What is metabolism?
Metabolism is the sum of all the chemical reactions that take place inside the cells of a living organism. These reactions never stop; they allow an organism to grow, move, repair damage and respond to its surroundings. Almost every metabolic reaction is controlled by enzymes, which speed the reactions up so they happen fast enough to support life at body temperature.
Metabolism is divided into two opposite but linked processes: anabolism and catabolism.
Anabolism and catabolism
- Anabolism (building up) joins small simple molecules into larger complex ones. It absorbs energy. Examples include photosynthesis and the joining of amino acids to form proteins.
- Catabolism (breaking down) splits large complex molecules into smaller simple ones. It releases energy. Examples include cellular respiration and digestion.
Key idea
Anabolism stores energy in new chemical bonds; catabolism releases energy by breaking bonds. The energy released by catabolism is often used to drive anabolism.
Why metabolic rate matters
The metabolic rate is the speed at which these reactions occur. It changes with body size, activity, age and temperature. When you exercise, catabolic reactions such as respiration speed up to release more energy, so oxygen demand rises. When you rest, the rate falls.
Example
Photosynthesis in a leaf is anabolic: carbon dioxide and water are combined into glucose, storing light energy. Respiration in the same leaf is catabolic: glucose is broken down to release that stored energy as ATP. Together they show how one organism runs both types of reaction at once.
Metabolism and enzymes
Almost none of these reactions would happen fast enough on their own. Each metabolic step is catalysed by a specific enzyme, and the reactions are usually linked together into metabolic pathways, where the product of one reaction becomes the substrate for the next. By switching enzymes on or off, a cell can control which pathways run and how quickly. This is how a single cell keeps building, breaking down and recycling molecules in an organised way instead of in a chaotic rush.
Remember
AnaBOLic = Builds up + absorbs energy. CataBOLic = Cuts down + releases energy. Both are controlled by enzymes.