Theory question 104
Photosynthesis and Plant Nutrition(a) Describe an experiment to show that green plants require sunlight for photosynthesis. (Details of test for starch not required) (b) What other conditions are necessary for photosynthesis?
(c) Name the part of a dicotyledonous leaf where most food is manufactured, giving reasons.
(d) What happens to the manufactured food?
Show the model answer
(a) Destarch a potted plant by keeping it in the dark for 24–48 hours; cover part of one leaf on both sides with black paper or foil (or a stencil); place the plant in bright sunlight for some hours; remove the leaf and test it for starch with iodine; the uncovered part turns blue-black (starch present) while the covered part stays brown (no starch); conclusion: light is needed for photosynthesis. (A destarched plant left uncovered in light can act as the control.)
(b) Carbon dioxide; water; chlorophyll; suitable temperature (enzymes).
(c) The palisade mesophyll (layer). Reasons: its cells contain the most chloroplasts; they are just below the upper epidermis and receive the most light; they are long, closely packed and arranged at right angles to the surface to trap light; nearby air spaces in the spongy layer supply carbon dioxide.
(d) Glucose formed is changed into starch and stored temporarily in the leaf; it is converted to soluble sugar (sucrose) and translocated in the phloem to other parts; it is used in respiration to release energy; it is used to make cellulose, proteins, fats and oils; excess is stored in storage organs (seeds, fruits, tubers, roots).
Describe the destarching step and include a control and conclusion in the experiment.