How do tree gets energy? How is it different from the mosquito and the koala?

Trees and other green plants are the source of energy for all animal life to live and grow. Through the process of photosynthesis plants change light energy from the sun into chemical energy that is stored in the plant as carbohydrates (sugars) as it grows. Mosquitoes don't suck blood as an energy source for themselves; they get their food comes from plant sugars. The blood they suck is used to feed their offspring, as protein in blood nourishes their eggs. Koalas sleep so much because they need time and energy to digest their food. It takes a lot of energy to digest the fibrous leaves of the eucalyptus. Koalas are herbivores who eat only eucalyptus.

all eukaryotic cell have mitochondria : )

a corroding iron, if that's one of your choices