Plankton is at the bottom of a food chain. Think of a pond. Mosquito larvae are in a pond. They eat the plankton. Small fish eat the larvae. Larger fish eat those smaller fish and the chain keeps going..BUTTT it all started with the PLANKTON. Without the plankton the whole food chain would be messed up and everything would start dying.. No plankton, no larvae, no small fish..and so on..
School is an example like you have a load of work and its hard and u keep going cuz u know it will get you someone good adn another is running a marothon even though its hard you just keep going. hope this helps and please rate below :)
I think the respiratory system and circulatory system