3.9: Summary

An atom is a fundamental unit—it can’t be broken apart without losing its identity. If carbon and oxygen atoms are broken apart into their component protons, neutrons, and electrons, we can’t tell which atoms they came from. Although there are more than 100 kinds of atoms, living substances are made mainly of only four—carbon, hydrogen, oxygen, and nitrogen.

An atom has protons and neutrons in its center (nucleus), and electrons that orbit the nucleus. Elements differ from each other by their number of protons—a different number of protons means a different element, e.g., magnesium has 12 protons; aluminum has 13 protons. Elements with the same numbers of protons but different numbers of neutrons are called isotopes.

Protons have a positive charge; electrons have a negative charge; and neutrons are neutral. An atom has the same number of protons (positive charges) as electrons (negative charges), making the atom itself neutral.

Protons and neutrons have “weight” (the weight of electrons is relatively trivial), and account for the atomic weight. Atomic weights are useful in calculating the amount of a mineral in a supplement, e.g., the amount of calcium in calcium carbonate.

Electrons orbit an atom’s nucleus in electron shells. The number of electrons in an atom’s outer shell is important in chemical reactions. An atom is stable only if its outer shell is completely filled with electrons. Helium and neon atoms are stable in this way and aren’t chemically reactive.

Chemically react” to fill it and become stable. An atom does this by sharing electrons with other atoms (forming covalent bonds), or by losing or gaining electrons. When an atom loses or gains electrons, it becomes an ion. Because electrons are negatively charged, an atom becomes a positively charged ion when it loses electrons, or becomes a negatively charged ion when it gains electrons.

Molecules consist of two or more atoms held together by chemical bonds. In a covalent bond, atoms are held together by shared electrons. In an ionic bond, a positively charged ion bonds to a negatively charged ion by the attraction of their opposite charges. Water can cause the ions to separate. Ions play crucial roles in the watery fluids of our blood and tissues.

Covalent bonds are much stronger than ionic bonds. The body uses enzymes (biological catalysts) to break apart covalent bonds. In breaking apart bonds and forming new ones, one molecule is made from another. For example, digestive enzymes break apart starch into sugar (starch can’t be absorbed from the intestine, but sugar can). In our cells, enzymes can break apart the sugar and join the fragments to make fat. Without enzymes, most of the reactions necessary for life can’t proceed.

When talking about how acid a fluid is, we’re technically referring to its content of hydrogen ions—the more hydrogen ions, the more acid the fluid. A fluid’s acidity or alkalinity is expressed as pH, on a scale of 0 (most acid) to 14 (most basic). pH 7 is neutral, below pH 7 is acid, above pH 7 is basic.

Calories are a measure of energy. Energy in food is measured in a bomb calorimeter—we burn the food, measure the heat released, and give the measurement in calories. The energy-providing nutrients are fat (9 calories per gram), carbohydrate (4 cal/g), protein (4 cal/g), and alcohol (7 cal/g). These values can be used to calculate the amount of calories in a food and the percent of calories from fat, to make “fair” comparisons of the fat in foods.

We not only expend energy as heat, but use energy for work, such as the contractions of the heart muscle or the repair of a wound. For these activities, we use a chemical form of energy called ATP. Our cells make ATP only as needed. We don’t store energy as ATP. We store energy (reserve source of calories) mostly as body fat.

We can measure the amount of energy we use for various activities by measuring the amount of heat we produce (ATP is used and heat is produced simultaneously). More commonly, we measure energy use by measuring the amount of oxygen the body uses, because we use oxygen in direct proportion to the amount of energy (calories) we use. Measuring by either method gives equivalent values.