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NEET UG · Biology

Biomolecules

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NEET UG · Biology

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Connect biomolecule units to their linkages, distinguish activation energy from overall energy change, and check your reasoning before Biomolecules MCQs. Match biomolecule bonds and read enzyme energy diagrams

About 4 min · NEET UG

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Match biomolecule bonds and read enzyme energy diagrams

Connect biomolecule units to their linkages, distinguish activation energy from overall energy change, and check your reasoning before Biomolecules MCQs.

What you will understand

Answer two different Biomolecules question types: which units are joined by a named bond, and what changes when an enzyme catalyses a reaction. Keep structural identification separate from energy reasoning.

Quick answer: identify the connection being asked about

Peptide linkages join amino-acid residues in polypeptides. Glycosidic linkages join sugar residues in polysaccharides. Phosphodiester linkages connect nucleotide residues along a nucleic-acid strand. Hydrogen bonds between complementary DNA bases describe a different connection. For enzyme questions, compare the activation barrier with the overall substrate-to-product energy change.

Read the unit-to-linkage map

Amino-acid residues in polypeptides are joined by peptide linkages; monosaccharide residues in polysaccharides by glycosidic linkages; nucleotide residues along a DNA or RNA strand by phosphodiester linkages. Hydrogen bonds between complementary DNA bases are distinct from covalent backbone linkages. Boxes are abstract residue symbols.
Amino-acid residues in polypeptides are joined by peptide linkages; monosaccharide residues in polysaccharides by glycosidic linkages; nucleotide residues along a DNA or RNA strand by phosphodiester linkages. Hydrogen bonds between complementary DNA bases are distinct from covalent backbone linkages. Boxes are abstract residue symbols.

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Worked example: count links in a chain

A linear, unbranched peptide contains five amino-acid residues and has no cyclic closure. There are four peptide linkages: draw five dots in a row and count the four gaps. Complete hydrolysis of those four peptide linkages consumes four water molecules in this simplified bookkeeping model.

The answer changes if the question changes the structure; do not reuse the “number of residues minus one” shortcut for a closed ring.

A polymer is not just any large-looking assembly

Polymers contain covalently linked repeating units. Membrane lipids can occur in the acid-insoluble fraction because membrane fragments are insoluble; this does not make each lipid molecule a polymer. For nucleic acids, distinguish base + sugar (nucleoside) from base + sugar + phosphate (nucleotide). A nucleotide is a building block; it is not the same thing as a whole DNA strand.

Compare barriers and endpoints

An illustrative free-energy diagram starts substrate S at 10 units and ends product P at 4. The uncatalysed peak is 30 and the catalysed peak 18, giving activation barriers 20 and 8. The overall change is negative 6 for either route. The enzyme changes the barrier, not the endpoint free-energy difference. Arbitrary numbers and schematic pathways, not experimental data.
An illustrative free-energy diagram starts substrate S at 10 units and ends product P at 4. The uncatalysed peak is 30 and the catalysed peak 18, giving activation barriers 20 and 8. The overall change is negative 6 for either route. The enzyme changes the barrier, not the endpoint free-energy difference. Arbitrary numbers and schematic pathways, not experimental data.

Open full-size diagram ↗

Worked example: subtract from the correct baseline

Use the arbitrary units in the diagram. Substrate S is at 10; product P is at 4. The uncatalysed peak is 30, so its forward barrier is 30 − 10 = 20. The catalysed peak is 18, giving 18 − 10 = 8. The overall free-energy change is 4 − 10 = −6 in both routes. The enzyme lowers the barrier; it does not move these endpoints.

For your next visit, redraw both routes with identical endpoints and explain why peak height alone is not activation energy.

Common mistakes to catch

A faster catalysed reaction does not mean a more negative overall free-energy change or a shifted equilibrium. Enzymes do not supply a reaction’s net energy. Most enzymes are proteins, but catalytic RNA is an exception. Avoid the statement “every enzyme denatures at the same temperature”: stability depends on the enzyme and its conditions.

Try it yourself 1: name the right bond

Which linkage connects successive nucleotide residues along one DNA strand?

A. Hydrogen bond between complementary bases

B. Peptide linkage

C. Phosphodiester linkage

Reveal the answer

C. The question asks about the covalent backbone of one strand. A concerns base pairing between strands; B joins amino-acid residues in a polypeptide.

Try it yourself 2: a second energy model

A schematic reaction starts at 12 units and ends at 5. Its peak falls from 32 to 20 with an enzyme. Which is correct?

A. The new forward barrier is 8; overall change remains −7.

B. The new forward barrier is 20; overall change is −12.

C. The enzyme changes the product energy to 20.

Reveal the answer

A. Subtract the substrate level: 20 − 12 = 8. The endpoint difference is still 5 − 12 = −7. B uses peak height as the barrier; C confuses the transition-state region with the product. These are illustrative units, not measured data.

A useful check on your next visit

Reconstruct the worked example from memory before looking at the diagrams. If a self-check exposed a mix-up, explain why your original answer failed, then try chapter practice.

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Sources and review

Technical review: Codex (automated). Sources checked on 23 October 2026. Diagrams, numerical models, worked examples and self-checks are original teaching material. Human subject review is not claimed.

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