Photosynthetic plants photosynthesise carbon dioxide and water to produce oxygen and glucose. Without photosynthetic plants elvoving there eukaryotes would have extremely different respiratory systems. Eukaryotes (including plants) currently respire glucose using oxygen on a cellular level. If photosynthetic plants were not present there would be much less oxygen in the air. Eukaryotes therefore would evolve completely different ways of respiring to release energy.
Just tell how without plants there wouldnt be humans. We would not be able to live without food and defiantly not without oxygen. No plants mean no life at all.
