Photosynthesis uses those products from cellular respiration as its reactants and in turn produces glucose and oxygen- the reactants needed for cellular respiration.
Energy Transformation: Both Cellular Respiration and Photosynthesis need to transform energy into different forms in order for their reactions to initially take place and continue onward.
Cellular Respiration and Photosynthesis
ATP is made via respiration at all times in plants and animals.
It is true that during the day when there is available solar energy, the so-called light reactions of photosynthesis are what the plant is "focusing" on.
Plants Need Photosynthesis & Cellular Respiration?
And to reward you for reading through all that, here's a cute little song I found on youtube about the relationship between cellular respiration and photosynthesis.