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- Carrier protein

Inside of cell

Lower concentration

■ Figure 6.14 A model of the facilitated diffusion of glucose. A

carrier—with characteristics of specificity and saturation—is required for this transport, which occurs from the blood into cells such as muscle, liver, and fat cells. This is passive transport because the net movement is to the region of lower concentrations, and ATP is not required.

Fox: Human Physiology, I 6. Interactions Between I Text I I © The McGraw-Hill

Eighth Edition Cells and the Extracellular Companies, 2003

Environment

Chapter Six

Carrier protein

Carrier protein

Stimulus

Carriers are intracellular

Vesicle

Stimulus

Carriers are intracellular

Vesicle

Carriers are inserted into plasma (cell) membrane

Carriers are inserted into plasma (cell) membrane

■ Figure 6.15 The insertion of carrier proteins into the plasma (cell) membrane. In the unstimulated state, carrier proteins (such as those for glucose) may be located in the membrane of intracellular vesicles. In response to stimulation, the vesicle fuses with the plasma membrane and the carriers are thereby inserted into the membrane.

In unstimulated muscles, the GLUT4 proteins are within the membrane enclosing cytoplasmic vesicles. Exercise—and stimulation by insulin—causes these vesicles to fuse with the plasma membrane. This process is similar to exocytosis (chapter 3; also see fig. 6.20), except that no cellular product is secreted. Instead, the transport carriers are inserted into the plasma membrane (fig. 6.15). During exercise and insulin stimulation, therefore, more glucose is able to enter the skeletal muscle cells from the blood plasma.

Transport of glucose by GLUT carriers is a form of passive transport, where glucose is always transported down its concentration gradient. However, in certain cases (such as the epithelial cells of the kidney tubules and small intestine), glucose is transported against its concentration gradient by a different kind of carrier, one that is dependent on simultaneous transport of Na+. Since this is a type of active transport, it will be described shortly in a different section.

The rate of the facilitated diffusion of glucose into tissue cells depends directly on the plasma glucose concentration. When the plasma glucose concentration is abnormally low—a condition called hypoglycemia— the rate of transport of glucose into brain cells may be too slow for the metabolic needs of the brain. Severe hypoglycemia, as may be produced in a diabetic person by an overdose of insulin, can thus result in loss of consciousness or even death.

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