Biological Molecules
Living matter is built from a small set of recurring molecular families: water, carbohydrates, lipids, proteins, and nucleic acids. The PMDC MDCAT 2026 syllabus expects you to recognise their monomers, polymers, bonds, and biological roles. This is one of the most heavily tested chapters — expect 5-7 MCQs.
Biological Importance of Water
Water (H2O) makes up 65-75% of cellular mass and is the medium of every metabolic reaction. Its unique properties arise from polarity and hydrogen bonding between molecules.
- Polarity
- Oxygen is more electronegative than hydrogen, giving water a partial negative pole on O and partial positive poles on H.
- Hydrogen bond
- Weak electrostatic attraction (~5 kcal/mol) between the δ+ H of one water molecule and the δ− O of another.
- Specific heat capacity
- Water requires 1 cal/g/°C — very high — allowing organisms to buffer temperature changes.
- Heat of vaporisation
- 540 cal/g — sweating dissipates large amounts of body heat with little water loss.
- Universal solvent
- Polar molecules and ions dissolve readily, enabling transport, digestion, and metabolism.
- Cohesion & adhesion
- Cohesion drives capillary action in xylem; surface tension supports small organisms walking on water.
Biological Molecules — Overview
Biomolecules are classified by structure and function. Most large biomolecules are polymers built from repeating monomers by condensation (dehydration synthesis), and broken down by hydrolysis.
- Carbohydrates — monomer = monosaccharide; main polymer = polysaccharide.
- Lipids — not strictly polymers; built from glycerol + fatty acids.
- Proteins — monomer = amino acid; polymer = polypeptide.
- Nucleic acids — monomer = nucleotide; polymer = DNA or RNA.
Trace and macro elements: C, H, O, N, P, S form ~99% of biomass. Trace minerals (Fe, Mg, Ca, Zn, Cu, I, etc.) are essential cofactors and structural components.
Carbohydrates
Carbohydrates have the general formula (CH2O)n. They serve as the cell's primary fuel, short-term energy store, and structural element.
Single sugar units (3-7 carbons). Examples: glucose, fructose, galactose (all hexoses, C6H12O6); ribose, deoxyribose (pentoses). Reducing sugars (Benedict's positive). Glucose is the universal cellular fuel.
Two monosaccharides joined by a glycosidic bond (formed by condensation, broken by hydrolysis).
- Maltose = glucose + glucose
- Sucrose = glucose + fructose (table sugar; non-reducing)
- Lactose = glucose + galactose (milk sugar)
Long polymers of monosaccharides.
- Starch — plant storage; amylose (linear, α-1,4) + amylopectin (branched).
- Glycogen — animal storage in liver and muscle; highly branched.
- Cellulose — structural in plant cell walls; β-1,4 bonds; humans cannot digest it.
- Chitin — in fungal cell walls and arthropod exoskeletons; contains nitrogen.
Conjugated Molecules
Conjugated molecules are hybrid biomolecules in which a carbohydrate, lipid, or other group is covalently attached to another biomolecule.
- Glycoproteins
- Protein + carbohydrate. Examples: mucins, ABO blood group antigens, antibodies, hormones (FSH, LH, hCG).
- Glycolipids
- Lipid + carbohydrate. Found on the outer leaflet of plasma membrane — cell-cell recognition.
- Lipoproteins
- Lipid + protein complexes that transport cholesterol and triglycerides in blood (HDL, LDL, VLDL, chylomicrons).
- Nucleoproteins
- Nucleic acid + protein, e.g., chromatin (DNA + histones), ribosomes (rRNA + ribosomal proteins).
- Phosphoproteins
- Proteins with phosphate group attached, e.g., casein in milk.
Lipids
Lipids are a heterogeneous group of nonpolar, hydrophobic biomolecules. They store ~9 kcal/g (more than twice the energy density of carbohydrates) and form biological membranes.
One glycerol + three fatty acids joined by ester bonds. Saturated fatty acids (no C=C) are solid at room temperature (animal fats). Unsaturated fatty acids contain C=C double bonds and are liquid (vegetable oils). The body cannot synthesise essential fatty acids (linoleic, linolenic, arachidonic).
Glycerol + 2 fatty acids + phosphate group (and usually a head group such as choline). Amphipathic — hydrophilic head, hydrophobic tails. Form the lipid bilayer of all biological membranes.
Four-fused-ring backbone (three 6-carbon + one 5-carbon ring). Examples: cholesterol (membrane fluidity, precursor of steroid hormones), testosterone, oestrogen, progesterone, cortisol, aldosterone, vitamin D, bile salts.
Esters of long-chain fatty acids with long-chain alcohols. Waterproof coatings on leaves (cuticle), feathers, fur, and the human ear canal (cerumen).
Proteins
Proteins are polymers of 20 standard amino acids joined by peptide bonds. They are the workhorses of the cell — enzymes, structural fibres, transporters, hormones, antibodies, contractile fibres, and receptors.
