Which of the following sugars is absorbed from the small intestine by facilitated diffusion?
## Core Concept
The small intestine absorbs various nutrients, including sugars, through different transport mechanisms. Facilitated diffusion is a type of passive transport that involves carrier proteins or channels to move molecules across the cell membrane without energy expenditure. This process is crucial for the absorption of certain sugars.
## Why the Correct Answer is Right
Fructose is absorbed from the small intestine by facilitated diffusion. This process involves the GLUT5 transporter, a type of facilitated diffusion transporter specific for fructose. The absorption of fructose does not require energy and is concentration-gradient dependent.
## Why Each Wrong Option is Incorrect
* **Option A:** Glucose is primarily absorbed through co-transport with sodium ions (Na+/glucose cotransporter 1, SGLT1), which is an example of secondary active transport, not facilitated diffusion.
* **Option B:** Sucrose is not directly absorbed; it must be broken down into glucose and fructose by intestinal enzymes like sucrase before absorption.
* **Option D:** Lactose must be hydrolyzed into glucose and galactose by lactase before its components can be absorbed, and glucose and galactose are absorbed through active transport mechanisms.
## Clinical Pearl / High-Yield Fact
A key point to remember is that individuals with certain conditions or deficiencies may have impaired sugar absorption. For example, some people have a deficiency in sucrase-isomaltase, leading to difficulties in digesting sucrose and other related sugars.
## Correct Answer: . Fructose