Metabolism
What Is Metabolism?
Metabolism is the sum of all the chemical reactions that take place in the cells of an organism. It includes reactions that build up larger molecules (anabolic) and reactions that break down molecules (catabolic).
Metabolism depends on enzymes to catalyse the reactions.
Anabolic vs Catabolic Reactions
| Type | Definition | Examples |
|---|---|---|
| Anabolic (building up) | Small molecules are joined to make larger ones; requires energy | Making proteins from amino acids; making glycogen from glucose; making lipids from fatty acids and glycerol; photosynthesis (making glucose) |
| Catabolic (breaking down) | Large molecules are broken into smaller ones; releases energy | Respiration (breaking down glucose); digestion (breaking down food molecules) |
Key Metabolic Reactions
1. Glucose metabolism
- Glucose from digestion can be used in respiration (catabolic — releases energy)
- Excess glucose is converted to glycogen for storage in liver and muscle cells (anabolic)
- Glucose can also be converted to starch in plants (anabolic) or cellulose for cell walls
- Glucose can be converted to lipids (fats) for long-term energy storage
2. Protein synthesis
- Amino acids (from digestion of proteins) are joined together to form new proteins (anabolic)
- Proteins are needed for growth, repair, enzymes, antibodies, and hormones
- This requires energy from respiration
- Excess amino acids cannot be stored, so they are broken down in the liver by deamination: the amino group is removed and converted to urea (which is excreted by the kidneys), and the remaining carbon compound is used in respiration or converted to fat
3. Lipid metabolism
- Lipids are made from three fatty acid molecules and one glycerol molecule (anabolic)
- Lipids are broken down in digestion by lipase into fatty acids and glycerol (catabolic)
- Lipids are used for energy storage, insulation, cell membranes, and hormone production
4. Respiration and energy
- Respiration (catabolic) breaks down glucose to release energy
- This energy drives all other metabolic processes in the cell
Enzymes and Metabolism
Enzymes are biological catalysts — they speed up metabolic reactions without being used up. They are proteins with a specific 3D shape.
Lock and key model:
- Each enzyme has an active site with a specific shape
- Only a substrate (reactant) with the complementary shape can fit into the active site
- The substrate binds to the active site, forming an enzyme-substrate complex
- The reaction occurs, and the products are released
- The enzyme is unchanged and can be reused
Factors affecting enzyme activity:
| Factor | Effect |
|---|---|
| Temperature | Increasing temperature increases rate up to the optimum (about 37°C for human enzymes). Above the optimum, the enzyme denatures — the active site changes shape and the substrate can no longer fit |
| pH | Each enzyme has an optimum pH. Too far above or below this, the enzyme denatures. Most human enzymes work best at pH 7 (neutral), but pepsin (in the stomach) works best at pH 2 |
| Substrate concentration | Increasing substrate concentration increases rate until all active sites are occupied (saturated), then the rate plateaus |
Metabolism in the Liver
The liver is a key metabolic organ:
- Deamination of excess amino acids → urea (excreted by kidneys) + carbon compounds
- Conversion of lactic acid back to glucose (after anaerobic exercise)
- Detoxification of alcohol and drugs
- Storage and release of glycogen (converting to/from glucose as needed)
- Production of bile (stored in gall bladder, used to emulsify fats in digestion)
Exam Tips
- Metabolism = all chemical reactions in the body — not just respiration
- Know examples of both anabolic (building up) and catabolic (breaking down) reactions
- Deamination happens in the liver and produces urea — a common exam question
- Enzymes denature at high temperatures — the active site changes shape, it does NOT "die" (enzymes are not alive)