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The Plasma Membrane

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The Plasma Membrane By Dupinder Saggu 12J Every living thing is made of many tiny cells. The cell has an outer barrier called the 'Cell Membrane' but is more commonly known as the 'Plasma membrane'. The plasma membrane constitutes the external surface of the cell and is in contact with many other cells. It is about approximately 6-10nm thick, this is too thin to be seen with the naked eye or even a light microscope, to see it you need the aid of an electron microscope. Even with the aid of an electron microscope you still would not be able to see the detail of the plasma membrane because its too thin and that is the reason why most of the knowledge about the plasma membrane are just theories. The plasma membrane has three major functions, the uptake of nutrients, sensing external stimuli (changing response to the environment) and cellulose synthesis. Almost all of the mass of biological membranes consist of polar lipids and proteins, the lipids are arranged in a bilayer shell around the cell. The plasma membrane is partially permeable so therefore some substances can cross more easily than others. A cell must also excrete waste products made in the cell through the cell's plasma membrane. There are many ways in which substances pass through the plasma membrane, these are by diffusion, facilitated diffusion, osmosis, active transport, endocytosis and exocytosis. In prokaryotes and plants, the membrane in the inner layer of protection surrounded by a rigid cell wall, these membranes also regulate the passage of nutrients in and the waste products out. ...read more.


A normal lipid molecule has three fatty acid chains attached to a glycerol. Below is a diagram (Figure 3) on how a phospholipid molecule looks like. Figure 3 Saturated Fatty Acid Chains Glycerol (Hydrophobic) Phosphoric acid (Hydrophilic) Every phospholipid is made up of a 'head' (phosphate), which has a negative charge to it and therefore will mix with water but not with fats, this is called hydrophilic and therefore has a 'water loving' property. A phospholipid is also made up of a 'tail', that will mix with fats but not with water, these are called hydrophobic and therefore has a 'water hating' property. That means in the phospholipids bilayer of a membrane, the hydrophilic heads are always on the outside and the hydrophobic tails are always on the inside. Therefore the arrangement of the phospholipids produces a barrier against water and water soluble substances, the reason for this membrane structure is because the inside and outside of the cell are mostly water and therefore the membrane needs to be arranged in a suitable way for the hydrophobic molecules to be away from the water. So a phospholipid bilayer acts as a barrier between two aqueous environments. The next set of diagrams show the arrangement of the phospholipids with water (figure 4, 5 and 6). Figure 4 Biomolecular film Phospholipids Hydrophobic Monolayer Hydrophilic Water H2O Figure 5 Water H2O Hydrophilic Phospholipids Monolayer Hydrophobic In Figure 4 and 5, the phospholipids monolayer is on the outside of the water, and you can see that the hydrophilic part (water loving head) ...read more.


Exocytosis is the opposite, the membrane is made in the cytoplasm and substances are carried in there (from the cell) till the vesicle gets to the edge of the plasma membrane and the substance is released out of the cell. So for both endocytosis and exocytosis membranes are needed to let substances in and out of the cell. I got all the information about the transports from Class notes and books published by Albert Et Al in 1994, McGraw-Hill in 2001 and Glenn and Susan Toole in 1987 and also three websites http://www.Cell-biology.org.html , http://gwis2.circ.gwu.edu/atkins/Neuroweb/plasmalemma.html and http://www.cytochemistry.net/cell.html There are a few factors that affect the rate of diffusion across the plasma membrane. Firstly the concentration gradient affects the rate of diffusion and so does the thickness of the plasma membrane, if it is think it would take longer and if it is thin it would be quicker, therefore the thinner the quicker. The temperature also affects the rate of diffusion because if the particles are hotter they would have more energy and therefore would be able to diffuse faster. The calculation to work out the rate at which a substance will diffuse we can use 'Fick's law'. Rate of Diffusion = Surface area of membrane X Difference in concentration Length of diffusion path As would be expected fro Fick's law, cellular diffusion is a very slow process unless there is a large concentration gradient over a short distance. Tissues such as those in the lungs and small intestines are especially adapted to maximum rate of diffusion by maintaining a steep concentration gradient, having a high surface area to volume ratio and being thin (minimizing the distance over which the diffusion takes place). ...read more.

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