Question

Which of the following sugars is absorbed from the small intestine by facilitated diffusion:
A.Fructose
B.Glucose
C.Sucrose
D.Lactose

Answer 1

Hint: Facilitated diffusion is the transport of substances across a membrane using cell transport molecules without the use of energy

Complete step by step answer:
-Facilitated diffusion is the transport of substances across a membrane using cell transport molecules without the use of energy. It is along the concentration gradient (from a region of higher concentration to a region of lower concentration)
Glucose and galactose are absorbed by active transport. Fructose is absorbed by facilitated diffusion
-GLUT5 is a type of glucose transporter that allows fructose to be transported from the intestinal lumen into the enterocyte by facilitated diffusion due to fructose's high concentration in the intestinal lumen. Fructose being an isomer of glucose is easily transported by the GLUT5 transporter. GLUT5 is seen in skeletal muscle, testis, kidney, fat tissue (adipocytes), and brain

Additional Information:
-Glucose is transported by a special type of active transport known as the secondary active transport. Here, the glucose molecules enter the enterocytes (small intestine cells) from the intestinal lumen along with sodium (through the SGLT2 transporter). Then, the sodium enters the bloodstream by the sodium potassium ATPase pump using ATP (active transport) and the glucose leaves the enterocytes through the GLUT2 transporter. Hence, ATP is used indirectly in the transport of glucose
-Galactose also enters the bloodstream similarly

Note:
There are many types of glucose transporters (GLUT)
-GLUT1 is seen in the placenta, blood brain barrier (BBB), tissue culture cells, etc.
-GLUT2 is seen in the hepatocytes, beta cells of islets of Langerhans, proximal tubules of the kidney, small intestine, etc.
-GLUT3 is seen in the Betz cells of spinal tracts, neurons, etc.
-GLUT5 is for fructose transport.