Answer the following questions using the diagrams provided, which represent cross sections of bacterial cell walls.

a. Which diagram represents a gram-positive bacterium? How can you tell? <IMAGE>

b. Explain how the Gram stain works to distinguish these two types of cell walls.

c. Why does penicillin have no effect on most gram-negative cells?

d. How do essential molecules enter cells through each wall?

e. Which cell wall is toxic to humans?