The theory of Donnan eqilibrium explains the presence of some

Options

A. Fixed diffusible cations on the inner side
B. Fixed non-diffusible anions on the inner side
C. Non-fixed diffusible anions on the inner side
D. Non-fixed diffusible cations on the inner side

Answer

### Step-by-Step Solution:

1. **Understanding Donnan Equilibrium**:
- Donnan equilibrium refers to the distribution of ions across a semipermeable membrane that separates two solutions. In the context of red blood cells (RBCs), it explains how certain ions are distributed inside and outside the cell.

2. **Membrane Characteristics**:
- The RBC membrane is selectively permeable. It allows the movement of negatively charged ions (anions) like chloride (Cl⁻) and bicarbonate (HCO₃⁻) but restricts the movement of positively charged ions (cations) like sodium (Na⁺) and potassium (K⁺).

3. **Chloride Shift**:
- The process where bicarbonate ions (HCO₃⁻) move out of the RBC into the plasma while chloride ions (Cl⁻) move into the RBC is known as the chloride shift. This exchange helps maintain ionic balance and pH levels in the blood.

4. **Evaluating the Options**:
- **Option 1**: Fixed diffusible cations on the inner side - Incorrect, as cations cannot diffuse across the membrane.
- **Option 2**: Fixed non-diffusible anions on the inner side - Incorrect, since bicarbonate ions are not fixed; they move out.
- **Option 3**: Non-fixed diffusible anions on the inner side - Correct, as anions like bicarbonate are moving out.
- **Option 4**: Non-fixed diffusible cations on the inner side - Incorrect, as the membrane does not allow cation movement.

5. **Conclusion**:
- The correct answer is **Option 3: Non-fixed diffusible anions on the inner side**.