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Biomolecules – Class 11 Biology notes, important questions and MCQs

NCERT Class 11 Biology, chapter 9. Notes, key terms, important questions with answers and MCQs.

Chapter overview

Living tissues contain carbohydrates, proteins, lipids and nucleic acids. Enzymes are proteins that speed up biochemical reactions.

Macromolecules

Carbohydrates: monosaccharides (glucose), disaccharides (sucrose), polysaccharides (starch, cellulose, glycogen, chitin).
Proteins: polymers of 20 amino acids joined by peptide bonds; primary to quaternary structure.
Lipids: fatty acids + glycerol; nucleic acids: polymers of nucleotides (sugar + phosphate + base).

Enzymes

Enzymes lower activation energy; they are specific to substrates (active site).
Factors: temperature, pH, substrate concentration; Michaelis constant Km.
Inhibition: competitive (inhibitor resembles substrate, e.g. malonate for succinic dehydrogenase); cofactors include prosthetic groups, coenzymes (NAD, from vitamins) and metal ions.

Key terms

Enzyme: biological catalyst, usually a protein
Competitive inhibition: inhibitor competes with substrate for the active site
Holoenzyme: complete active enzyme made of a protein part (apoenzyme) and a non-protein cofactor

Important questions with answers

Why are proteins called heteropolymers?2 marks

They are made of different kinds of monomers — up to 20 types of amino acids — joined in specific sequences.

How do temperature and pH affect enzyme activity?3 marks

Each enzyme works best at an optimum temperature and pH. Below the optimum temperature activity is low; high temperature denatures the protein and stops activity. Changes in pH away from the optimum alter the charge and shape of the active site, reducing activity.

Explain the mechanism of enzyme action using the enzyme–substrate complex and explain the meaning of Km in the Michaelis–Menten curve.5 marks

The substrate binds to the enzyme's active site to form an enzyme–substrate (ES) complex, which is short-lived. The active site changes slightly to fit the substrate more tightly (induced fit), weakening the substrate's bonds; the ES complex changes into an enzyme–product (EP) complex, which releases the product, and the free enzyme is ready again. This lowers activation energy. In the Michaelis–Menten curve, reaction velocity rises with substrate concentration and levels off at Vmax when all enzyme molecules are busy. Km is the substrate concentration at which velocity is half of Vmax; a low Km means high affinity of the enzyme for its substrate.

Quick revision

Enzymes lower activation energy.
Cellulose is the most abundant organic compound.
Malonate inhibits succinic dehydrogenase.

Exam tips

Glycosidic bond links sugars; peptide bond links amino acids; phosphodiester bond links nucleotides.
Remember enzyme classes in order: oxidoreductases, transferases, hydrolases, lyases, isomerases, ligases.

Practice MCQs

Enzymes are mostly
1. lipids
2. proteins
3. carbohydrates
4. vitamins
Show answer
Answer: (b) proteins. Proteins (some RNA).
Amino acids are joined by
1. peptide bonds
2. glycosidic bonds
3. hydrogen bonds only
4. phosphodiester bonds
Show answer
Answer: (a) peptide bonds. Peptide bond.
Starch is a polymer of
1. fructose
2. nucleotides
3. amino acids
4. glucose
Show answer
Answer: (d) glucose. α-glucose.
Enzymes speed up reactions by
1. raising temperature
2. changing equilibrium
3. lowering activation energy
4. adding energy
Show answer
Answer: (c) lowering activation energy. Lower Ea.
The bond joining monosaccharides in a polysaccharide is
1. peptide bond
2. glycosidic bond
3. ester bond
4. hydrogen bond only
Show answer
Answer: (b) glycosidic bond. Sugar–sugar link.
Malonate inhibiting succinic dehydrogenase is an example of
1. non-competitive inhibition
2. feedback activation
3. competitive inhibition
4. denaturation
Show answer
Answer: (c) competitive inhibition. Resembles succinate.
Most abundant organic compound in the biosphere is
1. cellulose
2. protein
3. glycogen
4. DNA
Show answer
Answer: (a) cellulose. Plant cell walls.
An enzyme that joins two molecules using energy from ATP belongs to class
1. ligases
2. hydrolases
3. lyases
4. isomerases
Show answer
Answer: (a) ligases. Ligases (synthetases).

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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.