Biomolecules ISC Class 11: Carbohydrates, Proteins, Lipids and Enzymes

What Biomolecules is about

Weightage: Biomolecules belongs to the unit Cell: Structure and Function, which CISCE weights at 15 of the 70 marks in the ISC Class XI Biology Paper I (Theory). Class XI is examined by the school, so your school sets the paper within this weightage.Source: CISCE ISC Biology syllabus, examination year 2028, Class XI unit weightage

Key concepts

Carbohydrates: classification and functions

Carbohydrates are polyhydroxy aldehydes or ketones (or compounds that give them on hydrolysis), usually fitting the general formula $C_x(H_{2}O{)}_y$ (deoxyribose, $C_5{H}_{10}{O}_4$, is an exception). Monosaccharides cannot be hydrolysed into simpler sugars. Classified by number of carbons: pentoses (5C), e.g. ribose $C_5{H}_{10}{O}_5$ (in RNA, ATP) and deoxyribose $C_5{H}_{10}{O}_4$ (in DNA; one oxygen fewer at carbon 2); hexoses (6C), e.g. glucose $C_6{H}_{12}{O}_6$, an aldose and the main respiratory fuel; fructose (a ketose) and galactose. Disaccharides are two monosaccharides joined by a glycosidic bond, losing one water: maltose = glucose + glucose (malt sugar, from starch digestion); lactose = glucose + galactose (milk sugar); sucrose = glucose + fructose (cane sugar, the form in which plants transport sugar in phloem; non-reducing). Polysaccharides are long chains: starch (plant storage; amylose and branched amylopectin; its helical chains hold iodine, giving a blue-black colour); glycogen (animal and fungal storage in liver and muscle; more highly branched; red-brown with iodine); cellulose (structural polymer of plant cell walls; straight, unbranched chains of glucose linked so that they form strong fibres; no colour with iodine; cannot be digested by humans); inulin (a polymer of fructose, stored in dahlia tubers and used to measure kidney filtration rate); chitin (a polymer of an amino sugar, N-acetylglucosamine; exoskeleton of arthropods and fungal cell walls). Functions: energy (glucose), storage (starch, glycogen), structure (cellulose, chitin), parts of nucleic acids (ribose, deoxyribose), transport (sucrose).

Worked example

Why does a drop of iodine turn a slice of potato blue-black but leave a piece of cotton unchanged?

Try it first, then show the solutionSolution

1. Potato stores starch, whose helical amylose chains trap iodine to give a blue-black complex
2. Cotton is almost pure cellulose, whose straight chains cannot form the helix, so no colour forms

Answer: Starch gives the iodine test; cellulose does not

Amino acids: structure, zwitterion and types

An amino acid has a central α-carbon bonded to a hydrogen, an amino group ($-N{H}_2$), a carboxyl group ($-COOH$) and a variable R group. Twenty kinds occur in proteins. Glycine: R = H (the simplest; no asymmetric carbon). Alanine: R = $-C{H}_3$ (methyl). Serine: R = $-C{H}_{2}OH$ (hydroxymethyl). Zwitterion: in solution near neutral pH the carboxyl group loses a proton and the amino group gains one, giving $H_3{N}^{+}-CHR-CO{O}^{-}$, a dipolar ion with both charges but no net charge; the pH at which this holds is the isoelectric point. Because they carry both acidic and basic groups, amino acids act as buffers. Types by R group: acidic (extra carboxyl group): glutamic acid, aspartic acid; basic (extra amino group): lysine, arginine; neutral (one amino and one carboxyl): glycine, alanine, valine, serine; sulphur-containing: cysteine and methionine. Essential amino acids cannot be made by the body and must come in the diet (e.g. lysine, leucine, isoleucine, valine, threonine, methionine, phenylalanine, tryptophan; histidine is also needed, especially by children). Non-essential amino acids can be made in the body (e.g. glycine, alanine, serine, glutamic acid).

Worked example

Write the structures of glycine, alanine and serine and state how they differ.

