Which is not true statement?

Options

A. `alpha`-carbon of `alpha`-amino acid is asymmetric
B. All amino acids are found in L-form
C. Human body can synthesize all amino acids they need
D. At pH = 7 both amino and carboxylic groups exist, in ionised form

Answer

To determine which statement is not true regarding amino acids, we will analyze each statement provided in the video transcript step by step.

### Step 1: Analyze the First Statement
The first statement claims that the general formula of an amino acid is NH2CHRCOOH, where R is an alkyl group. It also mentions that glycine, the most basic amino acid, has R as H.

- **Evaluation**: This statement is true. The general formula for amino acids is indeed NH2CHRCOOH, and glycine (NH2CH2COOH) is the simplest amino acid, which does not have a chiral center (asymmetric carbon).

### Step 2: Analyze the Second Statement
The second statement asserts that all amino acids are found in L-form.

- **Evaluation**: This statement is true. All naturally occurring amino acids in proteins are in the L-form configuration.

### Step 3: Analyze the Third Statement
The third statement claims that the human body can synthesize all amino acids.

- **Evaluation**: This statement is false. The human body can synthesize only 11 out of the 20 standard amino acids; the remaining 9 are essential amino acids that must be obtained through diet.

### Step 4: Analyze the Fourth Statement
The fourth statement discusses the behavior of amino acids at pH 7, indicating that they can exist as zwitterions (having both positive and negative charges).

- **Evaluation**: This statement is true. At physiological pH (around 7), amino acids exist in a zwitterionic form, where the amino group is protonated (NH3+) and the carboxyl group is deprotonated (COO-).

### Conclusion
Based on the evaluations:
- The first statement is true.
- The second statement is true.
- The third statement is false (this is the statement that is not true).
- The fourth statement is true.

Thus, the statement that is not true is the third one: "Human body can synthesize all amino acids."

### Final Answer
The statement that is not true is: "Human body can synthesize all amino acids."

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