H

When one pair of electrons is shared, the bond is said to be single. When two pairs of electrons are shared, the bond is referred to as double, and triple bonds are formed when three pairs of electrons are shared. Double bonds are shown in structural formulas with double lines (e.g., C=C), and triple bonds are shown with three lines (e.g., C=N). In covalent bonds involving molecules such as those of hydrogen (H2), where electrons are shared equally, the bonds are said to be nonpolar. However, polar covalent bonds (e.g., those of a water molecule) are formed when electrons are closer to one atom than to another and therefore are unequally shared. Because the electrons are shared unequally, parts of the molecule are not electrically neutral and are slightly charged. Covalent bonds are the strongest of the three types of bonds discussed here and are the principal force binding together atoms that make up some important biological molecules discussed later in this chapter (Fig. 2.6).

2. Ionic bonds. In nature, some electrons in the outermost orbital are not really shared but instead are completely removed from one atom and transferred to another, particularly between elements that can strongly attract or easily give up an electron. Molecules that lose or gain electrons become positively or negatively charged particles called ions. Ionic bonds form whenever one or more electrons are donated to another atom and result whenever two oppositely charged ions come in contact. Ions are shown with their charges as superscripts. For example, table salt (sodium chloride) is formed by ionic bonding between an ion of sodium (Na+) and an ion of

Figure 2.6 A covalent bond between two oxygen atoms. In a covalent bond, electrons are shared as outer shells of atoms overlap. In this instance, two pairs of electrons are shared between the two atoms, and the shared electrons are counted as belonging to each atom.

The Nature of Life 19

The Nature of Life 19

Figure 2.7 Ionic bonding between a sodium atom and a chlorine atom. The sodium becomes positively charged when it loses one of its electrons, which is gained by an atom of chlorine. The gained electron makes the chlorine ion negatively charged, and the two ions become bonded together by the attraction of opposite charges.

chlorine (Cl-). The sodium becomes a positively charged ion when it loses one of its electrons, which is gained by an atom of chlorine. This extra electron makes the chlorine ion negatively charged, and the sodium ion and chlorine ion become bonded together by the force of the opposite charge (Fig. 2.7).

Some ions, such as those of magnesium (Mg++), give up two electrons and therefore have two positive charges. Such ions can form ionic bonds with two single negatively charged ions such as those of chlorine (Cl-), forming magnesium chloride (MgCl2). Many biologically important molecules exist as ions in living matter.

3. Hydrogen bonds form as a result of attraction between positively charged hydrogen atoms in polar molecules and negatively charged atoms in other polar molecules. Negatively charged oxygen and/or nitrogen atoms of one molecule may attract positively but weakly charged hydrogen atoms of other molecules, forming a weak bond. Hydrogen bonds are very important in nature because of their abundance in many biologically significant molecules. They have, however, only about 7% to 10% of the strength of covalent bonds. Hydrogen bonds help cellular processes by maintaining the shapes of proteins such as enzymes, which make different compounds fit together precisely to complete a chemical reaction.

Acids, Bases, and Salts

Water molecules are held together by weak hydrogen bonds. In pure water, however, a few molecules sometimes dissociate into hydrogen (H+) and hydroxyl (OH-) ions, with the number of H+ ions precisely equaling the number of OH-ions.

Acids, which include things that taste sour, like cranberry or lemon juice, are chemicals that release hydrogen ions (H+) when dissolved in water, resulting in proportionately more hydrogen than hydroxyl ions being present. Some acids, such as the acetic acid of vinegar, release relatively few hydrogen ions and are said to be weak. Strong acids such as sulfuric acid dissociate almost completely into hydrogen and sulfate ions.

Bases (also referred to as alkaline compounds) usually feel slippery or soapy. They are defined as compounds that release negatively charged hydroxyl ions (OH-) when dissolved in water. Caustic soda, which is sodium hydroxide (NaOH), is a base that dissociates in water to positively charged sodium ions (Na+) and negatively charged hydroxyl ions (OH-). Bases can also be defined as compounds that accept H+ ions.

The acidity or alkalinity of the soil or water in which a plant occurs affects how it lives and grows or even if it can exist in a particular environment. Similarly, the acidity or alkalinity of the fluids inside cells has to be stable or various chemical reactions vital to life itself can't take place.

