NCERT Solutions for Class 11 Biology Chapter 9: Biomolecules

Concept — macromolecules. When a living tissue is ground in trichloroacetic acid and strained, the thick slurry separates into an acid-soluble pool (the filtrate) and an acid-insoluble fraction (the retentate). The compounds of the acid-insoluble fraction have very high molecular weights — of the order of ten thousand daltons and above — and are called macromolecules or biomacromolecules. They are, with the exception of lipids, polymers built of repeating building blocks.

• Proteins — polypeptides, heteropolymers of 20 types of amino acids joined by peptide bonds (e.g., enzymes, collagen, insulin, antibodies, haemoglobin).
• Polysaccharides — long chains of monosaccharides (e.g., cellulose, starch, glycogen, inulin, chitin).
• Nucleic acids — polynucleotides, polymers of nucleotides (DNA and RNA).
• Lipids — separate with the acid-insoluble fraction only because membranes break into vesicles during grinding; their molecular weight does not exceed about 800 Da, so lipids are not strictly macromolecules.

Extra depth. Macromolecules can be homopolymers (one kind of monomer repeated ‘n’ times, e.g., cellulose — only glucose) or heteropolymers (different monomers, e.g., proteins with 20 amino-acid types). Together with the acid-soluble pool (roughly the cytoplasmic composition) they represent the entire chemical composition of the tissue. The word ‘true macromolecular fraction’ in NCERT is used for polysaccharides, polypeptides and polynucleotides.

Conclusion: macromolecules are the high-molecular-weight (≥ 10⁴ Da) polymeric compounds of the acid-insoluble fraction — proteins, polysaccharides and nucleic acids (with lipids associated through membranes).

Concept — tertiary structure. A protein is a long heteropolymer thread of amino acids. Its primary structure is the sequence of amino acids, and parts of the thread are coiled as helices or folded into other forms in the secondary structure. In the tertiary structure the long protein chain is folded upon itself like a hollow woolen ball, giving the protein a 3-dimensional (three-dimensional) shape.

• It is the 3-D architecture of one polypeptide chain obtained by folding of the already helical/folded (secondary-structure) regions upon one another.
• The shape is held by hydrogen bonds, disulphide bonds and other interactions between the side chains (R groups) of amino acids that come close in space (Fig. 9.3 shows hydrogen and disulphide bonds).
• Folding of the chain makes crevices or pockets — in enzymes one such pocket is the active site into which the substrate fits.
• NCERT stresses that tertiary structure is absolutely necessary for the many biological activities of proteins; any change that alters it (heat, pH) destroys activity — the protein is denatured.
• When a protein has more than one polypeptide, the arrangement of the folded subunits is the quaternary structure (e.g., haemoglobin: 2α + 2β).

Extra depth. Only right-handed helices are seen in proteins. Because the tertiary structure creates the active site and binding pockets, a mutation that changes even one amino acid can change the folding and hence function. Proteins with a single polypeptide (monomeric) have no quaternary structure — their highest level is tertiary.

Conclusion: tertiary structure is the compact three-dimensional folding of a single polypeptide chain upon itself, essential for the biological activity of the protein.

Concept — approach. This is a project question. Small molecular weight biomolecules (micromolecules, below 1000 Da) are the compounds of the acid-soluble pool; Figure 9.1 already shows several of them. Below are ten such compounds with their classes and formulae; the structures of the first eight are drawn in Figure 9.1 (including adenine, uracil, glucose, ribose, glycine/alanine/serine, palmitic acid, glycerol, nucleosides and nucleotide, cholesterol, lecithin) and the structure of alanine is redrawn in Q7.

• How to find the structures: look up each compound in a biochemistry or organic-chemistry textbook, an encyclopaedia or a chemical database such as PubChem; draw the skeleton with all functional groups (–OH, –COOH, –NH₂, rings).
• Industries that isolate biomolecules: pharmaceutical companies (alkaloids, steroids, vitamins), food and beverage industry (sugars, amino acids, caffeine), cosmetic and soap industry (glycerol, fatty acids, lecithin), fine-chemical/biochemical suppliers (amino acids, nucleotides, bases).
• Buyers: pharmaceutical firms, food processors, cosmetic makers, hospitals (e.g., intravenous fluids), educational and research laboratories. Exact company names and prices must be collected by your own survey (web / local industry) — the formulae above are standard textbook values.

