TEACHER INFORMATION SHEET
TEACHER INFORMATION SHEET
A fun way to direct student thinking to concepts of 'The Living Cell' initially is to facilitate an interactive brainstorming. Below is one way a college biology professor gets her students to begin thinking (Dorothy Dunning, 2000, Biology professor at West Virginia University).
Props needed: One thing obviously alive (a dog), one thing that is not alive but embodies some of the properties of that which is alive (usually a motile plastic toy), and another thing which is also alive but very different from the first living thing. Stand the dog on a table and ask the students: "Is this alive?" Sometimes dead silence. I prod them as I pet the dog: "Is this object alive?" Eventually somebody says "Yes". "Super!" I exclaim as if this were a major scientific breakthrough, "Why?"
I pet the dog. If nobody responds in 10 sec I invite somebody to come up and pet the dog. Usually there's no shortage of eager participants, so while they're stroking away I ask them why they're doing this. "What is there about this object that attracts you? Do you rub other, similar-sized objects in the environment, e.g. fire hydrants? Park benches? Sticks (if it's a small dog)?"
"No, they don't". "Why not?" "What is it about this thing that makes them want to come stroke it?"
As I begin to extract something interesting about a dog, I immediately write it down on the board: because it's warm, it wags its tail, it's cute, it has fur, it licked me. No matter how stupid, all answers go on the board. Once the ice is broken, I get back to the question of whether the dog is alive. The answers come faster now: because it breathes, because it moves, because it licks. Usually somebody will remember that they heard something about this before, and the "scientific" answers start to appear: respiration, reproduction, energy, metabolism, eating, drinking. I don't dissect these at all, but put them up on the board, erasing the clumsy first tries as space becomes limiting. Before I erase anything I try to convert it to something useful. So 'because it moves' is combined with 'tail wagging' and 'licking' into "Movement."
When I've got a list of doggy characteristics of Life, I have the dog jump down and turn on the mechanical toy (I have an electric teddy bear that waves its arms and blows a whistle as it moves jerkily about.). I ask the same question: "Is this alive?" The answer comes immediately: "No". "Why not?" I say "It moves. It waves its arms. It makes noises. It has fur. Why is that thing (the dog) alive and this thing is not? What are the differences between them?" Somebody may say that the live dog was "organic." [I love that one, for the toy is plastic, covered with the fur of the wild orlon.] They quickly point out respiration and reproduction (and maybe metabolism), which are already on the board. "If I have a spayed bitch or a neutered dog, I point out that the dog cannot reproduce." "If I have a young pup, I'd say that she has never reproduced. I also point out that most of them (the students) have not reproduced. Does that make them not alive?" I ask a student in the back if they observed respiration in the dog. "Well, no." "How do you know the dog does it?" "Because I know that dogs breathe." "OK, fine. How do you know that teddy bears don't?" I don't beat the kid to death with this; just raise the issue and drop it.
I
put the toy away and haul up a philodendron in a pot. "Is this
alive?" They're into this now: "Yes." "Why?" "Breathing,
photosynthesis, energy, metabolism." "Is breathing the same thing in
this object as in the dog? Does this thing move? What effect does
respiration have on whether or not it's alive? Does it eat? Drink?
Did the dog do photosynthesis? Does that mean it's not alive?" Then I
similarly dissect "energy" and "metabolism" and whatever other
characteristics are up there. Praise them for metabolism.
Eventually I get to the main point: The characteristics of living things are non-obvious. They're not aspects we can observe directly without doing something to an object. We have to observe and MEASURE the RESULTS of metabolism to detect whether or not it's happening. Energy is all over; what we detect is the WORK that it does. I point out that "The toy did work, too. It also had energy. So why isn't it alive?" Ummm...things that aren't alive may also use energy and do work.
Then you can get into NECESSARY and SUFFICIENT characteristics. A feature may be NECESSARY (e.g. energy usage and doing work) but not SUFFICIENT to characterize Life. What we're looking for is characteristics that are both necessary and sufficient. "Is there any one characteristic that is both necessary and sufficient?" You won't get any responses to this one. Raise it and drop it. "Where is the energy that keeps the dog going? The philodendron? Where does it come from?" The energy comes from outside both organisms. Things that are alive must get energy from outside themselves. Somebody points out that the batteries in the toy are different from the energy that drives the dog and the philodendron. A gold star to that kid, if you can reach him/her. Are the batteries inside the toy? How did they get there? (Another necessary but not sufficient characteristic.)
Reproduction of a whole dog is not necessary for Life, but its component cells must reproduce to produce the dog they see before them. Then you get into cells. "Does anything that's alive have to have cells? How many? What's a cell? How do you know when you've got them? What are the minimum requirements for cellular life?" Somebody asks about viruses. Give that kid a gold star, too. Try to get into the imperfection of reproduction and differences between parents and offspring. (Adolescents are heavily into that, as they're convinced they and their parents are members of different species.) That leads naturally into evolution, another characteristic of Life. By this time the hour is pretty well shot.