Bing and Bong have just finished watching a fascinating film about the Blob, an interesting life form which was particularly fond of young human flesh as a food source. As with almost everyone who has seen a film, Bing and Bong begin to discuss the movie as they relax back in the three-bedroom apartment they share with Bing's pet alligator and Bong's pet duck (which always seems to be nervous for some reason).
Bing Well, Bong, that was a great movie. I haven't enjoyed a film as much since I saw "Day of the Triffids". What did you think?
Bong Are you kidding, Bing? I hated it. I always hate the movies you take me to. Giant monsters, screaming teenagers and stale popcorn are not my idea of a good time. Why didn't we go see "Age of Innocence" like I wanted to? Even "The Beverly Hillbillies" would have been better than the Blob.
Bing Wow, Bong. I didn't realize you were such a movie snob. You probably liked "Howard's End" and "Much Ado About Nothing" as well. In any case I wanted to see the Blob because it directly relates to what we are studying in Biology now.
Bong [Snapping the fingers] Of course, now I see the connection. You wanted to demonstrate intracellular or at least intrablobular digestion to me.
Bing Sorry Bong, a good guess but wrong again. We finished with digestion several days ago. You should realize I am talking about respiration, the exchange of gasses. You know [Bing breathes in and out heavily several times] oxygen, carbon dioxide, that kind of stuff.
Bong [Jumping up in excitement] Of course! My brain must have been temporarily numb from that horrible movie. You wanted to demonstrate the release of air in screams.
Bing [Motioning Bong to sit down] No Bong, your brain must still be numb. I wanted you to think about how the Blob was able to exchange oxygen and carbon dioxide with the atmosphere.
Bong [Sitting down] I see now. You wanted me to think about diffusion of gasses into and out of organisms. Oxygen diffused into the Blob across the Blob's surface membrane and carbon dioxide diffused out the same way.
Bing Good thinking. Diffusion is the way gases move across membranes in biological gas exchange. But if you think about it a little more deeply you will realize that if diffusion is how the Blob was getting oxygen then there is a big problem. In fact, I can show you that based on this problem it is impossible for an organism such as the Blob to be alive at all.
Bong Really? I am surprised that you would want to do that, Bing. I know you like to live in this fantasy world of monsters, spaceships, elves, and the like.
Bing True enough, I do enjoy that sort of thing but I am really a scientist and enjoy much more thinking scientifically about the universe. In fact, I recently learned of a calculation Einstein made which is of relevance to thinking about diffusion of gasses in living things.
Bong Einstein! I always like to hear about Einstein. It makes me feel so good to think about things I don't understand at all.
Bing But in this case you will understand it readily, I think. Einstein asked how far a molecule would travel in a given time due only to the random motion characteristic of all molecules. The derivation of this equation is really quite interesting, but let's be content with the equation he ended up with. If molecules are diffusing out from a point in three dimensions around that point, the distance squared (d2) that the molecule will go in a given time (t) is d2 = 6Dt.
Bong I see, the equation is really quite simple isn't it. And if we want to know the time in terms of distance, the equation can be rearranged to t = d2/6D. But what is D?
Bing D is the "diffusion coefficient", Bong. It is a characteristic of each molecule and depends on the size of the molecule and the diffusion medium. For oxygen D is .20 cm2/sec in air but only .000002 cm2/sec (2.0 x 10-6 cm2/sec) in water.
Bong [Grumpily] That's all very well, but what possible relevance is this to our discussion of the Blob?
Bing Oh don't be so grumpy, Bong. I promise we will go see a film at the Knickerbocker next time - those are the films you like. In any case you need to realize that since the Blob and other organisms are made up mostly of water the diffusion coefficient for oxygen for them is the same as it is for water. Think about what that means.
Bong OK, I'm thinking. I guess it means diffusion in organisms is pretty slow. What does that have to do with the Blob not existing?
Bing Let's do a calculation just to see what sort of relevance it has. Suppose you were an amoeba with a cell 50 micrometers in diameter. If the deepest part of your cell was 25 micrometers from the surface, how long would it take oxygen to get from the surface to that point by diffusion?
Bong [Bong does this calculation. In doing it,
Bong remembers that D = 2.0 x 10-6 cm2/sec, that
a micrometer is 1 millionth of a meter and thus 1 ten
thousandth of a cm. This means that 25 micrometers is 0.0025 cm or 2.5
X 10-3 cm.]
I see -- the time required is a mere _________. That's certainly a reasonable enough time, I should think.
Bing Yes, indeed, quite reasonable. But let's do that calculation for some bigger organisms including the Blob. Here are some diameters of different organisms at their thinnest point. We will divide the diameter by two to get the distance to the deepest cell and then calculate the time required for oxygen diffusion. [Bing and Bong do this now, thus filling in the chart]
Organism
Diameter Time Required
for Diffusion
Earthworm
.1 cm ___________________________________
Starfish
3.0 cm ___________________________________
Lizard
8.0 cm ___________________________________
Human
30 cm
___________________________________
Blob
200 cm
___________________________________
Bong Wow, that's a long time for the Blob. Well Bing, you have convinced me that the Blob could not exist, not that I was really worried about it. It seems to me that this is more important than just showing the Blob can't exist. Look at the times for the lizard and human. Those are too long too. How do animals get enough oxygen if they can't use diffusion from the surface?
Bing An excellent question. Of course we have been reading about that in the textbook and I think we are going to discuss it soon in class.
Bong I'm looking forward to that Bing; at least a lot more than going to another bad movie with you.
Donald Cronkie and Christopher Barney: October 7, 1994