Question

why plants don't need specialized respiratory system?

Answer 1

They do - but they don't have lungs, they have stomata.

Plants need carbon dioxide to get the energy out of the food they make. Obviously, plants don't have lungs or gills, so how do they breathe? They breathe through special little holes on the underside of their leaves called stomata.

The stomata allow fresh air to enter the leaf, where the plant's cells can take out the carbon dioxide. When the gas exchange is completed, the plant emits oxygen back outside through the stomata again.

Plants have special cells surrounding the stomata called guard cells. The guard cells can squeeze over the opening, to close up the hole. This is useful when it's very dry out, and the plant doesn't want to lose any water through the stomata. It's also useful as a defense against small insects, which can sometimes crawl through the opening and into the leaf.