ICSE Class 10 Answered

 (iii) Light is one of the major raw materials for photosynthesis. During photosynthesis, plants utilise carbon dioxide and release oxygen gas. The rate of photosynthesis depends on the intensity of light. As the light intensity is decreased, the rate of photosynthesis decreases and thus, the production of oxygen gas also decreases and hence, the bubbling of gas also stops eventually. The gas bubbles stop when there was still some light shining on the plant because although there was light, it was not sufficient for the process of photosynthesis to occur and produce a considerable amount of oxygen gas.

 

(iv) In the absence of light, exchange of gases takes place in the plant due to respiration or breathing. However, gas bubbles are not evolved because whatever oxygen is present in the medium is utilised by the plant to produce carbon dioxide during breathing. This carbon dioxide is then utilised by the plant in the dark reaction and converted into intermediates which will then be used to produce starch through photosynthesis in the presence of light.