Single-celled animals and plants must perform all seven processes within an individual cell. There is a limit to how large these cells can grow, as materials must reach all parts so making distance a problem. Large multi-cellular organisms need to organise their many cells to do particular jobs to overcome the problems of distance and complexity due to size. These common groups of cells are called tissues . Different tissues then combine to form specialised organs .
Organs have particular functions within living organisms.
Organs can be further organised as systems .