BIOS 208. Chapter 2. The Chemical Context of Life

BIOS 208/210: Fundamentals of Cell Biology

Chapter 2: The Chemical Context of Life

Key Concepts

2.1)            Matter consists of chemical elements in pure form and in combinations called compounds

·         Elements cannot be broken down chemically to other substances. A compound contains two or more different elements in a fixed ratio. Oxygen, carbon, hydrogen, and nitrogen make up approximately 96% of living matter.

2.2)            An element’s properties depend on the structure of its atoms

·         An atom, the smallest unit of an element, has the following components:

o   Nucleus

o   Protons

o   Neutrons

o   Electrons

·         An electrically neutral atom has equal numbers of electrons and protons; the number of protons determines the atomic number. The atomic mass is measured in daltons and is roughly equal to the sum of protons plus neutrons. Isotopes of an element differ from each other in neutron number and therefore mass. Unstable isotopes give off particles and energy as radioactivity.

·         In an atom, electrons occupy specific electron shells; the electrons in a shell have a characteristic energy level. Electron distribution in shells determines the chemical behavior of an atom. An atom that has a incomplete outer shell, the valance shell, is reactive.

·         Electrons exist in orbitals, 3D spaces with specific shapes that are components of electron shells.

2.3)            The formation and function of molecules depend on chemical bonding between atoms

·         Chemical bonds form when atoms interact and complete their valance shells. Covalent bonds form when pairs of electrons are shared.

·         Molecules consist of two or more covalently bonded atoms. The attraction of an atom for the electrons of a covalent bond is its electronegativity. If both atoms are the same, they have the same electronegativity and share a nonpolar covalent bond. Electrons of a polar covalent bond are pulled closer to the more electronegative atom.

·         An ion forms when an atom or molecule gains or loses an electron and becomes charged. An ionic bond is the attraction between two oppositely charged ions.

·         Weak bonds reinforce the shapes of large molecules and help molecules adhere to each other. A hydrogen  bond is an attraction between a hydrogen atom carrying a partial positive charge and an electronegative atom. Van der Waals interactions occur between transiently positive and negative regions of molecules.

·         A molecule’s shape is determined by the positions of its atoms’ valance orbitals. Covalent bonds result in hybrid orbitals, which are responsible for the shapes of H20, CH4, and many more complex biological molecules. Shape is usually the basis for the recognition of one biological molecule by another.

2.4)            Chemical reactions make and break chemical bonds

·         Chemical reactions change reactants into products while conserving matter. All chemical reactions are theoretically reversible. Chemical equilibrium is reached when the forward and reverse reaction rates are equal.