What Role Do Cell Membranes Play In Cell Transport

Discuss the role of proteins in cell membrane transport

Cell membranes are the barriers and gatekeepers of the cell; they provide overall structure, control the exchange of molecules between the extra and intercellular environments and are the site for many important processes. Cell membranes consist of a phospholipid bilayer where each repeating unit is comprised of two fatty acid tails bound to a phosphate head including a polar group attached to the glycerol region. Due to the amphipathic nature of the phospholipid, when in aqueous solution the hydrophilic heads are drawn towards the water causing a ring to form, as the tail is hydrophobic they point inwards within the layer where no water is present. As aforementioned, a primary role of

One example of this is the Carrier protein, which can be incorporated into the path of passive or active diffusion of substances. Carrier proteins exist in two states, one of which its binding sites are open to the extra cellular environment, as a cause of their folding and structure the binding sites within the protein are specific to certain solutes. As they are complementary the solute binds to the sites on the protein. The protein will then undergo a reversible conformational change; now the solute has been moved across the phospholipid bilayer by assistance of the carrier protein. In this particular example the flow of solute into the cell is considered passive, the reason for this is that no hydrolysis of ATP is required. The conformational changes in the carrier protein occur randomly and as solute is able to bind on either side of the membrane the net movement of solute into the cell is achieved due to a higher concentration of solute on the outside compared with the inside of the cell. Therefore a higher proportion of solute binds when the protein is of the conformation where its sites are exposed to the extra cellular space and it comes closer to reaching saturation compared to its other conformation.[2] The diffusion via carrier proteins described above can be split up into three separate classes, each slightly differing in the minutia of the process. Firstly the passive form mentioned is known as Uniport, where the carrier protein facilitates transport of a single type of solute. If two specific solutes are being cotransported unidirectionally then it is known as symport, usually the free energy provided by the flux of one solute down its respective gradient is used to drive the uphill transport of the other molecule. Lastly if the molecules are cotransported but travelling in different directions across the membrane it is known as antiport.[3]