Two green plants are kept separately in oxygen free containers, one in the dark and other in sunlight. It was observed that plant kept in dark could not survive longer. Give reason for this observation.

The plant kept in sunlight survives longer because it can perform photosynthesis. During photosynthesis, it absorbs light energy and converts it into chemical energy, splitting water molecules to release oxygen ($\text{O}_2$) as a by-product:

$$6\text{CO}_2 + 12\text{H}_2\text{O} \xrightarrow{\text{Chlorophyll, Sunlight}} \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 + 6\text{H}_2\text{O}$$

This released oxygen is then utilized by the plant for respiration (the breakdown of food sources for cellular energy needs).

Conversely, the plant kept in the dark cannot perform photosynthesis due to the absence of sunlight. Therefore, it cannot produce oxygen. Since the container is oxygen-free, the plant in the dark cannot carry out respiration to maintain its life processes, leading to its early death.