An Examination of the Structure and Function of the Cell Membrane

All cells have an outer protective casing, the cell membrane. It protects the cell from the outside world, but is also vital in the transfer of particles in and out of the cell. The cell membrane controls which particles can enter, and which particles can not, because it is selectively permeable. The membrane is made up of a bi-layer of lipid molecules, which have very small spaces between each other, and if a particle is small enough to fit in-between these gaps, then it can pass through the membrane. There are also proteins in the membrane that span its width, and form channels. These channels allow larger molecules to pass through.

The size of the particle, however, is not the only factor that determines whether it will pass through the membrane. There is also the concentration gradient, which relates to the amount of a certain substance inside the cell as opposed to outside the cell. The further down the gradient, the lower the concentration of a substance. This brings into play the processes of diffusion, osmosis, and active transport, all of which involve only small simple molecules.

Diffusion is the movement of a substance from a region of high concentration, to a region of low concentration of that substance. In other words, a movement down the concentration gradient. Osmosis is very similar to diffusion, however it involves the concentration of water. Osmosis, like diffusion, is a passive process, and does not require energy to occur. Active transport does require energy though, as it involves movement up the concentration gradient. This involves protein carrier molecules binding to the substance and moving it into or out of the cell, against the concentration gradient.

If a large molecule needs to be moved in or out of the cell, then two other processes, endocytosis and exocytosis are used. Endocytosis involves the cell membrane engulfing a foreign molecule and enclosing it in a vesicle inside the cell. Exocytosis involves the golgi body enclosing the substance to be exported into a vesicle, which then fuses with the cell membrane, and releases the substance outside the cell. Both endocytosis and exocytosis require energy.

Inside cells, inside the cell membrane, thousands of chemical reactions occur. It is chemical reactions that make the cells, and therefore life, work. However, the likelihood of these reactions occurring on their own is next to none. Therefore, enzymes are needed to catalyse the reactions. Enzymes are made up of proteins, and each enzyme is specifically designed for a particular reaction. This is due to the shape of the enzymes active site a groove on the enzyme where the substrates enter and the reaction occurs. Enzymes are never used up in reactions, and can continue to catalyse more and more reactions.

There are several environmental factors that can aid or hinder enzymes. Temperature is one of these, as in general the higher the temperature, the greater the rate of reaction. This is because at higher temperatures, molecules will move faster. However, as the temperature reaches and surpasses 45oC the rate will decrease. At this temperature, the bonds that hold the shape of the enzyme begin to break and the shape of the active site is destroyed. Another factor is the pH level, as each enzyme has an optimum pH, where the enzyme will perform optimally. The final factor is inhibitors, which always decrease the rate of reaction. Inhibitor molecules have a shape that is complementary to the active site of an enzyme, and therefore it will bond with an enzyme, blocking the active site, and rendering it useless.