PHYS 331
Diffusion of Biological Molecules Colombini
2016
The
structure of living organisms and the processes they use is constrained by
physical laws. Evolution finds viable
solutions but physics determines what solutions are possible. One problem is the limitation on the size of
cells. A process that limits cell size
is the rate of diffusion of molecules.
One might think that molecular motion is so fast that diffusion of
molecules from one place to another is not a problem. Yet, we should know better. Do we not stir our coffee in order to mix the
sugar that was added? Stirring is not
just to help the sugar dissolve but also to disperse it throughout the solution. In the same way, molecules generated in one
part of a cell may need to travel to another part in order to be used for some
purpose. If the distance travelled is
too long, the rate of diffusion slows down: metabolism, muscle response to
nerve impulses, the ability to catch a prey, the ability to run from a
predator, the ability to survive. Worse, the center of the cell could die from
lack of oxygen because oxygen must diffuse from the cell surface to the
mitochondria in the cell. If the cell is
too large, the mitochondria nearer the cell surface will consume all the oxygen
before it can reach the mitochondria located nearer the cell center. Dying mitochondria release factors that
trigger apoptosis, programed cell death.
The slow
pace of diffusion could be overcome by a mixing process like the spoon mixing
the coffee. Such a process does not
exist in cells, although there are transport processes that overcome the slow
rate of diffusion of larger structures like vesicles and mitochondria. Why is there no mixing process? Perhaps it is too energy consuming or it
would cause other negative effects. We
don’t know.
Since
diffusion is important it is also important to understand its properties. A good way to start is to make measurements
of the diffusion process.
1. Try to
design an experiment that could measure the rate of diffusion of molecules in
water.
2. What
problems do you foresee in performing such an experiment?
3. How could
you minimize those problems?
4. Once the
problems are minimized, what measurements could be made?
5. How often
should measurements be made?