Ions Electrolytes and Free Radicals

Ions are charged particles with unequal numbers of protons and electrons. Elements with one to three valence electrons tend to give them up, and those with four to seven electrons tend to gain more. If an atom of the first kind is exposed to an atom of the second, electrons may transfer from one to the other and turn both of them into ions. This process is called ionization. The particle that gains electrons acquires a negative charge and is called an anion (AN-eye-on). The one that loses electrons acquires a positive charge (because it then has a surplus of protons) and is called a cation (CAT-eye-on).

Consider, for example, what happens when sodium and chlorine meet (fig. 2.4). Sodium has three electron shells with a total of 11 electrons: 2 in the first shell, 8 in the second, and 1 in the third. If it gives up the electron in the third shell, its second shell becomes the valence shell and has the stable configuration of 8 electrons. Chlorine has 17 electrons: 2 in the first shell, 8 in

3Rolf Maximillian Sievert (1896-1966), Swedish radiologist

Saladin: Anatomy & I 2. The Chemistry of Life I Text I I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

60 Part One Organization of the Body

60 Part One Organization of the Body

11 protons

12 neutrons 11 electrons

Sodium atom (Na)

17 protons

18 neutrons 17 electrons

Chlorine "atom (Cl)

11 protons

12 neutrons 11 electrons

Sodium atom (Na)

17 protons

18 neutrons 17 electrons

Chlorine "atom (Cl)

17 protons

18 neutrons 18 electrons

Chloride "ion (Cl-)

11 protons

12 neutrons 10 electrons

17 protons

18 neutrons 18 electrons

Chloride "ion (Cl-)

Sodium chloride

Figure 2.4 Ionization. (a) A sodium atom donates an electron to a chlorine atom. (b) This electron transfer converts the atoms to a positive sodium ion (Na+) and a negative chloride ion (CF).

the second, and 7 in the third. If it can gain one more electron, it can fill the third shell with 8 electrons and become stable. Sodium and chlorine seem "made for each other"—one needs to lose an electron and the other needs to gain one. This is just what they do. When they interact, an electron transfers from sodium to chlorine. Now, sodium has 11 protons in its nucleus but only 10 electrons. This imbalance gives it a positive charge, so we symbolize the sodium ion Na+. Chlorine has been changed to the chloride ion with a surplus negative charge, symbolized Cl".

Some elements exist in two or more ionized forms. Iron, for example, has ferrous (Fe2+) and ferric (Fe3+) ions. Note that some ions have a single positive or negative charge, while others have charges of ±2 or ±3 because they gain or lose more than one electron. The charge on an ion is called its valence. Ions are not always single atoms that have become charged; some are groups of atoms—

phosphate (PO43~) and bicarbonate (HCO3~) ions, for example.

Ions with opposite charges are attracted to each other and tend to follow each other through the body. Thus, when Na+ is excreted in the urine, CP tends to follow it. The attraction of cations and anions to each other is important in maintaining the excitability of muscle and nerve cells, as we shall see in chapters 11 and 12.

Electrolytes are salts that ionize in water and form solutions capable of conducting electricity (table 2.2). We can detect electrical activity of the muscles, heart, and brain with electrodes on the skin because electrolytes in the body fluids conduct electrical currents from these organs to the skin surface. Electrolytes are important for their chemical reactivity (as when calcium phosphate becomes incorporated into bone), osmotic effects (influence on water content and distribution in the body), and electrical effects (which are essential to nerve and muscle function). Electrolyte balance is one of the most important considerations in patient care. Electrolyte imbalances have effects ranging from muscle cramps and brittle bones to coma and cardiac arrest.

Free radicals are chemical particles with an odd number of electrons. For example, oxygen normally exists as a stable molecule composed of two oxygen atoms, O2; but if an additional electron is added, it becomes a free radical called the superoxide anion, O2_v Free radicals are represented with a dot to symbolize the odd electron.

Free radicals are produced by some normal metabolic reactions of the body (such as the ATP-producing oxidation reactions in mitochondria, and a reaction that some white blood cells use to kill bacteria), by radiation (such as ultraviolet radiation and X rays), and by chemicals (such as carbon tetrachloride, a cleaning solvent, and nitrites, present as preservatives in some wine, meat, and other foods). They are short-lived and combine quickly with molecules such as fats, proteins, and DNA, converting them into free radicals and triggering chain reactions that destroy still more molecules. Among the damages caused by free radicals are some forms of cancer and myocardial infarction, the death of heart tissue. One theory of aging is that it results in part from lifelong cellular damage by free radicals.

Because free radicals are so common and destructive, we have multiple mechanisms for neutralizing them. An antioxidant is a chemical that neutralizes free radicals. The body produces an enzyme called superoxide dismutase (SOD), for example, that converts superoxide into oxygen and hydrogen peroxide. Selenium, vitamin E (a-tocopherol), vitamin C (ascorbic acid), and carotenoids (such as p-carotene) are some antioxidants obtained from the diet. Dietary deficiencies of antioxi-dants have been associated with increased incidence of heart attacks, sterility, muscular dystrophy, and other disorders.

Saladin: Anatomy & Physiology: The Unity of Form and Function, Third Edition

2. The Chemistry of Life

Text

Chapter 2 The Chemistry of Life 61

Table 2.2 Major Electrolytes and the Ions Released by Their Dissociation

Electrolyte

Cation

Anion

Calcium chloride (CaCl)

ca2+

2 Cl-

Disodium phosphate (Na2HPO4)

2 Na+

HPO42-

Sodium bicarbonate (NaHCO3)

Na+

hco3-

Sodium chloride (NaCl)

Na+

Cl-

Magnesium chloride (MgCl2)

mg2+

2 Cl-

Potassium chloride (KCl)

K+

Ethanol

Ethyl ether

Condensed structural Molecular formulae formulae

Structural formulae

Condensed structural Molecular formulae formulae

Ethanol

Ethyl ether

1 1 H H

CH3CH2OH

C2H6O

1 1 H H

CH3OCH3

C2H6O

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  • Kerry Macleod
    Is chloride ion a free radical?
    2 years ago

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