Cell Transport, Knowledge Organiser

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

Common Exam Mistakes

• Writing "osmosis is the movement of particles": Osmosis involves only water molecules. Mark schemes require "water" specifically — writing "particles" or "substances" loses the mark.
• Omitting "partially permeable membrane" from the osmosis definition: 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.

Memory Aid

• Osmosis = O — "O" looks like water droplets. Osmosis = water ONLY. "O for Only water."
• Active = Against — Both start with "A". Active transport goes Against the gradient.
• Turgid, Flaccid, Plasmolysed — "The Full Plant Plays" → Turgid (full of water), Flaccid (limp), Plasmolysed (pulled away from wall).
• Percentage change formula — "Final minus Initial, divided by Initial, times 100." Think: "FI-I over I times 100."

Common Mistakes

• ❌ "Osmosis is just diffusion of water" Incomplete — and incomplete gets no marks. Osmosis is specifically the movement of water molecules through a partially permeable membrane. Without mentioning the membrane, you will not get the mark. Osmosis is also NOT the movement of salt or solute — it is ONLY water.
• ❌ "Active transport and diffusion go in the same direction" They are opposite. Diffusion moves substances from high to low concentration (down the gradient). Active transport moves substances from low to high concentration (against the gradient), using energy from ATP. The body uses active transport when diffusion would move things the wrong way — e.g. absorbing minerals through root hair cells where soil mineral concentration is lower than inside the cell.
• ❌ "Diffusion requires energy" Diffusion and osmosis are both passive — they require NO energy. Only active transport requires energy (from ATP, produced by mitochondria). This is one of the most common exam errors.
• ❌ "Water always moves into cells" Water moves from high water concentration to low water concentration. It can move INTO cells (making them turgid) or OUT of cells (making them flaccid/plasmolysed), depending on the concentration of the surrounding solution.
• ❌ "Larger organisms have better diffusion" The opposite is true. Larger organisms have a smaller surface area to volume ratio, which makes simple diffusion less efficient. This is why they need specialised exchange surfaces (e.g. alveoli, villi, root hairs) to increase their surface area.