Plasma Membrane

The cell, which is the smallest unit of life, is surrounded by a plasma membrane. The plasma membrane functions somewhat like a wall, as it keeps the internal contents from the external environment. Just like a wall, the membrane is also somewhat permeable, except that the membrane takes a much more active role in determining what is allowed in to the cell and what is kept out. The plasma membrane is a very thin structure, which has some very important tasks. One of the main tasks that it performs is to control the movement of foreign substances in and out of the cell. Herein I will discuss the structure of this membrane and some functions it has as part of the living cell.

The plasma membrane, despite its simple appearance, is a very complex structure. It contains many different components that perform unique functions to keep the cell regulated. The two primary molecules that make up the structure of the membrane are the proteins and the lipids. The proteins are embedded within the lipid bilayer. Most, but not all, lipids are referred to as phospholipids. These lipids arrange themselves into the bilayer. Another lipid is the glyolipid, which are a protective lipid and performs a variety of functions within the membrane. Cholesterol is a lipid that is found in animal plasma membranes; this lipid reduces permeability, making it more difficult for unwanted items to get into the cell. Some of the proteins that are part of the membrane structure include transmembrane proteins, glyoproteins, and peripheral proteins.

One of the tasks performed by the plasma membrane is to control the transport molecules into and out of the cell. One form of transport is through diffusion, which is where molecules move from higher concentration to lower concentration. Simple diffusion is a form of passive transport that can be used; it is called passive, as it requires no energy to be expended during the process. This is when a small, non-polar molecule can pass right through the membrane without being rejected. When simple diffusion takes place proteins are not used. Some things that can easily be diffused include water, oxygen, carbon dioxide and ethyl alcohol. At times it is necessary for molecules to move against the concentration gradient, or from low to high concentration. This cannot be done through passive transport. This has to be done through an energy-requiring process. One form of this process is called active transport. Proteins take an active role in this process by regulating and controlling the transport of the molecules. In combination with proteins, energy is needed for the transport. An energy-carrier molecule called adenosine triphosphate (ATP) is binded to a transport protein, which enables it to carry the calcium ion across the membrane. Other active-transport proteins are used as pumps to help move the molecules against the concentration gradient.

Sometimes a particle is too large to be moved across the membrane by a passive or acttive transport, however, it must still be acquired by the cell. When this situation occurs, another process called phagocytosis can be used to perform this task. This is a form of endocytosis, which is another energy-requiring transport system. In this process, the cell has to extend portions of its surface membrane, when this happens the membrane surrounds the particle so that it can pull it into the cell. This extension is called a pseudopod. Once the particle is