Many cells, division of labour
A multicellular organism is made of many cells. Unlike a single-celled organism, its cells do not each do everything. Instead there is a division of labour: different cells become specialised for different jobs, so the whole body works more efficiently.
Why systems are needed
As an organism grows larger, its volume increases faster than its surface area, so the surface-area-to-volume (SA:V) ratio becomes small. Diffusion alone can no longer reach the innermost cells fast enough. To solve this, multicellular organisms develop:
- a transport system (such as blood in a circulatory system) to carry oxygen, food and wastes;
- a specialised respiratory system for efficient gaseous exchange;
- a digestive system to break down food for absorption.
Specialised cells
Cells take shapes suited to their task. A red blood cell is biconcave to carry more oxygen; a root hair cell is long and thin to absorb water; a nerve cell is long to carry messages.
Example
In humans, the lungs provide a large, moist surface for gaseous exchange, while the blood transports oxygen to cells deep inside the body — something a single cell would never need.
Working together
Specialised cells cannot survive alone; they depend on one another. A muscle cell can contract but cannot fetch its own oxygen, so it relies on red blood cells and the lungs. A nerve cell can carry a message but cannot digest food, so it relies on the digestive system. This cooperation, called interdependence, is only possible because the cells are organised into tissues, organs and systems that link the whole body together.
The result is a trade-off: a multicellular organism gives up the independence of a single cell but gains the ability to grow large, live longer and carry out complex tasks. This is why the largest and most advanced living things — including humans — are all multicellular.
Remember
Large body → small SA:V ratio → diffusion is too slow → specialised transport and exchange systems are needed.