What is the difference between reducing and non-reducing sugars?

Reducing sugars have a free aldehyde or ketone group (e.g. glucose, fructose, maltose, lactose) and give positive Fehling's/Tollens' test. Non-reducing sugars (e.g. sucrose) have both anomeric carbons locked in a glycosidic bond — no free reducing group.

Samacheer Kalvi 12th Chemistry Chapter 14 Exercise Solutions

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Chapter 14 — Biomolecules Exercise Solutions

Biomolecules is scoring when classification and examples are memorised clearly. Use tables to organise the information.

Key Concepts Quick Recap

Biomolecule	Monomer	Bond	Function
Carbohydrates	Monosaccharides	Glycosidic bond	Energy source, cell walls, DNA backbone
Proteins	Amino acids	Peptide bond (amide)	Enzymes, structure, transport, immunity
Nucleic acids (DNA/RNA)	Nucleotides	Phosphodiester bond	Genetic information, protein synthesis
Lipids	Fatty acids + glycerol	Ester bond	Energy storage, membranes, hormones
Vitamins	Small organic molecules	—	Coenzymes, essential metabolic functions

Carbohydrate Classification

Type	Definition	Examples
Monosaccharide	Cannot be hydrolysed; single sugar unit	Glucose, fructose, galactose, ribose
Disaccharide	2 monosaccharides; hydrolysed to 2 monomers	Sucrose, maltose, lactose
Oligosaccharide	3–10 monosaccharides	Raffinose (3 units)
Polysaccharide	100s–1000s of units	Starch, cellulose, glycogen

Important Vitamin Table

Vitamin	Chemical Name	Deficiency Disease	Solubility
A	Retinol	Night blindness, xerophthalmia	Fat
B₁	Thiamine	Beriberi	Water
B₂	Riboflavin	Cheilosis	Water
B₃	Niacin	Pellagra	Water
B₁₂	Cyanocobalamin	Pernicious anaemia	Water
C	Ascorbic acid	Scurvy	Water
D	Calciferol	Rickets (children), Osteomalacia (adults)	Fat
E	Tocopherol	Infertility, haemolysis	Fat
K	Phylloquinone	Impaired blood clotting	Fat

I. Choose the Best Answer (MCQs)

1. Glucose is classified as: (a) Disaccharide (b) Ketohexose (c) Aldohexose (d) Aldopentose

Answer(c) Aldohexose Glucose has 6 carbons (hexose) and an aldehyde functional group (−CHO) → aldohexose. Fructose is a ketohexose (has a ketone group at C-2).

2. The peptide bond in proteins is: (a) −NH−CO− (amide bond) (b) −O−CO− (ester bond) (c) −S−S− (disulphide) (d) −O−O− (peroxide)

Answer(a) −NH−CO− (amide bond) Peptide bond forms between the −COOH of one amino acid and the −NH₂ of another: −CO−NH− (water is eliminated).

3. Which of the following is a non-reducing sugar? (a) Glucose (b) Maltose (c) Sucrose (d) Lactose

Answer(c) Sucrose In sucrose, both the anomeric carbons of glucose and fructose are involved in the glycosidic bond → no free −CHO or ketone group → non-reducing.

4. The base present in DNA but NOT in RNA is: (a) Adenine (b) Guanine (c) Cytosine (d) Thymine

Answer(d) Thymine DNA bases: A, G, C, T. RNA bases: A, G, C, U (Uracil replaces Thymine). Sugar: deoxyribose in DNA, ribose in RNA.

5. Which of the following is a fat-soluble vitamin? (a) Vitamin C (b) Vitamin B₁₂ (c) Vitamin D (d) Vitamin B₂

Answer(c) Vitamin D Fat-soluble vitamins: A, D, E, K. Water-soluble: B-complex, C. Fat-soluble vitamins are stored in the body; excess can cause toxicity.

6. Enzymes are: (a) Inorganic catalysts (b) Lipid-based catalysts (c) Protein-based biological catalysts (d) Nucleic acid catalysts

Answer(c) Protein-based biological catalysts Most enzymes are proteins (some are ribozymes — RNA-based). They are highly specific, work at mild conditions, and can catalyse reactions 10⁶ to 10¹⁷ times faster than uncatalysed.

7. The secondary structure of protein is maintained by: (a) Peptide bonds (b) Disulphide bridges (c) Hydrogen bonds (d) van der Waals forces

Answer(c) Hydrogen bonds Secondary structure (α-helix, β-pleated sheet) is maintained by hydrogen bonds between C=O and N−H groups of the peptide backbone. Peptide bonds maintain the primary structure.

II. Short Answer Questions

Q1. Classify carbohydrates with examples. What are reducing sugars?

Answer

Classification:

Monosaccharides: single unit, cannot be hydrolysed. Example: glucose, fructose.
Disaccharides: two monosaccharide units linked by glycosidic bond. Example: sucrose (glucose + fructose), maltose (glucose + glucose), lactose (glucose + galactose).
Polysaccharides: many monosaccharide units. Example: starch (storage in plants), glycogen (storage in animals), cellulose (structural in plants).

Reducing sugars: Carbohydrates that can reduce Fehling's or Tollens' reagent — they have a free aldehyde or ketone group.

Reducing: glucose, fructose, maltose, lactose.
Non-reducing: sucrose (both anomeric carbons locked in glycosidic bond).

Q2. Describe the four levels of protein structure.

