Differentiate between the following: (2 × 1)
(i) Peptide linkage and Glycosidic linkage
(ii) Essential amino acids and Non-essential amino acids
Differentiate between the following: (2 × 1)
(i) Peptide linkage and Glycosidic linkage
(ii) Essential amino acids and Non-essential amino acids
(i) Peptide linkage vs Glycosidic linkage
- Peptide linkage: an amide bond (–CO–NH–) formed between the –COOH group of one amino acid and the –NH₂ group of another, with loss of a water molecule. It joins amino acids in proteins.
- Glycosidic linkage: an ether-type (C–O–C) bond formed between two monosaccharide units through an oxygen atom, with loss of a water molecule. It joins sugar units in di- and poly-saccharides.
(ii) Essential vs Non-essential amino acids
- Essential amino acids: cannot be synthesised in the body and must be obtained from the diet. Example: valine, leucine.
- Non-essential amino acids: can be synthesised within the body, so dietary intake is not compulsory. Example: glycine, alanine.
Marking Scheme
- 11 mark: peptide linkage identified as –CO–NH– amide bond in proteins vs glycosidic linkage as C–O–C bond between monosaccharides (0.5 + 0.5).
- 21 mark: essential = not synthesised by body/obtained from diet vs non-essential = synthesised by body, with valid examples (0.5 + 0.5).
Hint
State the bond type (–CO–NH– vs C–O–C), the reacting groups, and the biomolecule formed; 'essential' means the body cannot synthesise it.
Quick Oral Answer
A peptide linkage is the –CO–NH– amide bond joining amino acids in proteins, while a glycosidic linkage is the C–O–C oxygen bridge joining sugar units; essential amino acids must come from the diet whereas non-essential ones are made by the body.
Analysis & Explanation
Concept — two different condensation bonds
Both linkages form by elimination of water, but the groups involved differ. A peptide bond is a special amide bond between an acid and an amine, whereas a glycosidic bond is essentially an ether bridge between the –OH groups of two sugars.
Why the distinction matters
- Peptide bonds build the primary structure of proteins; glycosidic bonds build the backbone of carbohydrates such as maltose, sucrose and starch.
- The nitrogen in a peptide bond is what makes proteins reservoirs of nitrogen in the body.
Exam trap
- Students often write only 'amide' and 'ether' without stating the groups reacting or the class of biomolecule formed — both are needed for full marks.
- 'Essential' does not mean more important; it strictly means the body cannot make it.
Real-world link
A balanced diet must supply all essential amino acids (like the pulses-plus-cereal combination in Indian meals), because a deficiency of even one limits protein synthesis.
Common Mistakes
- 1Writing only the bond name ('amide' or 'ether') without mentioning which groups react or which biomolecule is formed.
- 2Thinking essential amino acids are chemically more important, rather than simply not being synthesised by the body.
- 3Confusing the two — attributing –CO–NH– to sugars or C–O–C to proteins.
Interesting Facts
Humans require nine essential amino acids from food (including valine, leucine, isoleucine and lysine); the other eleven are non-essential.
The peptide bond has partial double-bond character and is planar, which is why protein chains fold into rigid, well-defined shapes.
The glycosidic bond in starch (α-linkage) is digestible by humans, but the almost identical β-linkage in cellulose is not — a one-bond difference decides what we can eat.
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Frequently Asked Questions
Is a peptide linkage the same as an amide bond?
Yes. A peptide linkage is a specific amide bond (–CO–NH–) formed between the carboxyl group of one amino acid and the amino group of another. All peptide bonds are amide bonds, but in biochemistry the term 'peptide bond' is reserved for the amide bond between amino acids.
Why are some amino acids called 'essential'?
They are called essential because the human body lacks the enzymes to synthesise them, so they must be supplied through the diet. Non-essential amino acids can be manufactured internally from other molecules, so a dietary source, while helpful, is not strictly required.