Chemistry is the study of matter. Solid matter has a definite shape and volume. Liquid matter takes the shape of the container it is in. Gas has neither shape nor volume.
The logs in these illustrations show physical change and chemical change. The physical change only changes the physical appearance of the wood. The wood chips chopped off of the logs are still wood. A chemical change occurs when the logs are burned; the chemical composition of the wood changes to ashes. Ashes are of a different chemical composition from wood. Fig. 2-1
All matter is composed of elements. More than 100 elements exist, but only about 25 are required by living organisms. Have students refer to Table 2-1 and identify trace elements are found in the human body. Iodine, chromium, cobalt, copper, fluorine, selenium, and zinc are examples of trace elements found in the human body.
Using Table 2-1 have students identify the symbols for these four elements. The symbols are oxygen, O, carbon, C, hydrogen, H, and nitrogen, N. Other symbols commonly used in clinical settings are Ca (calcium), K (potassium), Na (sodium), Cl (chlorine), Fe (iron), and I (iodine).
Elements are composed of atoms. Atoms are the basic units of matter. The same symbol is used for the element and the atom. The element Na (sodium) is composed of millions Na atoms.
The arrangement of the subatomic particles (protons, neutrons, and electrons) resembles the sun and planets. The nucleus is represented by the sun. The shells, or orbits, surround the nucleus. Fig. 2-2, A
This illustration shows the nucleus surrounded by three electron shells, similar to the orbits of the planets around the sun. Fig 2-2, B
Protons and neutrons are located in the nucleus. Protons carry a positive electrical charge, and neutrons carry no electrical charge. Electrons carry a negative electrical charge and are located in the shells, or orbits, surrounding the nucleus. How do atoms differ? The difference between atoms is the numbers of protons and electrons in each atom. Fig. 2-2,C
What are the atomic number and atomic mass of hydrogen? Hydrogen has an atomic number of 1. The atomic mass of hydrogen is also 1 because its nucleus contains one proton and no neutrons. Radioactivity is the spontaneous breakdown, or decay, in which unstable isotopes release particles or energy waves. How are radioisotopes used clinically? They are used therapeutically to destroy cells and diagnostically for thyroid disease, e.g. 131 I.
Which electrons are important for chemical bonding? Only the electrons in the outermost shell are involved in bonding. Why are atoms attracted to each other? Atoms are attracted to each other because they want to achieve a stable outer electron shell by either filling or emptying the outer electron shell. The electrical attraction between atoms is a chemical bond. It is similar to the pull between magnets.
The sodium and the chlorine atoms in the diagram of NaCl (sodium chloride or table salt) are an example of an ionic bond. The single electron in the outer shell of the sodium atom makes it unstable. It achieves stability by donating an electron to the chlorine atom whose outer shell has seven electrons, making it unstable. This forms an ionic bond between the two atoms. Fig. 2-3, A
Covalent bonding is like joining hands. As shown in the illustration, water is formed by the sharing of electrons of two hydrogen atoms and one oxygen atom. Carbon atoms always forms covalent bonds, which are strong and do not break apart in an aqueous solution. In the body, hormones are often transported in blood, a mostly aqueous solution, and it is important that the hormones do not break down. Organic chemistry is the study of carbon-containing substances. Fig. 2-3, B
Ask students to find the two H atoms and the single O atom in the illustration of the water molecule. The weak attraction between water molecules illustrates a hydrogen bond. The weak positive charge around the hydrogen atom of one water molecule is attracted to the weak negative charge of the oxygen atom in another water molecule. A water molecule is also an example of a polar molecule. It has a lopsided charge, with a negative end and a positive end. This makes water an excellent solvent. Fig 2-3, C
What happens to an atom if negatively charged electrons are lost or gained from its outer shell? The electrical charge of the atom changes. Electrolytes are capable of conducting an electrical current.
Sodium is the principal extracellular cation. Clinically, it plays an important role in the body’s fluid balance.
Potassium is the chief intracellular cation. Clinically, it plays an important role in the electrical activity of nerves and muscles.
Combinations of atoms, such as bicarbonate, carry an electrical charge and are therefore ions. Along with sodium, chloride plays an important role in fluid balance.
