In primary structure a protein is imagined as a line, the left end represents :-

Options

A. The first amino acid and its carboxyl terminal
B. The last amino acid and its carboxyl terminal
C. The first amino acid and its amino terminal
D. The last amino acid and its amino terminal

Answer

To solve the question regarding the representation of the left end of a protein's primary structure, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Primary Structure**: The primary structure of a protein refers to the linear sequence of amino acids linked by peptide bonds. This sequence is crucial for determining the protein's overall structure and function.

2. **Identifying Ends of the Protein**: In the primary structure, the protein chain has two distinct ends:
- The **N-terminal** (amino terminal) end, which is the start of the protein chain.
- The **C-terminal** (carboxyl terminal) end, which is the end of the protein chain.

3. **Left End Representation**: The question specifically asks about the left end of the protein structure. By convention, the left end of a protein's primary structure represents the N-terminal, where the first amino acid is located.

4. **Conclusion**: Therefore, the left end of the primary structure of a protein represents the first amino acid and its amino terminal.

5. **Final Answer**: The correct option is "the first amino acid and its amino terminal."