Fig. 1.1 shows a leaf of a variegated Pelargonium plant. The leaf has green areas and pale (cream) areas. The plant was kept in a dark cupboard for two days...

Labelled answer diagram:

This is the classic starch test on a variegated (part-green, part-cream) leaf, used to show that chlorophyll is needed for photosynthesis. It tests measuring, ordering a practical method, safety reasoning, and interpreting the iodine result.

(a) Read the leaf length from base to tip with a ruler, e.g. about 9 cm (any sensible value). [1]

(b) Read the maximum width, e.g. about 6 cm (any sensible value). [1]

(c) The plant is kept in the dark for two days to remove / use up the starch already stored in the leaf (to destarch it), so that any starch found afterwards must have been made during the test. [1]

(d) The correct order is: 1 = heat the leaf in boiling water (kills the leaf and breaks down cell membranes), 2 = heat the leaf in hot ethanol (removes chlorophyll), 3 = dip the leaf in hot water to soften it (ethanol makes it brittle), 4 = add iodine solution. All four correct [3], two or three correct [2], one correct [1].

(e) The leaf is heated in ethanol to remove the chlorophyll / green colour so the iodine colour change is easy to see [1]. A hot water bath is used rather than a flame because ethanol is highly flammable and would catch fire over a naked flame, so a water bath is safer [1]. [2]

(f) After iodine is added, the green areas turn blue-black (starch present) [1], while the pale areas stay brown / orange-brown, the colour of iodine, showing no starch [1]. [2]

(g) Starch is found only in the green areas because those areas contain chlorophyll [1]; chlorophyll absorbs light for photosynthesis, which makes glucose that is stored as starch [1]; the pale areas have no chlorophyll, so they cannot photosynthesise and make no starch [1]. [3]

(h) Using a variegated leaf investigates whether chlorophyll is needed for photosynthesis (the variable being tested is the presence of chlorophyll). [1]

(i) Iodine solution changes from brown / orange to blue-black in the presence of starch. [1]

(j) A suitable control is a fully green, destarched leaf left in the light [1]; it should turn blue-black all over, confirming the test works and that only chlorophyll differed [1]. [2]

(k) Give a sensible estimate of the green fraction, e.g. about 60 % [1]. A squared grid improves it: lay the grid over the leaf and count the squares that are mostly green [1], then express these as a fraction of the total squares covered by the leaf for a more accurate figure [1]. [3]

Exam tip: the reason for each starch-test step is a favourite mark - boiling water stops reactions, ethanol removes chlorophyll, hot water re-softens, iodine tests for starch. Learn the "why" of each, not just the order.