question at the end before you open its answer.

The ATP balance sheet

• One NADH gives 3 ATP. One FADH$_2$ gives 2 ATP.
• For one glucose: 4 ATP made directly, 10 NADH (2 + 2 + 6) and 2 FADH$_2$.
• NCERT: net gain of 36 ATP. The NADH of glycolysis is counted as 2 ATP each. With 3 ATP each the total is 38. Newer values give 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• The count assumes an orderly pathway, no intermediates withdrawn and only glucose.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, so the figure is theoretical.
• Fermentation gives 2 ATP for one glucose.

The amphibolic pathway

• Carbohydrates are changed to glucose. Glycerol enters as PGAL. Fatty acids enter as acetyl CoA.
• Proteins give amino acids. After deamination they enter as pyruvic acid, acetyl CoA or a Krebs cycle intermediate.
• Acetyl CoA is also withdrawn to make fatty acids, and intermediates to make amino acids. So the pathway works both ways: it is amphibolic.

The respiratory quotient

• RQ = volume of CO$_2$ released / volume of O$_2$ taken in. It has no unit.

• A real tissue uses a mixture, so its RQ lies between the pure values.
• KOH takes in carbon dioxide, so the fall of volume in its tube is the oxygen taken in.

A worked example

Seeds take in 50 mL of oxygen and release 35 mL of carbon dioxide. Name the likely substrate.

1. $\text{RQ} = \dfrac{35}{50} = 0.7$.
2. This is the value of a fat.

Practice with new numbers

A program to try

The program works out the RQ of some readings.

Mistakes to avoid

• Dividing oxygen by carbon dioxide. The carbon dioxide goes on top.
• Giving a fat an RQ above 1. It is 0.7.
• Calling the pathway only catabolic. It is amphibolic.
• Giving the glycolysis NADH 3 ATP in the NCERT count. It is worth 2.

Check yourself

1. What is the NCERT net gain of ATP in aerobic respiration?

2. In what form does a fatty acid enter the pathway?

3. What is the RQ of a fat, and why is it below 1?

4. Why is the respiratory pathway called amphibolic?