Try it first, then show the solutionSolution

1. Common skeleton: $H_{2}N-CH(R)-COOH$
2. Glycine: R = H, giving $H_{2}N-C{H}_2-COOH$; alanine: R = $C{H}_3$; serine: R = $C{H}_{2}OH$

Answer: They differ only in the R group: H, methyl and hydroxymethyl

Proteins: peptide bond, four levels of structure and functions

Amino acids are linked by a peptide bond ($-CO-NH-$) formed between the carboxyl group of one and the amino group of the next, with loss of one water molecule (a condensation reaction). A chain of $n$ amino acids has $n-1$ peptide bonds. Primary structure: the sequence of amino acids from the N-terminal (free amino end) to the C-terminal (free carboxyl end); it decides all higher levels. Secondary structure: regular local folding held by hydrogen bonds between backbone groups: the right-handed α-helix (e.g. keratin of hair) and the β-pleated sheet (e.g. silk fibroin). Tertiary structure: the whole chain folds into a compact three-dimensional shape, stabilised by hydrogen bonds, ionic bonds, hydrophobic interactions and disulphide bridges between cysteines; this creates, for example, an enzyme's active site. Quaternary structure: two or more polypeptide chains (subunits) assembled together, e.g. haemoglobin with two α and two β chains. Functions: enzymes (catalysis), transport (haemoglobin carries oxygen; GLUT-4 carries glucose into cells), hormones (insulin), defence (antibodies), structure (collagen, the most abundant protein in animals; keratin), contraction (actin, myosin), receptors and storage. RuBisCO is the most abundant protein in the whole biosphere.

Worked example

Insulin has two chains of 21 and 30 amino acids. How many peptide bonds does it contain?

Try it first, then show the solutionSolution

1. Each chain of n amino acids has n − 1 peptide bonds: 21 − 1 = 20 and 30 − 1 = 29
2. Total = 20 + 29 = 49 (the chains are joined by disulphide bridges, not peptide bonds)

Answer: 49 peptide bonds

Lipids: fats and oils

Lipids are a diverse group of organic compounds that are insoluble in water and soluble in organic solvents such as chloroform. Classification: (1) simple lipids, esters of fatty acids with alcohols: fats and oils (with glycerol) and waxes (with long-chain alcohols); (2) compound lipids, which carry an extra group: phospholipids (with phosphate, e.g. lecithin, in membranes) and glycolipids; (3) derived lipids, such as steroids (cholesterol, sex hormones). Structure of fats and oils: a fatty acid is a hydrocarbon chain (often 12–20 carbons) ending in a carboxyl group, e.g. palmitic acid (16 carbons, saturated). Glycerol is a 3-carbon alcohol (trihydroxypropane) with three –OH groups. One glycerol joins three fatty acids by ester bonds, losing three water molecules, to form a triglyceride. Saturated fatty acids (no C=C double bonds; e.g. palmitic, stearic) pack closely, so fats rich in them are solid at room temperature (ghee, butter). Unsaturated fatty acids (one or more C=C; e.g. oleic, linoleic) have kinks, so their triglycerides are liquid oils (groundnut oil, gingelly oil). Functions: rich energy store (about 9 kcal per gram, more than twice that of carbohydrates), insulation under the skin, cushioning of organs, waterproofing (waxes), membranes (phospholipids) and hormones (steroids); they also help absorb fat-soluble vitamins.

Worked example

Why is coconut oil often solid in a cold room, while sunflower oil stays liquid?

Try it first, then show the solutionSolution

1. Coconut oil is rich in saturated fatty acids, whose straight chains pack tightly and solidify at a relatively high temperature
2. Sunflower oil is rich in unsaturated fatty acids with kinked chains that pack loosely and stay liquid

Answer: Degree of saturation of the fatty acids

Enzymes: properties, naming, classification and co-factors

Enzymes are biological catalysts; almost all are proteins (a few RNA catalysts, ribozymes, exist). Properties: (1) they speed up reactions without being used up, so small amounts suffice; (2) they are highly specific for their substrate and reaction; (3) they lower the activation energy; (4) they do not change the equilibrium, and many catalyse reactions in both directions; (5) they are sensitive to temperature and pH, and are destroyed (denatured) by heat. The substrate binds at the active site, forming an enzyme–substrate complex that changes to enzyme–product complex and then releases the product. Nomenclature: usually the suffix -ase is added to the substrate (maltase, lipase, protease) or to the reaction type (dehydrogenase, oxidase). Classification by reaction type (six classes): (1) oxidoreductases/dehydrogenases, oxidation–reduction between two substrates; (2) transferases, transfer a group (other than hydrogen) between substrates; (3) hydrolases, break bonds by adding water (ester, ether, peptide, glycosidic bonds); (4) lyases, remove groups without hydrolysis, forming double bonds; (5) isomerases, interconvert isomers; (6) ligases, join two molecules using energy from ATP (e.g. forming C–O, C–S, C–N, P–O bonds). Co-factors: many enzymes need a non-protein part. The protein part is the apoenzyme; with its co-factor it forms the active holoenzyme. Three kinds: prosthetic groups, organic and tightly bound (e.g. haem in peroxidase and catalase); coenzymes, organic and loosely bound for a short time, often made from vitamins (e.g. NAD and NADP contain niacin); metal ions, forming coordination bonds with the active site and substrate (e.g. zinc for carboxypeptidase).

