Monosaccharides are taken up into the enterocyte. Glucose and galactose are taken up by the sodium-glucose cotransporter 1 (SGLT1, active carrier transport). The cotransporter part of the name of this transporter means that it also transports sodium along with glucose or galactose. Fructose is taken up by facilitated diffusion through glucose transporter (GLUT) 5. There are 12 glucose transporters that are named GLUT 1-12, and all use facilitated diffusion to transport monosaccharides. The different GLUTs have different functions and are expressed at high levels in different tissues. Thus, the intestine might be high in GLUT5, but not in GLUT12. Moving back to monosaccharides, inside the enterocyte, all three are then transported out of the enterocyte into the capillary (absorbed) through GLUT2 as shown below1.
Inside of each villus there are capillaries and lacteals as shown below. Capillaries are the smallest blood vessels in the body, lacteals are also small vessels but are part of the lymphatic system, as will be described further in a later subsection.
The following video does a nice job of illustrating capillaries and lacteal and provides some basic details on uptake and absorption.
The capillaries in the small intestine join to the portal vein, which transports monosaccharides directly to the liver. The figure below shows the portal vein and all the smaller vessels from the stomach, small intestine, and large intestine that feed into it.
The video below shows how hepatic portal circulation flows.
At the liver, galactose and fructose are completely taken up through GLUT 2 and GLUT5, respectively, while only 30-40% of glucose is taken up through GLUT2. After the monosaccharides are taken up, they are phosphorylated by their respective kinase enzymes forming galactose-1-phosphate, fructose-1-phosphate, and glucose-6-phosphate as shown below.
Kinase enzymes normally phosphorylate substrates. Phosphorylation of the monosaccharides is important for maintaining the concentration gradient (by keeping nonphosphorylated monosaccharide levels within hepatocytes low) needed for facilitated diffusion through the GLUT transporters and for keeping monosaccharides in cells (so they do not move back out if the concentration gradient changes)3. In order, for the monosaccharide to leave the phosphate will need to be cleaved or removed.
- Stipanuk MH. (2006) Biochemical, physiological, & molecular aspects of human nutrition. St. Louis, MO: Saunders Elsevier.