It seems to be extraordinarily difficult to visualize molecules, the bonding between
atoms, and especially how and why water is so weird. Yet, so much of biology
depends on the chemical properties of water that it is really important to understand it.
Furthermore, a great deal of what we do in the kitchen depends on these properties as
I have found it quite helpful to begin with a discussion of Mayonnaise. This stuff is oil
and water, which through some kind of magic, don’t separate. Usually, oil and water
do not mix. What’s going on? To understand this, we need to visualize the shapes and
properties of water molecules, and visualize the H-bond interactions between
molecules. It is this H-bond network that makes water boil at such an anomalously
high temperature, compared to other molecules of similar mass. Similarly, the H-bond
network is responsible for water freezing at an anomalously high temperature as well.
The H-bond network is what makes lipid membranes work. The lipid tails cannot
participate in the H-bond network, and therefore do not dissolve in water. This is the
basic force that holds the lipid bilayer together.
The same lipids that form membranes, and are thus abundant in egg yolk, are
responsible for stabilizing the oil/water emulsion of mayonnaise. The oily tails of the
lipids dissolve in the droplets of oil, and the charged heads interact with the water
molecules. The droplets of oil stay suspended in the water.
Students can usually manage to see this--literally, to see the molecular interactions in
their minds’ eyes. The number of molecules is small. Their shapes are not difficult to
imagine. Therefore, it’s a manageable thing to imagine.
When we then encounter proteins, and consider protein folding, we already have a
mental image to relate proteins and amino acids to. The amino acids that do not
dissolve in water move to the center of the protein, and the amino acids that do dissolve
in water move to the outside--just like the lipid tails and their polar heads in
This simple image of protein folding makes it easier, in turn, to visualize how a
mutation in DNA can change the structure of a protein--it can make it fold incorrectly.
The following page and its discussion questions help lead students to a reasonable
model of water molecules. It also helps to look at the following animations:
Build your own water molecule--interactive animation of what is on the next page
A water molecule twirling in space--to see how the hydrogen atoms are joined to the
oxygen atom
A methane molecule twirling in space--to see what an analogous molecule looks like if
it can make four bonds rather than one.
Oil and water don’t mix--a molecular view of the water molecules forming their H-
bond network, and squeezing the oil out of solution
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