NCERT Solutions for Class 11 Biology Chapter 9: Biomolecules (NCERT 2026–27)
These Class 11 Biology Chapter 9 solutions cover Biomolecules — the chemistry of life. The chapter explains how living tissues are analysed, the difference between primary and secondary metabolites, the four classes of biomacromolecules (proteins, polysaccharides, nucleic acids and lipids), the four levels of protein structure, and the nature and action of enzymes. Every NCERT Exercise question is reproduced verbatim from the textbook and answered below in clear, exam-ready prose.
Chapter 9, Biomolecules, looks at living organisms from a chemical point of view. Although the elemental list obtained from living tissue is similar to that of the earth’s crust, the relative abundance of carbon, hydrogen and oxygen is far higher in living systems, and water is the most abundant chemical in any organism. Grinding a tissue in acid separates an acid-soluble pool (thousands of small biomolecules <1000 Da such as amino acids, sugars, nucleotides and fatty acids) from an acid-insoluble fraction (the macromolecules: proteins, nucleic acids and polysaccharides, plus lipids that travel here as membrane vesicles). The chapter classifies metabolites into primary (with known functions) and secondary (alkaloids, pigments, rubber, etc.), describes the four levels of protein structure (primary, secondary, tertiary, quaternary), and explains enzymes — their active site, the way they lower activation energy, factors affecting their activity, classification into six groups and the role of co-factors.
Key Concepts & Definitions
Biomolecule: any carbon compound obtained from living tissue. Micromolecules have molecular weights below ~1000 Da (acid-soluble pool); macromolecules (biomacromolecules) are found in the acid-insoluble fraction with weights of ten thousand Da and above.
Acid-soluble pool vs acid-insoluble fraction: grinding tissue in trichloroacetic acid gives a filtrate (acid-soluble pool, ~cytoplasmic small molecules) and a retentate (acid-insoluble fraction, the macromolecules).
Primary metabolites: biomolecules with identifiable functions and known roles in physiology (amino acids, sugars, nucleotides, etc.). Secondary metabolites: alkaloids, flavonoids, rubber, essential oils, pigments, toxins, antibiotics — many useful to human welfare though their roles in the host are not always understood.
Amino acids: α-amino acids carrying an amino group, a carboxyl group, a hydrogen and a variable R group on the same (α) carbon; 20 types occur in proteins; they can be acidic, basic or neutral.
Proteins: heteropolymers of amino acids joined by peptide bonds (polypeptides). Collagen is the most abundant protein in the animal world; RuBisCO is the most abundant protein in the biosphere.
Polysaccharides: long chains of sugars (e.g. cellulose, starch, glycogen, inulin, chitin); starch holds I2 to give a blue colour, cellulose does not.
Nucleic acids: polynucleotides; each nucleotide = a nitrogen base + a pentose sugar (ribose or 2′-deoxyribose) + phosphate. DNA and RNA store and transmit genetic information.
Enzymes: proteins (a few are RNA – ribozymes) that catalyse reactions by lowering activation energy via an active site; they show substrate specificity and need an optimum temperature and pH.
Average composition of cells (Table 9.4)
Component
% of total cellular mass
Water
70–90
Proteins
10–15
Carbohydrates
3
Lipids
2
Nucleic acids
5–7
Ions
1
NCERT Exercises — Solutions
All 11 exercise questions are reproduced verbatim from the NCERT textbook. Several are open-ended “find out / make a list” activities; for these the answers below give the expected information and worked examples a student can use.
1. What are macromolecules? Give examples.
ANSWERMacromolecules (biomacromolecules) are large, high-molecular-weight molecules — generally ten thousand daltons and above — that are found in the acid-insoluble fraction of living tissue. With the exception of lipids, they are polymers built from repeating smaller units (monomers).There are only three true polymeric classes: proteins (polymers of amino acids, e.g. collagen, insulin, trypsin), nucleic acids (polymers of nucleotides, e.g. DNA and RNA) and polysaccharides (polymers of sugars, e.g. cellulose, starch, glycogen). Lipids are not strictly macromolecules — their molecular weight does not exceed about 800 Da — but they appear in the macromolecular fraction because membrane fragments form water-insoluble vesicles that separate with the acid-insoluble pool.
