The Core idea and the Extended definition
At Core level, know that water moves into and out of cells by osmosis through a partially permeable cell membrane. For Extended, use the complete definition: the net movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane.
Do not write that “solute moves by osmosis”. Osmosis refers to water molecules. Do not describe the direction as high to low solute concentration without making clear what happens to water; that shortcut is where answers become ambiguous.
Plant cells in dilute and concentrated solutions
In a dilute solution, outside water potential is higher. Water enters the plant cell, the vacuole expands and the contents press against the cell wall. The cell is turgid. The wall resists further expansion and prevents bursting.
In a concentrated solution, water leaves. The cell becomes flaccid; with greater water loss, the cell membrane pulls away from the wall and the cell becomes plasmolysed. These terms belong to the Supplement layer.
How to explain a potato-cylinder result
Build the explanation in a fixed order: identify the water-potential difference, state the direction of water movement, name osmosis and the partially permeable membranes, then connect the movement to mass or length.
For example: “The solution had a higher water potential than the potato cells, so water moved into the cells by osmosis through their partially permeable membranes. The cylinders therefore gained mass.”
Percentage change and the isotonic point
Use (final value − initial value) ÷ initial value × 100. A negative result means a decrease. Percentage change allows fair comparison when starting masses differ.
On a graph, the concentration where percentage change is zero estimates the solution with the same water potential as the tissue. Read it by interpolation rather than choosing the nearest plotted point without explanation.