SolveItClass 11 · NCERT

NCERT Solutions · Class 11 Biology Biomolecules

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Exercises 9.1–9.11

  1. Exercise 9.1

    What are macromolecules? Give examples.

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    Macromolecules (or biomacromolecules) are the biomolecules found in the acid-insoluble fraction of a living tissue, with molecular weights in the range of ten thousand daltons and above.
    They are polymers — long chains built by repeating small building blocks.
    Examples:
    Proteins — heteropolymers of amino acids (e.g., collagen, trypsin, insulin, RuBisCO).
    Nucleic acids — DNA and RNA, polynucleotides built of nucleotides.
    Polysaccharides — long chains of sugars (e.g., cellulose, starch, glycogen, inulin, chitin).
    Against these, the compounds of the acid-soluble pool have molecular weights of only about $\displaystyle 18$ to $\displaystyle 800$ Da and are called micromolecules or simply biomolecules — amino acids, sugars, nucleotides, fatty acids, glycerol.
    Lipids separate along with the acid-insoluble (macromolecular) fraction only because grinding breaks membranes into water-insoluble vesicles. Their molecular weight does not exceed $\displaystyle 800$ Da, so lipids are not strictly macromolecules.

  2. Exercise 9.2

    What is meant by tertiary structure of proteins?

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    The tertiary structure is the third level of protein structure: the long protein chain, already carrying helices and other secondary folds, is folded upon itself like a hollow woollen ball.
    It gives the $\displaystyle 3$-dimensional view of a protein.
    Where the folded chain criss-crosses itself, many crevices or pockets are formed; in an enzyme one such pocket is the active site, into which the substrate fits.
    Tertiary structure is absolutely necessary for the many biological activities of proteins.
    It is shown in Figure $\displaystyle 9.3$ (c) of the chapter, between the secondary structure (b) and the quaternary structure (d).
    NCERT_Solution_Class11_Biology_Ch9_Q9-2

  3. Exercise 9.3

    Find and write down structures of 10\displaystyle 10 interesting small molecular weight biomolecules. Find if there is any industry which manufactures the compounds by isolation. Find out who are the buyers.

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    Ten small molecular weight biomolecules, with the structures the chapter prints in Figure $\displaystyle 9.1$:
    Glycine — an amino acid, a substituted methane with H, -COOH, \(\displaystyle -NH_2 \) and R = H on the α-carbon: \(\displaystyle NH_2-CH_2-COOH \).
    Alanine — the same skeleton with R = methyl: \(\displaystyle CH_3-CH(NH_2)-COOH \).
    Serine — R = hydroxymethyl: \(\displaystyle HOCH_2-CH(NH_2)-COOH \).
    Glucose \(\displaystyle (C_6H_{12}O_6) \) — a sugar, drawn as a six-membered ring carrying -OH groups and one \(\displaystyle CH_2OH \).
    Ribose \(\displaystyle (C_5H_{10}O_5) \) — a pentose sugar, a five-membered ring with \(\displaystyle HOCH_2 \) and -OH groups.
    Palmitic acid — a saturated fatty acid of $\displaystyle 16$ carbons including the carboxyl carbon: \(\displaystyle CH_3(CH_2)_{14}COOH \).
    Glycerol — trihydroxy propane, a three-carbon chain with an -OH on each carbon.
    Adenine — a nitrogen base with the double heterocyclic purine ring.
    Uracil — a nitrogen base with the single heterocyclic pyrimidine ring.
    Adenylic acid — a nucleotide: adenine joined to ribose (giving the nucleoside adenosine) with a phosphate esterified to the sugar.
    Other small molecules of Figure $\displaystyle 9.1$ you may draw instead: uridine, cholesterol, lecithin (a phospholipid) and a triglyceride.
    The industrial half of this exercise — which companies isolate such compounds and who buys them — is not in the chapter and has to be found from trade sources, catalogues or a visit to a manufacturer.
    NCERT_Solution_Class11_Biology_Ch9_Q9-3

  4. Exercise 9.4

    Find out and make a list of proteins used as therapeutic agents. Find other applications of proteins (e.g., Cosmetics etc.)

