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 of the following statements is not correct, we will analyze each statement one by one:

1. **Statement 1**: "Only alpha amino acids are obtained on hydrolysis of proteins."
- **Analysis**: Proteins are made up of amino acids linked by peptide bonds. Upon hydrolysis, proteins break down into their constituent amino acids, which are primarily alpha amino acids. This statement is correct.

2. **Statement 2**: "Amino acids which are synthesized in the body are known as non-essential amino acids."
- **Analysis**: Non-essential amino acids are indeed those that the body can synthesize and do not need to be obtained from the diet. This statement is also correct.

3. **Statement 3**: "There are 20 essential amino acids."
- **Analysis**: This statement is incorrect. There are only 9 essential amino acids that must be obtained from the diet. These include histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. Therefore, this statement is not correct.

4. **Statement 4**: "L amino acids are represented by writing the NH2 group on the left side."
- **Analysis**: In the Fischer projection, L-amino acids are indeed represented with the amino group (NH2) on the left side. This statement is correct.

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

### Final Answer:
The statement that is not correct is: **There are 20 essential amino acids.**