HOTS Science Primary 6 Energy

Priya conducts an experiment using a variegated leaf from a croton plant. The leaf has distinct green regions and white (non-green) regions. She destarch the plant for 48 hours in the dark, then exposes the plant to bright light for 6 hours. She then performs the standard starch test: boiling the leaf in water, decolourising it in ethanol, rinsing in water, and adding iodine solution. She also tests a second identical variegated leaf from the same plant that was kept in complete darkness for the entire 6-hour period instead of being given light. Which result correctly predicts the colour of ALL four regions — green/dark and white/dark?

A Leaf 1 (light): green regions turn blue-black, white regions remain orange-brown; Leaf 2 (dark): both green and white regions remain orange-brown — because photosynthesis requires both chlorophyll (absent in white regions) and light (absent for Leaf 2).
B Leaf 1 (light): both green and white regions turn blue-black because light provides enough energy to make starch throughout the whole leaf; Leaf 2 (dark): both regions remain orange-brown because no light is available.
C Leaf 1 (light): green regions turn blue-black, white regions turn pale blue because they receive some glucose by diffusion from the green parts; Leaf 2 (dark): green regions remain orange-brown, white regions turn blue-black due to stored starch from before destarching.
D Leaf 1 (light): white regions turn blue-black, green regions remain orange-brown because chlorophyll absorbs light and prevents starch from forming; Leaf 2 (dark): both regions turn blue-black because starch accumulates in the dark.
Show Worked Solution

Worked Solution

Step 1: Recall the two requirements for photosynthesis Photosynthesis requires: (1) chlorophyll — the green pigment that absorbs light energy; and (2) light — the energy source. Both must be present for photosynthesis to occur and starch to be produced. Step 2: Predict Leaf 1 (kept in light for 6 hours) Green regions: contain chlorophyll AND receive light → photosynthesis occurs → glucose produced → converted to starch → iodine turns blue-black. White regions: no chlorophyll → even with light, cannot photosynthesise → no glucose produced → no starch → iodine remains orange-brown. The plant was destarched, so no pre-existing starch is present in either region. Step 3: Predict Leaf 2 (kept in dark for 6 hours) Green regions: have chlorophyll but NO light → photosynthesis cannot occur → no new starch → iodine remains orange-brown. White regions: no chlorophyll AND no light → definitely no photosynthesis → no starch → iodine remains orange-brown. Both regions of Leaf 2 remain orange-brown. Step 4: Match to the correct option Option A is the only option that correctly applies both conditions: chlorophyll requirement (white regions never go blue-black) and light requirement (Leaf 2 stays orange-brown throughout). It correctly predicts all four regional outcomes.

Correct answer: Leaf 1 (light): green regions turn blue-black, white regions remain orange-brown; Leaf 2 (dark): both green and white regions remain orange-brown — because photosynthesis requires both chlorophyll (absent in white regions) and light (absent for Leaf 2).

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