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    The chapter gives you the proteins, not their uses as medicines - Table $\displaystyle 9.5$ lists insulin (a hormone), antibody (fights infectious agents), trypsin (an enzyme), collagen (intercellular ground substance), receptor and GLUT-$\displaystyle 4$, and section $\displaystyle 9.4$ says proteins act as transporters, infection fighters, hormones and enzymes. Which proteins are actually given to patients, and any use in cosmetics or industry, is not stated anywhere in the chapter - the exercise deliberately sends you outside it.
    From outside the book - proteins used as therapeutic agents:
    Insulin - injected in diabetes mellitus, where the body's own insulin is missing or ineffective; now made by recombinant bacteria/yeast.
    Human growth hormone (somatotropin) - treats dwarfism due to growth-hormone deficiency.
    Erythropoietin (EPO) - corrects the anaemia of chronic kidney disease by stimulating red blood cell formation.
    Clotting factor VIII and factor IX - replacement therapy in haemophilia A and B.
    Monoclonal antibodies - e.g. trastuzumab and rituximab in cancer therapy; antibody preparations are also the basis of antisera and antivenom given after snakebite, rabies exposure, diphtheria or tetanus.
    Interferons - antiviral proteins used in hepatitis B and C and in some cancers.
    Streptokinase and tissue plasminogen activator (tPA) - clot-dissolving enzymes given in a heart attack or stroke.
    Asparaginase - an enzyme used against acute lymphoblastic leukaemia; it starves the cancer cells of asparagine.
    Digestive enzyme preparations (pancreatin - trypsin, lipase, amylase) - given when the pancreas does not secrete enough of its own enzymes.
    Vaccine antigens - the hepatitis B vaccine is a protein (the surface antigen) manufactured in recombinant yeast.
    Botulinum toxin - a bacterial protein injected in minute doses to relax muscle spasms and squints.
    From outside the book - other applications of proteins:
    Cosmetics - collagen and elastin in anti-ageing creams and dermal fillers, keratin in hair conditioners and straightening treatments, hydrolysed silk, wheat and soy proteins in shampoos and lotions, and botulinum toxin for smoothing wrinkles.
    Food industry - rennet (chymosin) curdles milk to make cheese; papain from papaya and bromelain from pineapple tenderise meat and clarify beer; gelatin, obtained from collagen, sets jellies and forms capsule shells; whey, soy and egg albumin proteins are sold as food supplements.
    Detergents - proteases, along with lipases and amylases, are added to washing powders to digest protein stains such as blood, egg and gravy.
    Textiles and leather - silk (fibroin) and wool (keratin) are themselves protein fibres, and proteases are used to de-hair and soften hides.
    Diagnostics and research - antibodies in pregnancy test strips and ELISA; DNA polymerase in PCR and restriction enzymes in genetic engineering; glucose oxidase in the test strips of a blood glucose meter.
    Other industry - casein (milk protein) glues for wood and labels, gelatin-based adhesives and photographic emulsion, and collagen casings for sausages.

  5. Exercise 9.5

    Explain the composition of triglyceride.

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    A triglyceride is one molecule of glycerol esterified with three fatty acids.
    Glycerol is trihydroxy propane — a three-carbon chain carrying an -OH group on each carbon.
    A fatty acid is a carboxyl group attached to an R group; the R group may be methyl \(\displaystyle (-CH_3) \), ethyl \(\displaystyle (-C_2H_5) \) or a longer chain of \(\displaystyle -CH_2 \) groups ($\displaystyle 1$ to $\displaystyle 19$ carbons). Palmitic acid, for example, has $\displaystyle 16$ carbons including the carboxyl carbon.
    The fatty acids may be saturated (no double bond) or unsaturated (one or more C=C double bonds), and the three need not be the same — they are written as \(\displaystyle R_1 \), \(\displaystyle R_2 \) and \(\displaystyle R_3 \) in Figure 9.1.
    Each -COOH of a fatty acid is esterified to one -OH of glycerol. Esterifying one -OH gives a monoglyceride, two gives a diglyceride, and all three gives a triglyceride.
    Triglycerides are the fats and oils, told apart by melting point: oils have the lower melting point (e.g., gingelly oil) and so stay liquid in winter.

  6. Exercise 9.6

    Can you attempt building models of biomolecules using commercially available atomic models (Ball and Stick models).

