What does "2" mean in 2co2

It means how many of what is in the element. If the number is small and underneath the letters, its called a subscript :)

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The answer is water molecules are split.cellular respiration and photosynthesis are complement  reactions to each other. these are reverse reactions. in cellular respiration glucose and oxygen yield water and carbon dioxide. so, when respiration begins,  glucose molecules are split. in the opposite reaction - in photosynthesis, carbon dioxide and water yield glucose and oxygen. thus,  when respiration begins, water molecules are split.