As a slide presentation from San Diego State explains, glucose has to be taken in via a process known as facilitated diffusion. Facilitated diffusion is the way in which cells are able to import larger molecules through the cell membrane. The cell membrane is a barrier which surrounds all cells and only allows small and uncharged molecules to pass through freely. Larger molecules, such as glucose, require special transporting proteins. The transporters for glucose bind to glucose molecules and then facilitate their transfer into the cell. There are five different types of glucose transporters, named GLUT 1-5.
Muscle and Fat Tissue
As the hypertext library at Colorado State explains, muscle and fat (adipose) tissue utilize the GLUT4 transporter. This glucose transporter is unique in that it is not always present at the surface of the cells. GLUT4 transporters are kept on the inside of the cell until insulin binds to the muscle and/or adipose cell. This causes the cell to shuttle its GLUT4 transporters to the cell membrane where they can then be active. As a result, muscle and adipose cells only move glucose into the cell when insulin is present. Insulin is secreted by cells in the pancreas in response to heightened levels of blood glucose.
Other tissues throughout the body use the other four kinds of glucose transporters (GLUT 1-3 and GLUT 5). These glucose transporters allow glucose to be imported by neurons, cells in the liver and kidneys, as well as red blood cells. These kinds of glucose transporters ensure that cells that need a steady supply of glucose (because they are essential for life) are always able to maintain glucose levels within the cell.