Updated NCERT Solutions for Class 11 Biology Chapter 9: Biomolecules

Macromolecules are large, complex organic molecules found in the acid-insoluble fraction during the chemical analysis of living tissue. They have high molecular weights, typically ranging from ten thousand daltons and above. They are polymers formed by the linking of smaller monomer units.

The four major types of macromolecules are:

1. Proteins: Polymers of amino acids. Example: Collagen, Keratin, Hemoglobin.
2. Polysaccharides (Carbohydrates): Polymers of monosaccharides. Example: Starch, Cellulose, Glycogen.
3. Nucleic Acids: Polymers of nucleotides. Example: Deoxyribonucleic acid (DNA) and Ribonucleic acid (RNA).
4. Lipids: Although they have molecular weights less than 800 Da and are not strictly polymers, they are considered macromolecules because they are found in the acid-insoluble fraction due to their hydrophobic nature (forming vesicles). Example: Fats, Oils, Cholesterol.

1. Glycosidic Bond: This bond is formed between two adjacent monosaccharide units (like glucose) through a dehydration reaction (removal of a water molecule). It links carbohydrate monomers to form polysaccharides. (A diagram showing two glucose molecules joining to form maltose, highlighting the C-O-C glycosidic bond and the removal of H₂O, should be drawn here.)
2. Peptide Bond: This bond is formed between the carboxyl group (-COOH) of one amino acid and the amino group (-NH₂) of an adjacent amino acid, again through a dehydration reaction. It is the fundamental bond that links amino acids to form proteins. (A diagram showing two generic amino acids reacting to form a -CO-NH- peptide bond and releasing H₂O should be drawn here.)
3. Phosphodiester Bond: This bond is crucial for the structure of nucleic acids (DNA and RNA). It links the 3' carbon of one sugar molecule to the 5' carbon of another via a phosphate group. This creates the sugar-phosphate backbone of the nucleic acid. (A diagram showing two nucleotides being linked by a phosphodiester bond should be drawn here.)

The tertiary structure of a protein refers to the overall three-dimensional (3D) shape of a single polypeptide chain. It is formed by the further folding and coiling of the secondary structure (alpha-helices and beta-sheets).

• Formation: This 3D structure is stabilized by various interactions between the R-groups (side chains) of the amino acids. These interactions include:
  • Hydrogen bonds
  • Ionic bonds (salt bridges)
  • Hydrophobic interactions
  • Disulfide bridges (covalent bonds between cysteine residues)
• Importance: The tertiary structure is absolutely essential for the biological function of the protein. For example, the active site of an enzyme is formed by the specific folding of the polypeptide chain into its tertiary structure. A hollow, ball-like (globular) shape is a common tertiary structure for many enzymes.

Here are 10 interesting small molecular weight biomolecules:

Yes, numerous industries isolate or synthesize these compounds. The buyers range from large-scale food and pharmaceutical corporations to research institutions and the general public.

Proteins exhibit four levels of structural organization, each building upon the previous one.

1. Primary Structure: This is the linear sequence of amino acids in a polypeptide chain, linked by peptide bonds. It determines all subsequent levels of protein structure and function.
2. Secondary Structure: This refers to the local, regular folding of the polypeptide backbone into shapes like the α-helix and β-pleated sheet. This structure is stabilized by hydrogen bonds.
3. Tertiary Structure: This is the overall 3D shape of a single polypeptide chain. It is stabilized by interactions between the R-groups, including hydrogen bonds, ionic bonds, hydrophobic interactions, and disulfide bridges. This level is critical for the protein's biological activity.
4. Quaternary Structure: This structure is present only in proteins made of two or more polypeptide chains (subunits). It describes how these subunits are arranged. An example is Hemoglobin, which has four subunits.

The conversion of milk into curd (yoghurt) is a classic example of protein denaturation.

1. Milk Protein: The primary protein in milk is casein.
2. Adding Starter Culture: Lactic acid bacteria (*Lactobacillus*) are added to lukewarm milk.
3. Bacterial Action: These bacteria convert the lactose (milk sugar) into lactic acid.
4. Denaturation: The lactic acid lowers the milk's pH. This acidity disrupts the bonds holding the casein protein's tertiary structure.
5. Coagulation: The denatured casein proteins unfold and clump together (coagulate), forming the semi-solid curd.

Yes, building models of biomolecules using commercially available ball-and-stick model kits is an excellent way to understand their 3D structure. The kits have a standard color code for atoms (e.g., Black for Carbon, Red for Oxygen, Blue for Nitrogen).

Example: Building a Dipeptide:

1. Build two different amino acids using the colored balls.
2. Remove the -OH from the carboxyl group of the first amino acid and an -H from the amino group of the second.
3. Join the Carbon of the first amino acid to the Nitrogen of the second. This demonstrates the formation of a peptide bond and the release of a water molecule.

This hands-on activity helps visualize complex concepts like stereochemistry and bond angles.

When we titrate a simple amino acid (like glycine) with a weak base (e.g., NaOH), we can discover its ionizable groups.

1. Starting Point: In acid, the amino acid is fully protonated (net charge +1): R-CH(NH₃⁺)-COOH.
2. First Dissociation: As base is added, the carboxyl group (-COOH) loses a proton first. This is the first dissociating group.
3. Zwitterionic Point: At the isoelectric point (pI), the molecule is a zwitterion with a net charge of zero: R-CH(NH₃⁺)-COO⁻.
4. Second Dissociation: As more base is added, the amino group (-NH₃⁺) loses its proton. This is the second dissociating group.

