Question

(i) Define dark reaction.
(ii) Give an overall balanced equation of this process
(iii) Describe an experiment to prove that sunlight is necessary for photosynthesis.
(iv) Mention any two points in which photosynthesis and respiration in plants are opposite to each other.

Answer 1

(i) Dark reaction or biosynthetic reaction is also known as Calvin cycle. This reaction does not require direct light and occurs in the stroma of chloroplasts. During this phase, ATP and NADPH₂ (formed during light reactions) are utilised for the reduction of CO₂ to carbohydrates (food).

(ii) 6CO₂ + 12H₂O $\underset{\mathrm{Sunlight}}{\overset{\mathrm{Chlorophyll}}{\to }}$ C₆H₁₂O₆ + 6O₂ + 6H₂O

(iii) To prove that light is needed for photosynthesis, put a potted plant in dark for two days to destarch it.
Cover a portion of the leaf with a black paper so that it does not receive light. Keep the plant in light for a few hours. Pluck the leaf and remove the black paper. Boil it and treat it with alcohol to remove the colour. Now, add iodine solution to it. The entire area of the leaf will turn blue-black except the one covered with black paper, showing the presence of starch.
This shows that light is needed for photosynthesis.

(iv)

Photosynthesis	Respiration
Carbon dioxide is consumed and oxygen is released.	Carbon dioxide is released and oxygen is consumed.
It occurs in green plants only.	It is a characteristic of all living things.