Biomolecules

Welcome to HSLC Guru! In this article, we present a complete English-medium guide to Class 11 Biology Chapter 9 — Biomolecules based strictly on the ASSEB (Assam State School Education Board) syllabus. Living organisms are made of countless chemical substances called biomolecules. From the simplest sugar to the largest enzyme, biomolecules form the structural and functional basis of life. This chapter introduces you to the major biomolecules — carbohydrates, proteins, lipids, nucleic acids — and the role of enzymes that drive every life process. Each concept is explained in clear English with summaries, model questions, MCQs, fill-in-the-blanks, true/false, and a glossary so you can learn quickly and score well in your ASSEB Class 11 Biology examination.

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Summary

Chemical Composition of Living Tissues: When a living tissue is analysed, it is found to contain elements such as carbon, hydrogen, oxygen, nitrogen, phosphorus, sulphur and many minerals. Chemical analysis using a trichloroacetic acid extract divides the compounds into two pools — the acid-soluble pool (low molecular weight, less than 1000 daltons) and the acid-insoluble pool (high molecular weight macromolecules). The acid-soluble pool contains amino acids, sugars, nucleotides, organic acids and lipids; these are called primary metabolites. Plants, fungi and microbes also produce secondary metabolites like alkaloids (morphine), flavonoids, rubber, essential oils, antibiotics, pigments and toxins, which often have ecological or pharmacological importance. The acid-insoluble fraction contains biomacromolecules — proteins, polysaccharides and nucleic acids — molecules with molecular weight above 10,000 daltons. Lipids, although small, get included in the macromolecule pool because they form part of cell membranes and are isolated with insoluble debris.

Amino Acids and Proteins: Amino acids are organic compounds containing an amino group (–NH₂), a carboxyl group (–COOH), a hydrogen atom and a variable side chain (R) attached to the same alpha-carbon. Twenty-one amino acids form proteins. Based on the nature of the R-group they are classified as acidic (glutamic acid, aspartic acid), basic (lysine, arginine), neutral (glycine, alanine, valine) and aromatic (tyrosine, phenylalanine, tryptophan). Amino acids join through peptide bonds to form polypeptides. Proteins are heteropolymers of amino acids and show four levels of structural organisation. The primary structure is the linear sequence of amino acids; the secondary structure involves local folding into alpha-helices or beta-pleated sheets stabilised by hydrogen bonds; the tertiary structure is the three-dimensional folding that gives a protein its functional shape; and the quaternary structure is the assembly of two or more polypeptide subunits, as in haemoglobin which has two alpha and two beta chains.

Enzymes: Almost all enzymes are proteins (a few RNA molecules called ribozymes also act as enzymes). They are biocatalysts that increase the rate of biochemical reactions without being consumed. Enzymes have an active site that binds the substrate, forming an enzyme–substrate complex which lowers the activation energy. Enzyme kinetics shows that the rate of reaction increases with substrate concentration until a maximum velocity (V_max) is reached. Enzyme activity is affected by temperature, pH, substrate concentration and inhibitors (competitive and non-competitive). Many enzymes need non-protein helpers called cofactors: a tightly bound organic group is a prosthetic group (e.g., haem in peroxidase), a loosely bound organic molecule is a coenzyme (e.g., NAD, FAD derived from vitamins) and an inorganic ion is a metal ion cofactor (e.g., Zn²⁺ in carboxypeptidase). The International Union of Biochemistry classifies enzymes into six classes — oxidoreductases (transfer electrons), transferases (transfer functional groups), hydrolases (hydrolyse bonds), lyases (add or remove groups to form double bonds), isomerases (rearrange atoms) and ligases (join molecules using ATP).

Carbohydrates, Lipids and Nucleic Acids: Carbohydrates are polyhydroxy aldehydes or ketones. Monosaccharides (glucose, fructose, ribose) cannot be hydrolysed; disaccharides (sucrose, lactose, maltose) yield two monosaccharide units; polysaccharides (starch, glycogen, cellulose, chitin) are long chains and may be storage or structural. Glycosidic bonds link sugar units. Lipids are water-insoluble compounds — fatty acids, glycerol, triglycerides (fats and oils), phospholipids (membrane components) and steroids (cholesterol, hormones). Saturated fatty acids are solid at room temperature, unsaturated ones are usually liquid. Nucleic acids are polymers of nucleotides; each nucleotide has a nitrogenous base (adenine, guanine, cytosine, thymine or uracil), a pentose sugar (deoxyribose in DNA, ribose in RNA) and a phosphate group. DNA is a double helix that stores genetic information; RNA is usually single-stranded and helps in protein synthesis as mRNA, tRNA and rRNA. Together these biomolecules carry out structure, energy storage, catalysis, information transfer and regulation in every living cell.

