Biomolecules

1. Carbohydrates: Classification

Carbohydrates are optically active polyhydroxy aldehydes or ketones (or compounds that give these on hydrolysis). Aldehyde type = aldose; ketone type = ketose. Glucose is an aldohexose; fructose is a ketohexose.

• Monosaccharide: cannot be hydrolysed further (glucose, fructose, ribose).
• Disaccharide: two monosaccharides joined by a glycosidic linkage (an oxide/ether bridge, formed with loss of water). Sucrose, maltose, lactose.
• Polysaccharide: many units; not sweet (starch, cellulose, glycogen).
• Reducing sugar: reduces Tollens' or Fehling's reagent because it has a free aldehyde or a free hemiacetal (anomeric) carbon. All monosaccharides are reducing.
• D and L: D means the OH on the last chiral carbon (C5 in glucose) is on the right in the Fischer projection. It is NOT about the sign of rotation. (+) and (−) tell you the rotation.

2. Glucose: Structure and Reactions

Glucose (C₆H₁₂O₆) is prepared by hydrolysis of sucrose or starch with dilute acid. Its open-chain structure is CHO–(CHOH)₄–CH₂OH, proved by these reactions:

Reactions the open chain cannot explain: glucose does not give Schiff's test, does not add NaHSO₃, and its pentaacetate does not react with NH₂OH. Also, it exists as two crystalline forms (α and β). The answer to all: the CHO group is mostly locked in a ring.

The C1 aldehyde and C5–OH form a six-membered hemiacetal ring (pyran-like), so glucose is a pyranose. The two forms differ only at C1 and are called anomers (C1 is the anomeric carbon). In water each converts to the other through the open chain until a mixture of specific rotation +52.5° is reached. This is mutarotation.

3. Fructose, Disaccharides and Polysaccharides

Fructose is a ketohexose (C=O at C2). It forms a five-membered furanose ring, is laevorotatory, and is the sweetest sugar. Its formula is also C₆H₁₂O₆.

Disaccharides

• Sucrose (table sugar) is dextrorotatory (+66.5°). On hydrolysis it gives an equimolar mix of glucose (+52.5°) and fructose (−92.4°). The mix is laevorotatory, so the rotation inverts and the mixture is called invert sugar. Enzyme: invertase.
• Maltose hydrolyses to 2 glucose units (enzyme maltase). Lactose (milk sugar) hydrolyses to galactose + glucose.

Polysaccharides

• Starch = amylose (linear, α-1,4, soluble, gives blue colour with iodine) + amylopectin (branched, α-1,4 and α-1,6). It is the plant's food store.
• Cellulose is a straight chain of β-D-glucose joined by β-1,4 links. Humans lack the enzyme to digest it.
• Glycogen is called animal starch and is stored in liver and muscles. It is more highly branched than amylopectin.

Basic difference, starch and cellulose: starch has α-glucose units, cellulose has β-glucose units.

4. Amino Acids and Peptide Bond

Proteins are polymers of about 20 different α-amino acids (general formula R–CH(NH₂)–COOH).

• Essential amino acids cannot be made by the body (must come from food): valine, leucine, etc. (10 in total).
• Non-essential ones can be made in the body: glycine, alanine, etc.
• Zwitterion: in solution the COOH gives H⁺ to the NH₂, making a neutral dipolar ion (R–CH(NH₃⁺)–COO⁻). Hence amino acids behave like salts: high melting points and water-soluble. They are amphoteric (react with both acids and bases).
• All natural α-amino acids except glycine are optically active and have the L-configuration.
• Peptide bond: the –CO–NH– amide link between the COOH of one amino acid and the NH₂ of the next, with loss of H₂O.

5. Proteins, Enzymes and Denaturation

• Primary: the sequence of amino acids. Change one and you get a different protein.
• Secondary: α-helix (right-handed coil; N–H hydrogen-bonded to a C=O of the adjacent turn) or β-pleated sheet (chains side by side).
• Tertiary: overall folding of the chain. Quaternary: arrangement of two or more polypeptide subunits.
• Fibrous proteins (keratin, myosin): parallel chains, insoluble in water. Globular proteins (insulin, albumin): coiled, spherical, water-soluble.

Denaturation: a physical (temperature) or chemical (pH) change breaks the hydrogen bonds. The globule unfolds, the helix uncoils, and the biological activity is lost. Secondary and tertiary structures are destroyed while the primary structure stays intact. Examples: coagulation of egg white on boiling, curdling of milk (lactic acid). A native protein is one in its natural, active 3-D form.

Enzymes are biocatalysts, almost all globular proteins, very specific for one reaction and substrate. They work in mild conditions and lower the activation energy (sucrose hydrolysis: 6.22 kJ/mol without enzyme, 2.15 kJ/mol with sucrase). Names end in -ase (maltase, invertase).

6. Vitamins

Vitamins are organic compounds needed in small amounts for normal growth and health. Most cannot be made by the body. Fat-soluble: A, D, E, K (stored in liver and fat). Water-soluble: B group and C (excreted in urine, so needed daily; B₁₂ is the exception, it is stored).

7. Nucleic Acids (DNA and RNA)

Nucleic acids are polymers of nucleotides. Complete hydrolysis gives a pentose sugar, phosphoric acid and nitrogenous bases.

• Nucleoside = base + sugar (base joined at C1′).
• Nucleotide = base + sugar + phosphate (at C5′).
• Nucleotides link through phosphodiester bonds between C5′ and C3′ of sugars.

• The two DNA strands are complementary, held by hydrogen bonds: A pairs with T (two bonds) and G with C (three bonds). So the strands are not identical. In RNA the base amounts are unrelated, which suggests a single-stranded structure.
• DNA carries heredity and can self-replicate. RNA (m-RNA, r-RNA, t-RNA) carries out protein synthesis.

8. Hormones

Hormones are chemical messengers made by endocrine glands and carried by the blood. By chemical nature they are steroids (testosterone, oestrogen), polypeptides (insulin, glucagon) or amino acid derivatives (thyroxine, adrenaline). Insulin lowers blood glucose; glucagon raises it. Low iodine in the diet causes goitre (hypothyroidism), which is why table salt is iodised.

9. Quick Sheet and Last-Minute Checklist

Before the exam, check you can:

• Write what D-glucose gives with HI, Br₂ water, HNO₃, HCN and NH₂OH, and what each proves.
• Explain why sucrose is non-reducing and what invert sugar is.
• State the difference between starch and cellulose, and between amylose and amylopectin.
• Define peptide linkage and differentiate it from glycosidic linkage.
• Explain zwitterion and why amino acids have high melting points.
• Define denaturation and give two examples.
• Differentiate DNA and RNA, and nucleoside and nucleotide.
• Match six vitamins with their deficiency diseases.