Notes › Organic Chemistry

Biomolecules

1. Carbohydrates

Carbohydrates are polyhydroxy aldehydes or ketones, or compounds that give them on hydrolysis.

  • Monosaccharides: glucose (an aldohexose) and fructose (a ketohexose).
  • Disaccharides: sucrose (glucose + fructose), maltose (glucose + glucose, α-1,4), lactose (galactose + glucose, β-1,4).
  • Polysaccharides: starch (amylose about 15–20% and amylopectin about 80–85%), cellulose (β-D-glucose, β-1,4 links), glycogen (animal starch, more branched than amylopectin).

Glucose

  • Open-chain evidence: forms an oxime with $NH_2OH$ (C=O present); acetylation gives a pentaacetate (five −OH groups); HI gives n-hexane (straight chain); bromine water gives gluconic acid (it is an aldehyde).
  • It does not react with $NaHSO_3$ or give Schiff's test, and its pentaacetate does not react with $NH_2OH$. These failures point to the cyclic hemiacetal structure (pyranose).
  • α- and β-D-glucose are anomers, differing only at C-1. Their interconversion in solution is called mutarotation.

Reducing vs Non-reducing Sugars

All monosaccharides, maltose and lactose reduce Tollens' and Fehling's reagents. Sucrose does not, because both anomeric carbons are tied up in the glycosidic bond. Hydrolysis of sucrose (dextrorotatory) gives a laevorotatory mixture, hence the name invert sugar.

2. Proteins

Proteins are polymers of α-amino acids joined by peptide bonds ($-CO{-}NH-$). Amino acids exist as zwitterions ($H_3N^+{-}CHR{-}COO^-$) and are amphoteric. Except glycine, all natural α-amino acids are optically active (mostly L).

  • Essential amino acids must come from the diet (e.g. valine, leucine, lysine).
  • Primary structure: amino acid sequence. Secondary: α-helix and β-pleated sheet, held by H-bonds. Tertiary: overall folding (fibrous vs globular). Quaternary: arrangement of subunits, as in haemoglobin.
  • Denaturation (heat, pH change) destroys secondary and tertiary structure but leaves the primary structure intact. Example: coagulation of egg white on boiling.

3. Enzymes

Enzymes are biological catalysts, mostly globular proteins, highly specific to their substrate. They lower the activation energy; for example, maltase converts maltose to glucose and invertase converts sucrose to glucose and fructose.

4. Vitamins

  • Fat-soluble (stored in the body): A, D, E, K.
  • Water-soluble (must be supplied regularly): B group and C. Vitamin $B_{12}$ is an exception and can be stored in the liver.
  • Deficiencies: A — night blindness; $B_1$ — beriberi; C — scurvy; D — rickets; K — delayed blood clotting.

5. Nucleic Acids

DNA and RNA are polymers of nucleotides (base + pentose sugar + phosphate).

  • DNA: 2-deoxyribose; bases A, G, C, T; double helix with A=T (two H-bonds) and G≡C (three H-bonds).
  • RNA: ribose; bases A, G, C, U; usually single-stranded. Types: mRNA, rRNA, tRNA.

💡 Memory Hook

DNA's "T" and RNA's "U" are the only base difference. Purines (A, G) have two rings; pyrimidines (C, T, U) have one. Each pair always joins a purine to a pyrimidine.

Practice questions

  1. Which of the following is a non-reducing sugar?

    1. Glucose
    2. Maltose
    3. Lactose
    4. Sucrose
    Answer

    D. In sucrose, the anomeric carbons of both glucose and fructose are used in the glycosidic bond, so there is no free hemiacetal group.

  2. α-D-glucose and β-D-glucose are:

    1. Enantiomers
    2. Anomers
    3. Structural isomers
    4. Conformers
    Answer

    B. They differ only in the configuration at C-1, the anomeric carbon.

  3. Deficiency of vitamin C causes:

    1. Rickets
    2. Scurvy
    3. Beriberi
    4. Night blindness
    Answer

    B. Vitamin C (ascorbic acid) deficiency causes scurvy (bleeding gums).

  4. Which base is found in RNA but not in DNA?

    1. Adenine
    2. Guanine
    3. Thymine
    4. Uracil
    Answer

    D. RNA uses uracil in place of thymine.

  5. Denaturation of a protein destroys:

    1. Only the primary structure
    2. Secondary and tertiary structure
    3. Peptide bonds
    4. All levels including primary
    Answer

    B. Heat or pH changes break H-bonds and other interactions, but peptide bonds (primary structure) survive.

  6. The only common α-amino acid that is not optically active is:

    1. Alanine
    2. Glycine
    3. Valine
    4. Leucine
    Answer

    B. Glycine's α-carbon carries two hydrogens (R = H), so it is not chiral.

← Amines