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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.