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Very Short Answer Type Questions (1 Mark)

Q1. What is a biomolecule?

Answer: A biomolecule is any organic compound that is produced by and found in living organisms, such as carbohydrates, proteins, lipids and nucleic acids.

Q2. Name the four classes of biomacromolecules.

Answer: Proteins, polysaccharides, nucleic acids and lipids (functionally grouped with macromolecules).

Q3. Define primary metabolites.

Answer: Primary metabolites are small biomolecules that have identifiable functions and play known roles in normal physiological processes, e.g., amino acids, sugars and nucleotides.

Q4. Give two examples of secondary metabolites.

Answer: Morphine (alkaloid) and rubber (terpenoid) are common secondary metabolites.

Q5. What is the bond that links amino acids in a protein?

Answer: A peptide bond, formed between the carboxyl group of one amino acid and the amino group of the next with the elimination of a water molecule.

Q6. Name the sugar present in DNA and in RNA.

Answer: DNA contains 2′-deoxyribose, while RNA contains ribose.

Q7. What is an enzyme?

Answer: An enzyme is a biological catalyst, usually a protein, that speeds up a specific biochemical reaction without being consumed in the process.

Q8. Differentiate between a coenzyme and a prosthetic group in one line.

Answer: A coenzyme is loosely associated with the enzyme and can leave the active site after reaction, whereas a prosthetic group is permanently attached.

Q9. Name one storage polysaccharide in plants and one in animals.

Answer: Starch is the storage polysaccharide in plants and glycogen is the storage polysaccharide in animals.

Q10. What does V_max represent in enzyme kinetics?

Answer: V_max is the maximum velocity at which an enzyme-catalysed reaction can occur when all enzyme molecules are saturated with substrate.

Short Answer Type Questions (2-3 Marks)

Q1. Distinguish between primary and secondary metabolites with examples.

Answer: Primary metabolites are biomolecules essential for the normal growth, development and reproduction of an organism, e.g., amino acids, sugars, nucleotides, lipids and organic acids. Secondary metabolites do not have a direct role in primary metabolism but often serve in defence or signalling. Examples include alkaloids (morphine), flavonoids, rubber, essential oils, antibiotics (penicillin) and pigments (anthocyanin). Secondary metabolites are mostly seen in plants, fungi and microbes.

Q2. What are amino acids? Classify them based on the nature of the R-group.

Answer: Amino acids are organic acids containing an amino group, a carboxyl group, a hydrogen atom and a variable side chain (R) on the same alpha-carbon. They join through peptide bonds to form proteins. Based on the R-group they are classified as: (i) acidic — glutamic acid, aspartic acid; (ii) basic — lysine, arginine, histidine; (iii) neutral — glycine, alanine, valine, leucine; and (iv) aromatic — tyrosine, phenylalanine, tryptophan. There are 21 amino acids that occur in proteins.

Q3. Briefly describe the four levels of protein structure.

Answer: (i) Primary structure is the linear sequence of amino acids in a polypeptide chain. (ii) Secondary structure is the regular local folding of the chain into an alpha-helix or a beta-pleated sheet, stabilised by hydrogen bonds. (iii) Tertiary structure is the three-dimensional folding of the entire polypeptide producing the functional shape, stabilised by hydrogen, ionic, hydrophobic and disulphide bonds. (iv) Quaternary structure is the association of two or more polypeptide subunits, e.g., haemoglobin (two alpha + two beta chains).

Q4. Differentiate between competitive and non-competitive inhibition.

Answer: In competitive inhibition, the inhibitor structurally resembles the substrate and competes for the same active site of the enzyme; the effect can be reversed by increasing substrate concentration (e.g., malonate inhibits succinate dehydrogenase). In non-competitive inhibition, the inhibitor binds to a different site on the enzyme, changes its shape and stops the reaction; this cannot be reversed by adding more substrate.

Q5. What are cofactors? Give one example of each type.

Answer: Cofactors are non-protein components that bind to an enzyme and are essential for its catalytic activity. They are of three types: (i) Prosthetic group — tightly bound organic molecule, e.g., haem in catalase and peroxidase. (ii) Coenzyme — loosely bound organic molecule (often a vitamin derivative), e.g., NAD⁺ and FAD. (iii) Metal ion cofactor — inorganic ion, e.g., Zn²⁺ in carboxypeptidase, Mg²⁺ in hexokinase.

Q6. Compare DNA and RNA in three points.