Answer

Level	Description	Stabilised by
Primary	Sequence of amino acids in the polypeptide chain	Peptide (covalent) bonds
Secondary	α-helix or β-pleated sheet (local folding)	Hydrogen bonds
Tertiary	3D folding of the polypeptide chain	H-bonds, ionic bonds, hydrophobic interactions, disulphide bridges
Quaternary	Association of two or more polypeptide chains	Same as tertiary

Example: Haemoglobin has quaternary structure (2α + 2β subunits). Insulin has disulphide bridges stabilising tertiary structure.

Q3. Compare DNA and RNA.

Answer

Feature	DNA	RNA
Sugar	Deoxyribose	Ribose
Bases	A, G, C, T	A, G, C, U
Strands	Double-stranded (usually)	Single-stranded (usually)
Location	Nucleus (mainly)	Nucleus + cytoplasm
Stability	More stable	Less stable
Function	Stores genetic information	Protein synthesis (mRNA, tRNA, rRNA)
Winding	Right-handed B-helix	Various secondary structures

Base pairing in DNA: A=T (2 H-bonds), G≡C (3 H-bonds).

Q4. Write the functions of proteins with examples.

Answer

Function	Example
Enzyme	Amylase, pepsin, trypsin
Structural	Collagen (connective tissue), keratin (hair, nails)
Transport	Haemoglobin (O₂), serum albumin
Hormones	Insulin, glucagon
Immune protection	Antibodies (immunoglobulins)
Muscle contraction	Actin, myosin
Storage	Casein (milk), ferritin (iron)

Worked Examples

Example 1 — Classifying carbohydrates

Monosaccharide:    Glucose (C₆H₁₂O₆) — aldohexose
                   Fructose (C₆H₁₂O₆) — ketohexose
                   Ribose (C₅H₁₀O₅) — aldopentose (backbone of RNA)
                   Deoxyribose (backbone of DNA)

Disaccharide:      Sucrose = Glucose + Fructose (non-reducing)
                   Maltose = Glucose + Glucose (reducing)
                   Lactose = Glucose + Galactose (reducing; present in milk)

Polysaccharide:    Starch — storage in plants; α-glucose units
                   Cellulose — structural; β-glucose units (not digested by humans)
                   Glycogen — storage in animals (liver, muscle)

Example 2 — Reducing vs non-reducing sugars

Test: Fehling's solution (gives brick-red Cu₂O ppt) or Tollens' reagent (silver mirror)

Reducing sugars (positive test):
  Glucose, fructose, maltose, lactose ✓

Non-reducing sugar (negative test):
  Sucrose — both anomeric carbons engaged in glycosidic bond; no free −CHO ✗
  After hydrolysis: sucrose → glucose + fructose → both are reducing

Invert sugar = mixture of glucose + fructose obtained by hydrolysis of sucrose

Example 3 — DNA bases and complementary base pairing

DNA double helix — Chargaff's rules:
  %A = %T and %G = %C

Base pairs:
  A (Adenine) pairs with T (Thymine) → 2 hydrogen bonds
  G (Guanine) pairs with C (Cytosine) → 3 hydrogen bonds

If one strand of DNA is: 5'−ATGCCG−3'
Complementary strand:    3'−TACGGC−5'

RNA bases replace T with U:
  mRNA transcribed: 5'−AUGCCG−3'

Example 4 — Vitamin deficiency diseases

Memorise this table:
  A → Night blindness (inability to see in dim light)
  B₁ → Beriberi (nervous system damage)
  B₂ → Cheilosis (cracks at corner of mouth)
  B₃ → Pellagra (3D: Dermatitis, Diarrhoea, Dementia)
  B₁₂ → Pernicious anaemia (impaired red blood cell production)
  C → Scurvy (bleeding gums, poor wound healing)
  D → Rickets in children, Osteomalacia in adults (bone softening)
  E → Haemolysis, reproductive problems
  K → Impaired blood clotting (haemorrhage)

Example 5 — Enzyme specificity

Lock-and-key model (Emil Fischer, 1890):
  - The active site of an enzyme has a specific shape (the "lock").
  - Only a substrate with complementary shape (the "key") can bind.
  - Enzyme–substrate complex forms → reaction occurs → products released → enzyme recycled.

Induced fit model (Koshland):
  - Active site is flexible and adjusts its shape when substrate approaches.
  - More accurate for most enzymes.

Examples of enzyme specificity:
  Urease: only hydrolyses urea → NH₃ + CO₂
  Amylase: only hydrolyses starch (α-1,4 glycosidic bonds)
  Pepsin: cleaves peptide bonds in acidic stomach conditions only

Answer Pattern

1. Define the biomolecule. 2. State its classification. 3. Give one specific function. 4. Give one example with a real context.

Common Mistakes

Confusing reducing and non-reducing sugars — sucrose is non-reducing.
Mixing DNA and RNA bases — thymine is only in DNA; uracil only in RNA.
Saying all vitamins are water-soluble — A, D, E, K are fat-soluble.
Vague vitamin functions — always mention the deficiency disease.
Confusing secondary structure (H-bonds) with primary structure (peptide bonds).

Exam Checklist

Biomolecule classification is correct.
Examples are valid (not generic).
Vitamin deficiency disease is paired correctly.
DNA/RNA comparison covers bases, sugar, and strand structure.
Protein structure levels are described in order (1° → 4°).

Frequently Asked Questions

Yes. It is based on definitions, classification, structure, function, and examples. Memorise the key tables (carbohydrate types, vitamin deficiency diseases, DNA vs RNA).