### Theory:

Water:
Water is a source of hydrogen, which is used in the production of organic molecules. The water, releases the oxygen during the photosynthesis when light energy hits on it . Water is such an important component of photosynthesis, a lack of it could cause carbon dioxide absorption problems. This water is absorbed by the plant's roots.
Carbon dioxide:
The plants absorb the carbon dioxide from the atmosphere. First, small amounts are absorbed from the soil as carbonates by the roots. Then, with the help of their surfaces, the hydrophytes absorb carbon dioxide as bicarbonates.
The role of stomata:
Try to recall from class IX regarding stomata. Stomata are microscopic pores that are found on the leaf's surface. Plants obtain carbon dioxide through the gaseous exchange, which occurs in the leaves through the stomata.

Gases are exchanged across the surface of stems, roots, and leaves as well. Large volumes of water can be lost through these stomata in addition to the gaseous exchange. As a result, when carbon dioxide isn't needed, the plant closes the stomatal pores.

The guard cells control the pores opening and closing. For example,
• The guard cells enlarge when water rushes into them, causing the stomatal pore to open,
• In the same way, as the number of guard cells decreases, the pore closes.
The opening and closing of stomata

An experiment demonstrating the importance of carbon dioxide in photosynthesis:
• Take two healthy potted plants which are nearly the same size.
• Place them on separate glass plates in a dark room where sunlight cannot penetrate for three days.
• In a watch glass take potassium hydroxide and place it by the side of one plant. Potassium hydroxide is a substance that is used to absorb carbon dioxide.
• Now, cover both the plants with separate bell-jars, and seal the bottom of the jars with Vaseline so that the set-up is kept airtight.
• Place the plants in sunlight for about two hours.
• Pluck a leaf from each plant and check for starch with the help of an iodine solution.
Experiment shows that $$CO_2$$ is necessary for photosynthesis

Observation:

The leaves of the plant beside which potassium hydroxide was placed turned brown after the iodine test, and the leaves of the plant beside which potassium hydroxide was not placed turned blue-black.

Inference:

From this, we can say that the potassium hydroxide absorbed all the carbon dioxide. Hence, the leaf fails to make starch in the absence of $$CO_2$$, demonstrating that $$CO_2$$ is required for photosynthesis.
Reference:
https://commons.wikimedia.org/wiki/File:Opening_and_Closing_of_Stoma.svg