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Bromothymol blue, an acid indicator, goes from blue to yellow when reacting with acid. The color change in sunlight confirmed the use of CO2, dissolved within the solution as carbonic acid- its removal from the solution makes the more basic, and the solution goes from yellow (acidic) to blue (alkaline).

CO2 + H2O ↔ H2CO3

Further Explanation:

Photosynthesis is a chemical pathway that’s integral to producing energy in plants and other primary producers. Energy in the form of molecules of glucose is produced from light, water and carbon dioxide while oxygen is released.

Occuring in several complex steps, photosynthesis is a rate limited reaction, it depends on several factors including carbon dioxide concentration, ambient temperature and light intensity; the energy is retrieved from photons, I.e. particles of light, and water is used as a reducing agent. Water supplies chlorophyll in plant cell with replacement electrons for the ones removed from photosystem II.

Additionally,

water (H2O) split by light during photolysis into H+ and OH- acts as a source of oxygen along with functioning as a reducing agent; it reduces the molecule NADP to NADPH by providing H+ ions and produces molecules of the energy storage molecule ATP through an electron transport chain.
This occurs in the thykaloids, where pigment molecules like chlorophyll reside.
Later, in dark reactions, NADP and NADPH are used in the Calvin cycle where monosaccharides or sugars like glucose are produced after the modification of several molecules. These store energy in their bonds, which can be released in respiration in the mitochondria.

In all eukaryotic cells mitochondria are small cellular organelles bound by membranes, these make most of the chemical energy required for powering the biochemical reactions within the cell. This chemical energy is stored within the molecule ATP which is produced. Respiration in the mitochondria utilizes oxygen for the production of ATP in the Krebs’s cycle via the oxidization of pyruvate( through the process of glycoysis). The electron transport chain, in which oxygen functions as the terminal electron acceptor occurs in both plants and animals.