Animal cells do not have cell walls because they do not need them. Unlike plant cells, animal cells have evolved a different type of protection called the cell membrane. The cell membrane, also known as the plasma membrane, is a thin, flexible barrier that surrounds the cell and controls the movement of substances in and out of the cell.
Cell walls, which are found in plant cells, serve different purposes. They maintain cell shape, almost as if each cell has its own exoskeleton. This rigidity allows plants to stand upright without the need for bones. It also provides a barrier to protect the plant cell from its environment, and prevent the cell from bursting due to internal pressure.
The cell membrane, on the other hand, is composed of a lipid bilayer, a double layer of lipids (fats) that forms a barrier that is selectively permeable. This means that the cell membrane allows certain molecules to pass through while blocking others. This selective permeability is vital for the survival of the cell, as it enables the cell to take in nutrients and expel waste products.
The cell membrane also plays an important role in cell-to-cell communication, and in the movement of cells. Animal cells have the ability to change shape and move, which is vital for their survival. The flexibility of the cell membrane allows animal cells to move, and to change shape to adapt to their environment.