Concept 2.1   Atomic Structure Is the Basis for Life’s Chemistry

• Matter is composed of atoms. Each atom consists of a positively charged nucleus made up of protons and neutrons, surrounded by electrons bearing negative charges.
• The number of protons in the nucleus defines an element. There are many elements in the universe, but only a few of them (C, H, O, P, N, and S) make up the bulk of living organisms.
• Electrons are distributed in electron shells at varying energy levels away from the nucleus. The first shell can have a maximum of 2 electrons; the second shell, 8 electrons; the third shell, 18 electrons; and subsequent shells, 32 electrons. Review Figure 2.1

Concept 2.2   Atoms Interact and Form Molecules

• A chemical bond is an attractive force that links two atoms together in a molecule. Review ANIMATED TUTORIAL 2.1
• A covalent bond is a strong bond formed when two atoms share one or more pairs of electrons. Review Figure 2.2 and Figure 2.3
• When two atoms of unequal electronegativity bond with each other, a polar covalent bond is formed. The two ends, or poles, of the bond have partial charges (δ⁺ or δ^-). A hydrogen bond is a weak electrical attraction that forms between a δ⁺ hydrogen atom in one molecule and a δ^- atom in another molecule (or in another part of a large molecule). Hydrogen bonds are abundant in water. Review Figure 2.4
• Ions are electrically charged particles that form when atoms gain or lose one or more electrons in order to form more stable electron configurations. Anions and cations are negatively and positively charged ions, respectively. Ionic attractions form when ions with opposite charges attract. Review Figure 2.6
• Functional groups are covalently bonded groups of atoms that confer specific properties to biological molecules. Review Figure 2.7
• Macromolecules are formed by polymerization of smaller molecules call monomers. Review Figure 2.8, ANIMATED TUTORIAL 2.2, and ACTIVITY 2.1

Concept 2.3    Carbohydrates Consist of Sugar Molecules

• Carbohydrates contain carbon bonded to hydrogen and oxygen.
• Monosaccharides include pentoses (with five carbons) and hexoses (with six carbons). Review Figure 2.9 and ACTIVITY 2.2
• Glycosidic linkages are covalent bonds between saccharides. Disaccharides such as sucrose each contain two monosaccharides, whereas polysaccharides such as starch and cellulose contain long chains of monomers. Review Figure 2.10

Concept 2.4   Lipids Are Hydrophobic Molecules

• Fats and oils are triglycerides, composed of three fatty acids covalently linked to glycerol. Review Figure 2.11
• Saturated fatty acids have hydrocarbon chains with no double bonds. Unsaturated fatty acids contain double bonds in their hydrocarbon chains. Review Figure 2.12
• Phospholipids contain two fatty acids and a hydrophilic, phosphate-containing polar group attached to glycerol. They are amphipathic, with both polar and nonpolar ends. They form into a structural bilayer in water. Review Figure 2.13

Concept 2.5   Biochemical Changes Involve Energy

• A chemical reaction occurs when atoms have sufficient energy to combine or to change their bonding partners.
• Anabolic reactions require energy and are endergonic. Catabolic reactions release energy and are exergonic. Review Figure 2.14
• The laws of thermodynamics govern biochemical reactions. The first law states that in any transformation, energy is neither created nor destroyed. The second law states that useful energy tends to decrease. In other words, entropy (disorder) tends to increase. Review Figure 2.15

See ANIMATED TUTORIAL 2.3