Diffusion: get the definition and the factors
Define diffusion as the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient, as a result of their random movement. The word net matters. Particles move both ways, but the overall movement is down the gradient. No energy from respiration is needed; diffusion is passive.
Examiners love asking what increases the rate of diffusion. Bank these four: higher temperature (particles move faster), steeper concentration gradient, shorter distance across the membrane, and larger surface area. (Extended only) You should also explain how surface-area-to-volume ratio affects diffusion in larger organisms. Bigger organisms have a smaller ratio, so they need specialised exchange surfaces. Real exam contexts include oxygen and carbon dioxide moving across gas exchange surfaces, linking to transport in humans.
Osmosis: the definition examiners want word-for-word
This is the single most marked-down definition in the topic. Learn it exactly: osmosis is the net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane.
Three phrases must all appear for full marks: water molecules (not 'water' alone, and never solute), water potential (higher to lower. Extended candidates must use this term; Core candidates may use 'dilute to concentrated solution'), and partially permeable membrane. A frequent fail is writing 'from low to high concentration'. For water potential it is high to low, but for solute concentration it is the opposite, which is why examiners insist on the water-potential framing. Osmosis only moves water; if the particle is anything else, it is diffusion.
Turgor and plasmolysis in plant cells (Extended)
(Extended only) When a plant cell sits in a dilute solution, water enters by osmosis, the vacuole swells and pushes the cytoplasm against the cell wall. The cell becomes turgid. The cell wall stops it bursting, and turgor pressure supports the plant. The mark scheme rewards 'turgid' and 'the cell wall prevents the cell from bursting'.
In a concentrated solution, water leaves by osmosis, the cell becomes flaccid, and if enough water leaves the cell membrane pulls away from the cell wall. This is plasmolysis, and the cell is plasmolysed. Animal cells have no wall, so in a dilute solution they swell and burst (lyse), and in a concentrated solution they shrink/crenate. Wilting in plants is the visible result of cells losing turgor. Examiners often give a diagram and ask you to name the state. Learn turgid, flaccid and plasmolysed precisely.
Active transport: the one that needs energy
Define active transport as the movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration (against the concentration gradient) using energy from respiration. The marks live in three ideas: against the gradient, energy from respiration, and the involvement of carrier/transport proteins in the membrane (Extended only for the protein detail).
Classic contexts: root hair cells absorbing mineral ions from soil where ion concentration is already low inside the cell would be the wrong way for diffusion, and the gut absorbing glucose after most has been absorbed. The mark-killer here is saying active transport 'uses energy' without saying from respiration: that link is what they reward. Never write that it 'creates energy'. If you want this drilled against past-paper questions, you can take a free trial class with our team.
The potato osmosis practical (Paper 6 favourite)
The investigation almost always involves potato cylinders or chips left in sugar/salt solutions of different concentrations, measuring change in mass or length. You must be able to:
- Identify the independent variable (concentration of solution) and dependent variable (change in mass).
- Calculate percentage change in mass = (final − initial) ÷ initial × 100. Using percentage controls for different starting masses.
- Explain results: cylinders gain mass in dilute solutions (water enters by osmosis) and lose mass in concentrated solutions (water leaves).
- Identify the concentration where there is no change in mass: here the solution has the same water potential as the cell contents.
State control variables (temperature, time, volume of solution, size of cylinder) to score the 'fair test' marks. This practical is a near-guaranteed appearance on the alternative to practical paper.