Which of the following substances are absorbed by passive transport ?

Options

A. Glucose
B. Amino acids
C. `Na^(+)`
D. Bicarbonate ions

Answer

To determine which substances are absorbed by passive transport, we need to understand the concept of passive transport and analyze each option provided.

### Step-by-step Solution:

1. **Define Passive Transport**:
- Passive transport is a type of membrane transport that does not require energy. It occurs when substances move across cell membranes from an area of high concentration to an area of low concentration, following the second law of thermodynamics.

**Hint**: Remember that passive transport relies on concentration gradients and does not require ATP.

2. **Analyze Each Substance**:
- **Glucose**:
- Glucose is primarily absorbed in the small intestine. While it can diffuse, it generally requires active transport mechanisms (such as the sodium-glucose transport protein) to enter epithelial cells. Therefore, glucose is not absorbed by passive transport.

**Hint**: Consider how glucose is transported in the body—look for active transport mechanisms.

- **Amino Acids**:
- Similar to glucose, amino acids are absorbed in the small intestine and typically require active transport to be taken up by epithelial cells. They do not rely on passive transport.

**Hint**: Think about the digestion process and how proteins are broken down into amino acids.

- **Sodium Ions**:
- Sodium ions are also absorbed through active transport mechanisms, especially in the intestines. They do not passively diffuse across membranes.

**Hint**: Recall the role of sodium in cellular processes and its transport mechanisms.

- **Bicarbonate Ions**:
- Bicarbonate ions are produced as a waste product of mitochondrial respiration. They can undergo a pH-dependent conversion into carbon dioxide (CO2), which is a gas. Gases can diffuse freely across cell membranes without the need for energy, making bicarbonate ions the only option that can be absorbed by passive transport.

**Hint**: Focus on the conversion of bicarbonate to CO2 and how gases diffuse through membranes.

3. **Conclusion**:
- Among the options provided, **bicarbonate ions** are the only substances that can be absorbed by passive transport due to their ability to convert into a gas and diffuse across membranes.

### Final Answer:
**Bicarbonate ions** are absorbed by passive transport.