Cellular Biology

Structure of Cell Membranes

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"Structure of Cell Membranes"
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Cell membranes are crucial to cell proliferation and survival. The cell membrane is not only a protective barrier that surrounds the cell but it also allows passage of essential molecules to and from the cell. The cell membrane is made of two key components which are proteins and phospholipids. The membrane itself is roughly 5 nanometres thick, if we compare this to the macroscopic world it is about 20 000 times thinner than the average sheet of paper.

Phospholipids are the major constituents of the membrane, they are composed of a hydrophilic (likes water) head and a hydrophobic (hates water) tail. If I were to describe the general appearance or structure of a phospholipid, I would describe it as an egg shaped head with an attached tail composed of two strands.  

Since phospholipids have a side that is hydrophilic and another one that is hydrophobic it is called an amphipathic molecule. Water plays an important part in cell membrane structure. Water is distributed in humans as follows: two thirds of the water in our body is contained within the cells and the other one third is mixed with the liquids surrounding the cells. This will cause phospholipids to arrange themselves in order to meet with their water preferences. The head is hydrophilic and it will want to be oriented towards the intracellular or extracellular spaces which both contain water. The tail however is hydrophobic and will want to be away from both intracellular and extracellular spaces. The question is how do we obtain a structure that will meet the needs of phospholipid molecules?

The answer is forming a bilayer of phospholipids. In a bilayer there are two layers or sheets of phospholipids on top of one another. A single layer is composed of a row of phospholipids side by side with the same orientation (the heads all point in the same directions). This means that a single layer of phospholipids will have a hydrophilic side (containing only heads) and a hydrophobic side (containing only tails). Since the bilayer is composed of two layers of phospholipids, the individual layers will orient themselves to obtain a bilayer in which all heads face outward and all tails face inward. The tails then interact with each other and are directly responsible for membrane fluidity. The sides of a bilayer sheet are exposed to water, this results in exposure of the hydrophobic tails to water. This will cause the bilayer sheets to associate themselves with other bilayer sheets in order to form a spherical shape where it is made possible for tails not to come in contact with water. This is why all cell membranes have a spherical shape.

The other important constituents are membrane proteins which compose roughly 50% of the total weight of the membrane. It’s important to remember that protein molecules are heavier than phospholipids, so in a cell membrane there are 50 times more phospholipids than there are of proteins. Membrane proteins have many structures and functions within the cell membrane. Some proteins are attached to the outer or inner part of the cell membrane either via a lipid or another protein, some completely traverse the membrane and others are integrated within one layer of the membrane.

Proteins can form complex receptors that are specific to molecules and that let them permeate through the membrane. Other proteins form canals that let molecules of a certain size just slide into the cell. Some are also coupled with carbohydrates and act as markers or recognition sites for molecules or other cells. It is also possible for membrane proteins to act as enzymes to start a reaction such as the insertion of new proteins or protein translocation within the cell membrane.

The last important cell constituent which is only found on the surface of the outer layer of cell membranes are carbohydrates. Usually covalently bonded to proteins, they will form an envelope covering the membrane. This envelope of long carbohydrate chains is an extra barrier that helps reduce both physical and chemical damage to the cell membrane. It also helps with recognition among cells and cellular adhesion.

We can conclude that the cell membrane is fundamental to the cell, it protects the cell and allows nutrients to enter and exit the cell. The cell membrane is composed of phospholipids, proteins and minimal quantities of carbohydrates.  The cell membrane is a spherical bilayer with hydrophilic extremities and a hydrophobic midsection. Carbohydrates form an envelope around the cell membrane to protect it from physical and chemical damages.

More about this author: Catherine Boudreau

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