(a) Describe an experiment to show that green plants require sunlight for photosynthesis. (Details of test for starch not required) (b) What other condition...

Answer Details

(a) Experiment to show that green plants require sunlight for photosynthesis

• Take a potted green plant and place it in a dark cupboard for about 48 hours to destarch its leaves (so that any starch found later must have been newly made).
• Cover part of one leaf on both surfaces with a strip of black paper (or aluminium foil), leaving the rest of the leaf exposed. This screens the covered part from light.
• Place the plant in bright sunlight for several hours so that photosynthesis can take place in the exposed parts.
• Remove the covered leaf, take off the black paper, and test the whole leaf for starch.

Observation: the part of the leaf that was exposed to light turns blue-black with iodine (starch present), while the part that was covered does not (no starch).

Conclusion: starch (food) was made only where light reached the leaf, showing that sunlight is necessary for photosynthesis.

(b) Other conditions necessary for photosynthesis

1. Chlorophyll (in chloroplasts).
2. Carbon dioxide.
3. Water.
4. A suitable (warm) temperature.

(c) Part of a dicotyledonous leaf where most food is manufactured, with reasons

Most food is manufactured in the palisade mesophyll (palisade layer). This is because its cells are packed with numerous chloroplasts containing chlorophyll to trap light, and they are arranged closely at the upper part of the leaf, directly beneath the transparent epidermis, where they receive the most light.

(d) What happens to the manufactured food

The glucose made is used in respiration for energy; some is converted to starch and stored temporarily in the leaf; the rest is changed to sucrose and translocated through the phloem to other parts of the plant, where it is used for growth or stored (as starch, oils or proteins) in roots, stems, seeds and fruits. Some is also used to build cellulose and, with mineral salts, to form proteins.