Knowledge Organiser: Cell Transport

Key Terms

• Diffusion — the net movement of particles from a region of higher concentration to a region of lower concentration down a concentration gradient; passive, no energy needed
• Osmosis — the net movement of water from a dilute solution to a more concentrated solution through a partially permeable membrane; passive, no energy needed
• Active transport — the movement of substances against a concentration gradient (from low to high), using energy (ATP) released by respiration
• Turgid — plant cell swollen with water; cell membrane pressed hard against cell wall; gives the plant support
• Flaccid — plant cell with reduced water content; cell membrane no longer pressing against the wall; plant wilts
• Plasmolysed — plant cell in a very concentrated solution; cell membrane has pulled away from the cell wall due to water loss
• Partially permeable membrane — allows water molecules to pass through but not larger solute molecules

Must-Know Facts

• Diffusion and osmosis are passive — no ATP needed
• Active transport requires ATP released by respiration in mitochondria
• Osmosis involves only water molecules; it always requires a partially permeable membrane
• Active transport moves substances against the concentration gradient (low → high)
• Cells doing lots of active transport have many mitochondria (e.g. root hair cells absorbing mineral ions)
• Percentage change in mass = (Final mass − Initial mass) ÷ Initial mass × 100; use this to compare results from different-sized potato cylinders fairly
• Grade 7+: to increase rate of diffusion — increase concentration gradient, increase temperature, increase surface area, decrease diffusion distance

Quick Reference: Three Transports

(Final mass − Initial mass) ÷ Initial mass × 100

Common Exam Mistakes

• Writing "osmosis is the movement of particles": Osmosis involves only water molecules. The AQA mark scheme requires "water" specifically — writing "particles" or "substances" loses the mark.
• Omitting "partially permeable membrane" from the osmosis definition: AQA mark schemes require this phrase. "Osmosis is the movement of water from dilute to concentrated" is incomplete and will not score full marks.
• Confusing the direction of water movement: Water moves from the dilute solution (high water concentration) to the concentrated solution (low water concentration) — not the other way round. Think of it as water "trying to even things out".
• Not linking active transport to mitochondria/ATP: When asked to explain active transport, always state that energy (ATP) is needed and that this energy comes from respiration in mitochondria. Saying "it uses energy" alone is insufficient.
• Comparing raw mass change instead of percentage change: In osmosis investigations, always calculate percentage change in mass. Raw mass change is meaningless if cylinders started at different sizes — percentage change allows fair comparison.