Which of following statement is not correct ?

Options

A. Only `alpha` - amino acids are obtained on hydrolysis of proteins
B. The amino acids which are synthesised in the body are known as non - essential amino acids
C. There are 20 essential amino acids
D. L - amino acids are represented by writing the -`HN_2` group on the left side

Answer

To determine which statement is not correct, we need to analyze each option provided:

1. **Option 1**: "Only alpha amino acids are obtained on hydrolysis of protein."
- When proteins are hydrolyzed, they break down into their constituent amino acids, which are primarily alpha amino acids. This statement is correct.

2. **Option 2**: "The amino acids which are synthesized in the body are known as non-essential amino acids."
- Non-essential amino acids are indeed those that the body can synthesize. This statement is also correct.

3. **Option 3**: "There are 20 essential amino acids."
- This statement is incorrect. There are actually 10 essential amino acids that must be obtained from the diet. The essential amino acids include: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. Therefore, this statement is not correct.

4. **Option 4**: "L-amino acids are represented by writing the NH2 group on the left side."
- This statement is correct. In the Fischer projection of amino acids, the amino group (NH2) is placed on the left side, which is characteristic of L-amino acids.

Based on the analysis, the statement that is not correct is **Option 3**: "There are 20 essential amino acids."

### Summary of Steps:
1. Analyze each statement regarding amino acids and proteins.
2. Confirm the correctness of each statement based on biochemical knowledge.
3. Identify the incorrect statement.