Covalent bonds, the strongest interactions between atoms, are responsible for holding atoms together to form molecules. They form when two atoms come together and share a pair of electrons (Figure 2.1). For example, methane (CH4) is formed when four hydrogen atoms share electrons with one carbon atom (Figure 2.1A). The number of covalent bonds that an atom can form is determined by the number of unpaired electrons in its outer electron shell (its valence). Carbon has four unpaired electrons whereas hydrogen has one, so carbon can form covalent bonds with four hydrogen atoms. The other principal atoms of living organisms, oxygen and nitrogen, have two and three unpaired electrons, respectively.
Figure 2.1 Covalent bonds (A) Covalent bonds are formed by the sharing of electrons between two atoms, such as carbon and hydrogen. (B) Atoms can rotate around single bonds but not double bonds. (C) Polar bonds form between atoms that differ in their attraction for electrons. Water is a polar molecule, with a slight negative charge (δ–) on the oxygen atom and a slight positive charge (δ+) on the hydrogen atoms.
Most covalent bonds are single bonds, in which two atoms share a single pair of electrons. In some cases, however, atoms share two pairs of electrons, leading to formation of a double bond (Figure 2.1B). For example, carbon atoms can form either single (C-C) or double (C=C) bonds with one another. Double bonds are stronger than single bonds and also differ significantly in their effects on molecular structure. Atoms are free to rotate around a single bond, but not around a double bond. The resulting rigidity of double bonds can have a major effect on the structure of many of the macromolecules within cells.
Covalent bonds also differ in polarity, which depends on the extent to which electrons are attracted to the nuclei of the atoms forming the bond (Figure 2.1C). A covalent bond between two atoms of the same element (for example, a C-C bond) is nonpolar because the two identical nuclei attract electrons equally. Bonds between carbon and hydrogen are also nonpolar, because carbon and hydrogen nuclei attract electrons to similar extents and the electrons are shared equally between them. However, the nuclei of other atoms differ significantly in how strongly they attract electrons (electronegativity). Covalent bonds between atoms that differ in electronegativity are polar, because the shared electrons are unevenly distributed. Water is an important example because it is the most abundant molecule in cells, accounting for 70% or more of total cell mass. When oxygen bonds with hydrogen, the electrons are drawn closer to the more electronegative oxygen nucleus. As a result, water is a polar molecule, in which the hydrogen atoms have a slight positive charge and the oxygen has a slight negative charge.
Figure 2.2 Ionic bonds (A) Sodium (Na) atoms donate electrons to chlorine (Cl) atoms to form the charged ions Na+ and Cl–, which are held together by the attraction of opposite charges. (B) Ions are held closely together in solid salt crystals but dissociate in aqueous solutions because they interact with polar water molecules.
Strong differences in electronegativity lead to the formation of ionic bonds, in which electrons are completely transferred to one nucleus instead of being shared (Figure 2.2A). For example, sodium (Na) atoms donate electrons to much more electronegative chlorine (Cl) atoms to form sodium chloride (NaCl), which is composed of the charged ions Na+ and Cl−. Negatively charged ions are called anions and positively charged ions are called cations. The ions are held together by an ionic bond resulting from the attraction of opposite charges. Molecules in which ions are held together by ionic bonds are called salts. In solid form, for example a crystal of sodium chloride, the strength of ionic bonds is similar to covalent bonds. However, salts dissociate into individual ions in aqueous solutions because ions can also interact with water molecules (Figure 2.2B). In aqueous solution, the strength of ionic bonds is about twentyfold less than that of covalent bonds. A number of inorganic ions, including sodium (Na+), potassium (K+), magnesium (Mg2+), calcium (Ca2+), phosphate (HPO4 2–), chloride (Cl–), and bicarbonate (HCO3 –), play critical roles in cells.
Polar molecules can interact with one another through hydrogen bonds, a noncovalent bond formed between a positively charged hydrogen and a negatively charged nitrogen or oxygen (Figure 2.3). For example, water molecules can form hydrogen bonds with each other or with other polar molecules. In addition, hydrogen bonds can form between different organic molecules or between different parts of a single large molecule, such as a protein. Although they are approximately fiftyfold weaker than covalent bonds, hydrogen bonds play important roles in determining the structures of many macromolecules (including proteins and nucleic acids) and in governing the interactions of molecules within cells.
Figure 2.3 Hydrogen bonds (A) Water molecules form hydrogen bonds with each other or with other polar molecules. (B) Hydrogen bonds form between polar organic molecules, such as amino acids in different parts of a protein.
The polar nature of water is also responsible for the third type of noncovalent bond that plays important roles in cells, hydrophobic interactions (Figure 2.4). Ions and polar molecules are readily soluble in water (hydrophilic). In contrast, nonpolar molecules, which cannot interact with water, are poorly soluble in an aqueous environment (hydrophobic). Consequently, nonpolar molecules tend to minimize their contact with water by associating with other hydrophobic molecules. Although these interactions are frequently referred to as hydrophobic bonds, they really result from the absence of the formation of hydrogen or ionic bonds with the polar solvent (water). For example, covalent bonds between carbon and hydrogen atoms are nonpolar, so molecules containing only carbon and hydrogen are insoluble in the aqueous environment of the cell and interact with one another instead. Such hydrophobic interactions play an important role in protein folding and result in the formation of the lipid bilayers that are the basic structure of biological membranes.
Figure 2.4 Hydrophobic interactions (A) Polar molecules are readily soluble in water (hydrophilic). (B) Nonpolar molecules are poorly soluble in water (hydrophobic) and associate with one another instead.
The fourth type of noncovalent interactions, van der Waals interactions, occur when any two atoms are close together. They result from fluctuating electrical charges that are induced by the nearness of molecules. These interactions are transient and very weak, less than half the strength of hydrogen bonds. Although van der Waals interactions take place in both polar and nonpolar molecules, they are not generally significant in polar molecules because of the stronger effects of ionic and hydrogen bonds. However, the sum of many van der Waals interactions can enhance the association of large nonpolar molecules.