which of the following molecules are absorbed by only passive transport in small intestine?
1) glucose
2) amino acids
3) Na^+ ions
4) water

Options

A. glucose
B. amino acids
C. Na^+ ions
D. water

Answer

To determine which of the following molecules are absorbed by only passive transport in the small intestine, we will analyze each option based on the mechanisms of transport.

1. **Glucose**: Glucose is primarily absorbed in the small intestine through active transport. It utilizes a sodium-glucose co-transporter (SGLT) that requires energy to transport glucose against its concentration gradient. Therefore, glucose is not absorbed by passive transport only.

2. **Amino Acids**: Amino acids can be absorbed through both active and passive transport mechanisms. Similar to glucose, some amino acids are absorbed via active transport, while others can diffuse passively when the concentration gradient favors it. Hence, amino acids are not absorbed by passive transport only.

3. **Na^+ ions**: Sodium ions (Na^+) are absorbed mainly through active transport mechanisms. The sodium-potassium pump actively transports Na^+ ions out of the intestinal cells, creating a gradient that allows for the absorption of Na^+ from the intestinal lumen. Therefore, Na^+ ions are not absorbed by passive transport only.

4. **Water**: Water is absorbed in the small intestine primarily through passive transport mechanisms, specifically osmosis. It moves from an area of lower solute concentration to an area of higher solute concentration without the need for energy. Thus, water is the only molecule among the options that is absorbed exclusively by passive transport.

**Conclusion**: The correct answer is **water**.