Chapter 1: Macromolecules & Metabolism Overview
Cell biochemistry starts with a simple inventory: six categories of molecule cover essentially everything the body runs on. Three of them are macromolecules built from repeating micromolecule units, and only three of the six actually yield calories.
Macromolecules and their building blocks
| Macromolecule | Micromolecule unit |
|---|---|
| Protein | Amino acids |
| Carbohydrate | Monosaccharides |
| Lipid | Fatty acids + glycerol |
Alongside these three, vitamins, minerals, and water round out the six categories. Water is essential for every metabolic reaction, but — unlike carbohydrate, protein, and lipid — it yields no calories.
Catabolism and anabolism share one energy currency
Metabolism is really two opposite, linked directions of traffic:
- Catabolism breaks nutrients (carbohydrate, lipid, protein) down into small, energy-poor end products (CO2, H2O, NH3), releasing chemical energy captured as ATP.
- Anabolism spends that ATP to build the cell's own macromolecules (protein, polysaccharide, lipid) back up from small starting molecules (amino acids, sugars, fatty acids).
Digestion is the required first step before catabolism can even begin — dietary macromolecules must be broken down to their absorbable micromolecule units before cells can use them.
Three stages of catabolism
Where amino acids enter the TCA cycle
Unlike glucose and fatty acids, amino acid carbon skeletons can enter the TCA cycle at several different points — and this is exactly what determines whether an amino acid can be turned back into glucose:
- Glucogenic amino acids are catabolized to pyruvate or to a TCA-cycle intermediate (oxaloacetate, α-ketoglutarate, succinyl-CoA, or fumarate). Because the TCA cycle intermediates can feed gluconeogenesis, these amino acids can be converted into new glucose.
- Ketogenic amino acids are catabolized to acetyl-CoA or acetoacetyl-CoA instead. These cannot be used for net glucose synthesis — they can only be converted to ketone bodies or fat.
- Leucine and lysine are the only two purely ketogenic amino acids. Several others (isoleucine, phenylalanine, threonine, tryptophan, tyrosine) are both, because different carbons in their structure are degraded down different routes. All the rest are purely glucogenic.
Key teaching point
Every calorie-yielding nutrient you eat is broken down to the same small set of common intermediates — mainly pyruvate and acetyl-CoA — before entering the TCA cycle. This convergence is why the body can burn carbohydrate, fat, or protein interchangeably for energy, and why amino acid catabolism is inseparable from the same central pathway used for sugars and fats.