Aflea and dog are an example of , because the flea irritates the dog's skin and feeds on its blood. certain bacteria live in the digestive tract of humans, and their relationship is an example of because the bacteria humans to break down the food they eat. cattle egrets eat insects off the back of an elephant, which is an example of because the cattle egrets benefit without either harming or the elephants.

Animal bodies (and even plant bodies for that matter) are complex enough that life couldn't exist without enzymes. enzymes are usually large, complex biological catalysts that are expensive to make. enzymes speed up the rate of chemical reactions without being used up during the reaction (they can be reused many times). they are highly specific in the reactions they regulate. knowing what enzymes are and how they function, why does this explain why complex life forms could not exist if there were no enzymes.

The pharynx is the structure in the body that serves as a pathway of both air and food. how does the body make sure that food does not get into the lungs? the salivary glands secrete enzymes that pull food out of the air pathway. the small intestine pushes the air out of the digestive system. the pancreas breaks down food in the air pathway. the epiglottis closes the air pathway so that food will not enter it.

This is the nitrogenous base only found in rna