There are two types of proteins embedded into the phospholipids bilayer in different ways: some are extrinsic proteins, which occur on the surface of the bilayer and do not extend completely across the membrane, and there are intrinsic proteins which span the whole bilayer from one side to another and may also form hydrophilic channels through the membrane.
The main job of the intrinsic protein is to act as carriers to transport water-soluble material across the membrane; they are known as ‘channels’ in the cell surface membrane as the membrane itself maintains an electrochemical gradient. In facilitated diffusion, intrinsic proteins (or carrier proteins) are used to transport glucose. A transport protein (intrinsic protein) has a specific tertiary structure and will only transport particular substances across a membrane; in other words: it binds with a specific type of cell only. When the intrinsic protein binds with the glucose molecule, the transport protein changes shape and it causes the molecule to be taken through the protein to the other side of the membrane. The glucose is detached from the protein and the protein’s original shape is formed again; the glucose is also phosphorlated, which keeps the concentration of free glucose low, maintaining that ‘diffusion gradient’
Facilitated diffusion is a passive process. Both however, are the “net movement of molecules or ions from a region of high concentration to a region of low concentration down a concentration gradient”. Both do not require an energy source such as ATP, but reply on kinetic energy to diffuse molecules. In facilitated diffusion, the protein channels allow water-soluble molecules or ions to pass through and have a ‘gate’ for getting the solutes across.
Active process is the movement of substances, against a concentration gradient, from a region of higher concentration to a region of lower concentration. Active transport involves carrier proteins, and there are uniport carriers, symport carriers, and antiport carriers, which show the direction in which substances move. Active transport requires ATP (energy from cell metabolism) to move molecules. In active transport, the carrier proteins span the cell surface membrane and accept the molecules or ions to be transported on one side of it. The molecules or ions bind to receptors on the channels of the carrier protein. On the inside of the cell, the ATP binds to the protein, causing it to split into ADP and a phosphate molecule. As a result, the protein molecule changes shape and allows the molecules or ions on the outside of the cell to bind to the protein. Dephosphorylation is triggered and the shape of the protein is restored. This general process can be shown in the sodium/potassium pump where the exact same process occurs, however this pump controls cell volume because potassium ions leave the cell and because the membrane is more permeable to potassium, the tendency of water to enter the cell by osmosis is reduced.
There is also ‘co-transport’, in which the active transport of one substance indirectly drives the movement of another substance against a concentration gradient. The proton pump actively drives protons across the membrane, using energy from ATP; the high concentration formed of protons then diffuses back across the membrane via the symport carrier protein.
Osmosis is the movement of water from a region of high water potential to a region of low water potential through a partially permeable membrane down a water potential gradient. Water is small enough to pass between the phospholipid molecules, whereas ions do not easily pass through the hydrophobic part of the membrane. Water particles, in solutions containing very few solute molecules, can move freely and have a high water potential gradient. Therefore, the more solute is added, the lower its water potential will be.
A type of endocytosis includes phagocytic veciles which engulf solid material from the outside of the cell and bring it into the cytoplasm, forming a food vesicle, whereas a pinocytic vesicle ingests liquid material from the outside of the cell and brings it into the cytoplasm. Macromolecules and larger parcticles, such as bacteria are taken into cells by endocytosis. A bacterium is engulfed in a pocket of plasma membrane which then breaks off and forms a vacuole with the bacterium inside. The small pocket appeared in the plasma membrane is then pinched off to form a vesicle inside the cell.
Pinocytosis is similar to phagocytosis, but pinocytosis takes in small droplets of the external solution, forming vesicles. Receptor-mediated endocytosis is a kind of pinocytosis for specific molecules. In receptor-mediated endocytosis, particular molecules to be transported bind to clusters of protein receptors on the outside of the plasma membrane.
Large molecules such as proteins are transported out of cells by exocytosis.. The part of the membrane containing the cluster of receptors bulges inwards and pinches off, forming a vesicle containing the particular molecule. Vesicles containing proteins for export break away from the Golgi apparatus. Vesicles migrate to the plasma membrane and the vesicle membrane fuses with the plasma membrane. This process is known as “exocytosis”.
