IP Biology Upper Sec 03: Biological Molecules

These notes align with SEAB GCE O-Level Biology (6093) content used in IP programmes (exams from 2026).

Status: SEAB O-Level Biology 6093 syllabus (exams from 2026) checked 2025-11-30 - scope unchanged; remains the reference for this note.

What you must know

• Carbohydrates (C, H, O): glucose, sucrose, starch, glycogen; energy source/storage. Fats (C, H, O): energy storage, insulation; formed from glycerol + fatty acids. Proteins (C, H, O, N sometimes S): structure and enzymes.
• Food tests: iodine for starch (blue-black), Benedict for reducing sugars (brick-red ppt on heating), biuret for protein (violet), ethanol emulsion for fats (white emulsion).
• Enzymes: specific active site; lock-and-key/induced fit; lower activation energy; affected by temperature (optimum then denaturation) and pH (active site charges disrupted).

Detailed notes

• Structure → function: monosaccharides (glucose) are soluble and transport in blood; polysaccharides (starch, glycogen) are compact, osmotically inert stores; cellulose is unbranched with hydrogen bonding for high tensile strength in cell walls.
• Lipids: triglycerides pack densely for long-term energy and waterproofing; phospholipids form bilayers with hydrophilic heads/hydrophobic tails; unsaturated vs saturated chains influence membrane fluidity.
• Proteins: primary sequence dictates folding; hydrogen, ionic, and disulfide bonds stabilise tertiary structure; fibrous proteins (collagen) give tensile strength, globular (enzymes, antibodies) enable specificity.
• Enzyme kinetics: increasing temperature raises kinetic energy and collisions until optimum; beyond optimum, bond disruption alters active site. pH away from optimum changes charge states, reducing binding. Substrate concentration increases rate until saturation; enzymes are reusable.
• Food tests rationale: Benedict’s (reducing sugars donate electrons to Cu²⁺ → Cu₂O ppt); biuret detects peptide bonds; ethanol test relies on fat solubility in ethanol and precipitation on water addition.

Quick applications

• Design a test plan: include control with distilled water; use equal volumes, heat Benedict in water bath; add iodine after cooling starch sample.
• Explain denaturation: high temperature breaks hydrogen/ionic bonds → active site shape changes → substrate no longer fits.
• Link role to function: glycogen for short-term energy, cellulose for plant cell walls, adipose tissue for insulation and protection.

Worked examples

• Given three unknowns, design a flow of tests to identify which contains starch, which contains reducing sugar, and which contains protein. Include volumes, heating steps, and controls.