Final answer:
The main role of photosynthesis is to generate oxygen for the atmosphere and provide food for organisms that do photosynthesis.
Step-by-step explanation:
The main role of photosynthesis on Earth is to generate oxygen for the atmosphere.
During photosynthesis, plants and bacteria convert carbon dioxide gas, liquid water, and light into glucose and oxygen gas. Oxygen is released back into the atmosphere as a byproduct of photosynthesis, which is essential for cellular respiration and maintaining the ozone layer that protects Earth’s life from damaging UV radiation.
Plants also supply food to nearly all terrestrial organisms, either directly or indirectly, by being a primary food source for herbivores and omnivores. Without photosynthesis, there would be no oxygen in the atmosphere and the food web on Earth would collapse.