A single brick is not much use, but stack millions in the right way and you get a cathedral. Living things are built the same way: tiny cells are organised into ever-larger teams, each level doing a bigger job than the last. And to run on the food you eat, your body relies on a chemical workforce called enzymes.
Levels of organisation
Similar cells group together and, step by step, build a whole organism:
- Cell — the smallest unit of life (e.g. a muscle cell).
- Tissue — a group of similar cells working together (e.g. muscle tissue).
- Organ — several different tissues combined to do a job (e.g. the stomach).
- Organ system — a group of organs working together (e.g. the digestive system).
Picture an army: one soldier is a cell, a squad is a tissue, a company is an organ, and the whole division is an organ system. The order of increasing size is always cell → tissue → organ → organ system.
Digestion and enzymes
The food you eat is made of molecules far too big to pass into your blood. The digestive system breaks them down into small, soluble molecules the body can absorb. Doing this by chemistry alone would take years — so the body uses enzymes, which are biological catalysts: substances that speed up reactions without being used up themselves. Think of an enzyme as a pair of molecular scissors, snipping big food molecules into bite-sized pieces over and over.
The three key digestive enzymes
| Enzyme | Breaks down | Into |
|---|---|---|
| Amylase | Starch (a carbohydrate) | Sugars |
| Protease | Protein | Amino acids |
| Lipase | Fats (lipids) | Fatty acids + glycerol |
Memory hook: for the order, “Cute Tigers Order Snacks” — Cell, Tissue, Organ, System. For enzymes: protease eats protein, lipase eats lipids, and amylase handles the starch.