$(a)$ Explain why multicellular organisms need a transportation system for carrying food and oxygen.
$(b)$ Why does an organism require an excretory system?

(N/A) In multicellular organisms,cells are not in direct contact with the external environment. Specialized tissues are responsible for the uptake of food and oxygen at specific sites. Since these substances are required by all cells throughout the body,a transportation system is essential to move them from the site of absorption to all other parts.
$(b)$ Metabolic activities within an organism produce various waste products,many of which are toxic or harmful if allowed to accumulate. An excretory system is required to filter and remove these harmful by-products from the body to maintain homeostasis.