Cell Wall\'s and Cell Membranes

¶ … cells require a layer to separate the inside of the cell from the outside world. Cells have a semi-permiable membrane which controls what can and cannot enter the cell. In animal cells, as well as some others, this membrane is all that separates the cell from its surroundings. Plant cells, and many single-celled organisms such as bacteria have a rigid cell wall beyond the membrane that provides structure. (Buck) Cell membranes act as the cell's gatekeeper, but are not especially rigid or strong. Cell walls can limit the entry of certain types of molecules, but they do not provide the fine-grained traffic control function of the membrane. Cell walls, however are rigid and strong.

Cell membranes are semi-permeable; they only allow certain molecules to enter and exit. Some molecules, such as water can pass through the membrane through osmosis or diffusion. This process does not require any energy to be expended, or any action on the part of the cell. It is therefore referred to as "passive transport." (Wolf) Other molecules can enter and exit the cell through active transport, a process that requires energy. Active transport involves specialized structures in the cell membrane moving specific types of molecules in to and out of the cell when the pressure from osmosis or diffusion is trying to move them in the opposite direction. (Andreas)