Hippocampus Biology: The Nature of Water

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A key theme in biology is the relationship between structure and function. In order to appreciate the chemical and physical properties of water, we need to understand its molecular structure.

The chemical formula of water is H-2-O, meaning that each molecule of water contains two hydrogen atoms and one oxygen atom. Each hydrogen atom is connected to the oxygen atom with a covalent bond. Recall that this means the two atoms share two electrons, one contributed by the hydrogen atom and one contributed by the oxygen atom. The oxygen atom also has two pairs of electrons it doesn't use to form chemical bonds. Right now it looks like all four pairs of electrons sit in one plane, but that's not quite right. Bonding electrons and unshared electron pairs tend to repel each other. The electron pairs are arranged in the shape of a tetrahedron, which lets the electrons be as far apart from each other as possible. This diagram shows that the three atoms don't lie in a line. Instead, the molecule has a shape that chemists call "bent."

Water's bent shape has important implications for its chemical properties. One of these properties is polarity. "Polar" means having an uneven distribution of electron density, so "polarity" means a condition or state in which a substance has an uneven distribution of electron density.

We need to talk about two types of polarity: bond polarity and molecular polarity. Water exhibits both kinds of polarity. First, the bonds between oxygen and hydrogen are polar because the electrons aren't evenly distributed between the two atoms. As we see here, they're closer to the oxygen atom than to the hydrogen atom. A polar bond occurs when one atom in the bond is more electronegative than the other. Electronegativity is the tendency of an atom in a bond to attract shared bonding electrons. The bonds between hydrogen and oxygen are polar because oxygen is more electronegative than hydrogen.

Water also exhibits the second kind of polarity. Its bent shape plays an important role in its molecular polarity. Let's look again at the linear depiction of the water molecule. If the molecule were linear rather than bent, the electron distribution would be even, not uneven, so the molecule would NOT be polar.

Another way to think of this is that the "pull" of the electrons toward the oxygen from the bond on the left will be cancelled out by the "pull" of the electrons from the bond on the right. Without a net "pull" in any direction, the molecule won't be polar.

But remember, water is NOT linear, it's bent!

We already know the electrons in the two bonds are shifted toward the oxygen atom. More importantly, the two unshared electron pairs on oxygen mean that one region of the molecule has more electron density than the rest of the molecule. By definition the water molecule is polar, since there's an uneven distribution of electron density. We sometimes indicate the electron distribution by putting a partial positive charge, designated delta-plus, on the hydrogen atoms and a partial negative charge, designated delta-minus, on the oxygen.

The polar structure of water has important consequences for the interactions that occur between water molecules in biological systems. The most important interaction between water molecules is hydrogen bonding.

Recall that a hydrogen bond is a weak to moderate attractive force between a hydrogen atom bonded to oxygen, nitrogen, or fluorine, and an oxygen, nitrogen or fluorine atom on another molecule. Hydrogen bonds are usually represented by dotted lines. A hydrogen atom, at the partially positive end of the molecule, interacts with the partially negative oxygen atom on another water molecule. Hydrogen bonding also explains another unusual property of water: it's more dense as a liquid than as a solid.

Density is the ratio of a substance's mass to its volume. Most substances are more dense in the solid phase than in the liquid phase, but water is an exception. This is easily demonstrated by the fact that ice floats, so it must be less dense than water.

Here, the glass contains 300 milliliters of liquid water, which has a mass of 300 grams. This means its density is 1 gram per milliliter. Compare that to the mass of 100 milliliters of ice cubes. Their mass is just 91.7 grams, so the density of ice is 0.917 grams per milliliter. The definition of density tells us that a given mass of water molecules will have a larger volume in the solid phase. Therefore, the water molecules must be farther apart in ice than they are in liquid water. This results from the different hydrogen bonds in water and ice. In liquid water, on the left, the hydrogen bonds are constantly broken and re-formed as the molecules move around. When we look closely at the structure of ice, we see a lot more open space. The arrangement of hydrogen bonds means the atoms are farther apart, so ice is less dense than water.

Let's consider some of the biological consequences of ice's lower density. We already know that ice cubes float, but this also means that the ice on ponds, lakes, and oceans floats too. Fish, plants, and other organisms can survive, because the ice insulates them from the much colder air above the lake. If ice didn't float, then a lake would freeze from the bottom up, killing everything living in it.

We've saved the most important property of water for last: water as a solvent. A solvent is a liquid which dissolves another substance without any change in its chemical composition. Water is so universal in living systems because it's such a good solvent. Its polar structure lets it carry out one of its most important functions: the dissolving of ionic substances like table salt and small polar molecules like sugar. Let's use salt to explore the dissolving process at the molecular level.

Here's a grain of salt, made up of regularly spaced sodium ions and chloride ions. As the grain interacts with the water molecules, the partially positive ends of several water molecules are attracted to the negative chloride ions at the surface of the granule. The ion breaks off from the granule, and is immediately surrounded by several more water molecules. The same thing happens with the positive sodium ions in the granule, except that the water molecules orient so their partially negative ends are facing the positive ion. As before, the water molecules surround the ion and enable it to break off from the rest of the granule.

Ionic compounds like sodium chloride and small polar molecules like sugar that readily dissolve in water are called hydrophilic, or "water- loving." In contrast, hydrophobic or "water-fearing" substances - like oil - do not dissolve in water. We'll learn more about the structure of oil later in the course. The important point here is that oil molecules are nonpolar, and in the presence of water they pack together tightly to keep the water out.

The fact that oil and water don't mix has important biological consequences. First, consider what happens when a hazardous polar or ionic compound is accidentally spilled. Water molecules surround and dissolve the compound and dilute it, possibly enough so that it's no longer dangerous. But look what happens when a nonpolar substance like oil is spilled into the water. The oil can't be diluted by water in a lake or ocean. Since it's less dense than water, it floats on the surface where it can coat the feathers and fur of birds and mammals.

The polar structure of water has a huge influence on its function: it can either help prevent environmental damage by diluting polar compounds, or make it much more difficult to clean up damage caused by nonpolar substances like oil.