Organisms can be:
- Single-celled - where all life processes are carried out by one cell.
- Multi-celled - where cells differentiate and become specialised to carry out a particular function (e.g. red blood cells carry oxygen)
Specialised cells are organised. There are different levels of organisation:
- Cells with the same structure and function are known as tissue (e.g. muscle tissue).
- Several types of tissue that carry out a particular function is an organ (e.g. the heart organ contains muscle and nerve tissue).
- Organs that work together to carry out a particular function are known as an organ system. (e.g. the circulatory system.