2. What is meant by tertiary structure of proteins?
ANSWERThe tertiary structure of a protein is the overall three-dimensional shape produced when the long polypeptide chain (already showing regions of secondary structure such as α-helices and β-pleated sheets) is folded back upon itself, much like a hollow woollen ball.This folding brings distant parts of the chain close together and is stabilised by interactions such as hydrogen bonds, disulphide bonds and other side-chain interactions. The tertiary structure gives the true 3-D view of a protein and is absolutely necessary for its biological activity — for example, it creates the crevices and pockets, including the active site of an enzyme.
3. Find and write down structures of 10 interesting small molecular weight biomolecules. Find if there is any industry which manufactures the compounds by isolation. Find out who are the buyers.
ANSWERTen small molecular weight biomolecules (with their molecular formulae) are: glucose (C6H12O6), ribose (C5H10O5), glycerol / trihydroxypropane (C3H8O3), palmitic acid (C16H32O2), arachidonic acid (C20H32O2), glycine (C2H5NO2), alanine (C3H7NO2), serine (C3H7NO3), adenine (C5H5N5, a purine base) and uracil (C4H4N2O2, a pyrimidine base).Industry: Yes — pharmaceutical, food-processing, nutraceutical and biochemical-reagent companies isolate or synthesise such compounds; glucose and amino acids are made on a large scale by fermentation and starch hydrolysis, and fatty acids are obtained from oils.Buyers: hospitals and clinics (glucose for IV drips), the food and beverage industry, drug manufacturers, research laboratories, and companies making supplements, cosmetics and culture media. (This is an activity-based question; the list above is a model answer.)
4. Find out and make a list of proteins used as therapeutic agents. Find other applications of proteins (e.g., Cosmetics etc.)
ANSWERProteins used as therapeutic agents:insulin (treats diabetes), antibodies / monoclonal antibodies (treat infections and cancers), interferons (antiviral/anticancer), blood-clotting factors such as Factor VIII (haemophilia), thrombolytic enzymes like streptokinase (dissolve clots), growth hormone (growth disorders), vaccines containing protein antigens, and digestive enzymes (e.g. trypsin, papain).Other applications: in cosmetics, collagen and keratin are used in skin and hair products; in the food industry, gelatin, casein and whey proteins, and enzymes such as rennet (cheese) and proteases are used; in industry, proteins serve in leather processing, detergents (protease enzymes) and as biological glues. (Activity-based question; the list above is a model answer.)
5. Explain the composition of triglyceride.
ANSWERA triglyceride is a simple lipid (a neutral fat or oil) made by joining one molecule of glycerol with three molecules of fatty acid. Glycerol is trihydroxypropane and has three –OH groups.Each of the three hydroxyl groups of glycerol is esterified with the carboxyl (–COOH) group of a fatty acid, forming three ester linkages and releasing water. The three fatty acids may be the same or different and may be saturated or unsaturated. (Joining one fatty acid gives a monoglyceride and two gives a diglyceride.) Whether the triglyceride is a fat (solid) or an oil (liquid) depends on its melting point — oils, being richer in unsaturated fatty acids, have lower melting points.
6. Can you attempt building models of biomolecules using commercially available atomic models (Ball and Stick models).
ANSWERYes. In a ball-and-stick model the coloured balls represent atoms (commonly black = carbon, white = hydrogen, red = oxygen, blue = nitrogen) and the sticks represent the covalent bonds, with single, double or triple sticks for the bond order.A simple molecule to start with is water (H2O) — one oxygen ball with two hydrogen balls. Next try glycine (NH2–CH2–COOH) and other amino acids, then a monosaccharide such as glucose, and finally link units with peptide or glycosidic bonds to appreciate how monomers build polymers. The exercise helps visualise 3-D shape, bond angles and functional groups. (This is a hands-on activity; the description above explains how to do it.)
