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Biology · Unit 3

Biological Molecules: MDCAT Notes and MCQs

Classification

Micromolecules (low mol. mass): water, minerals, monosaccharides, amino acids, fatty acids, nucleotides. Macromolecules (polymers): polysaccharides, proteins, nucleic acids; lipids are large but not true polymers. Protoplasm ≈ 70–90% water; proteins are the most abundant organic compound (≈ 10–15%).

Water

  • Polar molecule → hydrogen bonds → excellent solvent ("universal solvent").
  • High specific heat (1 cal/g/°C) → resists temperature change; high heat of vaporization → cooling by sweating.
  • Maximum density at 4 °C; ice floats and insulates aquatic life.
  • Cohesion → water columns in xylem; reagent in hydrolysis and photosynthesis; slight ionization into H+ and OH−.

Carbohydrates — Cn(H2O)n

  • Monosaccharides: glucose (aldose, C6H12O6), fructose (ketose), galactose; ribose/deoxyribose (pentoses).
  • Disaccharides (glycosidic bond): sucrose / cane sugar = glucose + fructose (non-reducing); lactose = glucose + galactose; maltose = glucose + glucose.
  • Polysaccharides: starch (amylose α-1,4 unbranched + amylopectin with α-1,6 branches) — plant storage; glycogen — animal starch, highly branched, stored in liver & muscle; cellulose — β-1,4 glucose, unbranched, plant cell wall, most abundant organic compound on earth. Iodine: starch → blue-black, glycogen → red.

Proteins

  • Amino acid = central carbon + –NH2 + –COOH + H + R group. 20 common amino acids, joined by peptide bonds (condensation).
  • Structure: primary (sequence; Sanger sequenced insulin — 51 amino acids, 2 chains), secondary (α-helix, β-pleated sheet; H-bonds), tertiary (3-D folding; disulphide, ionic, hydrophobic, H-bonds), quaternary (more than one chain, e.g. haemoglobin — 4 chains).
  • Fibrous (keratin, collagen) vs globular (enzymes, antibodies, haemoglobin).

Lipids

  • Acylglycerols = esters of glycerol (alcohol) + fatty acids. Triglyceride = glycerol + 3 fatty acids (ester bonds). Saturated FA (no C=C) → solid fats; unsaturated → oils.
  • Phospholipids: glycerol + 2 fatty acids + phosphate group (+ choline in lecithin). Hydrophilic head, hydrophobic tails → form the cell-membrane bilayer.
  • Waxes (long-chain alcohol + fatty acid), steroids (cholesterol, hormones), terpenes.

Nucleic acids

  • Nucleotide = pentose sugar + phosphate + nitrogenous base. Purines: A, G. Pyrimidines: C, T (DNA), U (RNA).
  • DNA (Watson & Crick, 1953): double helix, two antiparallel strands, sugar-phosphate backbone outside, bases inside. A=T (2 H-bonds), G≡C (3 H-bonds). Diameter 2 nm; one turn = 3.4 nm with 10 base pairs (0.34 nm apart). Based on X-ray data of Rosalind Franklin & Wilkins and Chargaff's rule (A = T, G = C).
  • RNA: single-stranded, ribose, uracil. mRNA (≈ 3–4%, carries code), tRNA (≈ 10–15%, cloverleaf, carries amino acids, has anticodon), rRNA (≈ 80%, part of ribosomes).
  • Gene: a sequence of nucleotides in DNA that codes for one polypeptide.

Conjugated molecules

Glycolipids (carbohydrate + lipid) and glycoproteins (carbohydrate + protein) on the outer cell surface act in cell recognition, receptors and blood-group antigens. Others: lipoproteins, nucleoproteins.

PMDC learning outcomes for this unit
  • Biological molecules: 3.1 Define and classify biological molecules.
  • Biological molecules: 3.2 Discuss the importance of biological molecules
  • Biological Importance of Water: 3.3 Describe biologically important properties of water (polarity, hydrolysis, specific heat, water as solvent and reagent, density, cohesion/ionization)
  • Carbohydrates: 3.4 Discuss carbohydrates: monosaccharides (glucose), oligosaccharides (cane sugar, sucrose, lactose), polysaccharides (starches, cellulose, glycogen)
  • Proteins: 3.5 Describe proteins: amino acids, structure of proteins
  • Lipids: 3.6 Describe lipids: phospholipids, triglycerides, alcohol and esters (acylglycerol)
  • Ribonucleic acid (RNA): 3.7 Give an account of structure and function of RNA
  • Conjugated molecules: 3.8 Discuss conjugated molecules (glycolipids, glycoproteins)
  • Structure of DNA: 3.9 Explain the double helical structure of DNA as proposed by Watson and Crick.
  • Structure of DNA: 3.10 Define gene as a sequence of nucleotides as part of DNA, which codes for the formation of a polypeptide.

