IP Biology Upper Sec 02: Movement of Substances

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

• Diffusion: net movement of particles down concentration gradient; no energy needed; faster with higher temperature/steeper gradient.
• Osmosis: net movement of water across partially permeable membrane from higher to lower water potential (dilute → concentrated).
• Active transport: movement against gradient using energy from respiration and carrier proteins (e.g., mineral ions into root hair, glucose in ileum).
• Osmosis effects: plant cells become turgid in dilute solutions, flaccid/plasmolysed in concentrated; animal cells lyse in very dilute, crenate in concentrated.

Detailed notes

• Diffusion drivers: random molecular movement; faster with higher temperature, steeper gradient, larger surface area, smaller molecules, and shorter distance. Occurs in liquids/gases.
• Osmosis language: water potential (higher → lower) across partially permeable membrane. Plant cells: turgid in hypotonic; flaccid then plasmolysed in hypertonic. Turgor supports non-woody plants.
• Active transport: uses ATP and carrier proteins to move substances against gradient. Root hairs uptake nitrate/minerals; ileum absorbs glucose/amino acids even when lumen concentration is low.
• Surface area:volume: small cells and extensions (root hairs, microvilli) increase SA:V for faster diffusion/absorption relative to volume.

Worked walkthroughs

• Predict plant cell appearance in distilled water vs concentrated sucrose: firm/turgid vs plasmolysed (membrane pulls from wall); note reversibility when returned to dilute solution.
• Root hair uptake: water via osmosis, ions via diffusion then active transport; relate to many mitochondria (ATP) and long protrusion (high SA).
• Small intestine: diffusion/facilitated diffusion/active transport of nutrients; villi/microvilli increase area, blood flow maintains gradient.

Pitfalls and fixes

• Using “water concentration” instead of “water potential.”
• Forgetting membrane in osmosis definition.
• Saying active transport works without energy—state ATP from respiration and carrier proteins.
• Mixing osmosis (water) with diffusion of solutes.

Practice drills

• Explain why amoeba rely on diffusion but humans need transport systems.
• Compare RBCs vs plant cells in distilled water and concentrated salt; name the outcomes.