Physiological importance of some transport systems

Transport into the cell is vitally important in the physiology of the cell. Some examples are given. 

There are three major fuels for cells glucose, fatty acids and glutamine (Chapter 9). Transporter molecules are present in the membrane for each of these fuels. 

In skeletal muscle, glucose transport is non-equilibrium, so that an increase in activity of the transporter increases glucose utilisation. Factors that increase the activity of the transporter (e.g. the number of transporter molecules) in the membrane are insulin and sustained physical activity. In contrast, the hormone cortisol decreases the number of transporters in the membrane. This decreases glucose uptake and is one of the effects of cortisol that helps to maintain the normal blood glucose level (Chapter 12). 

Fatty acids Despite the fact that fatty acids are lipid soluble, so that they will diffuse across membranes without a transporter, one is present in the plasma membrane to speed up entry into the cells, so that it is sufficient to meet the demand for fatty acid oxidation. Triacylglycerol transport into cells also depends on the fatty acid transporter. Since it is too large to be transported per se, it is hydrolysed within the lumen of the capillaries in these tissues and the resultant fatty acids are taken up by the local cells via the fatty acid transporter (Chapter 7). Hence the fatty acid transporter molecule is essential for the uptake of triacylglycerol. 

Several transporters that are specific for transport of groups of amino acids are present in aU tissues. They are of particular physiological importance in some tissues. 

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