Biochemistry For Dummies. John T. Moore
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Water acts as a solvent (carrying dissolved chemicals) in the digestive and waste excretion systems.
Healthy humans have an intake/loss of about 2 liters of water per day. The intake is about 45 percent from liquids and 40 percent from food, with the remainder coming from the oxidation of food. The loss is about 50 percent from urine and 5 percent from feces, with the remainder leaving through evaporation from the skin and lungs. A water balance must be maintained within the body. If the water loss significantly exceeds the intake, the body experiences dehydration. If the intake significantly exceeds the water loss, water builds up in the body and causes edema (fluid retention in tissues).
In the following sections, we touch on the basic properties of this must-have liquid, as well as its most important biochemical function.
Let’s get wet! The physical properties of water
The medium in which biological systems operate is water, and the physical properties of water influence the biological systems. Therefore, it’s important to review some water properties from general chemistry.
Water is a polar molecule
Water is a bent molecule, not linear (see Figure 2-1). The hydrogen atoms have a partially positive charge
FIGURE 2-1: Structure of a water molecule.
Water has strong intermolecular forces
Normally, partial charges such as those found in a water molecule result in an intermolecular force known as a dipole-dipole force, in which the positive end of one molecule attracts the negative end of another molecule. The very high electronegativity of oxygen combined with the fact that a hydrogen atom has only one electron results in a charge difference significantly greater than you’d normally expect. This charge difference leads to stronger-than-expected intermolecular forces (stronger than the dipole-dipole forces), and these forces have a special name: hydrogen bonds.
Hydrogen bonds in oxygen- and nitrogen-containing molecules are very important in biochemistry because they influence reactions between such molecules and the structures of these biological molecules. The interaction between water and other molecules in which there may be an opportunity for hydrogen bonding explains such properties as solubility in water and reactions that occur with water as a solvent (more on that in a minute).
Water has a high specific heat
Specific heat is the amount of heat required to increase the temperature of a gram of water by 1 degree Celsius. The high specific heat of water means that changing the temperature of water isn’t easy. Water also has a high heat of vaporization, the quantity of heat required at a specified temperature to convert a specified mass of liquid into vapor. Humans can rid their bodies of a great deal of heat when their sweat evaporates from their skin, making sweat a very effective cooling method. We’re sure you’ll notice this cooling effect during your biochem exams.
Water’s most important biochemical role: The solvent
The polar nature of water means that it attracts (soaks up) other polar materials. Water is often called the universal solvent because it dissolves so many types of substances. Many ionic substances dissolve in water because the negative ends of the water molecules attract the cations (positively charged ions) from the ionic compound (a compound resulting from the reaction of a metal with a nonmetal) and the positive ends attract the anions (negatively charged ions). Covalently bonded (resulting from the reactions between nonmetals) polar substances, such as alcohols and sugars, also are soluble in water because of the dipole-dipole (or hydrogen-bonding) interactions. However, covalently bonded nonpolar substances, such as fats and oils, aren’t soluble in water. Check out Chemistry For Dummies (written by this book’s coauthor, John T. Moore, and published by Wiley) for a discussion of chemical bonding.
Figure 2-2 shows the structure of a typical amphipathic molecule. The molecule appears on the left of Figure 2-2, with its hydrophilic and hydrophobic regions shown. The right side of Figure 2-2 is a symbolic way of representing the molecule. The round head is the hydrophilic portion, and the long tail is the hydrophobic portion.
FIGURE 2-2: Structure of a typical amphipathic (both water-loving and water-hating) molecule.
Certain amphipathic molecules, such as soap molecules, can form micelles, or very tiny droplets that surround insoluble materials. This characteristic is the basis of the cleaning power of soaps and detergents. The hydrophobic portion of the molecule (a long hydrocarbon chain)