Question

Why water is considered as an essential requisite for photosynthesis

Answer 1

Thank you for asking this interesting question. water is an essential requisite because it plays an important role in photosynthesis mainly in the light reactions. the water provides and compensates for the electrons lost in chlorophyll, this electron binds the hydrogen atom(from the water molecule) to the carbon(from carbon dioxide molecule) in order to make sugar. 
water also acts as an oxygen provider as it releases oxygen(from the water molecule) into the atmosphere in the form of oxygen gas.
hope this answers your question. 

Answer 2

A simple answer

because O2 is liberated to water so they are more essential

At a fundamental level, water provides electrons to replace those removed from chlorophyll in photosystem II. Also, water produces oxygen as well as reduces NADP to NADPH (required in the
Calvin cycle) by liberating H+ ions.

Water as Oxygen Provider

During the process of photosynthesis, six molecules of carbon dioxide and six molecules of water react in the presence of sunlight to form one glucose molecule and six molecules of oxygen. The role of water is to release oxygen (O) from the water molecule into the atmosphere in the form of oxygen gas (O2).

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