What is a substrate and give an example

Idon’t know the answer and i’ve been stuck on it for a while now skskskskks

Photosynthesis uses co2 and cellular respiration produces co2. we call the point when the two processes are in balance--when there is no net production of co2--the compensation point. how might you limit one of the processes in order to achieve a compensation point?

Guard cells control which event? the growth of plants capture of solar energy gas exchange in leaves water absorption in roots