Biological Molecules MCQs with answers

Practise and track your score →
Q1
Which property of water allows it to resist sudden changes in body temperature?
AHigh density
BLow viscosity
CIonization
DHigh specific heat
Show answer
Answer: D (High specific heat). Water's high specific heat (1 cal/g/°C) means it absorbs much heat with little temperature change.
Q2
Water has its maximum density at:
A100 °C
B0 °C
C37 °C
D4 °C
Show answer
Answer: D (4 °C). Water is densest at 4 °C, so ice floats and lakes freeze from the top.
Q3
Sucrose (cane sugar) is composed of:
AGlucose + fructose
BFructose + galactose
CGlucose + glucose
DGlucose + galactose
Show answer
Answer: A (Glucose + fructose). Sucrose = α-glucose + β-fructose; lactose is glucose + galactose; maltose is glucose + glucose.
Q4
Glycogen differs from cellulose in that glycogen:
AGives blue colour with iodine
BIs highly branched and stored in animals
CIs made of β-glucose
DIs a structural polysaccharide
Show answer
Answer: B (Is highly branched and stored in animals). Glycogen is highly branched animal storage polysaccharide; cellulose is unbranched β-1,4 structural polymer.
Q5
The bond between two amino acids is:
AEster bond
BPhosphodiester bond
CGlycosidic bond
DPeptide bond
Show answer
Answer: D (Peptide bond). A peptide bond (–CO–NH–) forms by condensation between –COOH and –NH2.
Q6
Haemoglobin, made of four polypeptide chains, shows which level of protein structure?
ATertiary
BSecondary
CQuaternary
DPrimary
Show answer
Answer: C (Quaternary). Quaternary structure arises when two or more polypeptide chains associate.
Q7
Alpha helix and beta pleated sheet are held together mainly by:
APeptide bonds
BHydrogen bonds
CDisulphide bonds
DIonic bonds
Show answer
Answer: B (Hydrogen bonds). Secondary structure is stabilised by hydrogen bonds between –C=O and –N–H groups.
Q8
A triglyceride is formed from glycerol and three fatty acids by:
AHydrogen bonds
BPeptide bonds
CGlycosidic bonds
DEster bonds
Show answer
Answer: D (Ester bonds). Each –OH of glycerol forms an ester linkage with a fatty acid.
Q9
Which molecule forms the basic bilayer of cell membranes?
ATriglycerides
BPhospholipids
CWaxes
DCholesterol only
Show answer
Answer: B (Phospholipids). Phospholipids have hydrophilic heads and hydrophobic tails, forming a bilayer in water.
Q10
In DNA, guanine pairs with cytosine by:
AOne hydrogen bond
BA covalent bond
CThree hydrogen bonds
DTwo hydrogen bonds
Show answer
Answer: C (Three hydrogen bonds). G≡C pairs have 3 H-bonds; A=T pairs have 2.
Q11
The distance of one complete turn of the DNA double helix is:
A0.34 nm
B34 nm
C3.4 nm
D2 nm
Show answer
Answer: C (3.4 nm). One turn = 3.4 nm containing 10 base pairs, each 0.34 nm apart; diameter = 2 nm.
Q12
The most abundant type of RNA in a cell is:
AsnRNA
BmRNA
CtRNA
DrRNA
Show answer
Answer: D (rRNA). rRNA makes up about 80% of cellular RNA and is a structural part of ribosomes.
Q13
The anticodon is present on:
AtRNA
BrRNA
CDNA
DmRNA
Show answer
Answer: A (tRNA). tRNA carries an anticodon that pairs with the codon on mRNA.
Q14
Glycoproteins on the cell surface mainly function in:
AProtein synthesis
BCell recognition
CEnergy storage
DOxygen transport
Show answer
Answer: B (Cell recognition). Glycoproteins and glycolipids act as recognition markers and receptors.
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