Cells are specialised to carry out particular functions. Different cells and different systems carry out different functions. As a result co-ordination of the different cells is needed for the organism to work as a whole. Some cells are reliant on other cells to function properly. For example, muscle cells produce lactate during exercise. The lactate is carried away in the blood and broken down by the liver using oxygen transported by red blood cells.Plants need to co-ordinate flowering, pollination and germination with the seasons.