Extra depth. Primary metabolites such as amino acids and sugars occur in all organisms, while secondary metabolites such as alkaloids (morphine, caffeine) are characteristic of particular plants and are often valuable drugs or stimulants (Table 9.3 of NCERT lists alkaloids such as morphine and codeine). Whether a compound is isolated or made by chemical/biological synthesis depends on cost: isolation is used when the compound is abundant in a natural source.

Conclusion: small molecular weight biomolecules such as sugars, amino acids, fatty acids, bases and alkaloids are routinely isolated or manufactured for the pharmaceutical, food, cosmetic and research markets.

Concept — approach. Proteins serve a variety of cellular functions: many are enzymes, some are antibodies, some are receptors, some are hormones and some are structural proteins (NCERT Table 9.5). Because of these functions, purified proteins are used as medicines and in industry.

Extra depth. NCERT notes that some proteins carry nutrients across membranes, fight infection, act as hormones or enzymes. Therapeutic proteins must be produced in pure form; today many are made by genetic engineering in microbes rather than only by extraction (as you will study in biotechnology). The list above contains well-established uses; for local sources and prices consult pharmacy and industry catalogues, as the question asks you to find out.

Conclusion: purified proteins — insulin, antibodies, interferons, clotting factors and enzymes — are therapeutic agents, while collagen, keratin, enzymes and casein have cosmetic, food, detergent and other industrial uses.

Concept — triglyceride. Lipids are generally water-insoluble compounds. Many lipids contain both glycerol and fatty acids, the fatty acids being esterified with glycerol. A glyceride in which all three hydroxyl groups of glycerol are esterified with fatty acids is a triglyceride (Figure 9.1 shows a triglyceride with fatty acids R₁, R₂ and R₃).

• Glycerol — a simple lipid, trihydroxy propane (C₃H₈O₃), with three –OH groups.
• Fatty acids — a carboxyl group attached to an R group; R may be –CH₃, –C₂H₅ or 1 to 19 –CH₂ groups. Palmitic acid has 16 carbons (including the carboxyl carbon); arachidonic acid has 20. They may be saturated (no double bond) or unsaturated (one or more C=C).
• Linkage: the –COOH of each fatty acid combines with an –OH of glycerol to form an ester bond with loss of water. One fatty acid → monoglyceride; two → diglyceride; three → triglyceride.
• Triglycerides are also called fats and oils on the basis of melting point — oils have lower melting point (e.g., gingelly oil) and stay liquid in winter.

Extra depth. R₁, R₂ and R₃ may be identical or different fatty acids, so a great variety of triglycerides is possible. Phospholipids (such as lecithin) differ because one fatty-acid position is replaced by a phosphorylated organic compound. Lipids are not true macromolecules (molecular weight below about 800 Da) but come into the acid-insoluble fraction through membrane vesicles.

Conclusion: a triglyceride is composed of one molecule of glycerol esterified with three fatty-acid molecules (R₁, R₂, R₃), formed with the loss of three water molecules.

Concept — yes, it can be done. Ball-and-stick kits represent atoms as coloured balls and bonds as sticks (or springs for double bonds), so the 3-D shape of a molecule can be built and handled. Starting from small molecules (water, glucose, an amino acid) one can progress to a short peptide.

• Material: a molecular-model kit with carbon (4 holes, usually black), hydrogen (1 hole, white), oxygen (2 holes, red), nitrogen (3 holes, blue), phosphorus and sulphur balls, and sticks/springs for single and double bonds. The colour code is the usual convention; check the colours supplied in your kit.
• Step 1 — decide the structure: take the structural formula from Figure 9.1 (e.g., alanine, glycine, glycerol, glucose, ribose, palmitic acid).
• Step 2 — skeleton: join the carbon balls with sticks to form the backbone or ring (the α-carbon of an amino acid is a tetrahedral carbon).
• Step 3 — substituents: attach –OH, –NH₂, –COOH and R groups to the right carbons; use springs for C=O.
• Step 4 — fill valencies: put hydrogens on every remaining hole so each atom has its proper valency (C = 4, N = 3, O = 2, H = 1).
• Step 5 — check: compare the model with the structure and count atoms against the molecular formula.
• Step 6 — link units: join two amino-acid models by removing –OH from one carboxyl and H from the next amino group to show a peptide bond; similarly link glycerol with three fatty acids to show a triglyceride.

