Carbon dioxide is essential to sustain plants, as the carbon fixed to carbon dioxide during photosynthesis is used to synthesise glucose. This glucose is later used during cellular respiration to make ATP, the energy molecule. In addition, glucose is also used by plants as an important source of carbon to produce large parts of the plant.
The importance of carbon dioxide for plants can be proved with a small experiment;
Take two healthy potted plants.
Keep them in the dark for 2-3 days, so that all the carbon dioxide settles down to GT to be used by the plant.
Take the plants and keep both of them in two separate closed bell jar containers which are air tight. Place an open beaker with the plant in a jar containing a solution of potassium hydroxide (KOH). KOH can absorb carbon dioxide, so the jar containing KOH is void of carbon dioxide.
Keep both the jars in the sun for 4 hours.
Break off the leaves from the plants in both jars and boil them in ethanol to remove the chlorophyll.
Apply iodine solution on the leaves. The leaves of the plant present in the jar without KOH solution turn bluish-black in colour because the starch of the leaves reacts with iodine. However, the leaves of the plant in a jar containing KOH do not change colour as the plant cannot produce starch in the absence of carbon dioxide.