The plasma membrane separates a cell from its surroundings. It protects the interior of the cell from entry by unwanted substances and it contains the contents of the cell so that they can interact with one another.

The membrane also has a primary function of transporting materials into the cell to meet its nutritional needs and removing wastes. Much of the architecture of the plasma membrane is devoted to the movement of material in and out of the cell.

Transport
The plasma membrane has a primary function of regulating what enters and exits the cell. It does this by transporting materials into the cell to meet nutritional needs and removing wastes.

Transport is divided into two primary types: passive, requiring no additional energy, and active, which do require energy. The molecules are moved either into or out of the cell in three ways:

Passive Transport
The passive transport of material across the plasma membrane depends on the permeability of the membrane to various molecules and on conditions and concentrations on either side of the membrane. There are three primary types of passive transport, diffusion, facilitated diffusion, and osmosis.

Diffusion
Diffusion is the movement of molecules from a place of higher concentration to a place of lower concentration. Material will move along the concentration gradient (the difference in the concentration of the molecules over a distance) until equilibrium is reached.

Osmosis

Osmosis is the net movement of watermolecules through a selectively permeable membrane from an area of high concentration to an area of lower concentration. Selectively permeable means that the membrane will allow certain molecules to pass across it and will prevent others from doing so. Osmosis occurs when the water in a solution can cross a membrane but the molecules that are dissolved in the water, reducing its concentration, cannot pass.

The following terms are used to describe the conditions that affect osmosis in and out of cells:

Isotonic– the concentration of water and dissolved materials (solutes) in a cell equals that of its surroundings. No net movement of water.
Hypertonic– the inside of the cell has a lower concentration of dissolved materials (solutes) and a higher concentration of water than the surrounding environment. Water moves out of the cells and the cells shrivel.
Hypotonic– the inside of the cell has a higher concentration of dissolved materials (solutes) and a lower concentration of water than the surrounding environment. Water moves into the cells and they burst.

Facilitated Diffusion
Facilitated diffusion occurs when molecules move across a membrane by combining with specific carrier proteins, increasing the rate of diffusion. Movement is still from an area of high concentration to one of low concentration. This form of transport is used to move molecules that are large or carry a charge and are unable to move via simple diffusion.

Active Transport
Active transport requires energy because it moves molecules against the concentration gradient, from areas of low concentration to areas of high concentration. The process uses carrier proteins to move molecules through the plasma membrane. Active transport is very specific – it only moves certain molecules, and each type of molecule transported requires a unique carrier protein.

Two of the most important examples of active transport are the sodium-potassium pump, which is involved in the transmission of nerve impulses, and the proton pump, which functions in the generation of ATP.

Endocytosis and Exocytosis
Besides individual molecules, cells may also need to move large particles or groups of molecules through the plasma membrane. These processes require the membrane itself to move – the membrane expands or indents to engulf the material to be transported, then pinches the protrusion off either inside or outside the cell.

Endocytosis moves material into a cell. It can be broken into phagocytosis, or the ingestion large “food” molecules, and pinnocytosis, the ingestion of liquids.

Exocytosis removes wastes and cell products from the cell.