Biomolecules — Class 11 Biology NCERT Solutions (Free)
Free step-by-step NCERT solutions for Class 11 Biology chapter "Biomolecules" — 7 important questions with detailed answers for CBSE board exam preparation.
TL;DR: Free step-by-step NCERT solutions for Class 11 Biology chapter "Biomolecules" — 7 important questions with detailed answers for CBSE board exam prepar…
Written & reviewed by the Syllab.in Academic Team (CBSE/NCERT subject experts) · Updated
NCERT Solutions for Class 11 Biology — Biomolecules. Step-by-step answers to all 7 textbook questions from this chapter, written for CBSE board preparation and free to use.
Biomolecules — All 7 Questions Solved
Q1. Define carbohydrates and classify them with examples.
Carbohydrates are organic compounds composed of carbon, hydrogen, and oxygen in the ratio Cx(H2O)y. They are primarily synthesized by photosynthetic organisms and serve as energy sources and structural components.
Classification:
1. Monosaccharides (simple sugars; 3-7 carbons):
- Trioses (3C): Glyceraldehyde, dihydroxyacetone.
- Pentoses (5C): Ribose, deoxyribose, ribulose (in photosynthesis).
- Hexoses (6C): Glucose, fructose, galactose.
- Function: Energy metabolism; components of nucleic acids.
2. Disaccharides (two monosaccharide units joined by glycosidic bond):
- Sucrose (glucose + fructose): Transport form in plants; sweetener.
- Maltose (glucose + glucose): Malt sugar; intermediate in starch digestion.
- Lactose (glucose + galactose): Milk sugar.
- Function: Quick energy; transport.
3. Polysaccharides (many monosaccharide units):
- Starch: Storage polysaccharide in plants; amylose and amylopectin.
- Glycogen: Storage polysaccharide in animals; branched structure.
- Cellulose: Structural polysaccharide in plant cell walls; linear glucose chains.
- Chitin: Structural polysaccharide in arthropod exoskeleton and fungal cell walls.
Functions: Energy storage and metabolism, structural support, osmotic regulation.
Q2. What are proteins? Describe the structure of amino acids and peptide bonds.
Proteins are polymers of amino acids; they are fundamental molecules performing diverse functions: enzyme catalysis, structural support, transport, immunity, regulation. Amino acid structure:
All amino acids have:
1. Carboxyl group (-COOH): Acidic group; donates proton.
2. Amino group (-NH2): Basic group; accepts proton.
3. Hydrogen atom (-H).
4. Radical group (R group or side chain): Variable structure distinguishing different amino acids.
5. Central carbon (alpha carbon): Bonds all four groups.
There are 20 common amino acids classified by R group properties:
- Polar (hydrophilic): Serine, threonine, cysteine, tyrosine, asparagine, glutamine.
- Nonpolar (hydrophobic): Alanine, valine, leucine, isoleucine, methionine, phenylalanine, tryptophan, proline.
- Charged (basic): Lysine, arginine, histidine.
- Charged (acidic): Aspartic acid, glutamic acid.
Peptide bond:
- Covalent bond formed between carboxyl group (-COOH) of one amino acid and amino group (-NH2) of another.
- Formation involves condensation reaction (dehydration), releasing water molecule.
- Bond linking is C-N bond called peptide bond.
- Creates dipeptide (2 amino acids) or polypeptide (multiple amino acids).
- Many peptide bonds link amino acids forming protein backbone; R groups project from backbone.
Significance: Amino acid sequence determines protein structure and function; proteins essential for all life processes.
Q3. Explain the levels of protein structure with examples.
Protein structure has four hierarchical levels:
1. Primary structure:
- Linear sequence of amino acids in polypeptide chain.
- Held by peptide bonds.
- Example: Insulin has 51 amino acids in specific sequence.
- Determines all higher levels of structure; small changes (mutations) can drastically affect function.
2. Secondary structure:
- Repetitive, local folding caused by hydrogen bonds between backbone atoms (not R groups).
- Common structures:
a) α-helix: Coiled structure; found in keratin (hair), collagen (connective tissue).
b) β-pleated sheet: Extended, zigzag structure; found in silk fibroin, β-keratin (feathers).
c) Random coil: Irregular regions connecting helices and sheets.
- Held by hydrogen bonds between C=O and N-H groups of backbone.
3. Tertiary structure:
- Overall three-dimensional shape of single polypeptide chain.
