Re: How do very small organisms survive Brownian motion?

How do very small organisms survive Brownian motion?

Interesting Question!  First, what is Brownian motion?  When small objects, 
like bacteria, are observed under the microscope, they appear to jiggle 
around.  Robert Brown first observed this effect in 1828 when he was 
watching pollen grains.  Brownian motion was initially thought to be a 
property of living things.  But it was soon observed that even dust 
particles undergo Brownian motion, so another explanation needed to be 
found.  It wasn't until 1905 that Albert Einstein gave a mathematical 
description of Brownian motion.  What happens is that water molecules, which 
are about 0.3 nM in diameter, are constantly bumping into the bacteria.  The 
bacteria are much bigger, perhaps 2000 nM across.  The collisions of the 
water molecules are random, sometimes more molecules hit on one side of the 
bacteria than on the other side.  Thus there is a net displacement of the 
bacteria and it appears to jiggle around.  That's Brownian motion.

So it's the water molecules bumping into the bacteria that cause the 
Brownian motion. Although some will go faster, some will go slower, the 
average speed of a water molecule at room temperature is 650 meters/second!  
That's as fast as a bullet! 

Ouch!  Doesn't that hurt the bacteria?  

I turns out that the bacteria are tough enough.  The bacteria are mostly 
made of proteins and the chemical bonds that make up proteins need about 4 
eV (electron volts) of energy to break (that's 100 Kcal/mole for the 
chemists).  The speed of the average water molecule corresponds to an energy 
0.026 eV, not enough to break a chemical bond.  If you calculate the 
distribution of the speeds of the water molecules (a Boltzman distribution), 
you will find that fewer than one in 10+40 of them have an energy of 4eV.  
That's way less than the number of water molecules around the bacteria.  
There are only 6x10+23 water molecules in 18 mls of water.
  
But what about other types of bonds?  The proteins that make up the bacteria 
are held together (and also held in connection to each other) by a 
collection of hydrogen bonds, dipole interactions, and salt bridges.  These 
bonds have an energy of about 0.1 to 0.2 eV (2 to 5 Kcal).  About 2% to 0.3% 
of the water molecules will have energy in that range.  Occasionally these 
bonds will break, but the protein contains many of those bonds, so the 
structure of the protein remains intact.  

There's another type of much weaker bond, called the Van der Waals bond.  
Van der Waals bonds have an energy about the same as the energy of a water 
molecule at room temperature (0.025 eV).  Thus Van der Waals bonds are 
constantly breaking and reforming from the motion of water molecules and by 
the thermal motion in the protein molecule too.  Since the protein contains 
many Van der Waals bonds, most of them remain intact.  That's important 
since the Van der Waals force is important in maintaining the structure of a 
protein.  

I've gone a bit beyond bacteria, but that's what Brownian motion can do.  As 
you see, Brownian motion would not be important in damaging the bacteria.  
But Brownian motion can break some of the bonds that maintain the structure 
of the protein molecules, thus contributing to rate at which proteins lose 
their structure and must be replaced.  But don’t worry about the bacteria, 
they are constantly making new proteins to replace the old ones.  

The world of a bacteria in water is quite different from what we experience.  
An interesting discussion of this can be found in "Life at a low Reynolds 
Number" at  http://brodylab.eng.uci.edu/~jpbrody/reynolds/
lowpurcell.html

A bacteria swimming in water is something like us trying to swim in 
molasses!  Mike Conrad