For multicellular organisms to stay alive, all cells perform certain basic functions. No one cell can provide the best conditions for all functions, therefore the cells of a multicellular organism are each specialised in different ways to perform a particular role. Each cell has evolved more or fewer of certain organelles and structures to suit the role it carries out.
As an embryo matures the group of cells that are initially identical, take their own characteristics that suit it to the function it will perform when it is mature. All the cells in an organism are produced by mitotic divisions from the fertilised egg, it is because of this that they all contain the same genes. So , every cell contains the genes needed for it to develop into any one of the many different cells in an organism. But, only some of these genes are expressed in any one cell at any one time, the rest are switched off.
It is not just the shape of different cells that vary but also the number of each organelle, for example muscle cells have many mitochondria whereas bone cells have very few. As each cell is adapted to its own particular function to perform it most effectively, the whole organism functions effectively.
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