Biological Membranes

The theory was that, as it was known lipids were made up of three fatty acids joined to one molecule of glycerol by an ester bond. Glycerol was known to be hydrophilic (dissolves in water) and the fatty acids, which were long chains of hydrocarbon was hydrophobic (does not dissolve in water). When a film of lipid is spread out on water, the water-soluble side that in this case is the glycerol points towards the water, and the hydrophobic side points away. But the catch was that cytoplasm has water in it and the solution the cell is placed in also has water. This implied that there has to be a double layer of lipid. Phosphorous was also found attached to the lipid and therefore the name phospholipid bi-layer was derived. Phosphorous was later found out to be a factor controlling cell membrane viscosity. Cholesterol in the membrane also influences the membrane viscosity, it may become gel-like at times and at sometimes it may be almost solid. Scientists had roughly calculated with a crude electron microscope that the cell membrane thickness would be around 7.5-8 nm in width. It was found out that the other hydrophilic sides attracted each other due to polarity. Theory said again that protein formed a layer around the lipid. But with the help of better electron microscopes, it was found that the proteins existed as globules throughout the cell membrane. The membrane is not thought to be as rigid as before, it is more fluid like. That's why we call it a fluid mosaic structure. Another interesting feature found was that the lipids were found to only have two fatty acid molecules as opposed to the three fatty acid structures that were normally found in most common types of lipids. Pores in the cell membrane was predicted early on because it was observed that certain molecules insoluble in lipids were able to pass through. These pores are surrounded by protein and are lined with hydrophilic groups; this makes them penetrable by water-soluble substances. It has been established that these pores are less than 1.0 nm in diameter, which makes them invisible even to a powerful electron microscope. The plasma membrane also contains polysaccharides. Most carbohydrates are attached to the protein. They are called glycoproteins. The pattern of these polysaccharides differs from one cell to another. These glycoproteins enable the cells to identify different types of cells, for example in the process of healing a wound. They are the key to signaling and recognition. A glycolipid is formed when the carbohydrates attaches itself with the lipid. The function is not fully understood, but in red blood cells they affect the blood group. They act as receptors, as in the case of disease, the pathogens first attach to the glycolipids. The picture above shows exactly how complex the plasma membrane is. The yellow spheres being the hydrophilic glycerol side and the two strands represent the fatty acids. The red globs are proteins and the blue one being a glycoprotein.

As the membrane is extremely small, a very strong electron microscope is needed to view it properly. The freeze fracture technique has been able to show details up more clearly. The plasma membrane forms when one of the vesicles of the Golgi apparatus empties its contents to the outside, the membrane lining of the vesicle fuses with the existing cell membrane. The main function of the plasma membrane is to, firstly bind all cell organelles and cytoplasm within a cell. The other function is to act as a controlled gate that allows certain substances in and certain ones out. The process is very complex involving all four techniques, osmosis, diffusion, facilitated diffusion and also active transport.

Firstly water passes in and out of the cell through the membrane by osmosis. Water will move from where there is higher water potential to an area of lower water potential. An example of this is in the case of root cells taking in water. Secondly substances move from an area of higher concentration to an area of lower concentration. Thirdly during facilitated diffusion substances that are even though at a higher concentration cannot pass through the pores of the membrane, therefore have to be helped by proteins that are present in the cell membrane, but all this at the expense of ATP energy. Lastly during active transport as substance that is at a lower concentration will go to an area of high concentration again by the help of proteins using ATP energy. An example of the last technique is in the human nerve cells. What happens is that sodium ions are pumped out the cell agains the concentration gradient, at the same time potassium being pumped in. The proteins of the cell membrane work just like and enzyme, having a specific shape to hold on to a specific substrate. It is also possible for certain gasses to diffuse in and out of cells. Another thing that happens is that the cell membrane also can perform endophagocytosis. This is when a substance is surrounded by a part of the membrane and is brought into it. Enzymes act upon the substances and the digested products diffuse into the cell membrane whereas undigested matter is extruded by exophaocytosis. Pinocytosis can also occur but in this case its is water that is brought in by the cell membrane.

Certain environmental variable will affect the effectiveness of the membranes. Firstly temperature plays a role. If temperatures are raised too high beyond a certain point, the plasma membrane ceases to function. This is because it contains proteins and proteins are denatured above 45 degrees Celsius. They then lose their shape and therefore function. The concentration of substances also affects the speed with which substances are transported in and out. A person can be in a state of paralysis if his core temperature is raised too high because one reason being that his nerve cells cannot transport the potassium and sodium correctly due to the cell membrane proteins losing their shape and function.

Organelles themselves have membranes around them. Basically this is to prevent their contents from spilling out into the cytoplasm. A key point is that some organelle, which have very important and very vigorous cell reactions are enclosed by not just one membrane but by two. The mitochondria for example are a double membrane bound organelle. The reaction is so vigorous that there is a possibility that if not contained, the reaction would burn up the whole cell. The chloroplast is another example, where again it is a double bound organelle. The nucleus is also another example of such organelles. Many confuse between the cell wall and cell membrane. A cell membrane is not readily visible whereas the cell wall is. During plasmolysis, it is the cell membrane pulled inwards and not the cell wall.

Therefore at the end we can now appreciate the complexity and importance of the plasma membrane. It is so well regulated that if even one small irregularity were made, the cell would die out. Every part of the membrane is important and contributes to maintaining the right balances necessary inside a cell and organelle. The cell membrane is the initial pathway a cell can take in what is needed and extrude what is not.