Worked example

Name the class of enzyme for: (i) sucrase splitting sucrose into glucose and fructose with water (ii) an enzyme converting glucose-6-phosphate into fructose-6-phosphate.

Try it first, then show the solutionSolution

1. (i) Water is added to break a glycosidic bond: hydrolase
2. (ii) Glucose-6-phosphate and fructose-6-phosphate are isomers: isomerase

Answer: (i) Hydrolase (ii) Isomerase

Factors affecting enzyme activity and competitive inhibition

Temperature: activity rises with temperature up to an optimum (around 37 °C for most human enzymes) because molecules collide more often. Above it the enzyme's shape is disturbed and it is denatured; activity falls rapidly. At low temperatures the enzyme is only temporarily inactive and recovers when warmed. pH: each enzyme has an optimum pH (pepsin in the stomach about 2; trypsin in the small intestine about 8); away from it, changes in the charges of the active site reduce activity. Substrate concentration: at low substrate concentration, rate rises almost in proportion as more active sites are occupied; at high concentration all active sites are busy, the enzyme is saturated and the rate levels off at a maximum (Vmax). Adding more substrate then has no effect. Competitive inhibition: a molecule that closely resembles the substrate competes for the active site; while bound, it blocks the substrate, so the rate falls. Example: malonate inhibits succinic dehydrogenase, the enzyme that acts on succinate. Because binding is reversible, raising the substrate concentration can overcome the inhibition. Such inhibitors are used to control bacterial pathogens (e.g. sulpha drugs).

Worked example

A competitive inhibitor halves the rate of an enzyme reaction. What happens to the rate if substrate concentration is greatly increased?

Try it first, then show the solutionSolution

1. The inhibitor and substrate compete for the same active site
2. With much more substrate, the substrate wins most encounters and the rate returns close to the uninhibited maximum

Answer: The inhibition is overcome and the rate approaches Vmax

Secondary metabolites

Primary metabolites (sugars, amino acids, fatty acids, nucleotides) take part in basic, universal processes such as respiration and growth. Secondary metabolites are compounds made by plants, fungi and microbes that are not needed for basic metabolism; their roles are often ecological (defence, attraction, protection), and many are useful to people. Groups with examples: pigments (carotenoids, anthocyanins); alkaloids (morphine, codeine); terpenoids (monoterpenes, diterpenes); essential oils (lemongrass oil); toxins (abrin, ricin); lectins (concanavalin A); drugs (vinblastine, curcumin); polymeric substances (rubber, gums). For example, a bitter alkaloid deters herbivores, a bright pigment attracts pollinators, and rubber latex seals wounds.

Worked example

Why might a plant spend energy making bitter alkaloids that it does not need for growth?

Try it first, then show the solutionSolution

1. Alkaloids taste bitter and are often toxic to animals
2. Grazers and insects avoid plants rich in them, so the plant loses fewer leaves

Answer: As chemical defence against herbivores, an ecological role of secondary metabolites

Biomolecules in real life

Each concept, met in an everyday situation. Read the situation, answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Topics in this chapter

• Carbohydrates, Amino Acids, Proteins and Lipids
• Enzymes: Properties, Classification and Kinetics

Biomolecules: key facts

Formation of a disaccharide (condensation)

$2C_6{H}_{12}{O}_6\to C_{12}{H}_{22}{O}_{11}+H_{2}O$

Glucose + glucose → maltose; glycosidic bond.

General amino acid and zwitterion

$H_{2}N-CH(R)-COOH\rightleftharpoons H_3{N}^{+}-CH(R)-CO{O}^{-}$

Glycine R = H; alanine R = CH₃; serine R = CH₂OH.

Peptide bonds in a chain

$\text{peptide bonds}=\text{water molecules released}=n-1$

For each separate chain of n amino acids.

Triglyceride formation

$\text{glycerol}+3\text{fatty acids}\to \text{triglyceride}+3H_{2}O$

Ester bonds.

Holoenzyme

$\text{apoenzyme (protein)}+\text{co-factor}=\text{holoenzyme (active)}$

Co-factors: prosthetic group, coenzyme, metal ion.

Biomolecules: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

How the school exam asks Biomolecules

The question patterns to expect, and the method that works for each.

Tricks to remember Biomolecules

Short memory aids. Say each one aloud once, then use it on a question the same day.

