Biomolecules Class 11 Notes

Chemical Composition of Living Tissue

When living tissue is ground and filtered, the filtrate contains small molecules — amino acids, sugars, fatty acids, nucleotides, and ions — collectively called micromolecules. The residue contains large macromolecules. This analysis shows that living tissue is built from a handful of chemical elements: C, H, O, N, P and S, organized into a surprisingly small number of molecular types.

• Micromolecules: water, mineral ions, monosaccharides, amino acids, nucleotides, fatty acids, vitamins.
• Biomacromolecules: proteins, polysaccharides, nucleic acids, lipids (large but non-polymeric).
• Primary metabolites have direct roles in growth and reproduction — amino acids, sugars, lipids, proteins, nucleotides.
• Secondary metabolites (alkaloids, flavonoids, terpenoids, essential oils) have ecological roles like defence but are not directly involved in basic metabolism.

Proteins

Proteins are the most abundant organic molecules in living cells and serve as enzymes, structural components, transport molecules, hormones and antibodies. Proteins are polymers of 20 different types of amino acids linked by peptide bonds. Each amino acid has an amino group (–NH₂), a carboxyl group (–COOH), a hydrogen atom, and a variable side chain (R group) attached to the same α-carbon.

Levels of protein structure

• Primary structure: the linear sequence of amino acids in the polypeptide chain. It determines all higher structures.
• Secondary structure: local folding into α-helices (hydrogen bonds between C=O and N–H of the backbone) and β-pleated sheets (hydrogen bonds between adjacent strands).
• Tertiary structure: overall three-dimensional folding of a single polypeptide driven by hydrophobic interactions, hydrogen bonds, ionic bonds, disulfide bridges (–S–S–) and van der Waals forces. Functional proteins have this compact globular shape.
• Quaternary structure: arrangement of multiple polypeptide subunits. Haemoglobin has four subunits (two α and two β chains). Not all proteins have quaternary structure.

Polysaccharides

Polysaccharides are long chains of monosaccharide units linked by glycosidic bonds. They may be linear or branched. The monomeric unit in most plant polysaccharides is glucose, but the glycosidic linkage differs, giving each polysaccharide distinct properties.

• Starch: the storage polysaccharide in plants; consists of amylose (linear, α-1,4-glycosidic bonds) and amylopectin (branched, α-1,4 and α-1,6-glycosidic bonds).
• Glycogen: the storage polysaccharide in animals; similar to amylopectin but more highly branched — found in liver and muscle cells.
• Cellulose: the structural polysaccharide in plant cell walls; linear chain of glucose with β-1,4-glycosidic bonds. Humans cannot digest cellulose (lack cellulase enzyme) — it is dietary fibre.
• Chitin: a structural polysaccharide of N-acetylglucosamine units with β-1,4 linkages; forms the exoskeleton of arthropods and fungal cell walls.

Nucleic Acids

• DNA bases: adenine (A), guanine (G), cytosine (C), thymine (T). Double-stranded; base pairing: A = T (two hydrogen bonds), G ≡ C (three hydrogen bonds).
• RNA bases: adenine, guanine, cytosine, uracil (U replaces T). Usually single-stranded.
• Types of RNA: mRNA (messenger), tRNA (transfer, clover-leaf shape), rRNA (ribosomal).
• Nucleotides also serve as energy carriers (ATP), coenzymes (NAD⁺, FAD), and signalling molecules (cAMP).

Lipids

Lipids are a heterogeneous group of molecules that are insoluble in water but soluble in organic solvents such as chloroform and ether. Unlike true macromolecules, lipids are not polymeric. Lipids serve as structural components of cell membranes, energy reserves, insulation, and signalling molecules.

• Fatty acids: long hydrocarbon chains with a terminal carboxyl group. Saturated fatty acids (no double bonds, e.g. palmitic acid, stearic acid) and unsaturated fatty acids (one or more cis double bonds, e.g. oleic acid, linoleic acid).
• Triglycerides (simple lipids): one glycerol esterified with three fatty acids. They are the main storage lipids in adipose tissue.
• Phospholipids: glycerol with two fatty acids and one phosphate group; form the lipid bilayer of cell membranes. The phosphate head is hydrophilic; the fatty acid tails are hydrophobic.
• Steroids: four fused carbon rings; cholesterol is a precursor for steroid hormones (testosterone, oestrogen, cortisol) and bile salts.

Enzymes

Mechanism of enzyme action

The lock-and-key model explains specificity: the substrate fits into the active site like a key into a lock. The induced-fit model refines this — the active site changes shape slightly upon substrate binding, ensuring a tighter fit and stabilising the transition state. Enzymes lower the activation energy (Ea), making the reaction faster without changing the equilibrium constant.

Factors affecting enzyme activity

• Temperature: enzyme activity increases with temperature up to an optimum (usually 37–40 °C in humans) and then drops sharply as the protein denatures.
• pH: each enzyme has an optimum pH (e.g. pepsin at pH 2, trypsin at pH 8). Extreme pH disrupts ionic bonds in the protein.
• Substrate concentration: at low [S], rate increases linearly; at high [S], rate plateaus at Vmax when all active sites are occupied (Michaelis–Menten kinetics).
• Enzyme inhibition: competitive inhibitors compete for the active site; non-competitive inhibitors bind elsewhere (allosteric site) and change the enzyme's shape. Irreversible inhibitors permanently inactivate the enzyme.

Cofactors, coenzymes and prosthetic groups

Many enzymes require non-protein helpers called cofactors to be active. Cofactors can be inorganic metal ions (Zn²⁺, Mg²⁺, Fe²⁺) or organic molecules called coenzymes (derived from vitamins, e.g. NAD⁺ from niacin, FAD from riboflavin, coenzyme A from pantothenic acid). Prosthetic groups are coenzymes tightly bound to the enzyme (e.g. haem in catalase).

Metabolism and Living State

Metabolism is the sum of all biochemical reactions in an organism. It is divided into catabolism (breakdown of complex molecules into simpler ones, releasing energy) and anabolism (synthesis of complex molecules from simpler ones, consuming energy). ATP is the universal energy currency. Living cells maintain a dynamic steady state — the concentrations of metabolites are kept constant through feedback regulation, not by equilibrium.

Solved Examples

Example: How many peptide bonds are present in a polypeptide chain of 120 amino acids?

Solution: A chain of n amino acids contains (n − 1) peptide bonds. For 120 amino acids: 120 − 1 = 119 peptide bonds. Each peptide bond is formed by a condensation reaction releasing one water molecule, so 119 water molecules are released during synthesis.

Example: Why does the rate of an enzyme-catalysed reaction stop increasing after a certain substrate concentration?

Solution: At high substrate concentrations, all active sites of the enzyme molecules are occupied — the enzyme is saturated. Adding more substrate cannot increase the rate further because there are no free active sites. The reaction has reached Vmax. Increasing enzyme concentration would raise Vmax.