Diffusion is similar to this in the sense that it is also passive, meaning no energy required and it also takes place through the phospholipid bilayer. Diffusion is the movement of molecules from a higher concentration region to a lower concentrated region. This will continue until equilibrium is achieved. This is when the substance is evenly spread throughout the whole volume. Hydrophobic or small uncharged molecules diffuse across the cell membrane, passing between the lipid molecules as they move down the concentration gradient.
A different process of diffusion is called facilitated diffusion. Hydrophilic molecules and ions that are larger than carbon dioxide cannot diffuse through the bilayer, instead they cross the membrane with help of transport proteins. This is the process of facilitated diffusion. The transport protein involved is essential, it completely spans the membrane. It also has a binding site for the specific molecule or ion to be transported. After binding the molecule, the protein changes shape and carries the molecule across the membrane, where it is released. The protein then returns to its original shape, to wait for more molecules to transport. Glucose, sodium ions and chloride ions are just a few examples of molecules and ions that must efficiently get across the plasma membrane but to which the lipid bilayer of the membrane is virtually impermeable. Their transport must therefore be "facilitated" by proteins and provide an alternative route. This is sometimes referred to as ‘Passive Transport’. This is due to no energy being needed.
This is a diagram showing the facilitated diffusion process.
If a substance requires to move across a membrane against a concentration gradient (i.e. from lower concentration to higher concentration) then energy is required. This energy comes from respiration and is supplied by ATP – an energy transfer molecule. Special proteins within the cell membrane act as specific protein ‘carriers’. The carrier proteins bind to specific molecules and transport them into the cell where they are released. Active transport is a process which takes place in the internal lining of the small intestine. Examples of substances that can be actively transported across membranes are sodium ions and glucose.
The larger molecules or particles still need to be transported across cell membranes sometimes and they are able to achieve this by exocytosis and endocytosis. This relies on the fluid nature of the membrane.
Exocytosis
This is the release of substances from the cell as vesicles (small membrane-bound sacs) fusing with the plasma membrane. It restores the normal amount of plasma membrane. Any molecules dissolved in the fluid contents of these vesicles are released into the extracellular fluid - this is called secretion. Exocytosis is used for bulk transport out of the cell.
Endocytosis
Endocytosis is the reverse process of exocytosis. Substances are taken into the cell by the creation of a vesicle. The cells absorb material (molecules such as proteins) from the outside by engulfing it with their cell membrane. It is used by all cells of the body because most substances important to them are large polar molecules, meaning they cannot pass through the hydrophobic plasma membrane or cell membrane. Endocytosis is used for bulk transport into the cell. The diagram below shows endocytosis and exocytosis happening.
In conclusion, these six processes have enabled our body to function correctly by cells being able to control transport across their surface membrane. These processes are very important as they get rid of the waste carbon dioxide as when it reaches the blood vessels around the airways of the lungs, it passes out of the blood and into the air. Other reasons why this transport is essential is to maintain a suitable pH and ionic concentration within the cell for enzyme activity and obtain certain food supplies for energy and raw materials. Finally, it is vital to excrete toxic substances and secrete useful substances. The processes I have discussed allow this to occur.
