Transport of monosaccharides over tissue membranes

Blood sugar (glucose) is the main energy source for the brain, red blood cells and many of our
muscles. Yet, blood glucose levels are more or less constant in spite of the very rapid and large
turnover. This constancy is essential for normal body function. Stabile sugar levels are the result
of precisely controlled chemical and hormonal feed-forward and feed-back information loops. Let
us look at the basic metabolism that underlies this system and try to answer some "simple"
questions. How does metabolism of sugars start? Are there differences in the metabolism of the
various sugars found in the diet? How do we start up storage of glucose after a meal? How do
we stabilize blood glucose levels between meals? Are the differences in metabolism of common
sugars in various organs?
The initial step in metabolism of sugars is their transport over the cell s outer membrane. "Small"
sugars (glucose, fructose and galactose) cannot cross cell membranes without "carriers". Sugar
carriers are proteins embedded in the cell s outer membrane that provide transport systems for
monosaccharides. The glucose transport protein family (called GLUT) is discussed elsewhere in
MedBio.