The Glucose Transporter: An Example of Facilitated Diffusion
Glucose is the body’s primary source of direct energy, and most mammalian cells contain a membrane protein that facilitates the diffusion of glucose from thebloodstream into the cell (as depicted in Figure 4.43).
A gradient favoring the continued diffusion of glucose into the cell is maintained by phosphorylating the sugar after it enters the cytoplasm, thus lowering the intracellular glucose
concentration.
Humans have at least five related proteins (isoforms) that act as facilitative glucose transporters.
These isoforms, termed GLUT1 to GLUT5, are distinguished by the tissues in which they are located, as well as their kinetic and regulatory characteristics.
Insulin is a hormone produced by endocrine cells of the pancreas that plays a key role in maintaining proper blood sugar levels.
An increase in blood glucose levels triggers the secretion of insulin, which stimulates the uptake of glucose into various target cells, most notably skeletal muscle and fat cells (adipocytes).
Insulin-responsive cells share a common isoform of the facilitative glucose transporter,
specifically GLUT4.
When insulin levels are low, these cells contain relatively few glucose transporters on
their surface.
Instead, the transporters are present within the membranes of cytoplasmic vesicles.
When insulin levels rise in response to elevated blood glucose levels, the hormone acts on target cells to stimulate the translocation of the vesicles from the cytoplasm to the cell surface, where the transporters become incorporated into the plasma membrane and transport glucose into the cell (see Figure 15.21).