Alongside that, there are protein molecules in the membrane and these protein molecules play the role of transporters, allowing transport of substances to pass through the membrane from either side. These proteins mostly tend to have chains of carbohydrates attached and these are known as glycoproteins. These glycoproteins are always positioned on the outer surface of the membrane. These glycoproteins help to stabilise the structure of the membrane as well as their involvement in cell recognition.
The cell membrane is referred to normally as ‘the fluid mosaic model.’ The reason for this name being kept is that the molecules in the cell surface membrane are constantly moving and may change their position in the membrane. So, the membrane behaves very much like a fluid. Secondly, the protein parts of the cell membrane were once believed to be part of an evenly distributed layer over the inside and outside of the phospholipid layer. However now it is thought that the irregular dispersal resembles that of a mosaic.
The cell membrane has many different functions. Here is a list of functions, which I shall explain in due course:
- The cell membrane is a selectively permeable barrier regulating the movement of substances, which enter and leave the cell by various different processes.
- One of the most important roles of the membrane is to do with cell recognition. This is the capability of cells within our bodies to distinguish between cells belonging to other organisms/individuals or cells belonging to our own bodies.
- It also isolates the cell contents from external environment.
The plasmamembrane as I mentioned separates the components of a cell from the surroundings. This means that metabolic reactions can take place and still be isolated from outside of the cell. This is an extremely important factor because without the membrane being present the components of the cells would merge with everything else that would be normally be outside of the cell membrane and therefore the cell would never be able to stay in existence.
However much the cells are in isolation cells still need a regular supply of substances required for structural functions and energy sources which are both part of cell life processes. Alongside that, unwanted waste materials need to be removed as well. Furthermore, the plasmamembrane acts a control mechanism controlling the entry and exit of materials and there are various methods of movement across the cell surface membrane and they are the following:
- Diffusion
- Osmosis
- Facilitated Diffusion
- Active Transport
- Cytosis – Endocytosis and Exocytosis
Diffusion is the net movement of molecules or ions from a region of higher concentration to a region of lower concentration. Diffusion occurs because of the constant movement of molecules and ions that bump into each other and then rebound in different directions. The energy needed for this movement is provided by the kinetic energy that each molecule or ion contains. Eventually this movement of molecules/ions will distribute them evenly.
Fat-soluble molecules, such as glycerol, can diffuse through the membrane easily. They dissolve in the phospholipid bilayer and pass through it in the direction of the concentration gradient. Water, oxygen and carbon dioxide can also diffuse through the bilayer, passing easily through the temporary small spaces between the 'tails' of the .
Ions and most large molecules cannot diffuse through the phospholipid bilayer. They have to go through the protein pores. The cell membrane contains various protein pores that can differ in terms of structure and size depending on which substance is passing through. Diffusion that occurs through these protein pores is referred to as facilitated diffusion. This process works by the binding of the solute changes the shape of the carrier so that its position within the membrane also changes and so the molecule is released on the other side of the membrane. The cell plays no role in the active movement of these substances through the pores so the process is passive and there is no energy required.
Another type of diffusion is Osmosis. This is the movement of net movement of water through the partially permeable membrane from a solution of less negative water potential to a solution of more negative water potential. Water potential being the ability of a solution to diffuse water molecules by osmosis. Water molecules are quite small and are present in large quantities inside and outside of a cell. Even though they are polar, it is very easy to diffuse through tiny gaps between phospholipid tails.
Active transport has the opposite function to diffusion. Molecules and ions that need to be removed from the cell, moving against the concentration gradient is done so by Active Transport. As I mentioned earlier there are protein molecules within the membrane. These proteins can be known as transporter proteins. This transporter protein carries a solute across the membrane working against the concentration which means high amounts of energy is required. The ATP is produced and binds to the phosphate group to the transporter protein and similarly to facilitated diffusion changes its conformation so that the solute can be transported to the other side of the membrane.
Cytosis is the final process and transport across the membrane that I shall explain. Cytosis is when substances are moved in or out of a cell with the help of vesicles. In Endocytosis, substances are brought into the cell. This is done by the cell surface membrane surrounding a material that needs to be taken in and fuses around it to form a vacuole. Exocytosis is the movement of substances out of a cell. A vesicle that contain unwanted material moves towards the plasmamembrane and fuses with the cell membrane releasing whatever the contents of that vesicle were to the surroundings. Cytosis adds or subtracts the surface membrane of a cell and a balance is kept to determine the surface area of them membrane.
Another important function carried out by the plasmamembrane is to do with cell recognition. Without the recognition of other cells within our body, we would be invaded by microorganisms so it is very important that our cells do so. The features of the cell’s membrane that differentiate between cells are the proteins, carbohydrates, glycoproteins or glycolipids. The cells within our body have a certain set of chemicals on the surface and our immune system refers to these as being ‘self’. Cells with different surface chemicals are regarded as being ‘non-self’ and are furthermore attacked. This whole system of cell recognition is yet to be fully discovered however, this is the basic knowledge that we know up to this point in time.
Overall it is actually quite amazing to see how an extremely minute component of cell which itself is so small it cannot be seen by the naked eye, can play such important yet complex roles. We are yet to discover more about this cell ultrastructure however it would be of much interest to see what more the cell surface membrane can do.
References:
- www.s-cool.co.uk/asrevision
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Jones, G. and Jones, M. (1997). Advanced Biology. Cambridge. Cambridge University Press. 57-58, 73-74, 78-79, 484
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Baker, M., Indge, B. and Rowland, M. (2000). A New Introduction to Biology. Italy. Hodder and Stoughton Educational. 24, 27, 30