Two petri dishes of radish seeds are prepared as follows: a disk of absorbent, but otherwise inert, filter paper is placed in the bottom of each dish and 5 ml of distilled water are added to each disk, then two grams of dry radish seeds are spread over each moist disk. one dish is put in a dark cupboard and the other is put on a sunny windowsill. the investigator checks the dishes daily and adds water equally as needed. seeds in both treatments germinate within 2 days. at the end of 10 days, the plant material on each petri plate is dried and weighed. the results are:

A. The increased biomass in the sunlit dish came from the carbon and oxygen atoms present in the carbon dioxide it took up during photosynthesis that it used to synthesize organic molecules.

Plants use photosynthesis in order to convert light energy (from the sun) into chemical energy, stored in carbohydrate molecules (such as sugars). This chemical energy can be further transformed to fuel some of the organism's activities. This chemical reaction (photosynthesis) is light-dependent and includes the use of carbon dioxide and water to produce sugars and oxygen. The seeds that were put on a sunny windowsill had plenty of light energy needed for their photosynthetic activity. This resulted in the accumulation of sugar molecules in the plant tissues and an increase in the biomass. These sugar molecules were synthesized by the carbon and oxygen atoms comprising the carbon dioxide molecules absorbed during photosynthesis.   

B. The most likely explanation for the why the dark-grown seedling lost biomass is that cellular respiration in the cells of the seedlings consumed the starch and oils present in the seed as sources of energy.

Plants use cellular respiration in order to constantly supply energy to their cells. This chemical reaction uses the sugar molecules produced by the photosynthesis and stored in the plant tissues to get energy. Seeds put in a dark cupboard did not have access to any light energy, therefore, they could not perform photosynthesis. As a result, they had to perform respiration and consume starch and oils which were present in the seed to get the necessary energy. This resulted in a reduction in their biomass. 

Blindness is your answer