That was cellular respiration: energy
is stored by burning sugars.
Photosynthesis works in a different way.
In photosynthesis, sugars are assembled
from gases and water. You might easily realize
that if burning sugars produces energy,
then assembling sugars requires energy inputs.
In this case, the energy input comes from the Sun.
Plants, unlike animals, undergo both
photosynthesis and respiration. Plants
make sugars through photosynthesis, and then
burn them in respiration in order to
produce the energy needed to grow,
reproduce, etc.
Animals can only carry out respiration.
We therefore must eat sugars in order
to power our functions.