Question

obsevation of the activity carbon dioxide is essential for photosynthesis

Answer 1

yes CO2 is most essential for photosynthesis,
because
in C3 plant for photosynthesis require 450 ppm concentration in atmosphere below 450 photosynthesis rate decreases.
while the C3 plant required 360 ppm of CO2 concentration in atmosphere .
the first product of photosynthesis is form in the presence of CO2.
hence CO2 is most important..

Answer 2

Carbon dioxide is needed for photosynthesis. The lower part of the leaf has loose-fitting cells, to allowcarbon dioxide to reach the other cells in the leaf. This also allows the oxygen produced in photosynthesis to leave the leaf easily. Carbon dioxide is present in the air we breathe, at very low concentrations.