- Formed by interactions of R groups (side chains):
a) Hydrogen bonds: Between polar R groups.
b) Ionic bonds (salt bridges): Between charged R groups (+ and -).
c) Disulfide bonds (covalent): Between cysteine residues; strong, permanent.
d) Hydrophobic interactions: Nonpolar R groups cluster away from water.
e) Van der Waals forces: Weak interactions between atoms.
- Determines overall protein shape and active sites.
- Example: Myoglobin (oxygen transport); hemoglobin monomer.
4. Quaternary structure:
- Assembly of multiple polypeptide chains (subunits) in proteins.
- Not all proteins have quaternary structure (only those with 2+ chains).
- Held by same forces as tertiary structure (but between different chains).
- Example: Hemoglobin (4 subunits: 2 α, 2 β chains); collagen (3 chains).
- Allows cooperativity (subunits affect each other's function).
Significance: Each level builds upon previous; disruption at any level (denaturation, mutation, misfolding) affects protein function and can cause disease.
Q4. Define lipids and describe the structure and functions of triglycerides.
Lipids are a diverse group of hydrophobic (nonpolar) or amphipathic molecules composed primarily of carbon, hydrogen, and some oxygen. They are poorly soluble in water but soluble in nonpolar solvents.
Triglycerides (triacylglycerols, fats and oils):
Structure:
- Composed of glycerol (3-carbon alcohol with 3 hydroxyl groups) + 3 fatty acids.
- Each fatty acid is a long hydrocarbon chain (14-18 carbons typically) with a carboxyl group at one end.
- Fatty acids joined to glycerol via ester bonds (formed by condensation between carboxyl of fatty acid and hydroxyl of glycerol), releasing water.
- Saturated fatty acids: All C-C bonds are single bonds; straight chain; solid at room temperature.
Examples: Palmitic acid, stearic acid.
- Unsaturated fatty acids: Contain one or more C=C double bonds; kinked structure; liquid (oils) at room temperature.
Examples: Oleic acid, linoleic acid.
Functions:
1. Energy storage: Most efficient (9 kcal/gram); can be oxidized via β-oxidation for ATP production.
2. Insulation: Layer of fat under skin prevents heat loss in warm-blooded animals.
3. Cushioning: Protects organs from mechanical damage.
4. Vitamin carrier: Fat-soluble vitamins (A, D, E, K) dissolve in fats and are transported/stored.
5. Buoyancy: In aquatic mammals (blubber).
6. Signaling: Prostaglandins derived from fatty acids regulate inflammation, reproduction.
Significance: Triglycerides are major energy reserves in organisms; high-calorie density makes them ideal for long-term energy storage (more efficient than carbohydrates and proteins).
Q5. Describe the structure and functions of nucleic acids (DNA and RNA).
Nucleic acids are polymers of nucleotides; they store and express genetic information. Structure:
Nucleotide components:
1. Nitrogenous base (purine or pyrimidine):
- Purines (2 rings): Adenine (A), Guanine (G).
- Pyrimidines (1 ring): Cytosine (C), Thymine (T), Uracil (U).
2. Five-carbon sugar: Ribose (RNA) or Deoxyribose (DNA; lacks oxygen at 2' position).
3. Phosphate group: Connects sugars of adjacent nucleotides.
DNA (Deoxyribonucleic acid):
- Double helix structure (discovered by Watson, Crick, Franklin).
- Two antiparallel polynucleotide chains held by hydrogen bonds between bases: A-T (2 bonds), G-C (3 bonds).
- Bases stack inside helix; sugar-phosphate backbone outside.
- More stable than RNA due to deoxyribose and thymine.
- Mostly located in nucleus (some in mitochondria and chloroplasts).
- Functions: Storage of genetic information; enables heredity and variation; directs protein synthesis.
RNA (Ribonucleic acid):
- Single-stranded polynucleotide chain.
- Contains uracil instead of thymine.
- Ribose sugar (less stable than deoxyribose).
- Types:
a) mRNA (messenger RNA): Carries genetic information from DNA to ribosomes; directs protein synthesis.
b) rRNA (ribosomal RNA): Structural and catalytic component of ribosomes.
c) tRNA (transfer RNA): Brings amino acids to ribosomes; anticodon pairs with mRNA codon.
- Functions: Protein synthesis; gene regulation; catalysis (some RNAs are enzymes, ribozymes).
Significance: Nucleic acids are essential for life; DNA maintains genetic continuity; RNA translates genetic information into proteins.
Q6. Explain the structure of cellulose and its importance in plant cell walls.
