Metabolism — building up and breaking down in the body
Metabolism is the sum of all chemical reactions in the body. Every cell does some metabolism; the liver is the major centre, performing many roles that GCSE biology examines.
Anabolic vs catabolic reactions
-
Anabolic reactions BUILD large molecules from small ones — they need energy. Examples:
- amino acids → proteins
- glucose → glycogen (animal storage) or starch (plants)
- fatty acids + glycerol → lipids
- simple sugars → cellulose (plants)
-
Catabolic reactions BREAK DOWN large molecules — they tend to release energy. Examples:
- glucose → CO₂ + water (respiration)
- proteins → amino acids (digestion)
- excess amino acids → urea (deamination — see below)
Liver — the metabolic hub
The liver:
- Stores glycogen (and reverses it back to glucose when needed)
- Detoxifies poisons (e.g. alcohol → less harmful products)
- Produces bile for fat digestion (B2.2)
- Breaks down old red blood cells
- Performs deamination of excess proteins
Deamination — getting rid of excess protein
You can't store excess protein in the body (unlike fat or carbohydrate). When you eat more than you need:
- Excess amino acids are transported to the liver.
- The liver removes the amino group (NH₂) — this is deamination.
- The amino group becomes ammonia (very toxic).
- Ammonia is converted to urea (less toxic).
- Urea is transported in blood to the kidneys, then excreted in urine.
The remainder of the amino acid (a carbon skeleton) can be respired or stored as fat.
Glucose conversions
After a meal, glucose is high in the blood. Insulin signals:
- Liver and muscle cells take up glucose.
- Glucose is stored as glycogen (anabolic).
Between meals or during exercise, glucose falls. Glucagon (HT) signals:
- Liver breaks glycogen back down to glucose (catabolic).
- Glucose released into blood.
(See B5.7 for insulin/glucagon.)
Lipids and amino acids
Lipids and oils are stored in fat cells. They can be broken down for energy when carbohydrate stores are low.
Amino acids cannot be stored — excess is deaminated. Essential amino acids (those the body cannot synthesise) must come from diet.
⚠Common mistakes— Common mistakes / exam traps
- "The liver excretes urea" — no; the liver makes urea; the kidneys excrete it.
- "Glycogen is the same as glucose" — glycogen is a polymer of glucose, made by anabolism.
- Confusing deamination with denaturation — deamination = removing amino group; denaturation = enzyme losing shape.
- "Excess protein is stored in the body" — wrong; it's deaminated and the energy stored as fat.
Links
Connects to B2.2 (digestion makes amino acids), B4.2 (respiration), B5.7 (insulin / glucagon control), and B5.8 (kidney excretion of urea).
AI-generated · claude-opus-4-7 · v3-deep-biology