7. Draw the structure of the amino acid, alanine.
ANSWERAlanine is an α-amino acid in which the central (α) carbon carries four groups: a hydrogen atom (–H), an amino group (–NH2), a carboxyl group (–COOH) and a methyl group (–CH3) as its R group. Its structural formula can be written as:
H2N–CH(CH3)–COOH (molecular formula C3H7NO2)
Expanded around the α-carbon: the central C is bonded to –NH2, –COOH, –H and –CH3. In solution it can exist as a zwitterion, H3N+–CH(CH3)–COO−.
8. What are gums made of? Is Fevicol different?
ANSWERGums are natural polysaccharides (complex carbohydrates) — long chains of sugar units, often containing modified sugars and uronic acids. They are secondary metabolites obtained from plants (for example gum arabic, guar gum) and are water-soluble or water-swellable, sticky substances.Yes, Fevicol is different. Fevicol is a synthetic adhesive — a man-made polymer emulsion (polyvinyl acetate). It is not a natural biomolecule, so although both are sticky, gums are biological polysaccharides while Fevicol is a chemically manufactured polymer.
9. Find out a qualitative test for proteins, fats and oils, amino acids and test any fruit juice, saliva, sweat and urine for them.
ANSWERQualitative tests:
Biomolecule
Test
Positive result
Proteins
Biuret test (add NaOH + dilute CuSO4)
Violet / purple colour
Amino acids
Ninhydrin test (warm with ninhydrin solution)
Purple / blue colour
Fats & oils
Translucent-spot (grease) test on paper, or Sudan III stain
Expected results on samples: fruit juice — usually negative for protein/fat (mainly sugars); saliva — gives a positive Biuret/ninhydrin test because it contains the enzyme amylase (a protein); sweat — gives a weak positive for amino acids/protein (it contains traces); urine — normally negative for protein, but a positive Biuret result indicates abnormal protein in urine. (This is a laboratory activity; results above are the expected outcomes.)
10. Find out how much cellulose is made by all the plants in the biosphere and compare it with how much of paper is manufactured by man and hence what is the consumption of plant material by man annually. What a loss of vegetation!
ANSWERCellulose is the most abundant organic polymer on Earth. Plants in the biosphere are estimated to produce on the order of hundreds of billions of tonnes of cellulose every year (estimates of the order of 1011 tonnes annually).By comparison, the world manufactures only a few hundred million tonnes of paper per year — a very small fraction of the total cellulose produced. However, paper is just one use; humans also consume large quantities of plant material as food, fuel, timber and fibre. The point of the activity is to realise how heavily we draw on plant resources and why conserving vegetation matters. (This is a survey/estimation activity; figures are approximate.)
11. Describe the important properties of enzymes.
ANSWER1. Protein nature: Almost all enzymes are proteins with a definite three-dimensional (tertiary) structure; a few RNA molecules act as enzymes and are called ribozymes.2. Catalytic power: Enzymes are biocatalysts that greatly speed up reactions — for example carbonic anhydrase accelerates the formation of carbonic acid about 10 million times — while remaining chemically unchanged at the end.3. Lowering of activation energy: They work by binding the substrate at the active site, forming a transient enzyme-substrate (ES) complex and a transition state, thereby lowering the activation-energy barrier for the conversion of substrate (S) to product (P).4. Active site & specificity: Each enzyme has a specific active site (a crevice/pocket) into which only its substrate fits, giving enzymes high substrate specificity.5. Sensitivity to temperature and pH: Each enzyme has an optimum temperature and optimum pH; activity falls above and below these values and high temperature denatures the protein (most are damaged above ~40°C, though thermophilic enzymes work up to 80–90°C).6. Effect of substrate concentration: Reaction velocity rises with substrate concentration until a maximum velocity (Vmax) is reached, when all enzyme molecules are saturated.7. Inhibition and co-factors: Activity can be reduced by inhibitors (e.g. competitive inhibition of succinic dehydrogenase by malonate), and many enzymes need non-protein co-factors — prosthetic groups, coenzymes (often vitamin-derived) or metal ions — for activity.
