Why is it impossible for carnivores to eat plants?

Because carnivores only eat meat.  Or as the dictionary says, "carnivores feed on flesh" Flesh is meat. Not plants.

If a carnivore did eat plants instead of meat, then they'd be an herbivore.
If a carnivore ate plants and meat, they'd be an omnivore.

The light is great source of energy

By water evaporation from the leaves is created a negative pressure at the top of the tree that promotes the ascension  of the water through channels that are called xylem. there are also two molecule forces that are involved: the intra-water molecules cohesion (water molecules stick to each other) and the adhesion when the water molecules stick to the channels they are running in. this is what enables water to go from the roots to the top of a tall tree.