It's a beautiful cycle. Plants produce stored accessible energy from water+co2+sunlight. Carbohydrates aka sugar. O2 is also produced. Animals consume the energy which gives off c02 and the cycle continues Without plants all o2 becomes bound up. An reducing atmosphere because o2 is so reactive. But you have to also know that sugar isn't just energy for plants but also their structure. Roots trunks leaves bark. It's all made up of sugar created by photosynthesis. Which means plants bind up c02 unless plants get eaten. And if too much co2 gets bound up life on earth dies A beautiful cycle.