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    Yes — the biomolecules of this chapter are small and simple enough to be built with a ball-and-stick set, and Figure $\displaystyle 9.1$ gives the structures to copy.
    Start with the amino acids, which the chapter describes as substituted methanes: put four sticks on one central α-carbon and attach hydrogen, the carboxyl group \(\displaystyle (-COOH) \), the amino group \(\displaystyle (-NH_2) \) and the R group.
    R = H gives glycine, R = \(\displaystyle -CH_3 \) gives alanine, R = \(\displaystyle -CH_2OH \) gives serine.
    Glycerol — a three-carbon chain with an -OH on each carbon (trihydroxy propane).
    A fatty acid — a carboxyl group on a chain of \(\displaystyle -CH_2 \) groups ending in \(\displaystyle -CH_3 \); build a short one first, since palmitic acid needs $\displaystyle 16$ carbons.
    Ribose and glucose — five- and six-membered sugar rings with -OH groups and a \(\displaystyle CH_2OH \).
    The nitrogen bases — one ring for a pyrimidine (uracil, cytosine, thymine), two fused rings for a purine (adenine, guanine).
    Then join the pieces to make the bigger molecules the chapter names:
    base + sugar = a nucleoside; add a phosphate esterified to the sugar = a nucleotide.
    glycerol + three fatty acids = a triglyceride.
    two amino acids joined by a peptide bond = a dipeptide; repeat to get a polypeptide.
    Building the model makes visible what a drawing hides — that these are three-dimensional shapes, which is why the tertiary structure of a protein matters for its activity.

  7. Exercise 9.7

    Draw the structure of the amino acid, alanine.

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    Alanine is the α-amino acid whose R group is a methyl group: \(\displaystyle CH_3-CH(NH_2)-COOH \).
    What the drawing must show — a substituted methane, one central α-carbon with four bonds radiating from it:
    Hydrogen (-H) written above the α-carbon.
    Amino group \(\displaystyle (-NH_2) \) to the left of the α-carbon.
    Carboxyl group (-COOH) to the right of the α-carbon.
    R group, here a methyl \(\displaystyle (-CH_3) \), written below the α-carbon.
    Label the central carbon as the α-carbon, and label the three functional groups: amino group, carboxyl group, R group (methyl).
    The point the diagram must make is that the amino group and the acidic group sit on the same carbon — that is why these are called α-amino acids.
    You may add the zwitterionic form alongside, in which the -COOH has lost a proton \(\displaystyle (-COO^-) \) and the \(\displaystyle -NH_2 \) has gained one \(\displaystyle (-NH_3^+) \), since both groups are ionizable and the structure changes with pH.
    NCERT_Solution_Class11_Biology_Ch9_Q9-7

  8. Exercise 9.8

    What are gums made of? Is Fevicol different?

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    Gums are plant secondary metabolites — the chapter lists them in Table $\displaystyle 9.3$ under polymeric substances, together with rubber and cellulose.
    What a gum is chemically made of is not stated anywhere in the chapter. From outside the book: gums are polysaccharides, long chains of sugar units. The table row does not establish this — rubber sits in the same row and is not a chain of sugar units.
    Secondary metabolites such as gums are found in plant, fungal and microbial cells and, unlike primary metabolites, their role in the host is not fully understood — but many, gums among them, are useful for human welfare.
    Fevicol is different — it is a manufactured synthetic adhesive, not a substance isolated from a plant, so it is not a secondary metabolite at all.

  9. Exercise 9.9

    Find out a qualitative test for proteins, fats and oils, amino acids and test any fruit juice, saliva, sweat and urine for them.

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    The only qualitative test printed in this chapter is the one for starch: starch holds \(\displaystyle I_2 \) molecules inside its helical secondary structure and the starch-\(\displaystyle I_2 \) complex is blue. Cellulose has no such complex helices, cannot hold \(\displaystyle I_2 \), and gives no colour — so the test also distinguishes the two.
    The chapter gives no test for proteins, for fats and oils, or for amino acids, and no expected results for fruit juice, saliva, sweat or urine.
    The tests usually performed in the laboratory (these come from the chemistry practical course, not from this chapter) are:
    Proteins — the biuret test, which gives a violet colour.
    Amino acids — the ninhydrin test, which gives a purple colour.
    Fats and oils — the translucent-spot test on paper, or staining with Sudan dye.
    What the chapter does justify is why such tests exist at all: proteins are polypeptides with ionizable amino and carboxyl groups, fats and oils are glycerides that are water insoluble, and free amino acids carry the same two reactive groups on the α-carbon.
    The testing itself has to be done in the school laboratory under supervision, and the results recorded from your own observation.