Extra depth. NCERT notes that physicists think in three-dimensional views while organic chemists draw two-dimensional structures and biologists describe proteins at four levels; physical models bridge these views. Large molecules like proteins are modelled with computer software, but small biomolecules are ideal for kits.

Conclusion: yes — with ball-and-stick atomic models, small biomolecules such as amino acids, glycerol, fatty acids and sugars can be built by assembling atoms according to their valencies and checking against the structural formula.

Concept — α-amino acid. Amino acids are organic compounds containing an amino group and an acidic (carboxyl) group on the same carbon, the α-carbon. The four valency positions of this carbon are occupied by hydrogen, a carboxyl group, an amino group and a variable R group. In alanine the R group is a methyl group (–CH₃).

• Formula: H₂N–CH(CH₃)–COOH; molecular formula C₃H₇NO₂; IUPAC name 2-aminopropanoic acid.
• R group: R = H gives glycine, R = methyl gives alanine, R = hydroxy methyl gives serine (the three amino acids of Figure 9.1).
• Class: a neutral amino acid (one amino and one carboxyl group); NCERT lists valine as another neutral example.
• Ionisation: the –NH₂ and –COOH are ionizable, so in solutions of different pH the structure changes; in the zwitterionic form the amino group is –NH₃⁺ and the carboxyl group is –COO⁻.

Extra depth. Amino acids are substituted methanes: the central carbon carries four different groups, so (except glycine, where R = H) the α-carbon is asymmetric. Twenty types of amino acids occur in proteins (e.g., alanine, cysteine, proline, tryptophan, lysine); some are essential (to be supplied in the diet) and some are non-essential.

Conclusion: alanine is the α-amino acid H₂N–CH(CH₃)–COOH, with a methyl group as its R group.

Concept — gums. NCERT lists gums along with rubber and cellulose under the polymeric substances among the secondary metabolites (Table 9.3). Chemically, natural gums are polysaccharides — long chains of sugar units (monosaccharides) — obtained as exudates from plants. Polysaccharides are the ‘threads’ of sugars described in section 9.5.

• Natural gum: a plant-derived polysaccharide (polymer of sugars); it dissolves or swells in water to give a sticky, viscous mass. Examples of gum-yielding plants are the acacia/babul-type trees (gum arabic).
• Fevicol is a commercial synthetic adhesive — a white glue that is an aqueous emulsion of a synthetic polymer, polyvinyl acetate (PVA/PVAc), not a plant polysaccharide. Hence it is different from natural gum.
• Similarity: both are polymers that act as adhesives. Difference: gum is a biological secondary metabolite made of sugar units, while Fevicol is a man-made polymer made of vinyl acetate units.

Extra depth. Rubber (a polymeric secondary metabolite, from the latex of rubber plants) and cellulose are the other polymeric substances of Table 9.3. Natural gums are used as thickeners and stabilisers in foods and medicines; synthetic glues replaced many gums where stronger bonding or water resistance is needed. The statement that Fevicol is a PVAc emulsion is general industrial knowledge — check the label of the product you have.

Conclusion: gums are plant polysaccharides made of sugar units, whereas Fevicol is a synthetic polymer adhesive (polyvinyl acetate) — so yes, it is different.

Concept — qualitative tests. A qualitative test shows only the presence or absence of a class of biomolecule through a colour or visible change. The standard school-laboratory tests are given below (the exact observations depend on the sample you take).

1. Collect each sample in a clean test tube and label it.
2. Run the biuret, ninhydrin and grease-spot/Sudan tests on each sample; include distilled water as a negative control and a known protein/oil as a positive control.
3. Record the colour of each tube in a table and compare with the controls.
4. Draw conclusions: which sample contains which biomolecule.

Extra depth. Use controls so that colour changes are real. Biuret detects peptide bonds (so it needs at least two peptide bonds — free amino acids do not give it), which is why amino acids need the separate ninhydrin test. These are class tests, not quantitative; NCERT only asks you to find and carry out such tests, so the table of observations is the expected pattern, not a fixed result.

