Polar substances have little ability to diffuse across cell membrane. This is because the main component in the cell membrane is the lipid and polar substances are not lipid soluble. But non polar substances cross easily. Substances that are polar have several other mechanisms to compensate for their deficiency. They include facilitated diffusion across channel, active transport which is energy consuming, receptor mechanism that allows the substance no access to the interior of the cell but could influence cellular behavior by proxy through attachment to its receptor. Finally enzymatic activation of the cell membrane could cause chemical reactions within the cell (e.g. second messenger systems) or the release of metabolites that can cross the cell membrane barrier easily.
Transport of protons across the inner and outer lamellae of inner mitochondrial membrane by the electron transport system (flavoprotein-cytochrome system), with return movement of protons down the protein gradient generating ATP, is shown
The following are the functions of cell membrane
1. It maintains the shape of the cytoplasm and the entire cell
2. It bears receptors which are points of attachment by molecules that cause reactions in the cytoplasm or influence the nuclear behavior.
3. It controls cell transport and determines what enters the cell and what is not allowed into the cell.
Carrier Mediated Transport
This is an active transport. The mechanism involves saturability. It follows Michaelis-Menten kinetics. It reaches zero order kinetics in conditions of saturation. It is also energy dependent. The carriers are usually protein- may be integral protein. Some drugs are absorbed by this mechanism especially in the kidneys and the liver.
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Carrier mediated transport
Microanatomy
Histochemistry
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School of Medicine