• GaGa-GluGlu-GluFru

  Lactose = galactose + glucose; maltose = glucose + glucose; sucrose = glucose + fructose.

  Use it when: Disaccharides.

• Gly-H, Ala-Methyl, Ser-OH

  Glycine has just H; alanine adds a methyl; serine adds CH₂OH.

  Use it when: Amino acid structures.

• OTHLIL

  Oxidoreductases, Transferases, Hydrolases, Lyases, Isomerases, Ligases.

  Use it when: Six enzyme classes in order.

• Pro-Co-Me

  Prosthetic group (tight, haem), Coenzyme (loose, NAD), Metal ion (zinc).

  Use it when: Co-factors.

• Look-alike blocks the lock

  A competitive inhibitor is a fake key in the same lock; more real keys win.

  Use it when: Competitive inhibition.

Biomolecules: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section D style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 5-mark question split into 1 + 1 + 1 + 2 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Biomolecules

The slips examiners see most often, and what to write instead.

• ✗Mistake: Wrong sugar units in disaccharides

  Lactose and sucrose are swapped.

  Wrong working

  Sucrose = glucose + galactose.

  Right working

  Sucrose = glucose + fructose; lactose = glucose + galactose.

  ✓Do this: Lactose is in milk, like galactose (gala = milk).

  Usually costs: 1 mark.

• ✗Mistake: Counting peptide bonds as n

  Students count amino acids.

  Wrong working

  A decapeptide has 10 peptide bonds.

  Right working

  It has 9; bonds lie between neighbouring amino acids.

  ✓Do this: Fence posts: n posts, n − 1 gaps.

  Usually costs: 1 mark.

• ✗Mistake: Saying cold denatures enzymes

  Heat and cold are treated alike.

  Wrong working

  At 0 °C the enzyme is destroyed.

  Right working

  At low temperature the enzyme is temporarily inactive; high temperature denatures it.

  ✓Do this: Cold = sleep; heat = death.

  Usually costs: 1 mark.

• ✗Mistake: Confusing prosthetic group and coenzyme

  Both are organic co-factors.

  Wrong working

  NAD is a prosthetic group.

  Right working

  NAD is a coenzyme (loosely bound); haem in catalase is a prosthetic group (tightly bound).

  ✓Do this: Prosthetic = permanently fitted.

  Usually costs: 1 mark.

• ✗Mistake: Drawing a wrong R group

  Serine and alanine are mixed up.

  Wrong working

  Serine R = CH₃.

  Right working

  Alanine R = CH₃; serine R = CH₂OH.

  ✓Do this: Serine has the OH (like serum water).

  Usually costs: 1 mark.

Practice questions on Biomolecules

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQExam level

   Lactose is made of:
   (a) glucose + glucose
   (b) glucose + fructose
   (c) glucose + galactose
   (d) fructose + galactose

   Hint

   Milk sugar.

   Answer

   (c)

2. Q2 · 1 mark · MCQExam level

   A sulphur-containing amino acid is:
   (a) glycine
   (b) cysteine
   (c) lysine
   (d) serine

   Hint

   Forms disulphide bridges.

   Answer

   (b)

3. Q3 · 1 mark · Very short answerExam level

   Name the most abundant protein in animals.

   Hint

   Connective tissue.

   Answer

   Collagen

4. Q4 · 1 mark · Very short answerExam level

   Name the monomer of chitin.

   Hint

   An amino sugar.

   Answer

   N-acetylglucosamine

5. Q5 · 2 marks · Short answerExam level

   How does deoxyribose differ from ribose?

   Hint

   Carbon 2.

   Answer

   Deoxyribose ($C_5{H}_{10}{O}_4$) has one oxygen fewer: it has H instead of OH at carbon 2. Ribose ($C_5{H}_{10}{O}_5$) is found in RNA; deoxyribose in DNA.

6. Q6 · 2 marks · Short answerHarder exam level

   Distinguish between fats and oils.

   Hint

   Saturation and state.

   Answer

   Fats are triglycerides rich in saturated fatty acids and are solid at room temperature (ghee, butter); oils are rich in unsaturated fatty acids and are liquid at room temperature (groundnut, mustard oil).

7. Q7 · 2 marks · Short answerHarder exam level

   Give two differences between starch and cellulose.

   Hint

   Function and iodine.

   Answer

   Starch is a storage polysaccharide with helical (amylose) and branched (amylopectin) chains and gives a blue-black colour with iodine; cellulose is a structural polysaccharide of straight unbranched chains forming fibres and gives no colour with iodine. Humans digest starch but not cellulose.

