one joule (J) is the amount of energy required to move an object a distance of one meter against a force of one newton. This measure of energy is derived from the si units (système internationale d’Unités) universally employed by physical scientists. a second unit of energy, often used by cell biologists, is the kilocalorie (kcal); one calorie is the amount of energy needed to raise the temperature of 1 gram of water by 1°C. one kJ is equal to 0.239 kcal (1 kcal = 4.18 kJ).
Figure 2–3 Schematic indicating how two macromolecules with complementary surfaces can bind tightly to one another through noncovalent interactions. noncovalent chemical bonds have less than  1/20 the strength of a covalent bond. They are able to produce tight binding only when many of them are formed simultaneously. although only electrostatic attractions are illustrated here, in reality all four noncovalent forces often contribute to holding two macromolecules together (Movie 2.1).
individual noncovalent attraction would be much too weak to be effective in the face of thermal motions, their energies can sum to create a strong force between two separate molecules. Thus sets of noncovalent attractions often allow the complementary surfaces of two macromolecules to hold those two macromolecules together (Figure 2–3).
Table 2–1 compares noncovalent bond strengths to that of a typical covalent bond, both in the presence and in the absence of water. Note that, by forming competing interactions with the involved molecules, water greatly reduces the strength of both electrostatic attractions and hydrogen bonds.
The structure of a typical hydrogen bond is illustrated in Figure 2–4. This bond represents a special form of polar interaction in which an electropositive hydrogen atom is shared by two electronegative atoms. Its hydrogen can be viewed as a proton that has partially dissociated from a donor atom, allowing it to be shared by a second acceptor atom. Unlike a typical electrostatic interaction, this bond is highly directional—being strongest when a straight line can be drawn between all three of the involved atoms.
The fourth effect that often brings molecules together in water is not, strictly speaking, a bond at all. However, a very important hydrophobic force is caused by a pushing of nonpolar surfaces out of the hydrogen-bonded water network, where they would otherwise physically interfere with the highly favorable interactions between water molecules. Bringing any two nonpolar surfaces together reduces their contact with water; in this sense, the force is nonspecific. Nevertheless, we shall see in Chapter 3 that hydrophobic forces are central to the proper folding of protein molecules.
One of the simplest kinds of chemical reaction, and one that has profound significance in cells, takes place when a molecule containing a highly polar covalent bond between a hydrogen and another atom dissolves in water. The hydrogen atom in such a molecule has given up its electron almost entirely to the companion atom, and so exists as an almost naked positively charged hydrogen nucleus—in *an ionic bond is an electrostatic attraction between two fully charged atoms. **Values in parentheses are kcal/mole. 1 kJ = 0.239 kcal and 1 kcal = 4.18 kJ.
(A) hydrogen bond ~0.3 nm long covalent bond ~0.1 nm long
Figure 2–4 Hydrogen bonds. (a) Ball-andstick model of a typical hydrogen bond. The distance between the hydrogen and the oxygen atom here is less than the sum of their van der Waals radii, indicating a partial sharing of electrons. (B) The most common hydrogen bonds in cells.