The Cell Membrane

The cell membrane is probably the most important organelle in the cell. It holds the cell together and keeps everything intact. It also regulates homeostasis which will be discussed later in this section. Cell membrane is mobile and moves along "grooves" which are set paths the membrane follow. The cell membrane is composed of a phospholipid bilayer, which has hydrophlic heads that are soluble in water and hydrophobic tails which are not soluble in water. The heads are lipid molecules with a phosphate group (PO₄) on the end. The top and bottom layers of the membrane have their "stems" facing each other. There are also proteins, some with carbohydrate side chains, some without, in and through the membrane. These proteins can stretch though the top, bottom, or both layers together of the phospholipid bilayer. These proteins have many purposes that are nessesary for the cell to continue life functions. The cell membrane itself has 4 main functions. It holds the cell together, controls which substances go in and out of the cell, and maintains homeostasis. Most of these functions are made possible by the proteins. These proteins are used for five main reasons:

1. They are used to join cells together in cell adhesion.
2. They attach the membrane to the cytoskeleton. This is important in keeping the shape of the cell and to hold the membrane in a certain place when needed.
3. Sometimes the proteins gather together as enzymes and carry out the different steps of the metabolic reactions which take place near the cell membrane.
4. Proteins act as receptors , or sites for messenger molecules which signal the cell to start or stop metabolic activity.
5. Most importantly the proteins make the membrane selectively permeable. That is, they control the movement of substances through the membrane.

The membrane will only let certain substances through it at certain times. It could let substance "x" through at a certain time, but later reject the same substance. There are 4 main factors that determine whether or not a substance can pass through a cell membrane:

1. If they are lipid or lipid slouble moelcules.
2. Smaller molecules will pass easier than large molecules
3. Molecules with a neutral charge will pass easier than molecules with a charge (Ions).
4. As a living substance, the cell membrane has the abilty to pass different molecule at any given time.

Another job of a select few proteins are to carry sugar molecules that are refered to as glycoproteins and act as signatures for the cell. Each cell has a different signature, and the forms of these signatures vary in animals and humans.

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