Conclusion: The titration curve shows two buffering regions, proving that a simple amino acid has two dissociating groups: the α-carboxyl group and the α-amino group.

Alanine is a simple amino acid with a methyl group (-CH₃) as its R-group.

(A clear chemical structure should be drawn here, showing a central alpha-carbon (Cα) bonded to four groups: an amino group (-NH₂), a carboxyl group (-COOH), a hydrogen atom (-H), and a methyl R-group (-CH₃). The zwitterionic form with -NH₃⁺ and -COO⁻ can also be shown.)

• Natural Gums: Natural gums (like Gum Arabic) are heteropolysaccharides. They are complex carbohydrates made of different types of monosaccharide units. They are biological polymers.
• Fevicol: Yes, Fevicol is completely different. It is a synthetic adhesive, not a biomolecule. Its primary component is polyvinyl acetate (PVA), a man-made polymer resin.

In summary: Natural gums are biological polysaccharides, whereas Fevicol is a man-made synthetic polymer adhesive.

Test Results for Various Samples:

• Fruit Juice: Generally negative for protein/fat; may be weakly positive for amino acids.
• Saliva: Positive for protein (enzymes); positive for amino acids.
• Sweat: Weakly positive for protein; positive for amino acids.
• Urine (Normal): Negative for protein; positive for amino acids (contains urea).

To break down a polymer into its constituent monomers, a process called hydrolysis ("to break with water") is used. It is the reverse of the dehydration reaction that forms polymers.

Methods of Hydrolysis:

1. Enzymatic Hydrolysis: This is how living organisms do it. Specific enzymes are used to catalyze the breakdown.
   • Amylase breaks starch into glucose.
   • Proteases break proteins into amino acids.
   • Lipases break fats into fatty acids and glycerol.
2. Acid/Alkaline Hydrolysis: This is a common lab method. The polymer is heated with a strong acid (like HCl) or a base (like NaOH), which catalyzes the breaking of bonds by adding water molecules across them.

Secondary metabolites are organic compounds produced by organisms that are not directly involved in normal growth or reproduction. For humans, they are a rich source of medicines, dyes, pigments, and perfumes.

A zwitterion is a molecule that has separate positively and negatively charged groups, resulting in a net charge of zero. Amino acids exist as zwitterions at their isoelectric point (pI).

(A diagram showing the structure of Glycine with -NH₃⁺ and -COO⁻ groups on the alpha-carbon should be drawn here.)

A cofactor is the non-protein part of an enzyme required for its activity. It is a broad term. If the cofactor is an organic molecule that is transiently bound to the enzyme, it is called a coenzyme (e.g., NAD⁺, FAD). If the cofactor is tightly bound to the enzyme, it is called a prosthetic group (e.g., heme group).

Lipids are not considered true macromolecules because:

1. Their molecular weight is generally less than 800 Daltons, which is below the threshold for macromolecules.
2. They are not polymers; they are not formed from repeating monomer units.

They are found in the acid-insoluble fraction only because of their hydrophobic nature, which causes them to form vesicles.

Two major functions of carbohydrates are:

1. Energy Source: They are the primary source of energy for cellular activities (e.g., glucose).
2. Structural Component: They form structural parts of organisms (e.g., cellulose in plant cell walls, chitin in the exoskeleton of arthropods).

• Lock and Key Hypothesis: Proposed by Emil Fischer, this model suggests that the active site of an enzyme has a rigid, specific shape (the "lock") that fits the substrate (the "key") perfectly.
• Induced Fit Hypothesis: Proposed by Daniel Koshland, this is a more dynamic model. It suggests that the active site is flexible. The binding of the substrate induces a conformational change in the enzyme's active site, causing it to fit more snugly around the substrate.

More Accepted Model: The Induced Fit Hypothesis is more widely accepted today. It better explains how the transition state of the reaction is stabilized and how some enzymes can bind to a range of related substrates. The lock and key model is seen as too rigid.

Enzyme activity is influenced by several factors:

1. Temperature: Enzymes have an optimal temperature. Activity is low at low temperatures and drops rapidly at high temperatures due to denaturation. (A bell-shaped curve with 'Enzyme Activity' vs. 'Temperature' should be drawn.)
2. pH: Each enzyme has an optimal pH. Extreme pH changes denature the enzyme. (Another bell-shaped curve with 'Enzyme Activity' vs. 'pH' should be drawn.)
3. Substrate Concentration: The reaction rate increases with substrate concentration until the enzyme is saturated (Vmax). (A hyperbolic curve with 'Reaction Velocity' vs. 'Substrate Concentration' should be drawn.)
4. Inhibitors: Chemicals that can decrease enzyme activity.

1. (i) What is the most likely identity of the major biomolecule in sample 'X'?
   Protein.
2. (ii) Why did the Biuret test give a positive result?
   Because it detects the presence of peptide bonds, which are fundamental to proteins.
3. (iii) Why was the Benedict's test negative?
   Because the sample does not contain reducing sugars. Proteins are not sugars.
4. (iv) What did the final step (hydrolysis followed by Ninhydrin test) confirm?
   This confirmed that the protein (sample 'X') is made up of amino acid monomers.