Scientists measure the rates of biochemical processes by monitoring either substrate depletion or product generation. considering this, what substrates or products might you monitor to determine the rate of photosynthesis? of cellular respiration?

The right answer is to monitor the increase or the decrease of gases which are O2 and CO2.

In photosynthesis:

It takes carbon dioxide and water to synthesize a molecule of glucose, releasing oxygen, thanks to the light energy. So we can monitor either the increase in oxygen or the decrease in carbon dioxide.

6 CO2 + 6 H2O + light energy → C6H12O6 (glucose) + 6 O2

In the cellular repsiration:

Breathing is a physiological process that supplies the body with oxygen and releases it from carbon dioxide. So we can monitor either the increase in CO2 or the decrease in O2.

C6H12O6 (glucose) + 6 O2 → 6 CO2 + 6 H2O + ATP

atp is used for energy.