Biology Study Guide

UNIT I

CHEMISTRY/BIOCHEMISTRY

I. CHEMISTRY: THE BASIS FOR LIFE

A. ELEMENTS

Almost everything around us can be broken down into simpler substances. These substances can be further broken down into other simpler substances. There is a point where substances can no longer be broken down into other substances while keeping their characteristic properties. These substances are called elements. There are currently 106 named elements (92 naturally occurring), but this number is increasing because more man made elements are being created in laboratories.

From these 92 naturally occurring elements, only 20 are necessary for life. Of these 20 life elements, 6 make up 99% of all living matter: Sulfur, Phosphorous, Oxygen, Nitrogen, Carbon and Hydrogen (SPONCH, pneumonic device). Living organisms still need the other 14 elements, but in smaller amounts.

An atom is the smallest indivisible unit of an element that still has the characteristic of the element. Two or more atoms can combine chemically and form a molecule. A compound is any pure substance that contains two or more different atoms.

Atom = H Molecule = H2 Compound = H2O

B. FORMS OF MATTER

Atoms, elements and compounds are forms of matter. Matter can come in one of three states on the earth:

1. Solid

Has definite shape and has a definite volume

2. Liquid

Has no definite shape but has a definite volume

3. Gas

Has no definite shape and has no definite volume

C. SUBATOMIC PARTICLES

Atoms can be broken down into smaller components called subatomic particles: protons, neutrons, and electrons. Protons and neutrons make up the nucleus of an atom. They are roughly equal in mass, one atomic mass unit (amu) or Dalton. Protons are positively charged and neutrons are not changed. Electrons are negatively charged, have relatively small mass. An atom can be described as having a small, very dense nucleus with a very low density electron cloud surrounding it. Therefore, most of the mass of the universe is made up of protons and neutrons.

Strong nuclear forces hold the protons and neutrons together, while the electrons are attracted to the positive charge of the protons. Protons and neutrons can be broken down into smaller particles called quarks. All atoms of the same element have the same number of protons. The number of protons is the atomic number (written in subscript to the left of the atomic symbol). Unless otherwise noted, the number of protons equals the number of electrons. An atom is usually neutral in charge since the positive and negative charges are equal.

We can determine the number of neutrons by using the mass number which is the sum of the protons and neutrons (written as a superscript to the left of the atomic symbol). The number of protons is fixed, but the number of neutrons can vary within the same element. Thus, the same element may have different atomic masses. Atoms of the same element that have different atomic masses are called isotopes:

Hydrogen: 1p, 1e- Deuterium:1p, 1n, 1e- Tritium: 1p, 2n, 1e-

1 amu 2 amu 3 amu

Some combinations of protons and neutrons are stable, but other combinations are internally unstable and break down spontaneously. When this happens, the atom releases various subatomic particles and radiation. These isotopes are called radioactive isotopes.

D. ELECTRON ORBITALS

Electrons move in undefined paths in regions around the nucleus, called orbitals (orbitals are merely a volume in which the electron is probably moving). Only two electrons can occupy the same orbital. Electrons move to the orbital that is lowest in energy, usually closet to the nucleus. There are other regions called energy levels that contain orbitals. The energy level closet to the nucleus contains one orbital. The second energy level holds four orbitals and the third level also contains four orbitals.

• The first energy level can hold up to two electrons. (1s)

• The second energy level can hold up to eight electrons. (2s, 2p)

• The third energy level can hold up to eight electrons. (3s, 3p)

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