8. Q8 · 3 marks · Short answerHarder exam level

   Classify amino acids on the basis of their R groups, giving two examples of each.

   Hint

   Four groups.

   Answer

   Acidic (glutamic acid, aspartic acid); basic (lysine, arginine); neutral (glycine, alanine, valine); sulphur-containing (cysteine, methionine).

9. Q9 · 3 marks · Short answerHarder exam level

   List five properties of enzymes.

   Hint

   Catalysts.

   Answer

   Mostly proteins; speed up reactions without being used up; highly specific; lower activation energy; sensitive to temperature and pH (denatured by heat); work in small amounts; do not alter equilibrium.

10. Q10 · 3 marks · Short answerHarder exam level

    What are secondary metabolites? Give four groups with an example of each.

    Hint

    Not for basic metabolism.

    Answer

    Compounds not needed for basic metabolism, often with ecological roles. Alkaloids (morphine), pigments (anthocyanins), essential oils (lemongrass oil), toxins (ricin), also drugs (vinblastine), lectins (concanavalin A), polymers (rubber).

11. Q11 · 5 marks · Long answerHarder exam level

    (a) Describe the structure of a triglyceride. [2]
    (b) State three biological functions of lipids. [3]

    Hint

    Glycerol plus three.

    Answer

    (a) One glycerol molecule (three-carbon alcohol with three OH groups) is joined by ester bonds to three fatty acids, with release of three water molecules. (b) Energy storage (about 9 kcal per gram); thermal insulation and cushioning of organs; phospholipids form cell membranes; steroids act as hormones; waxes waterproof surfaces.

12. Q12 · 5 marks · Long answerHarder exam level

    (a) Explain the effects of temperature and pH on enzyme activity with graphs. [3]
    (b) What is competitive inhibition? Give an example. [2]

    Hint

    Optimum curves.

    Answer

    (a) Temperature: rate rises to an optimum (about 37 °C in humans) and then falls sharply as the enzyme denatures; low temperature inactivates temporarily. pH: bell-shaped curve with an optimum (pepsin about 2, trypsin about 8); away from it the active site charges change. (b) A substrate look-alike binds reversibly to the active site and blocks the substrate; e.g. malonate inhibits succinic dehydrogenase. It can be overcome by more substrate.

13. Q13 · 2 marks · Short answerHarder exam level

    A polypeptide chain of 150 amino acids is made in a cell. How many peptide bonds are formed and how many water molecules released?

    Hint

    n − 1.

    Answer

    149 peptide bonds; 149 water molecules.

14. Q14 · 3 marks · Short answerHOTS / competency

    A mutation replaces one cysteine in an enzyme with serine. The enzyme still has the correct amino acid sequence otherwise but no longer works. Suggest why.

    Hint

    Disulphide bridges.

    Answer

    Cysteine contains sulphur and can form disulphide bridges that hold the tertiary structure. Serine has an OH instead of SH and cannot form this bridge, so the chain folds differently and the active site loses its shape; the substrate no longer fits.

15. Q15 · 3 marks · Short answerHOTS / competency

    Pepsin works in the stomach but stops working when food reaches the small intestine, where trypsin takes over. Explain.

    Hint

    Optimum pH.

    Answer

    Pepsin's optimum pH is about 2, matching the acidic stomach. In the small intestine bile and pancreatic juice make the contents alkaline (about pH 8), which changes the charges at pepsin's active site and inactivates it, while trypsin's optimum is around pH 8.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.Sugar in RNA?Show answer

   Ribose.

2. 2.Bond between monosaccharides?Show answer

   Glycosidic bond.

3. 3.R group of glycine?Show answer

   H.

4. 4.Bond between amino acids?Show answer

   Peptide bond.

5. 5.Most abundant protein in the biosphere?Show answer

   RuBisCO.

6. 6.Enzymes joining molecules using ATP?Show answer

   Ligases.

7. 7.Metal co-factor of carboxypeptidase?Show answer

   Zinc.

8. 8.Competitive inhibitor of succinic dehydrogenase?Show answer

   Malonate.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Can you give the composition of maltose, lactose and sucrose?
• ☐Can you name five polysaccharides and their roles?
• ☐Can you draw glycine, alanine and serine?
• ☐Can you write a zwitterion?
• ☐Can you explain the four levels of protein structure?
• ☐Can you name the six classes of enzymes?
• ☐Can you distinguish the three kinds of co-factors?
• ☐Can you sketch the three enzyme-activity graphs?

Biomolecules: frequently asked questions