Water, Acids, and Bases
Water is the most abundant molecule in living organisms, usually making up about 60-70% of the total body weight. Even so, water is an inorganic molecule because it does not contain carbon atoms. Carbon atoms are common to organic molecules. In water, the electrons spend more time circling the larger oxygen (O) atom than the smaller hydrogen (H) atoms. This imparts a slight negative charge (symbolized as d-) to the oxygen and a slight positive charge (symbolized as d+) to the hydrogen atoms. Therefore, water is a polar molecule with negative and positive ends. The diagram on the left shows the structural formula of water, and the one on the right is called a space-filling model.
A hydrogen bond occurs whenever a covalently bonded hydrogen is positive and attracted to a negatively charged atom nearby. A hydrogen bond is represented by a dotted line because it is relatively weak and can be broken rather easily. In Figure 2.6, you can see that each hydrogen atom, being slightly positive, bonds to the slightly negative oxygen atom of another water molecule nearby.
Figure 2.6 Hydrogen bonding between water molecules. The polarity of the water molecules causes hydrogen bonds (dotted lines) to form between the molecules.
Structural formula of water
Properties of Water
Polarity and hydrogen bonding cause water to have many properties beneficial to life, including the three to be mentioned here.
1. Water is a solvent for polar (charged) molecules and thereby facilitates chemical reactions both outside and within our bodies. When ions and molecules disperse in water, they move about and collide, allowing reactions to occur. Therefore, water is a solvent that facilitates chemical reactions. For example, when a salt such as sodium chloride (NaCl) is put into water, the negative ends of the water molecules are attracted to the sodium ions, and the positive ends of the water molecules are attracted to the chloride ions. This causes the sodium ions and the chloride ions to separate and to dissolve in water:
Ions and molecules that interact with water are said to be hydrophilic. Nonionized and nonpolar molecules that do not interact with water are said to be hydrophobic.
2. Water molecules are cohesive, and therefore liquids fill vessels, such as blood vessels.
Water molecules cling together because of hydrogen bonding, and yet water flows freely. This property allows dissolved and suspended molecules to be evenly distributed throughout a system. Therefore, water is an excellent transport medium. Within our bodies, the blood that fills our arteries and veins is 92% water. Blood transports oxygen and nutrients to the cells and removes wastes such as carbon dioxide from the cells.
3. Water has a high heat of vaporization. Therefore, it absorbs much heat as it slowly rises, and gives off this heat as it slowly cools.
It takes a large amount of heat to change water to steam. (Converting one gram of the hottest water to steam requires an input of 540 calories of heat energy.) Water has a high heat of vaporization because hydrogen bonds must be broken before boiling occurs and water molecules vaporize-that is, evaporate into the environment. This property of water helps keep body temperature within normal limits. Also, in a hot environment, we sweat; then the body cools as body heat is used to evaporate the sweat, which is mostly liquid water.
Acids and Bases
When water molecules dissociate (break up), they release an equal number of
hydrogen ions (H+) and hydroxide ions (OH-):
Only a few water molecules at a time dissociate, and the actual number of H+ and OH- is very small (1 X 10-7 moles/liter).
Acids are substances that dissociate in water, releasing hydrogen ions (H+). For example, an important inorganic acid is hydrochloric acid (HCl), which dissociates in this manner:
HCl ® H+ + Cl-
Dissociation is almost complete; therefore, HCl is called a strong acid. If hydrochloric acid is added to a beaker of water, the number of hydrogen ions (H+) increases greatly. Lemon juice, vinegar, tomatoes, and coffee are all acidic solutions. Bases are substances that either take up hydrogen ions (H+) or release hydroxide ions (OH-). For example, an important inorganic base is sodium hydroxide (NaOH), which dissociates in this manner:
NaOH ® Na+ + OH-
Dissociation is almost complete; therefore, sodium hydroxide is called a strong base. If sodium hydroxide is added to a beaker of water, the number of hydroxide ions increases. Milk of magnesia and ammonia are common basic solutions.
The pH scale, which ranges from 0 to 14, is used to indicate the acidity and basicity (alkalinity) of a solution. pH 7, which is the pH of water, is neutral pH because water releases an equal number of hydrogen ions (H+) and hydroxide ions (OH-). Notice in Figure 2.7 that any pH above 7 is a base, with more hydroxide ions than hydrogen ions. Any pH below 7 is an acid, with more hydrogen ions than hydroxide ions. As we move toward a higher pH, each unit has 10 times the basicity of the previous unit, and as we move toward a lower pH, each unit has 10 times the acidity of the previous unit. This means that even a small change in pH represents a large change in the proportional number of hydrogen and hydroxide ions in the body.
The pH of body fluids needs to be maintained within a narrow range, or else health suffers. The pH of our blood when we are healthy is always about 7.4-that is, just slightly basic (alkaline). If the pH value drops below 7.35, the person is said to have acidosis; if it rises above 7.45, the condition is called alkalosis. The pH stability is normally possible because the body has built-in mechanisms to prevent pH changes. Buffers are the most important of these mechanisms. Buffers help keep the pH within normal limits because they are chemicals or combinations of chemicals that take up excess hydrogen ions (H+) or hydroxide ions (OH-).
Figure 2.7 The pH scale. The proportionate amount of hydrogen ions to hydroxide ions is indicated by the diagonal line. Any solution with a pH above 7 is basic, while any solution with a pH below 7 is acidic.
For example, the combination of carbonic acid (H2CO3) and the bicarbonate ion [HCO3] helps keep the pH of the blood relatively constant because carbonic acid can dissociate to release hydrogen ions, while the bicarbonate ion can take them up!
As we have seen, salts, acids, and bases are molecules that dissociate; that is, they ionize in water. For example, when a salt such as sodium chloride is put in water, the Na+ ion separates from the Cl- ion. Substances that release ions when put into water are called electrolytes, because the ions can conduct an electrical current. The electrolyte balance in the blood and body tissues is important for good health because it affects the functioning of vital organs such as the heart and the brain.