How Does Glucose Move into a Cell?
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Cells
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Our bodies are composed of trillions of cells, each with its own function. A kidney cell is different from a brain cell, which differs from a skin cell or a cell inside your bone marrow. However, all cells need to take in nourishment and to discard waste products. Glucose provides energy that all your cells require, but before they can use it, the glucose must pass through the cell wall or across the internal cell barriers that separate cell compartments. This requires a complicated interaction of proteins, enzymes and genes.
Simple Sugars
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The simple sugars glucose, fructose and galactose are called monosaccharides. They have the same chemistry, but the molecules of each are arranged differently. Monosaccharides can be absorbed directly by the bloodstream. Before they can cross cell barriers, however, fructose and galactose are converted by the liver into glucose.
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Passive Transport
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Cells are protected by lipid bilayers which form semi-permeable membranes. A few simple molecules, such as carbon, oxygen and water, can pass through by diffusion, or osmosis. This takes place when a molecule moves from an area of higher concentration to an area of lower concentration. When the inside of a cell has a low concentration of oxygen, for example, oxygen molecules from the blood supply, where there is a higher concentration, pass through the cell walls. The cell expends no energy to accomplish this transfer, so it is known as "passive transport." When the cell uses energy to exchange a molecule across the membrane, it is called "active transport."
Facilitated Diffusion
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The glucose molecule is generally too large to simply be diffused through the cell's lipid bilayer the way oxygen can. "Facilitated diffusion" is the process that allows glucose to pass through the cell membrane. As in osmosis, facilitated diffusion allows glucose to move from a higher to a lower concentration area, but the glucose molecules must be bound to specific "facilitator" molecules. It is a form of passive transport requiring no energy, but it does require "transmembrane proteins."
Transmembrane Proteins
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The transmembrane proteins are embedded in the cell membrane. They create a water-filled pore or channel that glucose can pass through by diffusion. Depending on what the cell needs, the channels can be opened or closed. Once the channel opens, molecules pass through using diffusion, moving from the area of higher to the area of lower concentration. The channels, or gates, are selective, meaning each channel only permits a certain molecule or type of molecule to pass through.
Five Subgroups
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The five glucose transporter protein subgroups are GLUT1, GLUT2, GLUT3, GLUT4 and GLUT5. They are located in the cell's plasma membrane and have receptors on both the inside and outside. GLUT1 and GLUT3 are found in almost all cells. GLUT2 is used by the cells of the liver, intestinal epithelia and pancreatic beta cells. GLUT4 is found in skeletal and fat cells, and GLUT5 prefers fructose and glucose.
Active Transport
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In cases in which the glucose has to go from an area of lower concentration to an area of higher concentration, active membrane transport is used. This requires transmembrane proteins and energy, usually in the form of ATP (adenosine triphosphate), to actively push the molecules across the bilipid layer.
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