Consider the following statements about peptides and proteins.
(A) A tripeptide contains three amino acids linked by two peptide linkages.
(B) When the number of amino acids linked is more than five, the products are called polypeptides.
(C) A polypeptide with more than hundred amino acid residues, having molecular mass higher than 10,000 u, is called a protein.
(D) Insulin, which contains 51 amino acids, is called a protein because it has a well defined conformation of a protein.
(E) Proteins are classified into fibrous and globular proteins on the basis of their molecular mass.
Which of the statements given above are incorrect? Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (B) and (E) only
Peptides are named by the number of amino acids they contain. A long polypeptide becomes a protein, though the line between the two is not very sharp, and proteins are then grouped by their shape.
(A) A tripeptide contains three amino acids linked by two peptide linkages. — Correct statement. Each linkage joins two neighbouring amino acids, so three amino acids need two linkages.
(B) When the number of amino acids linked is more than five, the products are called polypeptides. — Incorrect statement. Products are called polypeptides when the number of amino acids is more than ten; five or six amino acids give a pentapeptide or a hexapeptide.
(C) A polypeptide with more than hundred amino acid residues, having molecular mass higher than 10,000 u, is called a protein. — Correct statement. This is how a protein is told apart from a polypeptide.
(D) Insulin, which contains 51 amino acids, is called a protein because it has a well defined conformation of a protein. — Correct statement. A polypeptide with fewer amino acids may still be called a protein if it has a well defined conformation; insulin is the example.
(E) Proteins are classified into fibrous and globular proteins on the basis of their molecular mass. — Incorrect statement. This classification is on the basis of molecular shape: chains running parallel give fibres, and chains coiling around give a spherical shape.
NCERT (2026-27 edition):
10.2.3 Structure of Proteins – “…A tripeptide contains three amino acids linked by two peptide linkages. Similarly when four, five or six amino acids are linked, the respective products are known as tetrapeptide, pentapeptide or hexapeptide, respectively. When the number of such amino acids is more than ten, then the products are called polypeptides. A polypeptide with more than hundred amino acid residues, having molecular mass higher than 10,000u is called a protein. However, the distinction between a polypeptide and a protein is not very sharp. Polypeptides with fewer amino acids are likely to be called proteins if they ordinarily have a well defined conformation of a protein such as insulin which contains 51 amino acids.”
“Proteins can be classified into two types on the basis of their molecular shape.”