Which of the following statement are correct ?
1. `alpha` - amino acids present in protein are `alpha` - L amino acids
2. Amino acids have `-NH_2` as well as `- COOH` group
3. Number of amino groups and carboxyl groups are always same in all `alpha` - amino acids
4. Concentration of dipolar ion is maximum at isoeletric point
Select the correct answer from the codes given below

Options

A. Only 1 and 2
B. 1,2 and 3
C. 1,2 and 4
D. 1,2,3 and 4

Answer

To solve the question about the correctness of the statements regarding amino acids, we will analyze each statement one by one.

### Step-by-Step Solution:

1. **Statement 1: Alpha-amino acids present in protein are alpha-L amino acids.**
- **Analysis:** Alpha-amino acids are the building blocks of proteins. In biological systems, the amino acids that are incorporated into proteins are predominantly the L-form of alpha-amino acids. Thus, this statement is correct.
- **Conclusion:** This statement is **correct**.

2. **Statement 2: Amino acids have -NH2 as well as -COOH group.**
- **Analysis:** By definition, amino acids contain both an amine group (-NH2) and a carboxylic acid group (-COOH). This is a fundamental characteristic of amino acids.
- **Conclusion:** This statement is **correct**.

3. **Statement 3: Number of amino groups and carboxyl groups are always the same in all alpha-amino acids.**
- **Analysis:** All standard alpha-amino acids have one amino group and one carboxyl group. Therefore, the number of amino groups and carboxyl groups is indeed the same in all alpha-amino acids.
- **Conclusion:** This statement is **correct**.

4. **Statement 4: Concentration of dipolar ion is maximum at isoelectric point.**
- **Analysis:** The isoelectric point (pI) is the pH at which the amino acid exists predominantly in its zwitterionic form (dipolar ion), where the positive and negative charges balance out. At this point, the concentration of zwitter ions is indeed at its maximum.
- **Conclusion:** This statement is **correct**.

### Final Conclusion:
Since all four statements are correct, the answer to the question is that **all statements (1, 2, 3, and 4) are correct**.