Why do most plants, bacteria and fungi have cell walls while animal cells don ?
Well, animal cells don’t have cell walls because, um, they have to be flexible. You can see this best in muscle cells: they contract and relax, so the animal can move! Animals typically maintain an internal environment that is isotonic with respect to the cell interior. Plants and fungi don’t. The external environment is hypotonic. Water pushes into plant cells — until the cell is constrained rigidly by its membrane against its cell wall with physical pressure. Without the cell wall, the cell would burst.
