The amphibolic pathway

What else can be burnt?

Plants have starch, fats and proteins as well as glucose. Can a cell respire them?

It can, but not directly. Each substrate is first turned into a compound that enters the pathway at the right place.

Where the substrates enter

• Carbohydrates. All carbohydrates are changed to glucose before they are respired. Glucose is the favoured substrate, and it enters glycolysis.
• Fats. A fat is broken down into glycerol and fatty acids. Glycerol is changed to PGAL and goes into glycolysis. Fatty acids are broken down to acetyl CoA and go into the Krebs cycle.
• Proteins. A protein is broken down by proteases into amino acids. After deamination, which removes the amino group, an amino acid enters as pyruvic acid, as acetyl CoA or as an intermediate of the Krebs cycle, according to its structure.

Catabolic or anabolic?

Breaking down is called catabolism. Building up is called anabolism. Respiration breaks substrates down, so it has long been called a catabolic process, and its pathway a catabolic pathway.

But is this right? Look at what happens when the cell must make fatty acids. It takes acetyl CoA away from the respiratory pathway to build them. The same acetyl CoA that comes from breaking down fatty acids is used to make them.

The same is true for proteins. The cell builds its amino acids from intermediates that it withdraws from the Krebs cycle and from glycolysis. A cell that makes a protein uses the pathway for anabolism, and a cell that breaks one down uses it for catabolism.

Because the pathway takes part in both breaking down and building up, it is called an amphibolic pathway. The word means that it works both ways.

Two consequences

• The cell can choose. If a cell is short of glucose, it can burn fat or protein. If it has a surplus, it can build fat and protein from the intermediates.
• The substrate decides how much oxygen is needed. Fats and carbohydrates need different amounts of oxygen for each carbon dioxide. The next lesson uses this idea.

You should find that glucose enters at glycolysis, glycerol at PGAL, a fatty acid at acetyl CoA, and an amino acid at one of three places.

Try it in Python

The program lists where each substrate enters and picks out those that enter as acetyl CoA.

Practise with new numbers

Common mistakes

• Saying that the respiratory pathway is only catabolic. It also supplies the intermediates for building fats and proteins.
• Sending glycerol in as acetyl CoA. Glycerol enters as PGAL. Fatty acids enter as acetyl CoA.
• Forgetting deamination. An amino acid loses its amino group before it enters the pathway.
• Thinking that proteins enter at one place only. The place depends on the amino acid.
• Respiring starch directly. It is changed to glucose first.

Remember these

• Carbohydrates are changed to glucose. Glucose is the favoured substrate.
• Fat gives glycerol, which enters as PGAL, and fatty acids, which enter as acetyl CoA.
• Protein gives amino acids. After deamination they enter as pyruvic acid, acetyl CoA or a Krebs cycle intermediate.
• The pathway is used for breaking down and for building up, so it is amphibolic.

Check yourself

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

1. In what form does glycerol enter the respiratory pathway?

2. What do fatty acids become before they enter the pathway?

3. What does an amino acid lose before it enters the pathway?

4. When does a cell take acetyl CoA away from the respiratory pathway?

5. Why is the respiratory pathway called amphibolic?