A. Distinguish between inorganic and organic molecules.
B. Describe how the properties of O2, CO2, and watercontribute to their physiological functions.
Early scientists believed that inorganic substances came from non-living sources and that organic substances were extracted from living organisms. As the science of chemistry developed, however, it became apparent that the body also contains inorganic substances and that organic substances can be manufactured in the labora-tory. As currently defined, inorganic chemistry deals with those substances that do not contain carbon, whereas organic chemistry is the study of carbon-containing substances. These definitions have a few exceptions. For example, CO2 and carbon monoxide (CO) are classified as inorganic molecules, even though they contain carbon.
Inorganic substances play many vital roles in human anatomy and physiology. Examples include the O2 and other gases we breathe, the calcium phosphate that makes up our bones, and the metals that are required for protein functions, such as iron in hemo-globin and zinc in alcohol dehydrogenase. In the next sections, we discuss the important roles of O2, CO2, and water—all inorganic molecules—in the body.
Oxygen (O2) is a small, nonpolar, inorganic molecule consistingof two oxygen atoms bound together by a double covalent bond. About 21% of the gas in the atmosphere is O2, and it is essential for most living organisms. Humans require O2 in the final step of a series of chemical reactions in which energy is extracted from food molecules .
Carbon dioxide (CO2) consists of one carbon atom bound totwo oxygen atoms. Each oxygen atom is bound to the carbon atom by a double covalent bond. Carbon dioxide is produced when food molecules, such as glucose, are metabolized within the cells of the body. Once CO2 is produced, it is eliminated from the cell as a metabolic by-product, transferred to the lungs by the blood, and exhaled during respiration. If CO2 is allowed to accumulate within cells, it becomes toxic.
Water (H2O) is an inorganic molecule that consists of one atom ofoxygen joined by polar covalent bonds to two atoms of hydrogen. Water has many important roles in humans and all living organisms:
1.Stabilizing body temperature. Because heat energy causes not only movement of water molecules, but also disruption of hydrogen bonds, water can absorb large amounts of heat and remain at a stable temperature. Blood, which is mostly water, is warmed deep in the body and then flows to the surface, where the heat is released. In addition, water evaporation in the form of sweat results in significant heat loss from the body.
2.Providing protection. Water is an effective lubricant. For example, tears protect the surface of the eye from the rubbing of the eyelids. Water also forms a fluid cushion around organs, which helps protect them from damage. The fluid that surrounds the brain is an example.
3Facilitating chemical reactions. Most of the chemical reactions necessary for life do not take place unless thereacting molecules are dissolved in water. For example, NaCl must dissociate in water into Na+and Cl− before those ions can react with other ions. Water also directly participates in many chemical reactions. For example, during the digestion of food, large molecules and water react to form smaller molecules.
4.Transporting substances. Many substances dissolve in water and can be moved from place to place as the water moves. For example, blood transports nutrients, gases, and waste products within the body.