4.2.2—Diffusion and osmosis investigations

A transport investigation must connect a measurable change to movement across a boundary. Use matched samples, a controlled difference between the two sides, a fixed exposure and repeated measurements so that the effect of the transport variable can be separated from handling or environmental variation.

1. Define the question and variables: Choose the independent variable, such as solution concentration or concentration gradient, and the dependent readout, such as mass/length change, colour or solute concentration. Keep the transport process being tested explicit.
2. Prepare matched samples: Use plant-tissue pieces of comparable starting size and surface area, or a bounded model such as dialysis tubing or agar. Record the starting mass, length or concentration consistently.
3. Apply the treatment fairly: Place samples in the planned range of solutions or gradients for the same time and under the same relevant temperature, volume and handling conditions. Use a suitable control or comparison condition.
4. Measure the change: Remove and handle every sample in the same way, then record final mass or length, colour/penetration or concentration as appropriate. Calculate a change or percentage change when that is the chosen readout.
5. Repeat and compare: Use repeats at each condition, summarise the results consistently and show variation before inferring a trend. Compare like with like rather than selecting a convenient result.
6. Interpret the process: Diffusion is net movement of particles down a concentration gradient; osmosis is net movement of water across a partially permeable boundary down a water-potential gradient. In a plant-tissue mass experiment, mass change is an operational proxy for net water movement, not direct observation of individual molecules.

A steeper controlled gradient can change the net movement, but the conclusion is only defensible when sample geometry, time, temperature, solution volume and measurement handling are matched. Plant tissue can show osmosis through mass or length change; dialysis tubing or agar can model diffusion across a boundary, but the model does not automatically reproduce every property of a living cell.

Mass, length or colour change alone does not identify the transport process. State what crossed the boundary, whether the boundary was partially permeable, and which controls exclude evaporation, leakage, damaged tissue, unequal blotting or inconsistent handling. No net change means equal opposing net flows, not that molecules stopped moving.