Plants use photosynthesis to produce carbohydrates from inorganic substrates.
All organisms need energy to grow and reproduce. Many organisms (including all plants) use to produce which is used for cellular energy. The respiration reaction uses carbohydrates like glucose as a substrate. Whereas humans eat food to provide the fuel for respiration plants make their own through photosynthesis.
Without photosynthesis plants would not have carbohydrates for respiration.
In addition the carbohydrates produced in photosynthesis are also used to build plant cell structures like the cellulose cell wall.
