Inorganic Substances

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Chapter: Anatomy and Physiology for Health Professionals: Levels of Organization : Chemical Basics of Life

Inorganic substances in body cells include water, salts, and acids/bases.

Inorganic Substances

Inorganic substances in body cells include water, salts, and acids/bases.


The most abundant compound in the human body is water, accounting for nearly two-thirds of the body weight. Any substance that dissolves in water is called a solute. Because solutes dissolved in water are more likely to react with each other as they break down into smaller particles, most metabolic reactions occur in water. There are five major properties of water:

■■ Cushioning: Protection of certain organs from physical trauma (e.g., cerebrospinal fluid that cushions the brain)

■■ High heat capacity: Ability to absorb and release large amounts of heat before water itself actually changes temperature to any large degree; this pre-vents sudden temperature changes from external factors such as exposure to sun or wind or from internal conditions that quickly release heat suchas vigorous muscle activity; in the blood, water redistributes heat among tissues, maintaining homeostasis

■■ High vaporization heat: Water changes from liquid to gas (water vapor), which requires large amounts of heat to be absorbed in order to break hydrogen bonds that keep water molecules together; this is valuable as part of sweating—since mostly water evaporates from the skin, the body is efficiently cooled

■■ Polar solvent properties: Water is referred to as the universal solvent, since nearly all chemical reactions in the body require its solvent proper-ties. As a result, biochemistry is called wet chem-istry. Biological molecules only react chemically when they are in a solution. Water molecules are polar, and their slightly negative ends are oriented toward the positive ends of solutes, The reverse is also true. Solute molecules are attracted and then surrounded. This is why acids, bases, other small reactive molecules, and ionic compounds dissoci-ate in water. Their ions separate from each other and scatter throughout the water, forming true solutions.

Water also forms hydration layers of its mol-ecules around large charged molecules such as proteins. Hydration layers shield the mol-ecules from other nearby charged substances, preventing them from settling out of the solu-tion. These protein-water mixtures are known as biological colloids. Examples include blood plasma and cerebrospinal fluid.

The solvent properties of water make it the body’s major transport system. Metabolic wastes, nutrients, and respiratory gases are carried to be dissolved in the blood plasma.

Many metabolic wastes are excreted as urine. Lubricants such as mucus use water to dis-solve other substances.

■■ Reactivity: For many chemical reactions, water is an important reactant; an example is when foods are broken down to their components by adding a water molecule to each bond.


Salts are compounds of oppositely charged ions that are abundant in tissue fluids. A salt is an ionic com-pound that contains cations other than hydrogen ions and anions other than the hydroxyl ion. When salts are dissolved in water, they dissociate into their component ions. One example is when sodium sulfate (Na2SO4) dissociates into two sodium ions and one SO42– ion. It dissociates easily since its ions are alreadyformed. Water then easily overcomes the attraction between the oppositely charged ions.

All ions are electrolytes, which conduct electrical currents when in solution. Electrolytes release ions in water. As they dissolve in water, the negative and posi-tive ends of water molecules cause ions to separate and interact with water molecules instead of each other. The resulting solution contains electrically charged particles (ions) that conduct electricity. Groups of atoms that have an overall charge, such as sulfate, are known as polyatomic ions.

In the body, common salts include sodium chlo-ride (NaCl), calcium carbonate (CaCO3), and potas-sium chloride (KCl). The most plentiful salts are the calcium phosphates, which are utilized to harden the bones and teeth. The ionized form of salts is used for vital body functions. Salt ions are important for transporting substances to and from the cells, muscle contractions, and nerve impulse conduction. Ionic iron makes up part of the hemoglobin molecules transporting oxygen inside red blood cells. Certain enzymes require zinc and copper ions. Other import-ant functions of elements from body salts include:

■■ Calcium: Found as a salt in bones and teeth, itsionic form is needed for blood clotting, conduction of nerve impulses, and muscle contraction

■■ Chlorine: Its ion, chloride, is the most abundantanion in extracellular fluids

■■ Iron: A component of hemoglobin and someenzymes

■■ Phosphorus: Part of calcium phosphate salts inbones and teeth, also in nucleic acids, and is a partof adenosine triphosphate (ATP)

■■ Potassium: Its ion is the major positive cation incells; vital for conduction nerve impulses andmuscle contraction

■■ Sodium: Its ion is the major positive ion in extra-cellular fluids; important for conduction of nerveimpulses, muscle contraction, and water balance.

1. Distinguish between organic and inorganic chemicals.

2. Define the term “biochemistry” and list its other descriptive title.

3. What are the five major properties of water?

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