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Biomolecules – Class 12 Chemistry notes, important questions and MCQs

NCERT Class 12 Chemistry, chapter 10. Notes, key terms, important questions with answers and MCQs.

Chapter overview

Biomolecules — carbohydrates, proteins, nucleic acids, vitamins and lipids — build living systems and carry out life processes.

Carbohydrates and proteins

Monosaccharides (glucose, fructose), disaccharides (sucrose = glucose + fructose, non-reducing; maltose, lactose reducing), polysaccharides (starch, cellulose, glycogen).
Glucose: aldohexose, C₆H₁₂O₆; exists in α and β cyclic forms (anomers).
Proteins: polymers of α-amino acids joined by peptide bonds; structures: primary to quaternary; denaturation destroys higher structure.

Nucleic acids and vitamins

DNA (deoxyribose; bases A, G, C, T; double helix) and RNA (ribose; A, G, C, U; single strand).
Vitamins: fat-soluble A, D, E, K; water-soluble B and C. Deficiencies: A – night blindness, D – rickets, C – scurvy, B₁ – beriberi, K – poor clotting.
Enzymes are biological catalysts, mostly proteins.

Key terms

Peptide bond: –CO–NH– link between amino acids
Anomers: cyclic sugar forms differing only at C-1
Denaturation: loss of a protein's 3D structure and biological activity by heat or pH change, without breaking peptide bonds

Formulas

Sucrose → glucose + fructose (hydrolysis, invert sugar)

Important questions with answers

What are essential amino acids? Give two examples.2 marks

Amino acids that the body cannot make and must be taken in the diet, e.g. valine and lysine (also leucine, phenylalanine).

Give two pieces of evidence that glucose has an open-chain structure with an aldehyde group, and one fact it cannot explain.3 marks

With HI it gives n-hexane (straight chain of six carbons); with bromine water it gives gluconic acid (–CHO present); with acetic anhydride it forms a pentaacetate (five –OH). The open structure cannot explain why glucose does not give Schiff's test or react with NaHSO₃, which is explained by the cyclic hemiacetal form.

Describe the primary, secondary, tertiary and quaternary structures of proteins, and give the functions of any three vitamins with deficiency diseases.5 marks

Primary: sequence of amino acids joined by peptide bonds. Secondary: local folding into α-helix or β-pleated sheet held by hydrogen bonds. Tertiary: overall 3D folding by H-bonds, disulphide bridges, ionic and hydrophobic interactions (fibrous or globular). Quaternary: arrangement of two or more polypeptide subunits (haemoglobin has four). Vitamins: A – vision (deficiency: night blindness); C – healthy gums and skin (scurvy); D – calcium absorption (rickets); B₁₂ – red blood cell formation (pernicious anaemia).

Quick revision

Sucrose is non-reducing.
Vitamins A, D, E, K are fat-soluble.
Enzymes are protein catalysts.

Exam tips

Thymine is only in DNA; uracil is only in RNA.
Sucrose is non-reducing (both anomeric carbons are in the glycosidic bond); maltose and lactose are reducing.

Practice MCQs

Which base is found in RNA but not in DNA?
1. adenine
2. thymine
3. guanine
4. uracil
Show answer
Answer: (d) uracil. Uracil.
Deficiency of vitamin C causes
1. rickets
2. beriberi
3. scurvy
4. night blindness
Show answer
Answer: (c) scurvy. Scurvy.
Which is a non-reducing sugar?
1. glucose
2. maltose
3. lactose
4. sucrose
Show answer
Answer: (d) sucrose. No free carbonyl.
The bond linking amino acids in proteins is
1. peptide
2. glycosidic
3. hydrogen only
4. phosphodiester
Show answer
Answer: (a) peptide. Peptide bond.
The monomer of starch and cellulose is
1. fructose
2. ribose
3. galactose
4. glucose
Show answer
Answer: (d) glucose. α- and β-glucose.
Glucose with HI on prolonged heating gives n-hexane. This shows
1. a ring structure
2. all six carbons in a straight chain
3. presence of –CHO only
4. five –OH groups
Show answer
Answer: (b) all six carbons in a straight chain. Straight chain.
Which vitamin can be stored in the body for long?
1. vitamin C
2. vitamin B₁
3. vitamin A
4. vitamin B₂
Show answer
Answer: (c) vitamin A. Fat-soluble vitamins are stored.
Which base is present in RNA but not in DNA
1. adenine
2. thymine
3. uracil
4. cytosine
Show answer
Answer: (c) uracil. U replaces T.

All chapters of Class 12 Chemistry

Written by Kundali IQ from standard, well-established facts to help with revision. This is not an official NCERT publication; always read your textbook too.