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Chapter 7 · Part 8 of 9
  1. Chapter overview
  2. 1Why cells break down glucose
  3. 2The four stages at a glance
  4. 3Glycolysis splits glucose in two
  5. 4Pyruvate breakdown and the citric acid cycle
  6. 5The electron transport chain
  7. 6Chemiosmosis and the ATP payoff
  8. 7ATP synthase, a spinning machine
  9. 8Proteins and fats as fuel
  10. 9Making ATP without oxygen

Proteins and fats as fuel

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A meal contains more than glucose. Its proteins and fats are fuel too, and cells break them down using the same pathways they use for sugar.

One set of pathways for every fuel

Proteins and fats are first broken into smaller molecules. Those molecules then enter glycolysis or the citric acid cycle at different points, depending on their structure. From there, they follow the same route as glucose, ending with oxidative phosphorylation.

Proteins, carbohydrates, and fats at the top break down into amino acids, sugars, and glycerol plus fatty acids. A central pathway runs from glycolysis to pyruvate, acetyl CoA, the citric acid cycle, and oxidative phosphorylation. Sugars enter at the top of glycolysis, glycerol enters glycolysis at glyceraldehyde-3-phosphate, fatty acids enter at acetyl CoA, and amino acids enter at three points: lower glycolysis, acetyl CoA, and the citric acid cycle.
Figure 7.6 Acetyl CoA is a meeting point: sugars (by way of pyruvate), fatty acids, and some amino acids all reach the citric acid cycle through it.

Sharing pathways makes respiration efficient. Enzymes are proteins, and building each one costs the cell energy. Because one set of enzymes handles sugars, amino acids, and fats, the cell doesn’t have to make a separate set for each fuel.

Proteins

Proteins are chains of amino acids. Enzymes in digestive juices or inside cells first snip the bonds that hold the amino acids together. The 20 kinds of amino acids differ in their side chains, the part that varies from one amino acid to the next. Because of those differences, amino acids enter respiration at different points:

Fats

A fat molecule is built from glycerol, a small three-carbon molecule, joined to fatty acids, long chains of carbon and hydrogen. Cells first split fats into these two parts, and each part goes its own way:

Fuel Broken down into Where the pieces enter
Carbohydrates Sugars such as glucose Start of glycolysis
Proteins Amino acids Later steps of glycolysis, as acetyl CoA, or directly into the citric acid cycle
Fats Glycerol and fatty acids Glycerol: glycolysis, as glyceraldehyde-3-phosphate. Fatty acids: citric acid cycle, as acetyl CoA
Check yourselfWhat advantage do cells gain by breaking down carbohydrates, proteins, and fats through the same pathways?

The same enzymes can process many different fuels. Enzymes are proteins that cost energy to make, so sharing them saves the cell from building a separate set for each kind of molecule.

Check yourselfWhere do the pieces of a fatty acid enter cellular respiration?

Fatty acids are cut into two-carbon acetyl units that attach to CoA. They enter the citric acid cycle as acetyl CoA, skipping glycolysis and pyruvate breakdown.

Practice questions

  1. 1 of 2

    Cells break down sugars, fats, and proteins using many of the same pathways and enzymes. What is the main advantage of this arrangement?

  2. 2 of 2

    Before a protein from food can be used as fuel for cellular respiration, what must happen first?