Fructose is a simple sugar found in many natural foods such as fruits and honey. Absorption of fructose from the intestine into the bloodstream is an essential step in its metabolism and utilization by the body. Unlike glucose, which utilizes sodium-dependent co-transporters, fructose absorption follows a different pathway.

Mechanism of Fructose Transport into Enterocytes

Fructose transport into the intestinal cells (enterocytes) occurs predominantly through facilitated diffusion, a passive transport mechanism that uses specific transport proteins. The primary transporter responsible for fructose uptake at the apical membrane of enterocytes is Glucose Transporter 5 (GLUT5).

• GLUT5:
  GLUT5 is a member of the facilitative GLUT family, specialized in transporting fructose but is independent of sodium ions. It allows fructose molecules to move down their concentration gradient into the cells without the need for energy expenditure.

• Basolateral Transport:
  After entry, fructose exits the enterocyte at the basolateral membrane mainly via GLUT2, which can transport glucose, fructose, and galactose into the bloodstream.

Why Other Transporters Are Not Involved in Fructose Uptake

• Sodium-Dependent Glucose Transporter 1 (SGLT1):
  This transporter is responsible for the co-transport of glucose and galactose with sodium ions but does not transport fructose.

• SGLT2:
  Primarily functions in the kidney for glucose reabsorption, not in intestinal fructose transport.

• GLUT4:
  Insulin-regulated glucose transporter found mainly in muscle and adipose tissue, not involved in intestinal fructose absorption.

Summary Table: Fructose Transporters

Conclusion

The transport of fructose into enterocytes is primarily mediated by Glucose Transporter 5 (GLUT5) via facilitated diffusion. This mechanism is distinct from the sodium-dependent glucose transport system.

Therefore, the correct answer is:
(2) glucose transporter 5 (GLUT5)