A student investigated osmosis using strips of carrot tissue. Several strips were cut, each with an initial length of 50 mm, and each strip was left in a di...

This osmosis practical tests variables, a percentage-change calculation, and the water-potential reasoning behind turgor changes.

(a) Dependent variable [1] The variable the student measures is the length (change, or percentage change, in length) of the carrot strip. The independent variable is sucrose concentration; the dependent variable is what changes as a result.

(b) Complete the table [2] Percentage change \( = \frac{\text{final} - \text{initial}}{\text{initial}} \times 100 = \frac{46 - 50}{50} \times 100 = -8.0\% \) (1 mark working, 1 mark answer). The completed table is:

(c) Why the strip lengthened in distilled water [3] Distilled water has a higher water potential than the carrot cell contents; so water enters the cells by osmosis (down the water potential gradient, through the partially permeable membrane); the cells become turgid and the strip increases in length.

(d) Measuring length accurately [2] Lay the strip flat against the millimetre scale; read to the nearest millimetre with the eye directly above the scale to avoid parallax error.

(e) Why percentage change is better [2] The strips may not have had exactly the same starting length; expressing the result as a percentage allows a fair comparison between strips of different initial length, because each is judged relative to its own start.

(f) One source of error [1] Any one: strips not exactly 50 mm at the start; a bent or soft strip is hard to measure; surface water not blotted before weighing/measuring; different parts of the carrot used.

(g) Prediction for 1.0 mol/dm³ [3] The strip would decrease in length by more than 8.0% (become even shorter); because 1.0 mol/dm³ sucrose has an even lower water potential than the cell sap; so more water leaves the cells by osmosis and they lose more turgor.

(h) Name the process [1] Osmosis.