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...
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