Cellulose is a structural polysaccharide composed of glucose units linked by β-1,4-glycosidic bonds. Structure:
1. Monomer unit: β-glucose (differs from α-glucose in configuration at C1 carbon).
2. Polymer chain:
- Linear, unbranched polymer of β-glucose.
- Thousands of glucose units per molecule.
- Forms long, rigid fibers due to extended conformation.
- Polymer length determines mechanical properties.
3. Hydrogen bonding:
- Hydrogen bonds form between hydroxyl groups of adjacent glucose units.
- Extensive inter- and intra-molecular H-bonding creates strong, rigid structure.
- Multiple cellulose chains align and pack closely together, forming cellulose fibrils (microfibrils and macrofibrils).
4. Microfibrils:
- Basic structural unit visible under electron microscope.
- Bundles of parallel cellulose chains.
- Arranged in organized patterns within cell wall.
Importance in plant cell walls:
1. Structural support: Cellulose microfibrils provide tensile strength; prevents cell bursting under turgor pressure; allows upright growth.
2. Shape and rigidity: Gives plant cells definite shape (unlike animal cells).
3. Mechanical strength: Makes cell wall strong yet flexible; allows plants to withstand environmental stresses (wind, gravity).
4. Protection: Forms barrier against pathogens and physical damage.
5. Water absorption: Cellulose's hydrophilic nature allows cell wall to absorb and retain water, maintaining turgor.
6. Fiber organization: Parallel arrangement of microfibrils provides directional strength; affects plant growth patterns.
Note: Humans lack cellulase enzyme and cannot digest cellulose; however, cellulose (dietary fiber) is important for digestive health. Some animals (herbivores) have symbiotic bacteria producing cellulase for cellulose digestion.
Q7. What is ATP and how does it function as an energy currency in cells (5-mark answer)?
ATP (Adenosine Triphosphate) is the primary energy currency of cells; it captures, stores, and releases energy for cellular processes. Structure:
1. Components:
- Adenine: Nitrogenous base (purine).
- Ribose: 5-carbon sugar.
- Triphosphate: Three phosphate groups linked by two high-energy phosphoanhydride bonds.
2. High-energy bonds:
- Bonds between phosphate groups store chemical energy (~7.3 kcal/mol per bond under cellular conditions).
- When terminal phosphate is removed, large amount of energy is released (~30.5 kJ/mol).
Function as energy currency:
1. Energy release:
- ATP hydrolysis: ATP + H2O → ADP + Pi + Energy (30.5 kJ/mol).
- Energy released drives endergonic reactions (requiring energy).
- Also produces: ADP (adenosine diphosphate) and inorganic phosphate (Pi).
2. Coupling with cellular reactions:
- Exergonic reactions (ATP hydrolysis) coupled with endergonic reactions (biosynthesis, muscle contraction, active transport).
- Energy from ATP breakdown drives these reactions forward.
- Example: Na+/K+-ATPase uses ATP energy to pump ions against concentration gradient.
3. Energy regeneration:
- ATP continuously regenerated from ADP by:
a) Cellular respiration: Oxidation of glucose, fatty acids, amino acids.
b) Photosynthesis (in plants): Light reactions produce ATP.
- Each ATP molecule recycled ~500 times daily; provides rapid energy supply.
4. Muscle contraction:
- ATP powers myosin-actin interactions in sarcomeres.
- Myosin heads use ATP energy to pull actin filaments, causing contraction.
5. Biosynthetic pathways:
- ATP provides energy for synthesizing proteins, nucleic acids, lipids, polysaccharides.
- Enables organism to build complex molecules from simpler ones.
Significance: ATP is universal energy currency; its continuous production and hydrolysis enables all life processes. Without ATP regeneration, cells cannot survive.
Biomolecules — Practice MCQs with Answers
Attempt these 20 multiple-choice questions after working through the solutions above. Each carries the correct option and the reasoning behind it, so a wrong answer tells you which idea to revisit.
MCQ 1. Which of the following is NOT considered a primary metabolite?
- A. Amino acids
- B. Sugars
- C. Alkaloids
- D. Nucleotides
Answer: C. Alkaloids — Primary metabolites are directly involved in the normal growth, development, and reproduction of an organism, like amino acids, sugars, and nucleotides. Alkaloids are secondary metabolites, not directly involved in essential life processes.
MCQ 2. The analysis of chemical composition of living tissues reveals that inorganic compounds like water, minerals, and gases constitute a significant part. Which of these is the most abundant inorganic component in living organisms?
