Isoelectric point is

Options

A. the pH at which all molecular species are ionised and that carry the same charge
B. the pH at which all molecular species are neutral and uncharged
C. the pH at which half molecular species are ionised (charged) and other half unionised
D. the pH at which negataively and positively charged molecular species in equal concentration

Explanation

### Step-by-Step Solution:

1. **Definition of Isoelectric Point**:
- The isoelectric point (pI) is defined as the pH at which a molecule, such as an amino acid or protein, carries no net electrical charge. At this point, the concentrations of the positively charged (cationic) and negatively charged (anionic) forms of the molecule are equal.

2. **Understanding Zwitterions**:
- A zwitterion is a molecule that has both positive and negative charges but is overall neutral. For example, in the case of amino acids like glycine, the zwitterionic form has an amino group that is protonated (NH3+) and a carboxyl group that is deprotonated (COO-).

3. **Key Characteristics of Isoelectric Point**:
- At the isoelectric point, the concentration of zwitterions is at its maximum.
- The pH at this point does not mean that all species are ionized or uncharged; rather, it indicates a balance between the charged species.

4. **Evaluating Options**:
- **Option 1**: "The pH at which all molecular species are ionized and they carry the same charge." - This is incorrect because not all species are ionized at the isoelectric point.
- **Option 2**: "The pH at which all molecular species are neutral and uncharged." - This is incorrect because zwitterions are charged.
- **Option 3**: "The pH at which half molecular species are ionized and the other half unionized." - This is also incorrect as it does not accurately describe the isoelectric point.
- **Option 4**: "The pH at which negatively and positively charged molecular species are in equal concentration." - This is correct and aligns with the definition of the isoelectric point.

5. **Conclusion**:
- The correct answer is **Option 4**: The pH at which negatively and positively charged molecular species are in equal concentration.