Water is called the [1] universal solvent because it dissolves more substances than any other liquid. Substances that mix readily with water are called [2] hydrophilic , whereas those that do not are called [3] hydrophobic . In order for another molecule to dissolve in water, a shell of water molecules called a [4] hydration sphere must form around it. Water is retained in the body and forms films on organ surfaces due to [5] cohesion , the tendency for water molecules to be mutually attracted to each other. This property also gives water a high [6] heat capacity —the ability to absorb a large amount of heat energy with relatively little change in temperature.

[7] minerals are inorganic elements extracted from soil and passed through the food chain. The two most significant gases in human physiology are [8] oxygen and [9] carbon dioxide , but recently it has been discovered that many cells contain an enzyme that uses arginine to make [10] nitric oxide , a gas widely used as an intercellular messenger.

Compounds of carbon are called [11] organic . They get their unique functional properties mainly from small groups of atoms called [12] functional groups . For example, —COOH is called a/an [13] carboxyl group, —OH is called a/an [14] hydroxyl group, and —CH₃ is called a/an [15] methyl group. Some of these compounds are giant molecules called [16] macromolecules , with very high molecular weights. Usually these are [17] polymers , or chains of repetitive subunits called [18] monomers . Such chains are typically formed through [19] dehydration synthesis , which produces water as a by-product, and they are broken apart by [20] hydrolysis reactions, which consume water.

Carbohydrates are named for the fact that they are made essentially of carbon plus a 2:1 ratio of [21] hydrogen to oxygen to oxygen. The simplest of them are [22] monosaccharides , which literally means "single sugars." Sucrose, lactose, and maltose are the three most common dietary [23] disaccharides , or double sugars. Any compound made of thousands of glucose residues linked together is called a/an [24] polysaccharide . One of these, [25] cellulose , is indigestible plant fiber important as "bulk" or "roughage" in the diet. [26] glycogen is a compound in this category made by liver and muscle cells to store excess glucose. Carbohydrates may be covalently bonded, or [27] conjugated , with other macromolecules. One example is [28] glycoprotein , a protein with one to a few monosaccharide residues attached. Another is a [29] proteoglycan , which consists mainly of a long polysaccharide chain bonded to a core protein, and which often forms gels in the intercellular material of tissues. When two different classes of organic compounds are covalently joined to form a large molecule, each chemically different component is called a [30] moiety .

A [31] fatty acid is usually 4–24 carbon atoms long, with a carboxyl group at one end and a methyl group at the other. It is called [32] unsaturated if it has one or more double covalent bonds, and [33] saturated if all the carbon–carbon bonds are single covalent and it carries as much hydrogen as possible. When three of these are linked to a glycerol molecule, they form a neutral fat, or [34] triglyceride . If only two of these are linked to glycerol, and the third carbon of glycerol is linked to a hydrophilic phosphate-containing group, the molecule is called a/an [35] phospholipid . One of the most important properties of these molecules is that they are [36] amphiphilic , meaning that part of the molecule is attracted to water and part is repelled by it. This property helps determine the structure of cell membranes. Modified 31s with a ring in the middle of the carbon chain are called [37] prostaglandins ; they serve as chemical messengers from one cell to another, playing signaling roles in inflammation, blood clotting, and other processes. Lipids with four rings are called [38] steroids , and all of these are synthesized from a somewhat notorious compound called [39] cholesterol .

A protein is a chain of amino acids joined together by [40] peptide bonds. The order in which the amino acids occur is called the [41] primary structure of a protein. Most proteins also coil into a springlike shape called the [42] alpha helix , held together by hydrogen bonds from the C=O group of each amino acid to the –NH group of the third next one. This coiled molecule then tends to fold into a specific shape called the protein’s [43] tertiary structure. When a functional protein consists of two or more polypeptides associated with each other, it is said to have a [44] quaternary structure. The total three-dimensional shape of a protein, called its [45] conformation , is critical to its function. The process by which a structural change destroys a protein’s function is called [46] denaturation . When a protein is covalently bonded to a non-amino acid moiety, it is said to be a [47] conjugated protein. Proteins that function as catalysts are called [48] enzymes .

Enzymes work by reducing the [49] activation energy of a chemical reaction, enabling it to proceed more rapidly. The substance the enzyme acts upon is called its [50] substrate . The pocket or groove where an enzyme attaches to such a molecule is called the [51] active site . Just as only one key fits a particular lock, a given enzyme can usually bind only to certain 50s, a property called [52] enzyme-substrate specificity . This limits the chemical reactions an enzyme can catalyze. Most enzymes function best at the same [53] temperature because this variable is fairly uniform throughout the body, while [54] pH varies more from place to place, and different enzymes are adapted to function best at the 54 found in the organ where the enzyme occurs. Some enzymes require nonprotein ions called [55] cofactors or vitamin derivatives called [56] coenzymes in order to function. These vitamin derivatives function by transferring electrons from one reaction sequence, or [57] metabolic pathway , to another.

The primary purpose of adenosine triphosphate (ATP) is to transfer [58] energy from one reaction to another. ATP is composed of three phosphate groups linked to the sugar [59] ribose , which in turn is linked to a nitrogenous compound called [60] adenine . In a typical reaction involving ATP, an enzyme called an ATPase hydrolyzes the third [61] phosphate group from ATP and transfers it to another molecule. This process is known as [62] phosphorylation . The synthesis of ATP often begins with a glucose-splitting reaction pathway called [63] glycolysis . The pyruvic acid generated by this pathway is either converted to lactic acid by a process called [64] anaerobic fermentation or broken all the way down to CO₂ and H₂O by a process called [65] aerobic respiration . A major difference between these is that 65 requires [66] oxygen , whereas 64 does not, but 65 produces a great deal more ATP. The reactions of 65 are carried out in cellular structures called [67] mitochondria . Some energy transfer reactions use [68] guanosine triphosphate, a molecule similar to ATP. ATP is also closely related to the building blocks of the [69] nucleic acids, DNA and RNA.