Once upon a time Bob planted a seed in the soil in front of his barn. This seed will go through the process of photosynthesis. This position will give it the sunlight's energy. Underground the seed is collecting water and other minerals through the roots.
Bob then gives the seed water and waits a little while. The seed is also getting water and other minerals through it's roots underground.
Bob is surprised that his seed has sprouted a little sprout when he was away. There is now more mass in the seed.
Photosynthesis takes place in the chlorophyll or more specifically the chloroplasts of the plant cell. There are little holes called the stomata in the leaf which let in the reactants like CO2 and water, and let out the products like 02 and C6H1206 which is sugar.
Photosynthesis is endothermic because the plant absorbs all of the thermal energy from the sun. En means in so it is endothermic.
Even when it is nighttime the plant is just relaxing and is not in the process of photosynthesis because it cannot get the sun.