The reactions of life take place in aqueous solutions

A solution is produced when a substance (the solute) is dissolved in a liquid (the solvent). If the solvent is water, then the solution is called an aqueous solution. As you know by now, water is polar. Because many important molecules in biological systems are polar, they readily dissolve in water. Being soluble doesn’t mean that the molecules lose their identity and properties. They can still react, and indeed many important biochemical reactions occur in aqueous solutions.

Biologists who are interested in the biochemical reactions within cells identify the reactants and products and determine their amounts using two different types of analyses:

  1. Qualitative analyses focus on identifying the substances involved in chemical reactions. For example, a qualitative analysis would be used to investigate the steps involved and the products formed during respiration, when carbon-containing compounds are broken down to release energy in living tissues.

  2. Quantitative analyses measure concentrations or amounts of substances. For example, a biochemist would use a quantitative analysis to measure how much of a certain product is formed in a chemical reaction. What follows is a brief introduction to some of the quantitative chemical terms you will see in this book.

Fundamental to quantitative thinking in chemistry and biology is the concept of the mole. A mole is the amount of a substance (in grams) that is numerically equal to its molecular weight. So a mole of hydrogen gas (H2) weighs 2 g, a mole of sodium ion (Na+) weighs 23 g, and a mole of table sugar (C12H22O11) weighs about 342 g.

Quantitative analyses do not yield counts of molecules. Because the amount of a substance in 1 mole is directly related to its molecular weight, it follows that the number of molecules in 1 mole is constant for all substances. So 1 mole of salt contains the same number of molecules as 1 mole of table sugar. This constant number of molecules in a mole is called Avogadro’s number, and it is 6.02 × 1023 molecules per mole. Chemists work with moles of substances (which can be weighed in the laboratory) instead of actual molecules, which are too numerous to be counted. Consider 34.2 g (just over 1 ounce) of table sugar, C12H22O11. This is one-tenth of a mole, or one-tenth of Avogadro’s number: 6.02 × 1022 molecules.

A chemist can dissolve 1 mole of table sugar (342 g) in water to make 1 liter of solution, knowing that the mole contains 6.02 × 1023 individual sugar molecules. This solution—1 mole of a substance dissolved in water to make 1 liter—is called a 1 molar (1 M) solution. When a physician injects a certain volume and molar concentration of a drug into the bloodstream of a patient, a rough calculation can be made of the actual number of drug molecules that will interact with the patient’s cells. As you know, the dose is important.

The many molecules dissolved in the water of living tissues are not present at concentrations anywhere near 1 molar. Most are in the micromolar (millionths of a mole per liter of solution; µM) to millimolar (thousandths of a mole per liter; mM) range. Some, such as hormone molecules, are even less concentrated than that. While these molarities seem to indicate very low concentrations, remember that even a 1 µM solution has 6.02 × 1017 molecules of the solute per liter.