In this model, the membrane is comprised of a phospholipid layer and protein molecules, some of which have carbohydrate chains attached to them. The phospholipid layer has a hydrophilic phosphorus head and a hydrophobic lipid tail. The phosphorus head is on the outside with the lipid tails facing in towards each other. Water - soluble molecules are unable to pass through the membrane due to the hydrophobic nature of the lipids and their polar charges. As this means many essential molecules are unable to pass directly through the phospholipid layer, protein molecules present in the structure of the cell membrane enable such molecule to pass through.
The concentration of substances varies inside and outside the cell. Diffusion is the movement of such substances from an area of high concentration to a low concentration for even distribution. Consequently, the molecules are moving down its concentration gradient. The rate of diffusion is affected by other factors. A rise in temperature increase the kinetic energy that the molecules have, which means they have increased speed of movement resulting in faster diffusion. However temperature is not always changed and then other factor determine the rate. A greater surface area, eg, with microvilli, across which diffusion takes place increases rate, as does a greater difference in concentration between regions and a smaller length of diffusion path. A relation exist between these factors in the form of Fick’s Law, which states that the rate of diffusion is proportional to the surface area multiplied by the difference in concentration then divided by the length of diffusion path. Oxygen and carbon dioxide diffuse through the cell membrane by diffusion due to their small size and uncharged non – polar nature that enables them to pass freely through the small gaps that appear in the phospholipid layer.
While the phospholipid layer allows small molecules to pass through it, larger lipid – insoluble molecules are unable to do this. They travel across the membrane through carrier protein molecules present in the membrane. The lipid – insoluble molecules attach themselves to the carrier protein, which then changes shape and deposits the molecules inside the cell. This process is reversible and depends of the concentration gradient of the substance. As this is a form of diffusion, the cell does not have to provide energy for this reaction to take place.
Ions are unable to pass through the phospholipid layer as well. So that necessary ions, eg, Na and Cl, can be brought into the cell there are channel protein in the membrane as well. The hydrophilic water filled pores let ions and water - soluble molecules pass through. This always happen down as concentration gradient as this process is passive, ie, it require no energy to take place.
