The cells in this leaf are making their own energy by the process of

a
respiration, where oxygen and sunlight at Q and R are taken in and then converted into carbon dioxide and glucose at S and T.
b
photosynthesis, where carbon dioxide and sunlight at Q and R are taken in and then converted into glucose and oxygen at S and T.
c
photosynthesis, where glucose and oxygen at Q and R are taken in and then converted into water vapor and thermal energy at S and T.
d
respiration, where carbon dioxide and oxygen at Q and R are taken in and then converted into thermal energy and methane gas at S and T.