Biology · Biomolecules · NEET
It means a protein is one long chain made of many amino acids linked together in a line. NCERT states: 'Proteins are polypeptides. They are linear chains of amino acids linked by peptide bonds.' So 'polypeptide' = many amino acids joined by peptide bonds. For NEET, remember: protein = polymer of amino acids = polypeptide.
The peptide bond. It joins the carboxyl group (-COOH) of one amino acid to the amino group (-NH2) of the next amino acid. NEET often asks you to match Protein with 'peptide bond' in match-the-column questions, so memorise this pair.
By dehydration (removal of water). The -COOH of one amino acid reacts with the -NH2 of the next; a molecule of water (H2O) is removed and a -CO-NH- link forms. This is why it is also called a condensation reaction. Breaking it back needs water (hydrolysis), done by protease enzymes.
A peptide bond is a strong covalent bond. Do not confuse it with the weak hydrogen bonds or hydrophobic interactions that only fold the protein into shape. In NEET, the peptide bond holds the amino acid sequence (primary structure); weak bonds just fold that chain.
Each biomolecule has its own bond: proteins have peptide bonds (amino acids), polysaccharides have glycosidic bonds (sugars), and nucleic acids have phosphodiester bonds (nucleotides). NEET repeatedly tests this exact match, for example 'inulin (glycosidic), insulin (peptide)' in 2020.
The first amino acid is the N-terminal (free amino group), and the last amino acid is the C-terminal (free carboxyl group). NEET 2023 gave a trap that swapped these, so read carefully: first = N-terminal, last = C-terminal.
Identify the substances having glycosidic bond and peptide bond, respectively in their structure:
Match List-I with List-II. List-I: (A) Protein (B) Unsaturated fatty acid (C) Nucleic acid (D) Polysaccharide; List-II: (I) C=C double bonds (II) Phosphodiester bond (III) Glycosidic bonds (IV) Peptide bonds
Identify the correct statements about biomolecules. A. Lipids are generally water soluble. B. Proteins are polypeptides. C. Polysaccharides are long chains of sugars. D. Adenine and guanine are substituted pyrimidines. E. Almost all enzymes are proteins.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Yes. Every protein is a polypeptide, that is a chain of amino acids joined by peptide bonds. A protein may be one chain or several chains folded together, but each chain is a polypeptide.
A protein is a heteropolymer because it is made of 20 different types of amino acids. A homopolymer would have only one repeating monomer, but proteins use many different amino acids, so they are heteropolymers.
Protease enzymes (peptidases) break peptide bonds by hydrolysis, adding back a water molecule. NEET 2024 matched 'protease' with 'peptide bond', so remember this pair.
To join n amino acids you form (n-1) peptide bonds, so (n-1) water molecules are removed. Each peptide bond formation removes exactly one water molecule (dehydration).
The first amino acid of the chain, which keeps a free amino group, is the N-terminal. The last amino acid, which keeps a free carboxyl group, is the C-terminal. This order defines the primary structure.