  10. Exercise 9.10

    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!

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    No figure for this can be quoted from the chapter — it gives no data on how much cellulose the biosphere makes or how much paper is manufactured, so the comparison has to be built from outside sources (statistical yearbooks, forestry or paper-industry reports).
    What the chapter does establish, and what the exercise rests on:
    Cellulose is a polymeric polysaccharide made of only one type of monosaccharide, glucose — that is, a homopolymer.
    Plant cell walls are made of cellulose, so every plant in the biosphere is producing it continuously.
    Paper made from plant pulp is cellulosic, and so is cotton fibre — the paper we consume is plant cell wall material.
    Cellulose does not contain complex helices and hence cannot hold \(\displaystyle I_2 \), unlike starch.
    The way to do the exercise: find published estimates of annual global cellulose production by plants and of annual world paper production, express the second as a fraction of the first, and note that the plant material actually felled is larger than the paper finally made, because pulping and processing waste a part of it.

  11. Exercise 9.11

    Describe the important properties of enzymes.

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    They are proteins. Almost all enzymes are proteins; the exceptions are some nucleic acids that behave like enzymes, called ribozymes.
    They have primary, secondary and tertiary structure. An enzyme has a primary structure (its amino acid sequence) and a secondary and tertiary structure; where the folded chain criss-crosses itself, crevices or pockets form, and one such pocket is the active site.
    They are substrate specific. The substrate has to diffuse to the active site and fit into it, so an enzyme-substrate (ES) complex must form: \(\displaystyle E + S \rightleftharpoons ES \rightarrow EP \rightarrow E + P \). The enzyme is released unchanged and is ready to bind another substrate molecule.
    They lower the activation energy. The substrate must pass through a high-energy transition state; the enzyme brings this energy barrier down and makes the change of S to P easy.
    They give enormous reaction rates. Carbonic anhydrase forms about $\displaystyle 600,000$ molecules of \(\displaystyle H_2CO_3 \) every second, against about $\displaystyle 200$ molecules in an hour without the enzyme — roughly $\displaystyle 10$ million times faster.
    They are thermolabile, unlike inorganic catalysts. Inorganic catalysts work efficiently at high temperature and pressure, while enzymes are damaged above about $\displaystyle 40$ °C because heat denatures proteins. Enzymes from thermophilic organisms of hot vents and sulphur springs are an exception and stay active up to $\displaystyle 80$–$\displaystyle 90$ °C.
    They have an optimum temperature and an optimum pH. Each enzyme shows its highest activity at one particular temperature and pH, and activity declines both below and above it; low temperature only preserves the enzyme in a temporarily inactive state, while high temperature destroys the activity.
    They show saturation with substrate. Velocity rises as substrate concentration rises, then levels off at a maximum velocity \(\displaystyle V_{max} \), because the enzyme molecules are fewer than the substrate molecules and, once saturated, no free enzyme is left to bind more substrate.
    They can be inhibited. A chemical that binds the enzyme and shuts off its activity is an inhibitor. A competitive inhibitor closely resembles the substrate and competes for the substrate-binding site — e.g., malonate inhibits succinic dehydrogenase because it resembles succinate.
    Many need co-factors. The protein portion is the apoenzyme, and the non-protein co-factor may be a prosthetic group (haem in peroxidase and catalase), a co-enzyme (NAD and NADP, which contain the vitamin niacin) or a metal ion (zinc for carboxypeptidase). Catalytic activity is lost when the co-factor is removed.
    They are classified into $\displaystyle 6$ classes — oxidoreductases/dehydrogenases, transferases, hydrolases, lyases, isomerases and ligases — each with $\displaystyle 4$-$\displaystyle 13$ subclasses, and each enzyme is named by a four-digit number.
    They work in sequence. A multistep reaction in which each step is catalysed by an enzyme is a metabolic pathway, e.g., glucose becomes pyruvic acid through ten enzyme-catalysed reactions.