The process of photosynthesis (which occurs in chloroplasts) uses the energy of the sun to change water and carbon dioxide into sugar called glucose and so only occurs in the day.
Photosynthesis uses energy to make glucose
During the night and day (i.e. all the time) the process of respiration occurs (in the mitochondria) where glucose is used to create an energy molecule called ATP in each plant cell for continued survival and growth. Without ATP cells will die.
Respiration uses glucose to create energy (ATP).
Photosynthesis - Word Equations
Carbon Dioxide + Energy (Sunlight) + Water → Glucose + Oxygen
Glucose + Oxygen → Carbon Dioxide + Energy (Sunlight) + Water