My notes and thoughts from Biology 111, for Wednesday, September 3, 2008. The entire series can be found here.
We left off before the Labor Day break with polarity and the ability to form hydrogen bonds.
The polarity of the water molecule, having an oxygen to one side and the two hydrogen atoms to the other, gives the molecules a slight attraction to charged molecules, since the oxygen end is going to have a slight negative charge and the hydrogen end is going to have a slight positive charge. This is caused by the unequal sharing of valence e- between the oxygen and the two hydrogens. Because the oxygen pulls harder on the shared e-, they are going to spend more time toward the oxygen, increasing its negative charge a little, and away from the hydrogens, increasing their positive charge a little (actually decreasing their negative charge a little, to be accurate).
That little bit of polarity will cause the oxygen end of one water molecule to be attracted to the hydrogen end of another water molecule (or any other positively charged molecule), and though the effect is small in one pair of molecules, it adds up with millions of molecules.
(More below the fold)
This is what causes the meniscus in a test tube or a glass of water. The water toward the edge, closest to the glass, is attracted to the glass, pulling itself up a little to stick to the sides. It’s also the cause for the ability to fill a glass slightly over the edge.
Emergent Properties of Water
1. Cohesion and Adhesion
Cohesion is the ability of water (in this case) molecules to stick to each other by hydrogen bonding
Adhesion is the ability of water (in this case) to stick to other polar molecules.
In our previous example of the test tube or glass, cohesion would be responsible for the water overfilling the glass without spillage, and adhesion would be responsible for the meniscus.
They’ll also cause the water level inside a glass straw to be higher than the water level of a beaker in which the straw is placed. The thinner the straw, the greater the disparity between the water levels, as there are fewer water molecules in the center, far away from the glass. In the center, gravity will tend to overcome the hydrogen bonding, but at the sides, where the water is close to the glass, the hydrogen bond is strong enough to overcome the gravitational pull.
In another example, it’s the cause of a little bit of water acting like glue between two glass plates. The sum of all those hydrogen bonds makes it very difficult to separate two wet panes of glass.
In biology, this is how a plant gets water from the ground to the leaves. Little teeny veins in the tree attract the water molecules, which adhere their way up against gravity to the leaves, where it evaporates.
This lecture was rather short, due to the pop quiz Doc gave that day, and here is where it ended.
From whence came the art:
That image is of our textbook, Biology, Eighth Edition, by Campbell & Reese et al.