We also look at the properties of acids and bases, and the equilibria that they may undergo. Finally, we discuss the pH scale and buffers, including the infamous Henderson-Hasselbalch equation. So sit back, grab a glass of water (or your favorite water-based beverage), and dive in!
The Fundamentals of H2O
Water is essential to life; in fact, human beings are essentially big sacks of water. Water accounts for 60 to 95 percent of living human cells, and 55 percent of the water in the human body is in intracellular fluids. The remaining 45 percent (extracellular) is divided among the following:
Plasma (8 percent)
Interstitial (between cells) and lymph (22 percent)
Connective tissue, cartilage, and bone (15 percent)
Water also is necessary as a solvent for the multitude of biochemical reactions that occur in the body:
Water acts as a transport medium across membranes, carrying substances into and out of cells.
Water helps maintain body temperature.
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
