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A 45-year-old morbidly obese woman has been attempting to lose weight using a low- carbohydrate diet. After 2 months of little success, she confides in her son that she does add glucose to her coffee in the morning and after dinner but feels only some of this will be absorbed and should not be the cause of her limited success. Her son, a medical student, states that glucose is almost completely absorbed from the gut. What type of transport does glucose utilize for gastro intestinal absorption?

A. Active- Carrier mediated, against the concentration gradient and energy dependent

B. Facilitated- Carrier mediated, down the concentration gradient

C. Passive- Down the concentration gradient

D. Active and facilitated

E. Passive and facilitated

The correct answer iso D- Active and facilitated.

There are two separate mechanisms for the absorption of monosaccharides in the small intestine- Active and facilitated transport. Active transport is an energy dependent carrier mediated transport that involves transport of glucose against the concentration gradient. The carrier protein in active transport has two binding sites, one for glucose and the other for sodium. Thus it is a sodium dependent transport. Sodium transport is down the concentration gradient whereas the glucose transport is against the concentration gradient. Glucose and galactose are absorbed by a sodium-dependent process. They are carried by the same transport protein (SGLT 1), and compete with each other for intestinal absorption.

The second mechanism, facilitated transport also requires a carrier protein to speed up the process, but the energy is not invested in this transport and the flow is down the concentration gradient. Other monosaccharides (mainly) including glucose (but to a lesser extent) are absorbed by carrier-mediated facilitated diffusion. Fructose is mainly transported by Facilitated transport using GLUT-5 transport. Because they are not actively transported, fructose and sugar alcohols are only absorbed down their concentration gradient, and after a moderately high intake, some may remain in the intestinal lumen, acting as a substrate for bacterial fermentation.

Glucose is a polar molecule; the passive diffusion across the intestinal membrane is very-very slow. Thus only a small amount of glucose is absorbed by a passive diffusion.

 Passive versus Active transport

Figure- Passive diffusion versus active transport

Summary of Glucose transport

Features Passive diffusion Facilitated diffusion Active transport
Concentration gradient Down the concentration gradient from high to low. Down the concentration gradient from high to low. Against a concentration gradient from low to high
Energy expenditure none none Energy expenditure is in the form of ATP
Carrier protein/ transporter Not required required required
Speed Slowest mode Fast Fastest mode

Clinical significance

  • In SGLT- 1 deficiency, glucose is left unabsorbed and gets excreted in feces. Galactose is also malabsorbed since it is transported through the same transporter.
  • SGLT- 2, a similar glucose transporter is present in the renal tubular cells; the filtered glucose is reabsorbed back by this transporter. In its deficiency, glucose is not reabsorbed back, and is lost in urine, causing glycosuria.


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