The pH Scale

The concentration of H+ ions present is used to define degrees of acidity or alkalinity on a specific scale, called the pH scale. The scale ranges from 0 to 14, with each unit representing a tenfold change in H+ concentration. Pure water has a pH of 7—the point on the scale where the number of H+ and OH-ions is exactly the same, or the neutral point.1 The lower a number is below 7, the higher the degree of acidity; conversely, the higher a number is above 7, the higher the degree of alkalinity. Vinegar, for example, has a pH of 3, tomato juice has a pH of 4.3, and egg white has a pH of 8. Precipitation with a pH of less than 4.5 is now commonly referred to as acid rain (acid deposition). Acid rain (discussed in Chapter 25 ) is associated with industrial emissions and appears to be causing damage to vegetation, soil organisms, and buildings in some parts of the world, including North America.

When an acid and a base are mixed, the H+ ions of the acid bond with the OH- ions of the base, forming water (H2O). The remaining ions bond together, forming a salt. If hydrochloric acid (HCl) is mixed with a base—for example,

1. Note that although distilled water is theoretically "pure," its pH is always less than 7 because carbon dioxide from the air dissolves in it, forming carbonic acid (H2CO3); the actual pH of distilled water is usually approximately 5.7.

Chapter 2

sodium hydroxide (NaOH)—water (H2O) and sodium chloride (NaCl), a salt, are formed. The reaction is represented by symbols in an equation that shows what occurs:

Energy

Energy is the ability or capacity to do work or to produce a change in motion or matter. Energy exists in several forms and is required for growth, reproduction, movement, cell or tissue damage repair, and other activities of whole organisms, cells, or molecules. On earth, the sun is the ultimate source of life energy.

Thermodynamics is the study of energy and its conversions from one form to another. Scientists apply two laws of thermodynamics to energy. The first law of thermodynamics states that energy is constant—it cannot be increased or diminished—but it can be converted from one form to another. Among its forms are chemical, electrical, heat, and light energy.

The second law of thermodynamics states that when energy doesn't enter or leave a given system and is converted from one form to another, it (energy) flows in one direction. Furthermore, there will always be less energy remaining after the conversion than existed before the conversion. The total amount of energy in the universe, however, remains constant. For example, heat will always flow from a hot iron to cold clothing but never from the cold clothing to the hot iron. Such energy-yielding reactions are vital to the normal functions of cells and provide the energy needed for other cell reactions that require energy. Both types of reactions are discussed in Chapter 10.

Forms of energy include kinetic (motion) and potential energy. Potential energy is defined as the "capacity to do work owing to the position or state of a particle." For example, if a cart resting at the top of a hill rolls down the hill, the cart's potential energy is converted to kinetic energy. Some chemical reactions release energy, and others require an input of energy (Fig. 2.8).

Although all electrons have the same weight and electrical charge, their amount of potential energy varies. Electrons with the least potential energy are located within the single spherical orbital closest to the atom's nucleus, and electrons with the most potential energy are in the outermost orbital (Fig. 2.9). Some of the numerous energy exchanges and carriers that occur in living cells are discussed in later chapters.

Chemical Components of Cells

The living substance of cells consists of cytoplasm and the structures within it. The numerous internal structures, which vary considerably in size, are discussed in Chapter 3. About 96% of cytoplasm and its included structures are composed of the elements carbon, hydrogen, oxygen, and nitrogen; 3% consists of phosphorus, potassium, and sulfur. The remaining 1% includes calcium, iron, magnesium, sodium, chlorine, copper, manganese, cobalt, zinc, and minute quantities of other elements. When a plant first absorbs these elements from the soil or atmosphere or when it uses breakdown products within the cell, the elements are in the form of simple molecules or ions. These simple forms may be converted to very large, complex molecules through the metabolism of the cells.

The large molecules invariably have "backbones" of carbon atoms within them and are said to be organic. The structure and the myriad of chemical reactions of living organisms are

Anatomy Cell 3309 Uwo
Figure 2.8 A. The cart resting on top of the hill has potential energy (capacity to do work owing to its position). B. The potential energy is converted to kinetic energy when the cart rolls down the hill.

The Nature of Life 21

(third energy level) (second energy level) (first energy level)

nucleus

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