• Join over 1.2 million students every month
  • Accelerate your learning by 29%
  • Unlimited access from just £6.99 per month

The Plasma Membrane

Extracts from this document...


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.

The above preview is unformatted text

This student written piece of work is one of many that can be found in our AS and A Level Molecules & Cells section.

Found what you're looking for?

  • Start learning 29% faster today
  • 150,000+ documents available
  • Just £6.99 a month

Not the one? Search for your essay title...
  • Join over 1.2 million students every month
  • Accelerate your learning by 29%
  • Unlimited access from just £6.99 per month

See related essaysSee related essays

Related AS and A Level Molecules & Cells essays

  1. Marked by a teacher

    An investigation to examine the effects of temperature on membrane stability in beetroot, by ...

    3 star(s)

    All the samples should come from the same beetroot. If some samples came from one beetroot and others came from another, then the investigation would become invalid. This is because the cell membranes from one beetroot to another will differ in their permeability. Even within the same beetroot, the permeability of the membranes can differ.

  2. A Level Biology revision notes

    gradient o When pressure is equal on both sites net flow ceases (equilibrium) o The pressure is said to be hydrostatic (water-stopping) Water Potential * Measurement of ability or tendency of water molecules to move * Water potential of distilled water is 0, other solutions have a negative water potential * Hypotonic: solution with a lower conc.

  1. prove Boyles Law

    From my results, I can safely say that Boyle��s Law is correct. CRITICISMS OF EXPERIMENT There are criticisms of our experiment but they do not affect the general outcome of the experiment. Firstly, the graduations on the pressure gauge were too big to get an accurate result to two decimal places.

  2. The functions of proteins in cell membranes. ...

    Transport proteins are a sub-category of integral proteins. At each polar region, the protein has a binding site which attracts only specific molecules. When a molecule binds to the carrier protein, the transport protein alters its shape (moving the binding site to the opposite polar region) to allow the substance to travel freely through the membrane.

  1. Transport Across Plasma Membranes.

    For example, the biconcave disc of a red blood cell gives the cell a much larger surface area than if it had a spherical shape. This larger surface area means that the maximum amount of oxygen can diffuse into the cell.

  2. Transport across Plasma Membranes

    > The size of the molecules or ions. Large molecules require more energy to get them moving than small ones do, so substances with large molecules tend to diffuse more slowly than ones with small molecules. Water molecules can diffuse rapidly across the phospholipid bilayer because they are small enough.

  1. How would the power input to a solar cell change if it was not ...

    create an internal electric field, which allows electrons to move down the surface of the cell. At 0�C the average efficiency of a solar cell is 24%, however at standard room temperature (12�C) the efficiency is only 12%. Something contributing to this relatively low efficiency is the fact that there

  2. Technical documentation - Design specification.

    If its unchecked then cell F17 is left blank. Full Weeks Work Bonus Cell E18 returns TRUE if check box is checked, If unchecked = FALSE Cell F18 contains a similar formula to cell F17 but returns the value of FullWeekBonus (also found in the Rates sheet)

  • Over 160,000 pieces
    of student written work
  • Annotated by
    experienced teachers
  • Ideas and feedback to
    improve your own work