Final answer:
Plants release the oxygen produced during photosynthesis into the atmosphere, replenishing oxygen levels and supplying most of the oxygen in our atmosphere.
Step-by-step explanation:
Plants convert carbon dioxide and water into glucose (a sugar) and oxygen through the process of photosynthesis. The oxygen atoms produced during photosynthesis form oxygen gas, which is a waste product. This oxygen is released into the atmosphere, replenishing the oxygen levels and supplying most of the oxygen in our atmosphere. Without photosynthesizing organisms like plants, there would not be enough oxygen in the atmosphere for animals to survive.