The Plasma Membrane

Integral proteins, also known as transmembrane proteins, cross the phospholipid bilayer several times. These proteins have polar regions connected by non-polar segments. Some transmembrane proteins form membranes, whereas others are associated with the transmission of chemical signals across the plasma membrane (extracellular and intercellular protein filaments to the plasma membrane). Peripheral membrane proteins are not amphipathic and do not associate with non-polar regions of the phospholipids like the transmembrane proteins. They are located on the surface of the membrane where there are bound to be Polar regions of the integral membrane proteins. I got this information on proteins from a book by Bill Indge, Martin Rowland and Margaret Baker In 2000, a website http://www.Cell-biology.org.html and also from another website http://www.cytochemistry.net/cell.html

A second group of lipids included in the phospholipid bilayer is a class of non-polar sterols, such as cholesterol. The main functions of cholesterol are that they immobilize the first few hydrocarbon groups of the phospholipid molecules, making the phospholipid bilayer less deformable and decreases permeability. Therefore with cholesterol a cell would need a cell wall. Another main function of cholesterol is that it prevents crystallization of hydrocarbons and phase shifts in the membrane. The plasma membrane contains a fair amount of cholesterol whereas the intracellular membranes contain very little cholesterol. These are only slightly soluble in water in pure form, but when complexed with protein molecules, their solubility increases. They also associate tightly with the lipid arms of the phospholipids, this association increases the viscosity and the strength of the membrane. Therefore without the cholesterol the membrane would not hold together very well because without the cholesterol the plasma membrane would have more fluidity than necessary and may even break or tare open. So cholesterol reduces fluidity and keeps the cell (membrane) together. Cholesterol is slightly amphipathic because of a single polar hydroxyl group on its non-polar ring structure. Cholesterol forms organized clusters that function in pinching off the portions of the plasma membrane to form vesicles that deliver their contents to various intracellular organelles. I got the information on Cholesterol from a book published by Albert Et Al in 1994, McGraw-Hill in 2001 and Glenn and Susan Toole in 1987 and also from a website http://www.Cell-biology.org.html

Small amounts of carbohydrates are found on the extracellular surface of the plasma membrane, which are linked to membrane lipids and proteins. These surface carbohydrates play an important role in enabling cells to identify and interact with each other.

A membrane is made up of a phospholipids bilayer, this means it is made up of two layers of molecules called phospholipids. Phopholipids are like lipids but with phosphoric acid joined on to a glycerol. There are two fatty acid chains and a phosphoric acid joined onto a glycerol, but the phosphate part looks like a 'head' and the fatty acid chains look like 'tails'. 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) is against the water (H2O) and the hydrophobic part (water hating tail) is away from the water. But this only happens when the phospholipids monolayer is on the outside of the water, but when it is in water it has a different arrangement. It is arranged in a circle shape surrounded by water trying to Barrier the hydrophobic tail away from it. Below is a diagram of how a phospholipids monolayer would be arranged under water (figure 6)

Figure 6

Hydrophilic

Space

Hydrophobic (away

from water)

Water

(H2O)

Because the cell is surrounded by water, it needs to be a phospholipid bilayer rather than a monolayer so that the hydrophobic tail of the phospholipid is away from water. The phospholipid bilayer allows small molecules to cross the membrane and enter the cell, for example oxygen (O2) is small enough to squeeze through the membrane, but larger molecules need certain transportation. I received the above information on phospholipids from various sources, including class notes. I also got my information from a book published by Albert Et Al in 1994, McGraw-Hill in 2001 and Glenn and Susan Toole in 1987 and also from a website http://www.Cell-biology.org.html

Diffusion is the net movement of molecules of molecules or ions from a region of high concentration to a region of low concentration, this can also be described as a movement of molecules or ions down a concentration gradient. A concentration gradient occurs when one region has more of a molecule than its neighbouring region. So therefore in diffusion the concentration gradient moves down. The molecules would have to be relatively small to diffuse through the plasma membrane, for example oxygen would be able to diffuse through a plasma membrane. Figure 7 shows the rate of diffusion through different sized cells.

