IP Biology Upper Sec 01: Cell Structure and Organisation

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

• Key structures in plant vs animal cells: cell wall (plant, fully permeable), membrane (partially permeable), cytoplasm, nucleus, vacuoles (large central in plant; small temporary in animal), chloroplasts (plant only), centrioles (animal).
• Organelle functions: rough ER (protein transport), smooth ER (lipid synthesis, detox), Golgi (modifies/packages), mitochondria (aerobic respiration, ATP), ribosomes (protein synthesis).
• Specialised cells: root hair (long extension, large SA, thin wall), red blood cell (biconcave, no nucleus, haemoglobin), muscle cell (long, many mitochondria), xylem vessel (lignified, hollow).

Detailed notes

• Ultrastructure links: rough ER + ribosomes for protein synthesis and export; smooth ER for lipids/detox; Golgi modifies/packages into vesicles; lysosomes (animal) for digestion; tonoplast surrounds vacuole; plasmodesmata connect plant cells.
• Membrane as fluid mosaic: phospholipid bilayer with proteins; selectively permeable; transport via diffusion, osmosis, active transport, endo/exocytosis (qualitative).
• Turgor vs plasmolysis: plant cells in hypotonic solutions become turgid (wall prevents bursting); in hypertonic solutions cytoplasm pulls from wall (plasmolysis), reversible if rehydrated.
• Specialised cells: ciliated epithelium (cilia + mucus movement), palisade mesophyll (many chloroplasts, elongated), nerve cell (long axon, many mitochondria), sperm (flagellum, enzymes in acrosome).

Worked walkthroughs

• Predict appearance of plant cells in distilled water vs concentrated salt: turgid vs plasmolysed; relate to water potential.
• Trace a secreted protein: gene → mRNA → ribosome on rough ER → vesicle → Golgi → vesicle → exocytosis at membrane.
• Explain why muscle cells have many mitochondria and why red blood cells lack nucleus/mitochondria.

Pitfalls and fixes

• Calling cell wall “partially permeable”-it is fully permeable and non-living.
• Mixing chloroplasts (photosynthesis) with mitochondria (respiration).
• Using “diffusion” for water movement-use “osmosis” across partially permeable membranes.
• Forgetting animal cells lack large central vacuole, chloroplasts, and cell wall.

Practice drills

• Draw and label plant vs animal cell; list three differences and two similarities.