Part one of term paper
All cells have an outer protective casing, the cell membrane. It protects the cell from the outside world, but is also vital in the transfer of particles in and out of the cell. The cell membrane controls which particles can enter, and which particles can not, because it is selectively permeable. The membrane is made up of a bi-layer of lipid ...

Part two of term paper
... is not the only factor that determines whether it will pass through the membrane. There is also the concentration gradient, which relates to the amount of a certain substance inside the cell as opposed to outside the cell. The further down the gradient, the lower the concentration of a substance. This brings into play the processes of diffusion, osmosis, and active transport, all of which involve only small simple molecules. Diffusion is the movement of a substance from a region of high concentra ...