Cell WALL vs. MEMBRANE Demo; This one is cool. This will demonstrate the difference between a cell wall and a cell membrane - particularly in bacteria. I use this to show how penicillin actually "kills" bacteria. (More on that later) Make an analogy between the cell membrane being a balloon. Blow up a balloon till it pops in your face. (Wait for laughter....If there is none, your class has no sense of humor - fail them all...) Now, pull a pair of pantyhose out (preferably non-sexy ones, kids won't pay too much attention....), place the balloon INSIDE the pantyhose and proceed to inflate the balloon. It will amaze the kids that the balloon WILL NOT BURST!
WHY? Well, the wall is an interconnecting chain of polymer-like molecular chains, quite a bit like the structure of the nylon pantyhose upon close inspection - tight fibers that interweave each other reinforcing the entire structure. Without this tremendously tough and rigid cell wall, the membrane is left exposed to the environment. The membrane has very little strength and is there primarily to access and exchange certain meaningful items the cell needs. This is where a specific discussion of penicillin's function can come in handy. Penicillin actually doesn't KILL bacteria. It inhibits the full production of a new cell wall when the bacteria tries to reproduce. Bacteria reproduce by binary fission. This is where a cell will enlarge to about double its size and then splits into two "daughter" cells. During this enlarging stage, the cell must produce a larger cell wall in order to accommodate the increased size and more cytoplasmic stuff. After the one chromosome replicates, this new cell wall pinches shut in the middle, isolating one chromosome in each of the halves and thus, producing two cells where there was one.
This is where penicillin does its magic. It inhibits the completion of the new wall. A cell wall is made of long polymer chains that intertwine and connect - similar to the mesh of the pantyhose. In order for these long strings of polymers to get connected together a special enzyme is required. Penicillin blocks the action of this enzyme, transpeptidase. The long strings of polymers will still be made, they just can't cross-connect to reinforce each other. Thus the cell wall is very weak in some places. So, now, as water comes in, the membrane swells and bursts. This ends the life of the cell. For more in depth info for those of you who actually understand all that bio-chem speak, see Jack's Bugs in the News, a very informative biological site for anti-biotics and such.