Extra Practice Questions
Short Answer Type Questions
Q1. Why is water considered the most abundant biomolecule, and how is the dry weight of a tissue obtained?
ANSWERWhen the composition of living tissue is arranged class-wise, water makes up about 70–90% of the total cellular mass, more than any other component. To obtain dry weight, a known (wet) weight of tissue is dried so that all the water evaporates; the remaining material is the dry weight.
Q2. Distinguish between a nucleoside and a nucleotide.
ANSWERA nucleoside is formed when a nitrogen base is attached to a sugar (e.g. adenosine, guanosine, cytidine). A nucleotide is formed when a phosphate group is additionally esterified to the sugar of a nucleoside (e.g. adenylic acid, guanylic acid). Thus nucleotide = nucleoside + phosphate.
Q3. Why are lipids found in the acid-insoluble (macromolecular) fraction even though they are small molecules?
ANSWERLipids have molecular weights under ~800 Da, so they are not really macromolecules. But in cells many lipids are organised into membranes. When tissue is ground, the membranes break into water-insoluble vesicles that settle with the acid-insoluble pool, so lipids appear in the macromolecular fraction.
Q4. What is a zwitterion?
ANSWERBecause the –NH2 and –COOH groups of an amino acid are ionisable, in solution an amino acid can exist as a dipolar ion carrying both a positive (–NH3+) and a negative (–COO−) charge at the same time. This electrically neutral but doubly charged form is called the zwitterionic form, and it changes with the pH of the solution.
Q5. Differentiate between competitive inhibition and the effect of high temperature on enzymes.
ANSWERIn competitive inhibition an inhibitor that resembles the substrate competes for the active site (e.g. malonate vs succinate), reducing activity reversibly without destroying the enzyme. High temperature, on the other hand, denatures the enzyme by disrupting its tertiary structure, permanently destroying its catalytic activity.
Long Answer Type Questions
Q1. Describe the four levels of organisation of protein structure.
ANSWERPrimary structure is the linear sequence of amino acids in the polypeptide, with the first amino acid as the N-terminal and the last as the C-terminal end. Secondary structure arises when parts of the chain fold into regular shapes such as the right-handed α-helix or the β-pleated sheet. Tertiary structure is the overall 3-D folding of the whole chain upon itself (like a hollow woollen ball), stabilised by hydrogen and disulphide bonds, and is essential for biological activity. Quaternary structure describes how two or more folded polypeptide subunits are arranged together; for example, adult human haemoglobin has four subunits — two α and two β types.
Q2. Explain the mechanism of enzyme action with the concept of activation energy.
ANSWERAn enzyme has an active site into which the substrate (S) diffuses and binds, forming an enzyme-substrate (ES) complex. Binding induces the enzyme to alter its shape and fit tightly around the substrate (induced fit). In this state the substrate is converted into an unstable transition state, the bonds of the substrate are broken and new bonds form, giving an enzyme-product (EP) complex; finally the product (P) is released and the free enzyme is ready again. The reaction sequence is E + S → ES → EP → E + P. To change into product the substrate must cross a high-energy transition state; the energy difference between the substrate and this transition state is the activation energy. Enzymes work by lowering this activation-energy barrier, making the conversion of S to P far easier and faster than the uncatalysed reaction.
Q3. Classify enzymes into their six classes with the type of reaction each catalyses.
