Cellular Biology

Structure and Function of the Cell Membrane



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The cell membrane, or plasma membrane as it is sometimes called is possibly the most important aspect of the cell and without it life could not exist in any form. The membrane allows an organism to be enclosed and separated from the surroundings. The membrane is selectively permeable in that it only allows certain molecules through whilst others may not pass through it: the reason for this will be discussed later. The membrane means that concentrations of some molecules in the cell can be increased or decreased as well as the separation of certain reactions. A good example of this is the way that the mitocondrion is completely enclosed and so can perform catabolic reactions such as citric acid cycle and fatty acid degradation whereas in the cytoplasm other reactions, including fatty acid synthesis can take place and it is easy to see how this can be beneficial to the cell as a whole.

The membrane itself can be described as a bilayer as it contains two leaflets of phospholipids. Each phospholipid is an amphiphile as it contains both a hydrophilic and hydrophobic region. The hydrophilic region is more thermodynamically favourable when exposed to the aqueous solvent whereas the hydrophobic region is more stable when separated from the water. This means that a bilayer is formed so that the hydrophobic tails are touching and the hydrophilic head-groups are positioned in opposite directions. The reason for this amphiphilic behaviour is that each phospholipid contains a glycerol backbone, two hydrocarbon chains (fatty acids) and a head-group: the glycerol is polar so forms hydrogen bonds with water and the head-group, which can vary immensely from lipid to lipid is often charged and so is also hydrophilic; the fatty acid tails are hydrophobic due to their non-polar character and so are not solvent exposed.

It is not only phospholipids that are present in the plasma membrane: there are also numerous proteins. Proteins can either be intrinsic (spanning the whole membrane) or extrinsic (loosely associated with the membrane). A protein that spans a membrane can cross it in a few well-characterised ways: the first of which is alpha helical involving hydrophobic amino acid residues that are more thermodynamically stable on the inside of the membrane. Beta barrels are often used in addition as there are beta sheets that span the membrane and so form a cylindrical shape that can be used as a pore.

The proteins that span the membrane have various functions, perhaps the most obvious of these is allowing molecules that are polar to cross the membrane, either down a diffusion gradient (facilitated diffusion) or against a diffusion gradient (active transport) that requires an energy input, usually ATP hydrolysis. This allows a cell for instance to take up glucose against a gradient and so have a higher concentration inside than the extracellular concentration.
Another important function involves signalling between cells: as cells do not share cytoplasms there must be a way that one cell can communicate with another and it is at the cell membrane that this takes place. A molecule, such as insulin can be secreted from one cell and so cause a response in a second cell by binding to a specific receptor protein on the cell membrane. The binding of this molecule causes a response by causing the ligand to change its interactions with one or more other molecules within the cell: for example it can cause phosphorylation of another molecule.
The cell membrane is also the site of cell-cell interactions and in particular it is the glycoproteins that mediate this. Glycoproteins are proteins that have added carbohydrates that can be N or O linked. N-linked glycosylation involves attachment via an Asparagine and O-linked glycosylation occurs via the hydroxyl of serine or threonine: an example of glycoproteins in recognition are the blood type antigens. In some cases glycosylation can act as protection to a cell or protein.

There is one main hypothesis for the motion in a cell membrane, this is known as the fluid mosaic model and involves the movement of the lipids and proteins in a leaflet around each other in such a way that they look like a moving Mosaic. This is shown using the FRAP technique that shows how a bleached area of fluorescent antibodies recovers over time due to the movement of the proteins to which these antibodies are attached.
More recently it has been seen that not all proteins and lipids can traverse the whole cell, this has been hypothesised to be due to "lipid rafts" where the movement of membrane constituents is restricted so that they are not fully motile.

As you can see the cell membrane is hugely important in the cell as it leads to compartmentalisation and can affect interactions between cells and compartments and interestingly it is over a membrane that ATP synthesis occurs due to the movement of protons.

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