In simple terms, the cell membrane can be thought of as the skin or protective barrier of cell. It keeps all the internal components — or cytoplasm — of a cell inside of it, and the external items outside of it. Its main purpose is to maintain the integrity of the cell and determine what can enter and exit the cell.
The cell membrane is comprised of a lipid bilayer. The lipid molecules are formed in two rows, each row is a mirror image of the other row. Two parts make up the structure of the lipid molecules – a hydrophobic and a hydrophilic section. The hydrophobic section avoids water and water-like molecules and faces inwards making up the center of the lipid bilayer, while the hydrophilic section attracts water or water-like molecules and faces outwards. Thus, the basis of the fluid mosaic model can be established.
The fluid mosaic model of the cell membrane was discovered in 1972. It describes the arrangement of proteins within or on the lipid bilayer. The proteins are inserted into it in many different ways, forming a mosaic of proteins. Proteins that cross the lipid bilayer are important to the transportation of small molecules across the membrane.
Since the cell membrane is semi-permeable, only certain small molecules can pass back and forth across the membrane through a process called diffusion. However, before diffusion can occur the membrane must be permeable, or open, to the small molecule that is trying to cross — much like an open door. If the membrane is not permeable, the small molecule will not be allowed to pass — and the door can be considered closed. In addition, a small molecule can only pass from an area that is more crowded to an area that is less crowded. For example, if an amino acid wants to move across a membrane into a cell, and the cell membrane is permeable to the amino acid, there must be more amino acids outside of the cell and fewer amino acids inside the cell for the amino acid to enter into the cell via diffusion.
Another process that occurs through the cell membrane is facilitated transport. For some small molecules, the proteins that have situated themselves within the membrane can also help those molecules pass through the membrane. It is almost as if the proteins escort them across. Like diffusion, facilitated transport can only occur if the molecules are moving from an area of higher concentration to lower concentration — or, from an area with more molecules to an area with fewer molecules.
Lastly, molecules can cross a cell membrane via active transport. In active transport, the molecules travel from areas of lower concentration to higher concentration — opposite from the diffusion and facilitated transport processes — against the concentration gradient. To go against the gradient, energy is required in the form of adenosine triphosphate (ATP). ATP can be used for several different cellular activities ranging from movement to reproduction.