Answer: (i) DNA contains deoxyribose sugar, RNA contains ribose. (ii) DNA has thymine as one of its bases, RNA has uracil instead of thymine. (iii) DNA is mostly double-stranded forming a helix and acts as the genetic material; RNA is mostly single-stranded and works in the expression of genes (mRNA, tRNA, rRNA).

Long Answer Type Questions (5-7 Marks)

Q1. Describe the chemical composition of a living tissue and how the acid-soluble and acid-insoluble pools are separated.

Answer: A living tissue is mostly water, but its dry weight contains many organic and inorganic compounds. To analyse this composition, a small piece of tissue (vegetable, leaf or liver) is ground in trichloroacetic acid (TCA) using a mortar and pestle to form a thick slurry. The slurry is filtered through cheesecloth or a sieve. The filtrate is the acid-soluble pool, which contains low molecular-weight compounds (less than 1000 daltons) such as amino acids, sugars, nucleotides, organic acids and inorganic ions. These compounds, having identifiable metabolic roles, are termed primary metabolites. The retentate (the part that does not pass through the filter) is the acid-insoluble pool; on chemical analysis it is found to consist of large molecules — proteins, polysaccharides, nucleic acids and lipids — collectively called biomacromolecules. Lipids are technically small molecules (less than 800 daltons) but appear in the insoluble fraction because they are part of the cell membrane that breaks into vesicles trapped on the filter. Inorganic compounds present include calcium, magnesium, phosphate, sulphate, sodium, potassium and chloride. Thus, the chemical composition can be summarised as: water (more than 70 per cent), proteins, carbohydrates, lipids, nucleic acids, inorganic salts and small organic molecules. This simple TCA extraction reveals the chemical complexity of a tiny piece of tissue and forms the foundation of biochemistry.

Q2. Explain the structure of proteins. How are the four levels of protein organisation different from one another?

Answer: Proteins are heteropolymers of amino acids joined by peptide bonds. They serve numerous functions — structural (collagen, keratin), catalytic (enzymes), transport (haemoglobin), defence (antibodies) and hormonal (insulin). The structural organisation of a protein is described at four levels.

(i) Primary structure: This is the linear sequence in which amino acids are arranged in the polypeptide chain. The N-terminus has a free amino group and the C-terminus has a free carboxyl group. The sequence is unique to each protein and is genetically determined. Even a single change in the sequence may produce a non-functional or harmful protein, e.g., sickle-cell haemoglobin.

(ii) Secondary structure: Some portions of the polypeptide fold into regular shapes — the right-handed alpha-helix or the beta-pleated sheet — held together by hydrogen bonds between –C=O and –N–H groups along the backbone. These shapes give the protein local rigidity.

(iii) Tertiary structure: The whole polypeptide folds upon itself into a compact three-dimensional shape, much like a hollow tube bent and twisted. It is stabilised by hydrogen bonds, hydrophobic interactions, ionic bonds and disulphide bridges between cysteine residues. The tertiary structure determines the biological activity of the protein.

(iv) Quaternary structure: When two or more polypeptide chains (subunits) come together to form a functional protein, the arrangement of these subunits is the quaternary structure. The classic example is haemoglobin, which has two alpha and two beta subunits arranged tetrahedrally, each holding a haem group that binds oxygen. Thus the four levels together explain how a linear chain of amino acids becomes a functional 3-D molecule.

Q3. What are enzymes? Describe their properties, mechanism of action and the factors affecting their activity.

Answer: Enzymes are biocatalysts, mostly proteinaceous, that increase the rate of biochemical reactions by lowering the activation energy. Some RNA molecules called ribozymes also show catalytic activity.

Properties: (i) They are highly specific for their substrate. (ii) They speed up reactions millions of times without being consumed. (iii) They have an active site to which the substrate binds. (iv) They function within a narrow range of temperature and pH. (v) They are sensitive to inhibitors and activators. (vi) Many enzymes need cofactors for activity.

Mechanism of action: The substrate binds to the active site of the enzyme to form an enzyme–substrate (ES) complex. This complex distorts the bonds of the substrate, lowering the activation energy. The substrate is converted into product, which then leaves the active site, freeing the enzyme to act again: E + S → ES → EP → E + P. The active site fits the substrate by an “induced-fit” model — the enzyme slightly changes shape when the substrate binds.

Factors affecting enzyme activity: (i) Temperature — activity increases with temperature up to an optimum (about 37 °C in humans), beyond which the enzyme denatures. (ii) pH — every enzyme has an optimum pH; pepsin works at pH 2 and trypsin at pH 8. (iii) Substrate concentration — rate increases with substrate concentration up to V_max, after which all active sites are saturated. (iv) Enzyme concentration — activity is directly proportional to enzyme concentration if substrate is in excess. (v) Inhibitors — competitive inhibitors mimic the substrate; non-competitive inhibitors bind elsewhere and change the shape of the enzyme.