- A. Proteins
- B. Water
- C. Carbohydrates
- D. Lipids
Answer: B. Water — Water is the most abundant chemical compound in living organisms, typically making up 70-90% of a cell's total weight. Proteins, carbohydrates, and lipids are organic macromolecules.
MCQ 3. All amino acids have a central carbon atom (alpha-carbon) to which four groups are attached. These groups are an amino group, a carboxyl group, a hydrogen atom, and a variable group denoted as 'R'. Which of these groups determines the unique properties of each amino acid?
- A. Amino group
- B. Carboxyl group
- C. R-group
- D. Hydrogen atom
Answer: C. R-group — The 'R' group, or side chain, varies among different amino acids and gives each amino acid its unique chemical and physical properties, such as polarity, charge, and size.
MCQ 4. Which type of bond is formed between two amino acids to create a dipeptide?
- A. Glycosidic bond
- B. Ester bond
- C. Peptide bond
- D. Hydrogen bond
Answer: C. Peptide bond — A peptide bond is a covalent bond formed between the carboxyl group of one amino acid and the amino group of another amino acid, with the removal of a water molecule (dehydration synthesis).
MCQ 5. The alpha-helix and beta-pleated sheet structures are examples of which level of protein organization?
- A. Primary structure
- B. Secondary structure
- C. Tertiary structure
- D. Quaternary structure
Answer: B. Secondary structure — Secondary structure refers to the local folding of the polypeptide chain into regular repeating structures like alpha-helices and beta-pleated sheets, stabilized by hydrogen bonds between backbone atoms.
MCQ 6. Denaturation of a protein primarily affects its ________ and ________ structures, often disrupting critical interactions such as ________ and ________ bonds.
- A. primary, secondary, peptide, disulfide
- B. secondary, tertiary, hydrogen, disulfide
- C. tertiary, quaternary, phosphodiester, glycosidic
- D. primary, tertiary, peptide, hydrogen
Answer: B. secondary, tertiary, hydrogen, disulfide — Denaturation involves the disruption of a protein's secondary, tertiary, and sometimes quaternary structures, often by breaking weak interactions like hydrogen bonds, ionic bonds, and disulfide bridges, without breaking the primary peptide bonds.
MCQ 7. Which of the following is a monosaccharide?
- A. Sucrose
- B. Lactose
- C. Glucose
- D. Starch
Answer: C. Glucose — Glucose is a simple sugar (monosaccharide) and is the primary source of energy for living cells. Sucrose and lactose are disaccharides, while starch is a polysaccharide.
MCQ 8. Cellulose is a major component of plant cell walls. Which type of glycosidic linkage is characteristic of cellulose, making it difficult for most animals to digest?
- A. α-1,4 glycosidic linkage
- B. β-1,4 glycosidic linkage
- C. α-1,6 glycosidic linkage
- D. β-1,6 glycosidic linkage
Answer: B. β-1,4 glycosidic linkage — Cellulose is a linear polymer of glucose units linked by β-1,4 glycosidic bonds. Most animals lack the enzyme cellulase to hydrolyze these bonds, unlike starch which has α-1,4 linkages.
MCQ 9. Which of the following is NOT a type of lipid?
- A. Triglycerides
- B. Phospholipids
- C. Steroids
- D. Glycogen
Answer: D. Glycogen — Glycogen is a polysaccharide that serves as an energy storage molecule in animals and fungi. Triglycerides, phospholipids, and steroids are all classes of lipids.
MCQ 10. A triglyceride is composed of one molecule of glycerol and three molecules of which other compound?
- A. Amino acid
- B. Fatty acid
- C. Monosaccharide
- D. Nucleotide
Answer: B. Fatty acid — Triglycerides are formed by an ester linkage between one glycerol molecule and three fatty acid molecules.
MCQ 11. Saturated fatty acids are characterized by the absence of double bonds between carbon atoms in their hydrocarbon chains. This structural feature typically leads to which of the following properties?
- A. They are usually liquid at room temperature due to more kinks in their chains.
- B. They pack more tightly together, making them solid at room temperature.
- C. They are less stable and more reactive than unsaturated fatty acids.
- D. They have a lower melting point compared to unsaturated fatty acids of similar chain length.
Answer: B. They pack more tightly together, making them solid at room temperature. — The straight chains of saturated fatty acids allow them to pack closely, increasing van der Waals interactions and raising their melting point, making them solid at room temperature (e.g., butter). Unsaturated fatty acids have kinks due to double bonds, preventing tight packing and making them liquid (e.g., olive oil).
