Medical brain notes 27

  ELEMENTS All matter, both living and not living, is made of ele-ments, the simplest chemicals. An  element  is a sub-stance made of only one type of atom (therefore, an atom is the smallest part of an element). There are 92 naturally occurring elements in the world around us. Examples are hydrogen (H), iron (Fe), oxygen (O), calcium (Ca), nitrogen (N), and carbon (C). In nature, an element does not usually exist by itself but rather combines with the atoms of other elements to form compounds. Examples of some compounds important to our study of the human body are water (H 2 O), in which two atoms of hydrogen combine with one atom of oxygen; carbon dioxide (CO 2 ), in which an atom of carbon combines with two atoms of oxygen; and glu-cose (C 6 H 12 O 6 ), in which six carbon atoms and six oxygen atoms combine with 12 hydrogen atoms. The elements carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur are found in all living things. If calcium is included, these seven ...

  ATOMS Atoms  are the smallest parts of an element that have   the characteristics of that element. An atom consists of three major subunits or particles: protons, neutrons, and electrons (Fig. 2–1). A  proton  has a positive elec-trical charge and is found in the nucleus (or center) of the atom. A  neutron  is electrically neutral (has no charge) and is also found in the nucleus. An  electron  has a negative electrical charge and is found outside the nucleus orbiting in what may be called an electron cloud or shell around the nucleus. Figure 2–1.  An atom of carbon. The nucleus contains   six protons and six neutrons (not all are visible here). Six electrons orbit the nucleus, two in the first energy level and four in the second energy level. QUESTION:  What is the electrical charge of this atom as   a whole?   The number of protons in an atom gives it its  atomic number . Protons and neutrons have mass and ...

  CHEMICAL BONDS A chemical bond is not a structure, but rather a force or attraction between positive and negative electrical charges that keeps two or more atoms closely associ-ated with each other to form a molecule. By way of comparison, think of gravity. We know that gravity is not a “thing,” but rather the force that keeps our feet on the floor and allows us to pour coffee with consis- often have physical characteristics different from those of the atoms of the original elements. For example, the elements hydrogen and oxygen are gases, but atoms of each may chemically bond to form molecules of water, which is a liquid. The type of chemical bonding depends upon the tendencies of the electrons of atoms involved, as you will see. Four kinds of bonds are very important to the chemistry of the body: ionic bonds, covalent bonds, disulfide bonds, and hydrogen bonds. IONIC BONDS An  ionic bond  involves  the loss of one or more elec-trons by one atom and the gain of th...

  CHEMICAL REACTIONS A chemical reaction is a change brought about by the formation or breaking of chemical bonds. Two general types of reactions are synthesis reactions and decom-position reactions. In a  synthesis reaction , bonds are formed to join two or more atoms or molecules to make a new com-pound. The production of the protein hemoglobin in potential red blood cells is an example of a synthesis reaction. Proteins are synthesized by the bonding of many amino acids, their smaller subunits. Synthesis reactions require energy for the formation of bonds. In a  decomposition reaction , bonds are broken, and a large molecule is changed to two or more smaller ones. One example is the digestion of large molecules of starch into many smaller glucose mole-cules. Some decomposition reactions release energy; this is described in a later section on cell respiration.

  INORGANIC COMPOUNDS OF IMPORTANCE Inorganic compounds are usually simple molecules that often consist of only one or two different ele-ments. Despite their simplicity, however, some inor-ganic compounds are essential to normal structure and functioning of the body. WATER Water makes up 60% to 75% of the human body, and is essential to life for several reasons: 1.   Water is a  solvent ; that is, many substances (called   solutes) can dissolve in water. Nutrients such as glucose are dissolved in blood plasma (which is largely water) to be transported to cells throughout the body. The sense of taste depends upon the sol-vent ability of saliva; dissolved food stimulates the receptors in taste buds. The excretion of waste products is possible because they are dissolved in the water of urine.   2.   Water is a lubricant , which prevents friction where   surfaces meet and move. In the digestive tract, swallowing depends upon the presence of saliva, and muc...