Plants, Soil, and the Dark Cupboard

The first conception is that soil is plant food; students conflate water and minerals with energy-rich food because everyday language calls fertiliser 'plant food' and the video used that phrase. The second is that respiration and photosynthesis are simply opposite processes; students miss that both occur in the plant, at different rates, and that respiration releases energy from glucose while photosynthesis stores energy in glucose. The third is a failure to connect light deprivation with reduced starch production. These ideas persist because they are linguistically plausible, because the diagram labels 'water and minerals' without contrasting them with 'food', and because the students have no evidence that soil mass does not turn into plant mass. The teacher's first priority is to elicit and confront the conceptions with evidence, not to repeat the correct definition. A correct sentence memorised after a lecture will coexist with the old idea.

Engage: ask, 'A tree gains hundreds of kilograms. Does that mass come from the soil?' Students predict and write reasons. Explore: students weigh a potted plant and its dry soil, plant a bean, and after some days weigh again; the soil mass barely changes while the plant gains mass. In parallel, cover part of a leaf with black paper, keep the plant in light, and test the covered and uncovered areas with iodine for starch. Explain: soil supplies water and minerals, not food; glucose is made in the leaf using light, water, and carbon dioxide. Elaborate: use Hydrilla in light and dark to show gas exchange, and compare respiration and photosynthesis as simultaneous processes. Evaluate: a predict-explain-observe-explain write-up. The prediction that soil mass becomes plant mass creates conflict when the balance shows otherwise. The teacher should require students to revise their original sentence, not just copy the correct one.

Use a transfer scenario: 'A potted plant is kept in a sealed transparent chamber with moist soil and water. After three weeks it has grown. Where did the new leaf mass come from? A classmate says it came from the soil. Write a reply using evidence from our investigation.' Criteria: the answer should state that most mass comes from carbon dioxide and water, that soil supplies water and minerals but not energy-rich food, and that light is required for glucose production. A strong answer also distinguishes respiration from photosynthesis and notes that both occur. A memorised answer names the correct inputs but cannot explain the sealed chamber or the soil-mass evidence. The teacher should score the explanation, not the keyword list, and compare it with the student's original prediction to see conceptual change.