Chemistry of Life

An atom is the smallest unit of an element participating in a chemical reaction.
A molecule consists of two or more atoms held by chemical bonds (e.g., $H2$ , $O2$ , $H_2O$ , $HCl$ ).
A compound is a molecule with two or more different elements held by chemical bonds (e.g., $H_2O$ , $HCl$ ).

Inorganic compounds: Do not contain carbon or a carbon chain (e.g., water, mineral salts).
Organic compounds: Always contain carbon and hydrogen; made by plants (e.g., carbohydrates, lipids, proteins).

Elements: C, H, and O with a H:O ratio of 2:1.
Three main groups:
- Monosaccharides: Simple sugars (monomers) like glucose ( $C6H{12}O6$ ) and fructose ( $C6H{12}O6$ ).
- Disaccharides: Two monosaccharides linked by glycosidic bonds (e.g., sucrose = glucose + fructose, maltose = glucose + glucose).
- Polysaccharides: Many glucose molecules bonded together (e.g., starch, cellulose, glycogen, chitin, lignin).
Physical Properties:
- Mono- and disaccharides: white, crystalline, water-soluble, sweet.
- Polysaccharides: non-crystalline, not sweet, insoluble in water.
Biological importance:
- Monosaccharides: immediate energy source.
- Polysaccharides: food storage (starch, glycogen), structural components (cellulose, chitin).
- Transport of food (monosaccharides & disaccharides are soluble)
Food tests:
- Glucose: Benedict’s solution turns orange/yellow when heated with glucose.
- Starch: Iodine turns blue-black in the presence of starch.

Elements: C, H, and O; H:O ratio is much greater than 2:1.
Monomers: glycerol and three fatty acids linked by ester links.
Saturated fats: solid at room temperature (e.g., animal fat, butter).
Unsaturated fats (oils): liquid at room temperature (e.g., olive oil, cod-liver oil).
Physical properties: insoluble in water (hydrophobic), soluble in other lipids (lipophilic).
Biological importance:
- Rich source of stored energy.
- Component of protoplasm and cell membranes.
- Heat insulation, water loss reduction.
Food tests:
- Milky emulsion test: alcohol added to fat results in a milky emulsion when water is added.
- Translucent stain test: ether evaporates, leaving a translucent stain on filter paper.

Elements: C, H, O, N; sometimes S, P, and Fe.
Monomers: amino acids linked by peptide bonds.
Polypeptide: < 50 amino acids, Protein: > 50 amino acids
Physical properties: insoluble in water, sensitive to heat and pH, denatured by high temperatures and pH changes.
Biological importance:
- Protoplasm and cell membrane components.
- Enzymes, haemoglobin, antibodies.
- Structural support (keratin, collagen).
- Reserve energy source.
Test for proteins: Biuret test—NaOH and $CuSO_4$ turn purple in the presence of protein.

Large protein molecules; organic catalysts.
Specific: act on specific substrates (lock and key principle).
Sensitive to temperature: have an optimum temperature; denature at high temperatures.
Sensitive to pH: have an optimum pH; denature with pH changes.
Reactions are reversible.
Examples: carbohydrases (amylase), proteases (pepsin), lipases.

Inorganic substances required for cell function and metabolic processes.
- Macronutrients: needed in large quantities (e.g., calcium).
- Micronutrients: needed in small quantities (e.g., iron, iodine).

Most important inorganic molecule for living organisms.
Functions include digestion, chemical reactions, temperature regulation, transport, waste dissolution, structural support, and photosynthesis.