Molecules such as glucose are transported across plasma membranes by special protein carriers. Carrier-mediated transport in which the net movement is down a concentration gradient, and which is therefore passive, is called facilitated diffusion. Carrier-mediated transport that occurs against a concentration gradient, and which therefore requires metabolic energy, is called active transport.
In order to sustain metabolism, cells must take up glucose, amino acids, and other organic molecules from the extracellular environment. Molecules such as these, however, are too large and polar to pass through the lipid barrier of the plasma membrane by a process of simple diffusion. The transport of such molecules is mediated by protein carriers within the membrane. Although such carriers cannot be directly observed, their presence has been inferred by the observation that this transport has characteristics in common with enzyme activity. These characteristics include (1) specificity, (2) competition, and (3) saturation.
Like enzyme proteins, carrier proteins interact only with specific molecules. Glucose carriers, for example, can interact only with glucose and not with closely related monosaccharides.
Test Yourself Before You Continue
1. Explain what is meant by simple diffusion and list the factors that influence the diffusion rate.
2. Define the terms osmosis, osmolality, and osmotic pressure, and state the conditions that are needed for osmosis to occur.
4. Explain how the body detects changes in the osmolality of plasma and describe the regulatory mechanisms by which a proper range of plasma osmolality is maintained.
Clinical Investigation Clues
Remember that Jessica is constantly thirsty, despite drinking large amounts of water.
■ What is stimulating Jessica's sense of thirst?
■ How is this related to the glucose in her urine and her frequent urination?
Fox: Human Physiology, 6. Interactions Between Text © The McGraw-Hill
Eighth Edition Cells and the Extracellular Companies, 2003
Interactions Between Cells and the Extracellular Environment 135
As a further example of specificity, particular carriers for amino acids transport some types of amino acids but not others. Two amino acids that are transported by the same carrier compete with each other, so that the rate of transport for each is lower when they are present together than it would be if each were present alone (fig. 6.13).
As the concentration of a transported molecule is increased, its rate of transport will also be increased—but only up to a maximum. Beyond this rate, called the transport maximum (Tm), further increases in concentration do not further increase the transport rate. This indicates that the carriers have become saturated (fig. 6.13).
As an example of saturation, imagine a bus stop that is serviced once an hour by a bus that can hold a maximum of forty people (its "transport maximum"). If there are ten people waiting at the bus stop, ten will be transported each hour. If twenty people are waiting, twenty will be transported each hour. This linear relationship will hold up to a maximum of forty people; if there are eighty people at the bus stop, the transport rate will still be forty per hour.
The kidneys transport a number of molecules from the blood filtrate (which will become urine) back into the blood. Glucose, for example, is normally completely reabsorbed so that urine is normally free of glucose. If the glucose concentration of the blood and filtrate is too high (a condition called hyperglycemia), however, the transport maximum will be exceeded. In this case, glucose will be found in the urine (a condition called glycosuria). This may result from the consumption of too much sugar or from inadequate action of the hormone insulin in the disease diabetes mellitus.
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