Q4. Classify enzymes into the six IUB classes. Give one example for each.

Answer: The International Union of Biochemistry has divided enzymes into six broad classes based on the type of reaction they catalyse:

(i) Oxidoreductases: Catalyse oxidation–reduction reactions by transferring electrons or hydrogen atoms. Example: lactate dehydrogenase, which converts lactate to pyruvate.

(ii) Transferases: Transfer a functional group (such as a methyl, amino or phosphate group) from one molecule to another. Example: hexokinase, which transfers a phosphate from ATP to glucose.

(iii) Hydrolases: Break bonds by adding water (hydrolysis). Examples: amylase, lipase, pepsin, trypsin.

(iv) Lyases: Add or remove groups from substrates to form double bonds without hydrolysis or oxidation. Example: aldolase in glycolysis.

(v) Isomerases: Catalyse the rearrangement of atoms within a molecule, producing isomers. Example: phosphoglucose isomerase, which converts glucose-6-phosphate to fructose-6-phosphate.

(vi) Ligases: Join two molecules together using energy from ATP. Example: DNA ligase joining DNA fragments during replication.

Q5. Write a detailed note on carbohydrates, lipids and nucleic acids.

Answer: Carbohydrates are polyhydroxy aldehydes or ketones (or compounds yielding them on hydrolysis). They are classified as: (i) Monosaccharides — simple sugars that cannot be hydrolysed further, e.g., glucose, fructose, ribose; (ii) Disaccharides — two monosaccharides linked by a glycosidic bond, e.g., sucrose (glucose + fructose), maltose (glucose + glucose), lactose (glucose + galactose); (iii) Polysaccharides — long chains of sugar units, e.g., starch and glycogen (storage), cellulose and chitin (structural). Carbohydrates serve as the main source of energy and as structural components such as the plant cell wall.

Lipids are organic compounds insoluble in water but soluble in organic solvents. They include simple lipids (fats and oils, formed from glycerol and three fatty acids), compound lipids (phospholipids and glycolipids — major components of cell membranes) and derived lipids (steroids such as cholesterol, hormones such as testosterone, fat-soluble vitamins A, D, E, K). Saturated fatty acids have only single bonds between carbons and are solid at room temperature; unsaturated fatty acids have one or more double bonds and are usually liquid (oils). Lipids store energy at twice the calorific value of carbohydrates and provide insulation.

Nucleic acids are polymers of nucleotides; each nucleotide consists of a nitrogenous base, a pentose sugar and a phosphate group. The nitrogenous bases are purines (adenine, guanine) and pyrimidines (cytosine, thymine in DNA; uracil in RNA). The phosphodiester bond links the 3′ carbon of one sugar to the 5′ carbon of the next. DNA is a double helix in which two anti-parallel strands are held together by hydrogen bonds between A=T (two bonds) and G≡C (three bonds); it stores hereditary information. RNA is usually single-stranded and exists as messenger RNA (carries the genetic message), transfer RNA (carries amino acids during protein synthesis) and ribosomal RNA (a structural part of ribosomes). Together carbohydrates, lipids and nucleic acids, along with proteins, run every chemical activity of life.

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Multiple Choice Questions (MCQs)

Q1. The compounds with a molecular weight less than 1000 daltons in a tissue extract are called:
(a) Macromolecules
(b) Micromolecules / primary metabolites
(c) Polymers
(d) Secondary metabolites
Answer: (b) Micromolecules / primary metabolites.

Q2. Which of the following is a secondary metabolite?
(a) Glucose
(b) Glycine
(c) Morphine
(d) ATP
Answer: (c) Morphine.

Q3. The bond that joins amino acids in a protein is:
(a) Glycosidic bond
(b) Peptide bond
(c) Phosphodiester bond
(d) Hydrogen bond
Answer: (b) Peptide bond.

Q4. The number of amino acids commonly found in proteins is:
(a) 16
(b) 18
(c) 21
(d) 24
Answer: (c) 21.

Q5. Haemoglobin shows which level of protein structure?
(a) Primary
(b) Secondary
(c) Tertiary
(d) Quaternary
Answer: (d) Quaternary.

Q6. Which class of enzymes catalyses hydrolysis reactions?
(a) Lyases
(b) Hydrolases
(c) Ligases
(d) Isomerases
Answer: (b) Hydrolases.

Q7. NAD and FAD are examples of:
(a) Prosthetic groups
(b) Coenzymes
(c) Metal ions
(d) Substrates
Answer: (b) Coenzymes.

