Biochemistry For Dummies. John T. Moore

       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 ; the oxygen atom has a partially negative charge . This charge distribution is due to the electronegativity difference between hydrogen and oxygen atoms (the attraction that an atom has for a bonding pair of electrons). The water molecule in Figure 2-1 is shown in its bent shape with a bond angle of about 105 degrees.

      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.

      

The term hydrogen bond doesn’t refer to an actual bond to a hydrogen atom but to the overall interaction of a hydrogen atom bonded to either oxygen, nitrogen, or fluorine atoms with an oxygen, nitrogen, or fluorine on another molecule (intermolecular) or the same molecule (intramolecular). Hence the term intermolecular force. (Note that although hydrogen bonds occur when hydrogen bonds to fluorine, you don’t normally find such combinations in biological systems.)

      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).

One environmentally important consequence of hydrogen bonding is that, upon freezing, water molecules are held in a solid form that’s less dense than the liquid form. The hydrogen bonds lock the water molecules into a crystalline lattice that contains large holes, which decreases the density of the ice. The less-dense ice — whether in the form of an ice cube or an iceberg — floats on liquid water. In nearly all other cases where a solid interacts with water, the reverse is true: The solid sinks in the liquid. So why is the buoyancy of ice important? Ask ice fishermen! The layer of ice that forms on the surface of cold bodies of water insulates the liquid from the cold air, protecting the organisms that live under the ice.

      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.

      

Because of water’s high specific heat and heat of vaporization, lakes and oceans can absorb and release a large amount of heat without a dramatic change in temperature. This give-and-take helps moderate the Earth’s temperature and makes it easier for an organism to control its body temperature. Warm-blooded animals can maintain a constant temperature, and cold-blooded animals — including lawyers and some chemistry teachers — can absorb enough heat during the day to last them through the night.

      

      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.

      

Polar molecules, because of their ability to interact with water molecules, are classified as hydrophilic (water-loving). Nonpolar molecules, which don’t appreciably interact with (dissolve in) water, are classified as hydrophobic (water-hating). Some molecules are amphipathic because they have both hydrophilic and hydrophobic regions.

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)