Figure 7

Diffusion

Tissue A

Diffusion

Tissue B

Diffusion

Extension of the plasma membrane

called Microvilli

Tissue C

The Cells in the Figure 7 are all the same type of tissues, but with different thicknesses. Diffusion across Tissue A would be quicker than the diffusion in Tissue B, the reason for this is that Tissue A is thinner than Tissue B. Although Tissue B and C are the same thickness, Tissue C has an extension called microvillis, basically Tissue B has a larger surface area and therefore the rate of diffusion would be quicker because it would be more likely for the molecules to be diffused because there is more surface to diffuse through. The rate of diffusion is affected by a number of factors, one of them are the differences in concentration, the surface area across which diffusion occurs and the thickness of the surface. Oxygen and water can diffuse through the plasma membrane, because the water is an uncharged particle and would not be attracted to any part of the phospholipids.

Another type of transportation is called 'Osmosis', this is when water (H2O) moves from a region of high concentration to a region of low concentration through a semi-permeable membrane. That means if there is higher concentration of water outside the cell, osmosis would take place and move through the plasma membrane and start to fill the cell until it eventually bursts. Plant cells are different because they have a cell wall and if water was to be absorbed into the cell, the cell would get bigger, fill and become turgid, but it would stay together because of the cell wall. The water molecules can move through a phospholipid bilayer because the phospholipids are always moving and have tiny gaps between them which the water molecules have to squeeze through. Intrinsic transmembrane proteins have a function to channel molecules or ions, therefore they allow water molecules to move through them, but not just water molecules but bigger molecules too, because the channels that transmembrane proteins make allow larger molecules to move through them as well.

Active transport is the movement of molecules from a region of low concentration to a region of high concentration through a semi-permeable membrane. But for active transport to take place energy is needed called ATP which is produced by the mitochondria in a process called respiration. So therefore the cell needs energy in the form of ATP to make substances cross their membrane. Therefore the rate of active transport depends on the rate of cell respiration, it is also slowed down by poison such as cyanide.

The reaction of Active Transport can be written as an equation.

ATPase

ATP ADP + Pi + Energy

Active transport involves carrier proteins in the phospholipid bilayer of a membrane. The carrier molecule can only change shape following an input of energy from the cell. Below is a diagram (Figure 8) of how Active transport works.

Figure 8

) Energy from cell 2) Transported substance binds

required to achieve binds with carrier protein.

carrier protein.

ATP ADP + Pi

3) Carrier protein changes shape, releasing transported substance into the cell.

Facilitated diffusion is the passive movement of molecules down a concentration gradient but it envolves carrier proteins in the cell membrane. Facilitated diffusion is like active transport, but it requires no energy. Instead of energy it depends on the concentration gradient. The carrier proteins bind to a certain type of diffusing molecule. When they have done so, they release the diffusing molecule on the other side of the membrane by changing shape.

Passive and active transport only move a few molecules or ions at a time. Endocytosis and exocytosis move substances in large amounts like a bulk. They do this by inserting the substance in small membrane bound sacs called vesicles. In endocytosis part of the plasma membrane moves sinks into the cell and then breaks off and becomes a little sac containing substances from the outside of the cell. When the vesicles bring in a solid it is called 'phagocytosis' and when it brings a fluid into the cell it is called 'pinocytosis'. 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).

BIBLIOGRAPHY

AS AQA BIOLOGY Specification A Bill Indge, Martin Rowland and Margaret Baker

HUMAN PHYSIOLOGY McGraw-Hill

MOLECULAR BIOLOGY OF THE CELL Albert Et Al

TEACHERS NOTES Mrs Welsh

UNDERSTANDING BIOLOGY (for Advanced Levels) Glenn and Susan Toole

http://www.cytochemistry.net/cell.html

http://www.Cell-biology.org.html

http://gwis2.circ.gwu.edu/atkins/Neuroweb/plasmalemma.html