Photosynthesis and respiration can be thought of as opposite process.

Respiration

During respiration, cells break down simple food molecules such as glucose and release the energy they contain.

C6H12O6
(Glucose)
+
6 O2
(Oxygen)
+
6 CO2
(Carbon Dioxide)
+
6 H2O
(Water)
+
Energy

Cellular respiration occurs in the mitochondria of an organism's cells.