22. Describe the four main characteristics of an ani- mal's respiratory surface.

23. Trace the pathway of air through the human res-
piratory system, from the nostrils to the lungs. At
what point within the lungs do the oxygen mol-
ecules really enter the body?

answerdendrites; blank(2) axons;

while the heads of the phospholipid structure attract water, the tails are hydrophobic. the model shows how the hydrophilic section folds around the fluid molecules to bring them into the cell so they don’t need to pass through the hydrophobic tails.