Question

give reason why multicellular organisms require special organ for exchange of gases between their body and their environment?​

Answer 1

This is because their bodies are bigger and more complex than unicellular organisms. The oxygen absorbed needs to reach all parts of the organism otherwise the organism will not be able to survive.

Answer 2

Answer:

In unicellular organisms the entire body of the organism is in contact with the environment hence exchange of materials can take place but, in multicellular organisms the entire body of the organism is not in contact with the environment and hence simple diffusion is not helpful.