What is metabolism?
Metabolism means all the chemical reactions that happen inside a living thing. A reaction is a change where some molecules turn into new ones.
These reactions keep you alive. They give you energy, build your body and remove waste. They happen in every cell, all the time, even when you sleep.
The molecule you start with is called the substrate. The new molecule made is the product.
Catabolism and anabolism
Metabolism has two sides.
Catabolism: breaking down
Big molecules are split into small ones. Energy is released. Examples: respiration (glucose → carbon dioxide + water), digestion of starch, breaking down fat when you are hungry.
Anabolism: building up
Small molecules are joined into big ones. Energy is used. Examples: making proteins from amino acids, photosynthesis in plants, storing glucose as glycogen in the liver.
How they link
Catabolism makes energy and small building blocks. Anabolism uses that energy and those blocks. They are two halves of one cycle. When you grow, anabolism is bigger. When you fast or exercise hard, catabolism is bigger.
| Catabolism | Anabolism | |
|---|---|---|
| Molecules | big → small | small → big |
| Energy | released | used |
| Example | respiration | protein making |
ATP: the energy coin of the cell
ATP stands for adenosine triphosphate. It has one adenosine part and three phosphate groups in a row.
When the last phosphate breaks off, energy is set free:
ATP → ADP + P + energy
ADP (adenosine diphosphate) has only two phosphates. Energy from respiration joins a phosphate back on, so ADP becomes ATP again. This is the ATP–ADP cycle.
Cells use ATP energy to: make big molecules, contract muscles, pump substances across membranes (active transport), send nerve signals and keep birds and mammals warm.
Why not use glucose directly? Glucose holds a lot of energy at once. ATP gives it out in small, handy amounts, right where it is needed. That is why it is called the energy "currency".
Enzymes and metabolic pathways
Most reactions would be far too slow at body temperature. Enzymes are proteins that speed them up. Each enzyme fits only one kind of substrate, like a key fits one lock.
Reactions are linked in chains called metabolic pathways: A → B → C → D. The product of one step is the substrate of the next. Each step has its own enzyme.
If one enzyme is missing or blocked, the chain stops at that step. Some inherited diseases happen this way. Many pathways are also controlled by their own end product: when there is enough D, D slows down the first enzyme (feedback inhibition), so the cell does not waste materials.
Metabolic rate and energy balance
Metabolic rate is how fast your body uses energy. Even at rest you use energy for breathing, heartbeat and keeping warm; this is the basal metabolic rate (BMR). An average adult uses roughly 6,000–8,000 kJ a day at rest.
Metabolic rate goes up with exercise, growth, fever and cold weather. It is higher in children per kilogram, and is controlled partly by the thyroid hormone thyroxine.
If energy from food is more than energy used, the extra is stored (fat, glycogen). If less, stores are broken down and body mass drops.
Key formulas and definitions
- Metabolism = catabolism + anabolism
- Catabolism: big molecule → small molecules + energy
- Anabolism: small molecules + energy → big molecule
- ATP → ADP + P + energy (about 30 kJ per mole)
- Aerobic respiration: glucose + oxygen → carbon dioxide + water + energy (ATP)
- Energy balance: energy in (food) − energy used = energy stored
Worked examples
1. Sort these into catabolism or anabolism: (a) respiration, (b) photosynthesis, (c) digestion of starch, (d) making muscle protein.
(a) catabolism: glucose breaks down. (b) anabolism: CO₂ and water are built into glucose. (c) catabolism: starch breaks into sugar. (d) anabolism: amino acids join into protein.
2. In a pathway A → B → C → D, enzyme 2 (B → C) stops working. What happens to the amounts of B and D?
B cannot turn into C, so B builds up. No C is made, so no D is made either. B goes up, D goes down.
3. A teenager eats food giving 10,000 kJ a day and uses 9,000 kJ. Is the body mainly in anabolism or catabolism overall? What happens to the extra?
Energy in is 1,000 kJ more than energy used, so building and storing (anabolism) wins overall. The extra 1,000 kJ is stored as glycogen and fat, or used for growth.
4. Why does a person shiver in the cold?
Shivering is fast muscle contraction. It breaks down more glucose (catabolism) and uses more ATP. Part of that energy is released as heat, which warms the body.
Common mistakes
- Thinking metabolism means only "how fast you burn fat". It means every chemical reaction in the body.
- Mixing up the names. Catabolism breaks down (energy out); anabolism builds up (energy in).
- Saying ATP stores energy for a long time. ATP is used within seconds; long-term stores are fat and glycogen.
- Thinking enzymes are used up. An enzyme is not changed by the reaction and can be used again and again.