The cell membrane is selectively permeable; it regulates what passes from one side to the other. A selectively permeable membrane will allow water to flow freely while limiting the passage of molecules, especially large molecules or those with a charge. A plasma membrane provides some protection to the insides of cells, but its main life-giving function is to control the passage of substances into and out of cells.

The mitochondira within the cell membrane convert energy from nutrients into ATP which is the useable energy form.
Organelles within the cytoplasm help eliminate waste, such as lysosomes, which break down bacteria and other waste products.

There are numerous types of membrane transport such as Diffusion. Simple diffusion through the plamsa membrane is mostly limited to lipid-soluble substances that can dissolve into the membrane such as gases and liquids. Filtration is another form of important membrane transport which is the passage of water and dissolved materials through a membrane as a result of a mechanical force on one side. For example, it occurs in the kidneys as materials are filtered out of the blood in the first step of urine formation.