When table salt is placed in water it dissociates, and the products of this dissociation are ions. This process is referred to as ionization. Only electrolytes ionize. Ask students to examine the diagram of ionization of saline solution. Have them sketch a similar diagram illustrating the ionization of potassium chloride (KCl). The diagrams should show complete KCl molecules dissociating into K+ and Cl. Fig. 2-4
Two identical atoms may bond to form a molecule, or atoms of different elements can combine to form a molecule. Molecule is a broader term than compound. Two oxygen atoms combine to form a molecule of oxygen. When one sodium atom and one chlorine atom combine, it forms table salt, a compound. Why is water both a molecule and a compound? It is a molecule because it is formed of two atoms; it is a compound because it is formed of two different atoms. This is also true of NaCl. In clinical settings, oxygen is referred to as O2. Fig. 2-5
Water constitutes approximately two thirds of an adult’s body. Why is water called the universal solvent? Water is known as the universal solvent because most substances dissolve in water. Water absorbs large amounts of thermal energy (heat) without the temperature of the water itself increasing dramatically. It is therefore important in regulating body temperature. Water is a major component of lubricating fluids. It also cushions the brain, the spinal cord, and the developing infant in the mother’s womb.
The rates of chemical reactions are important. Most chemical reactions require a catalyst. Outside the body, most chemical reactions take place so slowly that they would be incompatible with life. In the body, enzymes serve as catalysts to speed up chemical reactions. Some genetic diseases (e.g., PKU) are caused by enzyme deficiencies.
Acid-base balance is required for the chemical reactions in the body to occur. Imbalances cause life-threatening clinical problems. A strong acid, such as hydrochloric acid, dissociates completely into a hydrogen ion; however, a weak acid, such as vinegar, does not dissociate completely. A base is slippery like soap and has a bitter taste. Lye is an example. Both strong acids and strong bases can burn. A clinical example of an acid-base problem is ketoacidosis in an uncontrolled diabetic.
Direct students to examine the pH scale (Fig. 2-6) and note the colors used for acids (increasingly pink) and bases (increasingly blue). The color changes on the pH scale are the same as those that appear on litmus paper. A pH of less than 7 on the scale indicates that a solution has more hydrogen ions than hydroxyl ions and is acidic. A substance is said to be basic, or alkaline, if it contains fewer hydrogen ions than hydroxyl ions. This is indicated by a pH of more than 7. What is the pH of intestinal contents? Is it acidic or basic? The pH of intestinal contents is alkaline, or basic.
Ask students to find the pH range of normal blood in Figure 2-6. The normal pH range of blood is 7.35 to 7.45, or slightly basic. To students, a pH value signifying blood acidosis (less than 7.35) may appear alkaline when considered against the pH scale 7; however, blood acidosis and alkalosis are established against the normal pH range for blood. A clinical example is a blood pH of 7.2 in an uncontrolled diabetic, which signifies ketoacidosis. During many illnesses, the blood pH is closely monitored.
Even at rest, the body depends on a constant supply of energy. Without energy, the body cannot function. Mechanical energy is shown in the movement of legs. An example of chemical energy is the use of food as fuel. Electrical energy is used in nerve transmission. Radiant energy stimulates the eye for vision. An example of thermal energy is body temperature. Nuclear energy is not useful physiologically.
The most important part of the ATP molecule is the end phosphate group. The phosphate groups have unique high-energy chemical bonds. When they are broken, a large amount of energy is released.
The energy released by ATP can be used directly by the cells of the body to perform needed tasks. Fig. 2-7, B
The storing of energy by the high-energy bonds of the phosphate groups is similar to a loaded mouse trap. When energy is needed by the body, ATP is split and the energy is released; this is like the trap being set off by a mouse. Fig. 2-7, C
The food we eat provides the body with the energy it needs to function. The release of energy from ATP also powers the body at the level of the cell. This is called biological work. Fig. 2-7, A
When mixtures are separated, the substances that were combined to form the mixture retain their original properties. An example is a mixture of sugar and iron filings, which can be separated into its original components with a magnet.
The particles that are mixed together in a solution remain evenly distributed and a solution is always clear. Ask students to name solutions commonly used around the house. One example might be a weak solution of vinegar and water used to clean glass; many other household cleaners are used in solutions. A solute can be solid, liquid, or gas. The solvent is usually liquid or gas. In an aqueous solution, the solvent is water, and in a tincture it is alcohol. Tinctures are used externally. Does the solute settle to the bottom in a solution? No, the solute does not settle to the bottom; it is dissolved.
If a suspension is not shaken continuously, the particles settle at the bottom. Can you give some examples of suspensions? Examples might include sand in water and salad dressing. Some medications given in suspensions must be shaken prior to administration. A colloid is a gel-like substance resembling egg whites. Blood plasma is a colloidal suspension; proteins remain suspended in the plasma. Some drugs should not be given together because they form a precipitate, which in the body can become an embolus.
The Human Body in Health and
Illness, 4th edition