A plasma membrane is a cell organelle that is actively controlling the passage of substances moving in and out of a cell. The plasma membrane, as well as lipids, includes several proteins; the proteins that are within the membrane are found buried or embedded into the lipid bilayer. These proteins include enzymes, receptors and antigens.
There are four methods of transportation both in and out of a cell; diffusion, facilitated diffusion, osmosis, and active transport. Fick's law is used to measure the rate of diffusion:
Diffusion is the movement of atoms, molecules or ions from a region of high concentration to a region of low concentration (down the concentration gradient²). The energy for this to occur comes directly from the particle itself; this is defined as passive (not requiring energy). The reason for it not requiring energy from another substance is the fact that the particle is in constant movement and is able to use its kinetic energy for diffusion. Diffusion occurs in the cell membrane where the membrane is fully permeable. In the case of the plasma membrane, the membrane is permeable to non-polar substances such as steroids and glycerol, as well as oxygen and carbon dioxide in solution. Diffusion can also occur through pores in the membrane. Water diffuses through these protein lined pores or can diffuse through spaces between the phospholipid molecules. This situation is only available to those molecules, however, that are a ‘regulation’ size; meaning that the particles must be small enough to have the ability to travel through the small pores and thus diffuse.
In situations where a charged particle or large molecules are to be transported across the membrane which could be soluble or insoluble in lipids and is not able to diffuse through normal diffusion, molecules are able to do so through another form of diffusion called facilitated diffusion. This is where a large molecule is allowed to mover through a protein- lined pore; the movement of these substances requires two proteins: a channel protein and a carrier protein. The channel proteins line a water-filled pore in the membrane so water-soluble molecules can easily pass through. Different channels allow different substances to pass through (the channels are selective). Some channels are gated (they will only open when appropriately stimulated).A carrier protein is where the substance actually combines with a protein and is carried from one side of the membrane to the other. These proteins are specific for a particular substance. In both these cases, substances are moving down the concentration gradient¹ so no energy is required.
The diffusion of water molecules across a membrane that is partially permeable is called osmosis. When a solution is separated from water by a membrane which is permeable to water molecules, the water molecules will travel from the high concentration of water molecules to a lower concentration. Osmosis is the net movement of water from a high concentration to a lower concentration across a partially permeable membrane.
Unlike the previous diffused forms of movement across the membrane the active transport consists of a different type of movement. All the above types of transport occur from a high concentration to a lower concentration (down the concentration gradient), where as active transport occurs against this concentration gradient. Movement through active transport is very selective; it only takes in what is needed, for example, the transport of ions from the lower concentration in the soil to the higher concentration inside the plant cells. Because molecules are moved against the concentration gradient, it requires energy. It is thought that active transport uses carrier proteins similar to those involved in facilitated diffusion.
If very large molecules or groups of molecules need to enter or exit a cell, they do so using vesicles. The material to be transported out of the cell is surrounded by membrane. The vesicle will fuse with the cell surface membrane and the contents leave. This is called exocytosis. Materials entering the cell can do so when the plasma membrane encloses and thus surrounds the material. The membrane seals off to form a vesicle, which can then move into the cell. This is endocytosis.
The transportation of substances through the plasma membrane is very specific and the procedures that facilitate diffusion are what allow the different types of d transportation to take place. The transportation of substances is fundamental to life processes and therefore to existence.
