The centrally located nucleus contributes most of an atom's weight and contains positively charged particles called protons (p⁺) and may contain uncharged or neutral particles called neutrons (n^o). Together they are referred to as the nucleus. Because of the presence of the protons, the nucleus itself has a positive charge. It is the number of protons that distinguishes one type of atom from another. Electrons (e^-) are negatively charged particles that orbit the nucleus in unfixed three-dimensional variously shaped clouds called orbitals. This path is irregular and may occur anywhere in the cloud. This is called its path of probability, because there is no guarantee it will be found in this area, but there is a good chance it will be. In nature, most atoms exist in a charged state. If there are more protons than electrons, that atom has a net positive charge and is called a cation. Other atoms have a net negative charge; that is, they have more electrons than protons, and are called anions.

Molecules are groups of atoms bonded together. Molecules are made of the same type of atom (oxygen)O₂). A compound contains two or more different types of atoms. An example is water)H₂O, consisting of two atoms of hydrogen and one atom of oxygen. Molecules and compounds can have a positive or negative charge. For example, phosphate molecules have a negative charge (PO₄^-).

Chemical processes: When atoms combine with or break apart from other atoms, a chemical reaction occurs, which produces new products with different properties. These reactions are the foundation of all life processes on the physical plane.

The atoms in molecules and compounds are held together by forces of attraction called chemical bonds, which can be either ionic or covalent. Chemical reactions are the making or breaking of bonds between molecules.

Molecules and compounds are formed when atoms are able to take on or give up additional electrons and therefore combine to balance their electrons. Atoms which give up electrons are called electron donors. Those which tend to pick up electrons are called electron acceptors. Ionic bonding results from the electrostatic interaction between ions. When a positively charged atom and a negatively charged atom are attracted to each other, an ionic bond is formed.

A covalent bond occurs when neither of the combining atoms loses or gains an electron; instead they share one, two or three electron pairs. Two atoms of the same type may form a covalent bond. Covalent bonds are far more common in organisms than ionic bonds and are far more strong and stable.

A hydrogen bond consists of a hydrogen atom covalently bonded to one oxygen atom or one nitrogen atom, but attracted to another oxygen or nitrogen atom. This weak bond can serve as a bridge between or within molecules, but does not form a molecule by itself because of the weakness of the bond. A few elements already have the maximum number of electrons in their outermost energy level and therefore do not naturally seek to combine with other elements. These are called inert elements or noble gases; an example is helium.

Elemental atoms of the same type may have differing numbers of neutrons in their nucleus giving them different nuclear masses. Due to this variation, the atomic weight for a given element is only an average. Atoms of a given element with differing numbers of neutrons are called isotopes.

Radioisotopes are isotopes that are unstable because they continuously undergo alteration of their nuclear structure as they try to form a more stable configuration. This nuclear "decay" process causes the atoms to emit radiation. Radioisotopes are used in several medical tests, where their path is followed through the body by detecting their radiation emission.

Most of the chemicals in the body exist in the form of compounds. Inorganic compounds are small, ionically bonded molecules that lack carbon. Some are vital to bodily function; they include water, many of the salts, as well as acids and bases. Organic compounds are molecules which contain carbon.

Energy: Chemical reactions produce energy, which is the capacity to do work. Potential energy is inactive or stored, and kinetic energy is the energy of motion. Energy exists in several different forms:

Chemical energy is released or absorbed in the breakdown or formation of chemical bonds. This energy is utilized when we metabolize food.
Mechanical energy is that which is directly involved in movement.
Radiant energy, such as heat and light, travels in waves. Some heat is released during breakdown processes in the body; this helps maintain body temperature.
Electrical energy is the result of the flow of charges, electrons or charged atoms called ions. It is essential for the conduction of nerve impulses. Our muscles will not work without the electrical impulses from the attached nerves.

Acids, bases and salts: Bodily fluids are mostly water and must maintain a fairly constant balance of acids and bases to sustain life. A solution is acidic when it contains a large amount of the hydronium (H⁺) ion, a proton. An acid compound, such as hydrochloric acid (HCl), will tend to give up H⁺ ions. A solution is defined as basic or alkaline if it contains high concentrations of the hydroxyl (OH^-) ion. A basic compound, sodium hydroxide (NaOH) will tend to give up OH^- ions. A salt is a compound that contains a cation other than H⁺ and an anion other than OH^-. Acids and bases combine with one another to form salts. When molecules of inorganic compounds such as acids, bases, or salts are dissolved in water, they undergo ionization (i-on-i-ZA-shun); that is, they break apart into ions. Such atoms are called electrolytes (e-LECK-trow-lites) because they will conduct an electric current. Electrolytes in the body consist of essential minerals which help maintain normal fluid balance, conduct electrical impulses in the nerves and muscles, and make up the skeletal system among many other functions. Serum electrolytes consist of ions dissolved in the blood that are responsible for maintaining pH in the body.