ANSWEREnzymes are divided into six classes: Oxidoreductases (dehydrogenases) catalyse oxidation-reduction between two substrates; Transferases catalyse the transfer of a group (other than hydrogen) from one substrate to another; Hydrolases catalyse hydrolysis of ester, ether, peptide, glycosidic, C–C, C–halide or P–N bonds; Lyases catalyse the removal of groups by mechanisms other than hydrolysis, leaving double bonds; Isomerases catalyse interconversion of optical, geometric or positional isomers; and Ligases catalyse the joining of two compounds, e.g. linking C–O, C–S, C–N or P–O bonds. Each class has 4–13 subclasses and enzymes are named by a four-digit number.
MCQs
1. The most abundant chemical in living organisms is:
(a) protein (b) water (c) carbohydrate (d) lipid
2. The most abundant protein in the whole biosphere is:
(a) collagen (b) insulin (c) RuBisCO (d) trypsin
3. Which of the following is a secondary metabolite?
For each Assertion–Reason question, choose: (A) Both true and the Reason correctly explains the Assertion; (B) Both true but the Reason is not the correct explanation; (C) Assertion true, Reason false; (D) Assertion false, Reason true.
A-R 1. Assertion: Lipids appear in the acid-insoluble (macromolecular) fraction of tissue.
Reason: Lipids form water-insoluble membrane vesicles that separate with the acid-insoluble pool.
A-R 2. Assertion: The tertiary structure of a protein is essential for its biological activity.
Reason: Three-dimensional folding creates pockets such as the active site of an enzyme.
A-R 3. Assertion: Enzymes increase the activation energy of a reaction.
Reason: A higher energy barrier makes the conversion of substrate to product faster.
A-R 4. Assertion: A protein is a heteropolymer and not a homopolymer.
Reason: A protein is built from 20 different types of amino acids.
A-R 5. Assertion: High temperature permanently destroys the activity of most enzymes.
Reason: Heat denatures the protein by disrupting its tertiary structure.
Answer key: 1-(A), 2-(A), 3-(D), 4-(A), 5-(A).
Common Mistakes & Exam Tips
Common mistakes to avoid
Calling lipids true macromolecules — they are small (<800 Da) molecules that only travel in the macromolecular fraction as membrane vesicles.
Confusing a nucleoside (base + sugar) with a nucleotide (base + sugar + phosphate).
Writing that enzymes increase activation energy — they lower it.
Saying all 20 amino acids are essential — only some are essential (must come from diet); the rest are non-essential.
Mixing up collagen (most abundant in the animal world) with RuBisCO (most abundant in the biosphere).
Forgetting that a few enzymes are RNA (ribozymes), not proteins.
How to score full marks in this chapter
Memorise crisp definitions (macromolecule, metabolite, nucleoside vs nucleotide, zwitterion, activation energy) and be ready to give examples with each. For protein structure, learn all four levels in order with one example each (haemoglobin for quaternary). For enzymes, list properties as numbered points and always link “active site→ES complex→lowers activation energy.” Quote the textbook figures — water 70–90%, 20 amino acids, 5 nucleotides, six enzyme classes — to show accuracy.
Frequently Asked Questions
What is Class 11 Biology Chapter 9 Biomolecules about?
Chapter 9 studies living organisms chemically — how tissues are analysed, primary and secondary metabolites, the four classes of biomacromolecules (proteins, nucleic acids, polysaccharides and lipids), the four levels of protein structure, and the nature, action and classification of enzymes.
How many exercise questions are there in NCERT Class 11 Biology Chapter 9?
There are 11 questions in the end-of-chapter Exercises. Several are activity-based “find out / make a list” questions; all are reproduced verbatim and answered on this page.
What is the difference between primary and secondary metabolites?
Primary metabolites (amino acids, sugars, nucleotides, etc.) have identifiable functions in normal physiology, while secondary metabolites (alkaloids, pigments, rubber, essential oils, toxins, antibiotics) often have unknown roles in the host but are useful to humans.
Are these Class 11 Biology Chapter 9 solutions free?
Yes. All ClearStudy NCERT Solutions for Class 11 Biology are free and follow the official NCERT textbook for session 2026–27.