MCQ 12. The basic building block (monomer) of nucleic acids like DNA and RNA is a:
- A. Amino acid
- B. Nucleoside
- C. Nucleotide
- D. Fatty acid
Answer: C. Nucleotide — A nucleotide is composed of three components: a nitrogenous base, a pentose sugar (deoxyribose in DNA, ribose in RNA), and a phosphate group. Nucleosides lack the phosphate group.
MCQ 13. In a DNA strand, adjacent nucleotides are linked by which type of bond?
- A. Peptide bond
- B. Glycosidic bond
- C. Phosphodiester bond
- D. Ester bond
Answer: C. Phosphodiester bond — Phosphodiester bonds connect the 3' hydroxyl group of one nucleotide's sugar to the 5' phosphate group of the next nucleotide, forming the sugar-phosphate backbone of DNA and RNA.
MCQ 14. If a double-stranded DNA molecule contains 20% guanine, what is the percentage of adenine?
- A. 20%
- B. 30%
- C. 40%
- D. 60%
Answer: B. 30% — According to Chargaff's rules, in double-stranded DNA, the amount of guanine (G) equals cytosine (C), and adenine (A) equals thymine (T). If G = 20%, then C = 20%, making G+C = 40%. The remaining 60% must be A+T. Since A=T, then A = 60%/2 = 30%.
MCQ 15. Enzymes are biological catalysts that:
- A. Increase the activation energy of a reaction
- B. Are consumed during the reaction
- C. Speed up the rate of a biochemical reaction
- D. Change the equilibrium of a reaction
Answer: C. Speed up the rate of a biochemical reaction — Enzymes are proteins that act as biological catalysts, significantly increasing the rate of specific biochemical reactions by lowering their activation energy, without being consumed in the process.
MCQ 16. The "lock and key" model of enzyme action has been refined by the "induced fit" model. What is the key difference proposed by the induced fit model?
- A. The active site is rigid and unchanging.
- B. The enzyme's active site changes shape slightly upon substrate binding.
- C. The substrate changes its shape to fit the rigid active site.
- D. Enzymes can bind to any type of substrate.
Answer: B. The enzyme's active site changes shape slightly upon substrate binding. — The induced fit model suggests that the active site of an enzyme is not rigid but undergoes a conformational change to precisely fit the substrate, optimizing the enzyme-substrate interaction for catalysis.
MCQ 17. An enzyme exhibits maximum activity at a specific pH and temperature. If the temperature is significantly increased beyond the optimal level, what is the most likely consequence for the enzyme?
- A. Its activation energy will increase.
- B. It will become denatured, losing its three-dimensional structure and activity.
- C. Its affinity for the substrate will increase.
- D. It will start catalyzing a different reaction.
Answer: B. It will become denatured, losing its three-dimensional structure and activity. — Extreme temperatures (above optimal) can cause enzymes to denature, meaning their intricate three-dimensional structure, particularly the active site, unfolds and loses its catalytic function due to the disruption of weak bonds.
MCQ 18. Some enzymes require non-protein chemical components called cofactors to be active. If the cofactor is an organic molecule, it is specifically referred to as a:
- A. Apoenzyme
- B. Holoenzyme
- C. Coenzyme
- D. Prosthetic group
Answer: C. Coenzyme — Coenzymes are organic cofactors that are typically loosely bound to the enzyme. Prosthetic groups are also organic but are tightly bound. An apoenzyme is the protein part without the cofactor, and a holoenzyme is the complete active enzyme with its cofactor.
MCQ 19. The sum total of all chemical reactions occurring in a living organism is called:
- A. Catabolism
- B. Anabolism
- C. Metabolism
- D. Homeostasis
Answer: C. Metabolism — Metabolism encompasses all the biochemical processes that occur within a living organism to maintain life, including both catabolism (breaking down molecules) and anabolism (building up molecules).
MCQ 20. Identify the secondary metabolite among the following options:
- A. Glucose
- B. Insulin
- C. Rubber
- D. DNA
Answer: C. Rubber — Secondary metabolites are organic compounds that are not directly involved in the normal growth, development, or reproduction of an organism, but often have ecological functions. Rubber is a well-known example.
How to Use These Solutions
Attempt each question yourself first and only then compare with the worked answer. Marks in the CBSE board exam are awarded for the METHOD as much as the final result, so reproduce the steps rather than memorising the last line. Where a solution states a law, a formula or a definition, learn that wording — examiners look for it.
Related practice for this chapter: chapter MCQs, previous-year questions, revision notes and sample papers.