What is pH? The term pH (potential of hydrogen) is used to describe the degree of acidity or alkalinity (basicity) of a solution. pH is discussed by using a scale of 1 to 14, with 1 being the most acidic and 14 being most alkaline; 7.0, the midpoint of the scale, is neutral; that is, it is neither acid nor alkaline. A solution becomes more acidic as its hydrogen ion concentration rises. A pH of 5.0 is more acid (i.e., less alkaline) than one of 6.0. The difference between the two figures is negative and logarithmic; each number is a multiple of -10. Each value represents an enormous difference in hydrogen concentration. A pH of 7.20 is a 40% increase of hydrogen compared to 7.35. Each whole pH number represents a tenfold change from the previous value. The range of blood pH compatible with life is 6.8 to 7.8. When an organism's delicate acid/base ratio is altered, an excessively acid or alkaline state results.

pH Range: ACID!BASE/ALKALINE 1   2   3   4   5   6   7   8   9   10   11   12   13   14 Neutral

>--------->-------- increasing base/alkaline concentration--------->--------> <--------<--------increasing hydrogen ion/acid concentration-----<----------<

Biochemical reations (reactions that occur in living systems) are very sensitive to small changes in the pH balance. A buffer system is a reserve of molecules which are utilized as needed to help resist large swings in pH; its essential function is to react with strong acids or bases in the body and replace them with weak acids or bases that will change the pH values only slightly. Carbonic acid and bicarbonate are both produced by the body and are important buffers involved in the maintenance of normal blood pH.

Organic compounds: Organic compounds always contain carbon and hydrogen. Carbon is a unique life-supporting element. Because it has four electrons in its outer shell, it can combine with a variety of other atoms, including other carbon atoms, to form straight chains, or branched chains and ring-shaped molecules. Organic compounds, held together almost entirely by covalent bonds, include carbohydrates, fats, proteins, nucleic acids (DNA and RNA) and adenosine triphosphate (ATP).

    Carbohydrates are a large group of organic compounds known as sugars and starches. Their principal function is to provide the most readily available source of energy to sustain life. They are mainly composed of carbon, hydrogen and oxygen. There are three basic types of carbohydrates:

Monosaccharides (mon-oh-SACK-ah-rides) are simple sugars containing three to seven carbon atoms. Glucose and fructose are in this category. Although they have the same types of atoms, they are arranged differently, resulting in two different sugars.

Disaccharides (die-SACK-ah-rides) consist of two monosaccharides which are chemically joined. This combining process results in the loss of a water molecule and is called dehydration synthesis. Sucrose, or white table sugar, is an example of this carbohydrate; it is a combination of fructose and glucose. Disaccharides can be broken down into small molecules by adding water; this is called digestion.

Polysaccharides (pol-e-SACK-ah-rides) are long chains of mono-saccharides joined together through dehydration synthesis; they lack the sweetness of sugars. One of the main polysaccharides is glycogen.

A saturated fat contains no double bonds between any of its carbon atoms, and all the carbon atoms are bonded to the maximum number of hydrogen atoms; thus this fat is saturated with hydrogen atoms. Most of these are animal fats which remain solid at room temperature.

Unsaturated fat contains one or more double covalent bonds between its carbon atoms and is not completely saturated with hydrogen atoms. Examples are olive and peanut oil which remain liquid at room temperature.

Polyunsaturated fats contain two or more double covalent bonds between their carbon atoms. Corn, safflower and sunflower oils are examples of polyunsaturated fats.

Prostaglandins (pros-tah-GLAN-dens) are a large group of membrane-associated fats composed of 20 carbon fatty acids containing five carbon atoms joined to form a ring. They are called local hormones because they are produced by cells in certain localized areas of the body (such as the uterine musculature) and influence the functioning of their neighbor cells. Although synthesized in minute amounts, they simulate hormones and are involved in the regulation of many hormonal responses.

Nucleic (new-CLAY-ick) acids are large organic molecules containing carbon, hydrogen, oxygen, nitrogen and phosphorus. The basic units of nucleic acids are nucleotides. Each of these contains three basic parts:

1. A nitrogen base; these are ring-shaped structures containing carbon, hydrogen, oxygen and nitrogen. They can be one of five possible nitrogen bases: the double-ringed structures of adenine and thymine (called purines) or the single ringed structures of cytosine, guanine and uracil (called pyrimidines).
2. A pentose sugar (either deoxyribose or ribose)
3. A phosphate group

Deoxyribonucleic (dee-ox-see-rye-bow-new-KLAY-ick) acid ( DNA) is a molecule consisting of two strands of chemicals with crossbars (a ladder), which twist about each other to form a spiral staircase shape. The uprights of the ladder consist of alternating phosphate groups and deoxyribose portions of the nucleotides. The rungs of the ladder have two halves which consist of paired nitrogen bases. Adenine always pairs with thymine, and cytosine always pairs with guanine. Genes are segments of DNA molecules. They determine our inherited traits and control bodily activities throughout our lifetime. When a cell divides, its hereditary information is passed on to the next generation of cells via the genes.

Ribonucleic acid ( RNA) is single stranded and its sugar is ribose. It does not contain thymine, but does contain the nitrogen base uracil. RNA has a specific role to perform with DNA in protein synthesis reactions.

Adenosine (a-DEN-oh-sin) triphosphate (try-FOSS-fate) ( ATP) is essential to the life of the cell, because it stores energy for various cellular activities. It consists of the phosphate groups adenine and the ribose sugar. When its energy is released, another type of molecule, adenosine diphosphate (ADP), is formed. ADP can be converted back to ATP using the energy supplied by various breakdown reactions, especially that of glucose.