Chapter 2 Chemical Foundations for Cells

I. The Chemistry In and Around You
A. Life depends on chemical reactions.
B. The application of chemical knowledge has produced products that provide benefits and problems for our world and its inhabitants. Consider the pesticides, for example.

II. Organization of Matter
A. Matter is anything that occupies space and has mass.
1. It includes solids, liquids, and gases.
2. Matter is made of some ninety-two naturally occurring elements; the four most abundant in humans are: O, C, H, and N (in decreasing order).
a. Trace elements are present in very small, but critical, quantities.
b. The symbol designation for each element is the same the world over.
3. Compounds are substances in which two or more elements are combined in fixed proportions.
B. The Structure of Atoms
1. A molecule is a bonded unit of two or more (same or different) atoms.
2. An atom is the smallest unit of matter that is unique to a particular element.
3. Atoms are composed of three particles:
a. Protons (p+) are part of the atomic nucleus and have a positive charge. Their quantity is called the atomic number (unique for each element).
b. Neutrons are also a part of the nucleus; they are neutral. Protons plus neutrons = atomic mass.
c. Electrons (e–) have a negative charge. Their quantity is equal to that of the protons. They move around the nucleus.
C. Isotopes
1. Atoms with the same number of protons (for example, carbon with six) but a different number of neutrons (carbon can have six, seven, or eight) are called isotopes (12C,13C, 14C).
2. Some radioactive isotopes are unstable and tend to decay into more stable atoms.
a. They can be used to date rocks and fossils.
b. Some can be used as tracers to follow the path of an atom in a series of reactions or to diagnose disease.

III. Bonds Between Atoms
A. The Nature of Chemical Bonds
1. A chemical bond is a union between atoms formed when they give up, gain, or share electrons.
2. Energy Levels
a. Electrons are attracted to protons but are repelled by other electrons.
b. Orbitals permit electrons to stay as close to the nucleus and as far from each other as possible.
c. Each orbital contains one or two electrons.
d. Orbitals can be thought of as occupying shells around the nucleus.
1) The shell closest to the nucleus has one orbital holding a maximum of two electrons.
2) The next shell can have four orbitals with two electrons each for a total of eight electrons.
e. Atoms with “unfilled” orbitals in their outermost shell tend to be reactive with other atoms.
B. Ionic Bonding
1. When an atom loses or gains one or more electrons, it becomes positively or negatively charged—an ion.
2. In an ionic bond, (+) and (–) ions are linked by mutual attraction of opposite charges, for example, NaCl.
C. Covalent Bonding
1. A covalent bond holds together two atoms that share one or more pairs of electrons.
2. In a nonpolar covalent bond, atoms share electrons equally.
3. In a polar covalent bond, because atoms share the electron unequally, there is slight difference in charge between the two poles of the bond; water is an example.
D. Hydrogen Bonding
1. In a hydrogen bond, an atom or a molecule interacts weakly with a hydrogen atom already taking part in a polar covalent bond.
2. These bonds impart structure to liquid water and stabilize nucleic acids and other large molecules.

IV. Properties of Water
A. Water is a polar molecule because of a slightly negative charge at the oxygen end and a slightly positive charge at the hydrogen end. Water molecules can form hydrogen bonds with each other.
B. Polar substances are hydrophilic (water-loving); nonpolar ones are hydrophobic (water-dreading) and are repelled by water.
C. Water displays three properties:
1. Water tends to stabilize temperature because it can absorb considerable heat before its temperature changes. This is an important property in evaporative and freezing processes.
2. Hydrogen bonding of water molecules provides cohesion (capacity to resist rupturing), which imparts surface tension and helps pull water through plants, for example.
3. The solvent properties of water are greatest with respect to polar molecules because “spheres of hydration” are formed around the solute molecules.

V. Acids, Bases, and Salts
A. Acids and Bases
1. A substance that releases hydrogen ions (H+) in solution is an acid; for example, HCl.
2. Substances that release ions such as OH– that can combine with hydrogen ions are called bases.
B. The pH Scale
1. pH is a measure of the H+ concentration in a solution; the greater the H+ the lower the pH scale.
2. The scale extends from 0 (acidic) to 7 (neutral) to 14 (basic).
3. The interior of living cells is near pH = 7.
C. Buffers
1. Buffer molecules combine with, or release, H+ to prevent drastic changes in pH.
2. Carbonic acid is one of the body’s major buffers.
D. Dissolved Salts
1. A salt is an ionic compound formed when an acid reacts with a base; example: NaOH + HCl Æ NaCl + H2O.
2. Salts dissociate into useful ions (examples: Na+ and Ca++) in body fluids.