Why isn't photosynthesis catabolic reaction? Here is the chemical equation of photosynthesis $$CO_2 + H_{2}O \longrightarrow C_{6}H_{12}O_{6} + O_2 $$ We see that the water is in reactant side of the equation. By the way, It ought to be catabolic reaction. However, It seems anabolic reaction on my textbook. * What should we think for this reaction? * How do we get If energy turned out in product side of the equation? * Why isn't photosynthesis catabolic reaction?

Catabolic reactions are those that make larger/more complex molecules into those that are smaller/less complex, releasing energy.

Anabolic reactions are those that makes smaller/less complex molecules into those that are larger/more complex, costing energy.

Photosynthesis takes two of the least complex molecules from the perspective of biology, carbon dioxide and water, and forms glucose which is high-energy and complex - this is clearly an anabolic reaction.

The only way I can interpret your question is that you were misunderstanding the definitions of catabolic and anabolic.