Final answer:
The aortic and pulmonary semilunar valves lack chordae tendineae and papillary muscles, instead, they have pocket-like folds of endocardium which prevent backflow of blood into the ventricles.
Step-by-step explanation:
The aortic semilunar valve and the pulmonary semilunar valve differ from the atrioventricular valves like the tricuspid and mitral (or bicuspid) valves in that they lack chordae tendineae and papillary muscles. Instead, these semilunar valves are composed of pocket-like folds of endocardium reinforced with connective tissue. The semilunar valves function to prevent backflow from the arteries into the ventricles. When the ventricles relax, blood pushes against the valve cusps and seals the openings, preventing the backflow of blood into the heart. On the other hand, the atrioventricular valves are equipped with chordae tendineae and papillary muscles, which are critical to their function in preventing backflow from the ventricles into the atria.