The sealed container functions as its very own little world with various cycles where water, carbon dioxide and oxygen circulate. Plants grow because they receive light, water, carbon dioxide and nutrients. In our jars, solar energy is replaced by grow lights that shine through the glass of the jars.
Both photosynthesis and cellular respiration occur inside the jar. When it is light, plants can use carbon dioxide and water to produce carbohydrates and oxygen in a reaction called photosynthesis. Plant matter is broken down by small decomposers that use carbohydrates together with oxygen to obtain energy. This cellular respiration produces carbon dioxide and water, just like in the air we exhale. Do you see the water droplets on the glass? The light causes the air within the jar to heat up. Water can then evaporate from the plants and soil as invisible water vapour. This water vapour condenses into water droplets on contact with the colder glass. The droplets eventually run down towards the bottom of the jar and the cycle begins once again.