Involved in electron transport during respiration
atom electrically neutral. The number of neutrons in the atoms of an element can vary slightly, so the element may occur in forms having different weights but with all forms behaving alike chemically. Such variations of an element are called isotopes. The element oxygen (Fig. 2.3), for example, has seven known isotopes. The nucleus of one of these isotopes contains eight protons and eight neutrons; the nucleus of another isotope holds eight protons and ten neutrons, and the nucleus of a third isotope consists of eight protons and nine neutrons. If the number of neutrons in an isotope of a particular element varies too greatly from the average number of neutrons for its atoms, the isotope may be unstable and split into smaller parts, with the release of a great deal of energy. Such an isotope is said to be radioactive.
Figure 2.3 Isotopes of oxygen portrayed two dimensionally.
The Nature of Life 17
Molecules: Combinations of Elements
The atoms of most elements can combine with other atoms of the same or different elements; in fact, most elements do not exist independently as single atoms. When two or more elements are united in a definite ratio by chemical bonds, the substance is called a compound. Table salt (sodium chloride, NaCl), for example, is a compound consisting of sodium and chlorine atoms combined in a 1:1 ratio.
A molecule consists of two or more atoms bound together and is the smallest independently existing particle of a compound or element. The molecules of the gases oxygen and hydrogen, for example, exist in nature as combinations of two atoms of oxygen (O2) or two atoms of hydrogen (H2), respectively. Water molecules (H2O) consist of two atoms of hydrogen and one atom of oxygen (Fig. 2.4). Molecules are in constant motion, with an increase or decrease in temperature speeding up or slowing down the motion. The more molecular movement there is, the greater the chances are that some molecules will collide with each other. Also, the chances of random collisions increase in proportion to the density of the molecules (i.e., the number of molecules present in a given space).
Random collisions between molecules capable of sharing electrons are the basis for all chemical reactions. The reactions often result in new molecules being formed. Each chemical reaction in a cell usually takes place in a watery fluid and is controlled by a specific enzyme. Enzymes are organic catalysts (a catalyst speeds up a chemical reaction without being used up in the reaction. Enzymes are discussed on page 24).
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The use of dumbbells gives you a much more comprehensive strengthening effect because the workout engages your stabilizer muscles, in addition to the muscle you may be pin-pointing. Without all of the belts and artificial stabilizers of a machine, you also engage your core muscles, which are your body's natural stabilizers.