IP Biology Notes: Biological Molecules (Upper Sec 03)
26 Nov 2025, 00:00 Z
Reviewed by
Ezekiel Tan·Academic Advisor (Biology)
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Use this as a free IP Biology notes chapter on biological molecules for Year 3 to Year 4. It keeps the IP pacing while reinforcing the 6093 biology foundations most schools test through DBQs, diagrams, and practical explanations.
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.
