Essential Properties of Water - Biochemistry

Electrolytes readily dissolve and ionize in water. The term "electrolyte" refers to a molecule that will produce ions in solution, and can be synonymous with "salt" in certain contexts, as well as acidic and basic compounds.

When dealing with solutions, it is helpful to remember that solubility depends on polarity and that "like dissolves like." Polar solutes (ionic salts) dissolve well in polar solvents (water); the same goes for nonpolar solutes and solvents.

Hydrophobic interactions are driven by the increase in entropy (not enthalpy) that water molecules achieve by excluding a nonpolar solute. In the hydrophobic effect, water initially forms cage-like structures called clathrates around the nonpolar solute, which is entropically unfavorable. However, entropy is regained when the water molecules exclude the solute and interact with each other in a disordered manner.

Cohesion is defined as attraction between the same type of molecule. Water attracts other water molecules because it is polar and has partial charges. This attraction means that water has strong intermolecular forces even at room temperature and more thermal energy is required to vaporize it. Adhesion involves the attraction of a substance with a container or surface.

The electrons around water are not symmetrically positioned; rather, they are distributed toward the oxygen atom, which is highly electronegative. Hence, water's two hydrogen atoms have a net positive charge. Nonetheless, the total charge on water is neutral, with electropositive and electronegative regions, rendering it polar. This polarity allows water molecules to hydrogen-bond with each other.

The important point here is that biological polymers are basically all condensation polymers. In the case of proteins, polysaccharides (carbohydrates), and nucleic acids (like DNA and RNA), synthesis occurs via a loss of a water molecule. Non-biological molecules can also be formed via condensation, such as nylon, which also often has water as its by-product. Polystyrene is an addition polymer; upon formation, monomers do not lose any molecules. Bonds are only rearranged. Although it may seem counterintuitive, the fact is that in nature, polymers do not typically form this way.

Hypotonic solutions have a low solute concentration, and hypertonic solutions have high solute concentration. By the rules of osmosis, water moves in and out of cells along a concentration gradient, because membranes are only slightly permeable to water. Hence in a hypotonic solution, water will enter cells, causing them to swell and possibly break, while in a hypertonic solution, cells will lose water and shrink. Aquaporins are water channels that encourage osmosis, rather than hinder it.

A molecule of water is polar with the two hydrogen atoms being partially posistive and the oxygen atom being partially negative. The two hydrogens can therefore each act as a donor which accounts for two net hyrdogen bonds made by water. The oxygen contains a partial negative charge and two lone electrons pairs can each act as acceptors and from two more hydrogen bonds, making a total of 4.

Osmosis states that a solvent will move from a region of high concentration to a region of relatively lower concentration. In this case the solvent is water. The concentration of water inside the dialysis bag is higher than the concentration of water in the beaker because this water also contains 1 molar glucose. Therefore the water in the dialysis bag will flow out into the beaker to bring the concentration of water in both spaces toward equilibrium

Water is polar for all of these reasons. First, oxygen is more electronegative than hydrogen. This means that oxygen attracts electrons more strongly than hydrogen does. Thus, in the hydrogen-oxygen bond of water, the electrons will be distributed asymmetrically--they will be oriented more towards oxygen than hydrogen. The molecular geometry of the water molecule (bent) results from oxygen containing a steric number of four with two lone pairs, thus the molecule has a slightly positive end and a slightly negative end.

Interstitial water is the fluid on the outside of cells, and surrounds the cells of the human body. While estimates vary, the best answer is that 15% of the entire human body is composed of interstitial water. Overall, about 60% of the body (42L) is composed of water. Additionally, about two-thirds of the total body water is intracellular water. Finally, about 5% of the entire body is composed of plasma.

Remember that these percentages are estimates, and vary by individual. Sex and muscle mass are two factors that can significantly alter these estimates.

Oxygen is a more electronegative atom than hydrogen, and therefore attracts electrons within the molecule more strongly than hydrogen. Because electrons are negatively charged, oxygen must therefore contain a partial negative charge, while hydrogen must contain a partial positive charge.

Because water is neutral, the overall charge should add up to 0. However, there are two hydrogens in water, compared to one oxygen. Two positive charges of will cancel out a single negative charge of .

Hydrogen bonding occurs when a hydrogen atom covalently bonded to an fluorine, oxygen, or nitrogen interacts with a lone pair of electrons on another fluorine, oxygen, or nitrogen atom. So first, the two hydrogens in water can form hydrogen bonds, as they are connected to an oxygen atom. Next, the oxygen atom in water has two lone pairs of electrons, and those also can form hydrogen bonds. A single water molecule therefore has the capability of forming four hydrogen bonds.

Because of hydrogen bonding, water has a low high heat of vaporization. In other words, it takes more energy to free up water molecules from a hydrogen-bonded liquid state to a gaseous state.

An amphipathic molecule is one which contains both hydrophilic and hydrophobic regions. Benzene is made up of six carbon atoms joined together in a ring, with one hydrogen atom attached to each carbon (). This makes it a hydrocarbon, meaning that benzene is entirely hydrophobic. It does not have any hydrophilic regions and thus cannot be amphipathic.

Calcium chloride () is an ionic compound. This means that it is hydrophilic, not amphipathic. Acetic acid () is an interesting compound because it consists of a methyl group and a carboxylic acid. The carboxylic acid is hydrophilic. The methyl group, consisting of a carbon and three hydrogen atoms, is hydrophobic, but this group is too small to make the entire compound amphipathic. Thus, acetic acid is not amphipathic.

Lastly, palmitic acid () is definitely an amphipathic molecule. The carboxylic acid portion is hydrophilic, and the hydrocarbon chain is very long, making up the hydrophobic region.

Ammonia, NH3, will be the most soluble in water simply because polar substances dissolve in polar solvents. This follows the principle that "like dissolves like." Ammonia and water are both polar due to the presence of lone pairs of electrons combined with a lack of geometrical symmetry (water is bent and ammonia is trigonal pyramidal).

Carbon dioxide and nitrogen are linear, negating any potential polarity. Methane is tetrahedral, and lacks any polarized bonds. Carbon trioxide is tigonal planar, negating polar interactions due to symmetry.

Electrolytes readily dissolve and ionize in water. The term "electrolyte" refers to a molecule that will produce ions in solution, and can be synonymous with "salt" in certain contexts, as well as acidic and basic compounds.

When dealing with solutions, it is helpful to remember that solubility depends on polarity and that "like dissolves like." Polar solutes (ionic salts) dissolve well in polar solvents (water); the same goes for nonpolar solutes and solvents.

Hydrophobic interactions are driven by the increase in entropy (not enthalpy) that water molecules achieve by excluding a nonpolar solute. In the hydrophobic effect, water initially forms cage-like structures called clathrates around the nonpolar solute, which is entropically unfavorable. However, entropy is regained when the water molecules exclude the solute and interact with each other in a disordered manner.