Conclusion: biuret tests proteins, ninhydrin tests amino acids and Sudan / grease-spot tests fats and oils; each sample (fruit juice, saliva, sweat, urine) is tested and its colour change recorded against controls.

Concept — approach. Cellulose is a polysaccharide homopolymer of glucose; plant cell walls are made of cellulose, and paper from plant pulp and cotton fibre is cellulosic (section 9.5). The question asks for figures, which change with time; the values below are the commonly quoted order-of-magnitude estimates — you should check the latest data from reliable sources (FAO, paper-industry reports) before writing them in a project.

• Comparison: the paper made by humans is only a small fraction (well under one per cent) of the cellulose plants make each year — nature’s cellulose production is enormous.
• Consumption of plant material by man: includes paper and pulp, timber, fire-wood, cotton and other fibres, and the plants eaten as food and fodder; therefore the total annual consumption of plant material is much larger than paper alone.
• What a loss of vegetation: every tonne of paper needs plant pulp; cutting forests for pulp, timber and fuel reduces vegetation cover, which affects photosynthesis, carbon storage, soil protection and biodiversity.
• What can we do? use both sides of paper, recycle paper, use recycled or agro-waste based paper, reduce printing, plant trees and use sustainably managed pulp.

Extra depth. Because plants synthesise cellulose from glucose made in photosynthesis, using vegetation is also using stored solar energy and carbon. Recycling one lot of paper saves fresh pulp, so total plant consumption can fall without reducing paper use. The numerical estimates given are approximate; NCERT gives no figure, so state your source in a project.

Conclusion: plants make vastly more cellulose each year than humans turn into paper, but human use of plant material (paper, timber, fuel, food) is large enough to reduce vegetation, so conservation and recycling are necessary.

Concept — enzymes. Almost all enzymes are proteins (a few nucleic acids called ribozymes also act as enzymes). They are biological catalysts: each enzyme has a primary, secondary and tertiary structure and its tertiary folding creates crevices, one of which is the active site into which the substrate fits.

1. Protein nature: enzymes are proteins with a 3-D structure (exceptions — ribozymes); loss of the tertiary structure (denaturation) destroys activity.
2. Very high catalytic power: carbonic anhydrase forms about 600,000 molecules of H₂CO₃ per second against about 200 per hour uncatalysed — roughly 10 million times faster.
3. Lower the activation energy: the enzyme binds the substrate at the active site, forms the ES complex and passes through a transition state with a smaller energy barrier (Fig. 9.4).
4. Active site and specificity: the substrate fits the active site; binding induces a change in enzyme shape so that it fits more tightly around the substrate.
5. Not consumed: the enzyme is released unchanged (E + S → ES → EP → E + P) and is ready to bind another substrate molecule.
6. Optimum temperature and pH: each enzyme shows maximum activity at a particular temperature and pH; low temperature keeps it temporarily inactive and high temperature denatures it (Fig. 9.5).
7. Thermal sensitivity vs thermostability: most enzymes are damaged above about 40°C, unlike inorganic catalysts; enzymes of thermophilic organisms remain active up to 80–90°C.
8. Substrate concentration: velocity rises and reaches V_max after saturation of enzyme molecules.
9. Inhibition: a competitive inhibitor resembling the substrate (e.g., malonate for succinic dehydrogenase) competes for the active site and reduces activity.
10. Co-factors: many enzymes need a non-protein co-factor (prosthetic group, co-enzyme or metal ion); the protein part is the apoenzyme.
11. Classification: 6 classes — oxidoreductases, transferases, hydrolases, lyases, isomerases, ligases.

Extra depth. The enzyme does not change the energy difference between substrate and product (whether the reaction is exothermic or endothermic); it only lowers the activation energy. Rate roughly doubles or halves for every 10°C change, but above the optimum the loss of structure dominates. A multi-step metabolic pathway such as glucose to pyruvic acid needs ten enzyme-catalysed reactions; with one or two extra steps the same pathway gives lactic acid, or ethanol in yeast.

Conclusion: enzymes are mainly protein catalysts with an active site that lower activation energy, remain unchanged, work best at optimum temperature and pH, and are controlled by inhibitors and co-factors.