This section covers the chemistry of life — biological molecules, enzymes as biological catalysts, and the movement of substances across membranes by diffusion, osmosis, and active transport.
Carbohydrates: energy source. Simple sugars (glucose), complex (starch in plants, glycogen in animals). Test: iodine (starch → blue-black), Benedict\'s (reducing sugar → brick red). Proteins: made of amino acids, for growth and repair. Test: biuret reagent → purple. Lipids (fats/oils): energy storage, insulation. Test: ethanol emulsion test → white emulsion. Water: solvent, transport medium. Vitamins and minerals needed in small amounts.
Biological catalysts: proteins that speed up reactions. Lock-and-key model: substrate fits into active site (specific shape). Factors: temperature (increases rate up to optimum ~37°C for human enzymes, then denatures), pH (each enzyme has optimum pH, e.g. pepsin pH 2, amylase pH 7). Denaturation: high temperature or extreme pH changes active site shape → substrate no longer fits. Examples: amylase (starch → maltose), protease (protein → amino acids), lipase (lipids → fatty acids + glycerol).
Diffusion: net movement of particles from high to low concentration (down concentration gradient). No energy needed. Examples: O₂ into blood, CO₂ out of cells. Osmosis: movement of water molecules from dilute to concentrated solution through a partially permeable membrane. Turgid (plant cell in water), plasmolysed (plant cell in concentrated solution). Active transport: movement against concentration gradient, requires energy from respiration. Example: root hair cells absorbing mineral ions from soil.
Diffusion: any substance moves from high to low concentration through any medium — no energy required. Osmosis: specifically water moving from dilute to concentrated solution through a partially permeable membrane — no energy required (a special case of diffusion). Active transport: any substance moves from low to high concentration (against the gradient) — requires energy from respiration, uses carrier proteins. Cells use all three: diffusion for gas exchange, osmosis for water balance, active transport for absorbing minerals and ions when they are in low concentration externally.
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