1. Why are endocytosis and exocytosis considered active transport mechanisms if they do not necessarily move 1 molecule against a concentration gradient?2. What is clathrin and what is its role in receptor-mediated endocytosis?3. Compare and contrast receptor-mediated endocytosis with phagocytosis and pinocytosis.4. What are some other examples of proteins secreted by exocytosis that are essential for cell and tissue function?