There are a variety of ways that cells can signal to each other; 2 require direct contact while 3 use receptor-ligand interactions. Briefly describe each; the kind (s) of cell (s) or circumstances where you would find each type of communication. What is the ultimate goal of intracellular signal transduction? Why do cells do this? Why expend so much energy and material? What role does phosphorus play? Briefly explain the difference between necrosis and apoptosis? Why is the latter part of the cell’s program?