Biomolecules

Biomolecules are carbon-based molecules essential to life: carbohydrates, proteins, enzymes, vitamins, and nucleic acids.

Carbohydrates

Polyhydroxy aldehydes or ketones (or compounds yielding them on hydrolysis). Formula: Cx(H2O)y.
Monosaccharides (not hydrolysable): glucose, fructose, ribose.
Disaccharides (2 units): sucrose (non-reducing), maltose, lactose (both reducing).
Polysaccharides: starch, cellulose, glycogen.

Glucose (C6H12O6): Aldohexose; 4 asymmetric C atoms → 2⁴ = 16 stereoisomers. Exists mainly as 6-membered pyranose ring (cyclic hemiacetal). Alpha-D-glucose: –OH at C-1 below ring (Haworth); beta-D-glucose: –OH above ring. Reactions: reduces Tollens and Fehling (aldehyde group); pentaacetate with (CH3CO)2O (5 –OH groups). Mutarotation: alpha and beta forms interconvert in solution; equilibrium at +52.5° optical rotation.

Fructose: Ketohexose (C=O at C-2); furanose ring (5-membered). Reducing sugar — isomerises in alkaline conditions via enediol to aldoses.

Sucrose: Non-reducing — glycosidic bond uses both anomeric carbons (C-1 glucose + C-2 fructose); neither ring can open. Hydrolysis gives glucose + fructose (invert sugar; rotation changes from +66.5° to –20°).

Starch: Amylose (linear, alpha-1,4, blue-black with I2) + Amylopectin (branched, alpha-1,4 and alpha-1,6, purple-red with I2). Cellulose: beta-1,4 links; indigestible. Glycogen: animal storage; most branched.

Proteins

Built from alpha-amino acids (H2N–CHR–COOH). Zwitterion in solution: NH3^+–CHR–COO^-. Isoelectric point (pI): pH of zero net charge. Peptide bond: –CO–NH– (resonance → planar).

Structural levels: (1) Primary — amino acid sequence. (2) Secondary — alpha-helix (H-bonds every 4th residue, same chain) or beta-sheet (H-bonds between chains). (3) Tertiary — 3D fold via –S–S– bridges, H-bonds, hydrophobic interactions. (4) Quaternary — multiple polypeptide chains (e.g., haemoglobin: 4 subunits).

Denaturation: heat/pH/chemicals disrupt 2°/3°/4° structure; peptide bonds stay intact.

Enzymes

Globular protein catalysts; highly specific. Substrate binds at active site (lock-and-key or induced-fit model). Cofactors (metal ions) or coenzymes (vitamins) assist.

Vitamins

Fat-soluble: A (night blindness), D (rickets), E, K. Water-soluble: B1 (beriberi), B12 (pernicious anaemia), C (scurvy).

Nucleic Acids

Nucleotide = nitrogen base + pentose sugar + phosphate. DNA: deoxyribose, A/T/G/C, double helix; A–T (2 H-bonds), G–C (3 H-bonds); genetic storage. RNA: ribose, A/U/G/C, single-stranded; mRNA/tRNA/rRNA. Chargaff's rule: %A = %T; %G = %C.

Worked Examples

Example 1

How many asymmetric C in glucose?
C2, C3, C4, C5 are asymmetric; C1 (aldehyde) and C6 (–CH2OH) are not.
Answer: 4 asymmetric carbons; 2⁴ = 16 stereoisomers.

Example 2

Why is sucrose non-reducing?
Both anomeric carbons are in the glycosidic bond; neither ring can open to give free –CHO or –C=O.
Answer: No free carbonyl — does not reduce Tollens or Fehling.

Example 3

What is mutarotation?
Alpha-D-glucose and beta-D-glucose interconvert in water until equilibrium (+52.5°) is reached.
Answer: Change in optical rotation to equilibrium value when a pure anomer dissolves in water.

Example 4

Primary vs secondary protein structure.
Primary = amino acid sequence (peptide bonds). Secondary = alpha-helix (intrachain H-bonds) or beta-sheet (interchain H-bonds).
Answer: Primary is sequence; secondary is regular local folding.

Example 5

Differentiate DNA and RNA.
DNA: deoxyribose, T, double helix, genetic storage. RNA: ribose, U, single-stranded, protein synthesis.
Answer: DNA has deoxyribose and thymine; RNA has ribose and uracil.

Example 6

Chargaff rule: %A = 30%; find %C.
%T = 30%; %G + %C = 40%; so %C = 20%.
Answer: %C = 20%.

Example 7

Amylose vs amylopectin.
Amylose: linear (alpha-1,4), blue-black with I2. Amylopectin: branched (alpha-1,4 and alpha-1,6), purple-red with I2.
Answer: Amylose is linear (blue-black); amylopectin is branched (purple-red).

Common mistakes

> Watch out: Fructose is a ketose but still reducing (alkaline isomerisation). Denaturation does NOT break peptide bonds. Sucrose is non-reducing; maltose and lactose are reducing. DNA has T; RNA has U.

Summary

Carbohydrates: mono/di/polysaccharides. Glucose shows mutarotation; starch = amylose + amylopectin; cellulose has beta links. Proteins: 4 levels; denaturation disrupts 2°–4° only. Enzymes: specific protein catalysts. Vitamins: fat-soluble A/D/E/K; water-soluble B/C. DNA (deoxyribose, T, double helix) stores genes; RNA (ribose, U) enables protein synthesis.