Q8. Cellulose is a polymer of:
(a) Galactose
(b) Fructose
(c) Glucose
(d) Ribose
Answer: (c) Glucose.

Q9. The base unique to RNA is:
(a) Adenine
(b) Cytosine
(c) Uracil
(d) Guanine
Answer: (c) Uracil.

Q10. Tyrosine is an example of:
(a) Acidic amino acid
(b) Basic amino acid
(c) Aromatic amino acid
(d) Sulphur-containing amino acid
Answer: (c) Aromatic amino acid.

Fill in the Blanks

Q1. The acid-insoluble fraction of a living tissue contains __________.
Answer: biomacromolecules (proteins, polysaccharides, nucleic acids).

Q2. Glutamic acid is an example of __________ amino acid.
Answer: acidic.

Q3. The site on an enzyme where the substrate binds is called the __________.
Answer: active site.

Q4. The storage carbohydrate of animals is __________.
Answer: glycogen.

Q5. The two strands of DNA are held together by __________ bonds.
Answer: hydrogen.

True / False

Q1. All enzymes are proteins.
Answer: False (most enzymes are proteins, but some RNA molecules called ribozymes are also enzymes).

Q2. Lipids are technically small molecules but are placed in the macromolecule fraction.
Answer: True.

Q3. Cellulose is a storage polysaccharide.
Answer: False (it is a structural polysaccharide).

Q4. The primary structure of a protein is the linear sequence of amino acids.
Answer: True.

Q5. Adenine pairs with cytosine in a DNA double helix.
Answer: False (adenine pairs with thymine; guanine pairs with cytosine).

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Glossary

Term	Meaning
Biomolecule	An organic molecule produced by a living organism.
Primary metabolite	Small biomolecule with a known role in normal physiology, e.g., amino acid, sugar.
Secondary metabolite	Compound (e.g., alkaloid, rubber) not directly involved in primary metabolism.
Biomacromolecule	Large biomolecule of molecular weight more than 10,000 daltons.
Amino acid	Building block of proteins; has –NH2, –COOH, –H and an R group.
Peptide bond	Covalent bond between –COOH of one amino acid and –NH2 of the next.
Primary structure	Linear sequence of amino acids in a protein.
Secondary structure	Local folding of polypeptide into alpha-helix or beta-sheet.
Tertiary structure	Three-dimensional folding of the entire polypeptide chain.
Quaternary structure	Assembly of two or more polypeptide subunits, e.g., haemoglobin.
Enzyme	Biocatalyst, usually a protein, that speeds up a biochemical reaction.
Active site	Region of the enzyme where the substrate binds.
Vmax	Maximum velocity of an enzyme reaction at saturating substrate.
Cofactor	Non-protein component required for enzyme activity.
Prosthetic group	Tightly bound organic cofactor, e.g., haem.
Coenzyme	Loosely bound organic cofactor, e.g., NAD, FAD.
Metal ion cofactor	Inorganic ion needed by an enzyme, e.g., Zn2+, Mg2+.
Oxidoreductase	Enzyme catalysing oxidation–reduction reactions.
Transferase	Enzyme transferring a functional group between molecules.
Hydrolase	Enzyme catalysing hydrolysis of bonds.
Lyase	Enzyme adding or removing groups to form double bonds.
Isomerase	Enzyme rearranging atoms to form isomers.
Ligase	Enzyme joining two molecules using ATP.
Carbohydrate	Polyhydroxy aldehyde or ketone; main source of energy.
Monosaccharide	Simple sugar that cannot be hydrolysed, e.g., glucose.
Disaccharide	Sugar yielding two monosaccharides on hydrolysis, e.g., sucrose.
Polysaccharide	Long chain of sugar units, e.g., starch, cellulose.
Glycosidic bond	Bond linking two sugar units.
Lipid	Water-insoluble organic molecule, e.g., fat, phospholipid.
Saturated fatty acid	Fatty acid with no C=C double bonds, solid at room temperature.
Unsaturated fatty acid	Fatty acid with one or more C=C double bonds, usually liquid.
Phospholipid	Lipid with a phosphate group; component of cell membranes.
Steroid	Lipid with a four-ring carbon skeleton, e.g., cholesterol.
Nucleotide	Building block of nucleic acids: base + sugar + phosphate.
DNA	Double-stranded nucleic acid; stores genetic information.
RNA	Single-stranded nucleic acid; involved in protein synthesis.
Phosphodiester bond	Bond linking nucleotides through phosphate between sugars.
Ribozyme	RNA molecule that acts as a biological catalyst.