Amino acids and the peptide bond
Each amino acid has an α-carbon bonded to: (1) NH2 (amino group), (2) COOH (carboxyl group), (3) H, and (4) a variable R side chain. A peptide bond forms by condensation between —COOH of one and —NH2 of the next.
- Primary — linear amino acid sequence held by peptide bonds.
- Secondary — local folding into α-helix or β-pleated sheet held by hydrogen bonds.
- Tertiary — overall 3D shape stabilised by hydrogen, ionic, hydrophobic, and disulfide bonds (between cysteine residues).
- Quaternary — assembly of >1 polypeptide subunits, e.g., haemoglobin (2α + 2β).
Denaturation — loss of secondary, tertiary, or quaternary structure due to heat, extreme pH, or chemicals. The primary sequence remains intact, but biological activity is lost.
Ribonucleic Acid (RNA)
RNA is a single-stranded polymer of ribonucleotides. Each nucleotide consists of a ribose sugar, a phosphate, and a nitrogenous base — adenine, guanine, cytosine, or uracil (replacing thymine). RNA mediates the flow of genetic information from DNA to protein.
| Property | mRNA | tRNA | rRNA |
|---|---|---|---|
| Full name | Messenger RNA | Transfer RNA | Ribosomal RNA |
| Function | Carries the genetic code from DNA to ribosome | Brings amino acids to the ribosome | Structural & catalytic component of the ribosome |
| Shape | Linear, single-stranded | Clover-leaf (2D); inverted-L (3D) | Folded, complex 3D structure |
| Key feature | Read in codons of 3 bases | Has an anticodon; amino acid attached at 3′ CCA end | Forms peptide-bond active site (peptidyl transferase) |
| Approx. abundance | ~5% | ~15% | ~80% (most abundant) |
| Size | Variable (hundreds–thousands of nt) | ~75–90 nt (smallest) | 120 nt (5S) to 4700 nt (28S) |
| Stability | Short-lived in eukaryotes | Stable | Very stable |
Structure of DNA
Deoxyribonucleic acid (DNA) is the universal carrier of genetic information. Its double-helix structure was proposed by James Watson and Francis Crick (1953), building on Rosalind Franklin's X-ray diffraction images and Chargaff's rules.
- Two antiparallel polynucleotide strands twisted into a right-handed double helix.
- Sugar-phosphate backbone on the outside; nitrogenous bases face inward.
- Bases pair by hydrogen bonds: A=T (2 H-bonds), G≡C (3 H-bonds).
- 10 base pairs per turn; helix diameter 2 nm; pitch 3.4 nm.
- Sugar = deoxyribose (lacks OH on 2′ carbon).
Chargaff's rules: in any DNA, %A = %T and %G = %C. Total purines (A + G) = total pyrimidines (T + C).
Replication is semi-conservative — each daughter molecule contains one parental and one new strand (Meselson & Stahl, 1958).
Worked MCQs
Five MCQs that capture the high-yield testing patterns for this chapter.
Q1. Which property of water is responsible for the transport of water in the xylem of tall trees?
Cohesive forces between water molecules pull a continuous column up the xylem (transpiration pull), while adhesion to the xylem walls helps overcome gravity (capillarity).
Q2. Cellulose differs from starch primarily in:
Both are polymers of glucose, but cellulose has β-1,4 glycosidic bonds (straight, unbranched fibres) whereas starch has α-1,4 bonds (helical). Humans lack β-1,4 cellulase and cannot digest cellulose.
Q3. The bond that holds two amino acids together in a polypeptide is:
A peptide bond is formed by condensation between the —COOH of one amino acid and the —NH2 of the next, releasing H2O. Glycosidic bonds link sugars, ester bonds link fatty acids to glycerol, and phosphodiester bonds link nucleotides.
Q4. In a DNA molecule, if 30% of the bases are adenine, what is the percentage of cytosine?
By Chargaff's rule, A = T = 30%, so A + T = 60%. Therefore G + C = 40%, and since G = C, each is 20%.
Q5. Which RNA carries the amino acids to the ribosome during translation?
Transfer RNA (tRNA) has a clover-leaf secondary structure with an anticodon at one end and an amino acid attached at the 3′ CCA end; it reads codons on mRNA and delivers the corresponding amino acid.
Quick Recap
- Water: polar, hydrogen-bonded; high specific heat, universal solvent, cohesion-adhesion.
- Carbohydrates: monosaccharides → disaccharides (glycosidic bond) → polysaccharides (starch, glycogen, cellulose, chitin).
- Lipids: triglycerides (glycerol + 3 FA), phospholipids (membranes), steroids (cholesterol, hormones), waxes.
- Proteins: 20 amino acids, peptide bonds; primary → secondary → tertiary → quaternary structure.
- DNA: double helix, antiparallel, deoxyribose, A=T, G≡C; Watson-Crick 1953.
- RNA: single-stranded, ribose, uracil; types — mRNA, tRNA, rRNA.
- Conjugated: glycoproteins, glycolipids, lipoproteins, nucleoproteins.