*The particles actually can diffuse both ways but tend to move to a low concentrated region.
In some cases, molecules are not soluble in lipids and because of this they cannot diffuse through a membrane. When this happens, facilitated diffusion takes place. This is an ‘easier’ way for the particles to get through the phospholipid bilyer.
Between the phospholipids are carrier proteins that ‘carry’ the insoluble molecule over to the other side instead of it diffusing through the lipids. They are the exact shape of molecules to allow them to fit inside and travel through.
The rate of diffusion depends on the number of carrier proteins there are present and that are open to ‘carry’ the molecules.
The way by which water molecules are diffused is called Osmosis (through a semi-permeable membrane). A semi-permeable membrane allows the molecules of water to pass through, but does not allow other molecules to be diffused through it. Osmosis enables water molecules to diffuse from a region of high water potential to a region of low water potential. Water potential is the pressure that water molecules exert when they collide into the membrane. If water potential is high, it means there is a high volume of water molecules in that region. However if the water potential is low it would mean that there are not many water molecules in that particular area. Water potential is measured in Pascals (Pa). When solute is added to the solution, the water potential decreases and so the number will be negative. Pure water has a water potential of 0Pa.
A method that is very similar to facilitated diffusion, but uses energy and goes against a concentration gradient (from low concentration to high concentration), is called active transport. The way by which molecules are transported is again by carrier proteins within the bilayer of the membrane. Another way that this is different to facilitated diffusion is that the carriers only work ‘one way’ and so molecules cannot diffuse in both directions.
Finally, I will explain Endocytosis and Exocytosis.
In the other forms of diffusion, osmosis and active transport, molecules are diffused individually. In endo/exo-cytosis larger volumes of material are transported in and out of the cell. Also, the material can be a liquid or a solid.
In order for these materials to be transported, the cell makes vesicles, usually in the Golgi apparatus, to carry any products in or out of the cell membrane. A vesicle is like a sac that holds the material.
The formation and movement of the vesicles is called cytosis. Therefore, exocytosis is the outward movement of products and endocytosis is the inward movement.
There are two types of endocytosis: Phagocytosis – means ‘cell eating’. It is the taking up of solid objects, Pinocytosis – means ‘cell drinking’. It is the taking up of liquids.
During exocytosis, the substance to be removed from the cell is contained in a vesicle that is usually made in the Golgi body. The Golgi releases the vesicle and it moves towards the surface of the membrane where it fuses with it. It is then released outside of the membrane.
In conclusion, I can say that there are many ways in which molecules can enter and exit a cell through a plasma membrane, some of which also involve other organelles such as the Golgi body.
