Essentials of Biology Sylvia S. Mader Chapter 2 Lecture Outline Prepared by: Dr. Stephen Ebbs Southern Illinois University Carbondale Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

2.1 The Nature of Matter • Matter refers to anything that takes up space and has mass. Matter is composed of elements, substances that cannot be broken down into another substance. There are 92 natural elements. Living organisms are made primarily of six elements.

2.1 The Nature of Matter (cont.)

Atomic Structure The atomic theory states that elements are made of tiny particles called atoms. The name of an atom is represented by the atomic symbol. Hydrogen = H Sodium = Na Chloride = Cl

Atomic Structure (cont.) Atoms are composed of three smaller particles. – Neutrons, which have no electrical charge. – Protons, which have a positive charge. – Electrons, which have a negative charge. Atoms have a mass number determined by the weight of the neutrons and protons.

Atomic Structure (cont.)

Atomic Structure (cont.) All atoms of an element have the same number of protons, the atomic number. The atomic number and mass number are often included with the chemical symbol. 12 6 C Mass number Atomic number Chemical symbol

The Periodic Table The elements are organized to form the periodic table. The columns in the table are groups. The rows in the table are periods. Elements in groups have similar chemical and physical characteristics.

The Periodic Table

Isotopes The atomic number indicates the number of protons in an atom. The atomic mass is the sum of the number of protons and neutrons. If an atom has the same number of protons but a different number of neutrons it is an isotope.

Isotopes (cont.) The nucleus of an isotope can be unstable and may disintegrate, or decay. Decaying isotopes emit radiation. Radiation can be detected with instruments such as a Geiger counter.

Uses of Radioactive Isotopes Radioactive isotopes can be used as tracers to follow the movement of that element. Radioactive isotopes can be used to sterilize medical and dental instruments. Radioactive isotopes can also be used to kill cancer cells.

Uses of Radioactive Isotopes (cont.)

Uses of Radioactive Isotopes (cont.)

Arrangements of Electrons in an Atom Electrons encircle the nucleus of an atom at discrete energy levels called electron shells. In atoms with two or more shells, the outer shell follows the octet rule (8 electrons) The electrons in the outer valence shell determine the chemical reactivity of atoms.

Arrangements of Electrons in an Atom (cont.)

Types of Chemical Bonds A group of atoms bonded to one another form a molecule. If the molecule has more than one type of element present it is a compound. Different types of bonds hold molecules and compounds together.

Types of Chemical Bonds (cont.) Charged atoms, or ions, can form when atoms lose or gain electrons. Positive and negative ions are attracted to one another and bond together in ionic bonds. A salt is a dry solid composed of atoms connected by ionic bonds.

Types of Chemical Bonds (cont.)

Types of Chemical Bonds (cont.) A covalent bond results when two atoms share electrons, thereby completing their valence shells. When molecules contain covalent bonds, the structure of the molecule can be drawn with a formula or model.

Types of Chemical Bonds (cont.)

Chemical Reactions When molecules or compounds are chemically changed it is called a chemical reaction. Photosynthesis is an example of a chemical reaction.

Chemical Reactions (cont.) 6 CO2 + 6 H2O  C6H12O6 + 6 O2 carbon dioxide water oxygen glucose The chemical reaction for photosynthesis Molecules that participate in a reaction are reactants. Molecules formed by a reaction are products.

2.2 Water’s Importance to Life Water is the single most important molecule of earth. All organisms are 70-90% water. Water has unique properties that make it a life-supporting substance.

The Structure of Water Atoms differ in their electronegativity, or their affinity for electrons in a covalent bond. The unequal sharing of electrons in a molecule such as water makes the molecule polar. Polar water molecules are attracted to one another and can form hydrogen bonds.

The Structure of Water (cont.)

Properties of Water Water is a solvent that can dissolve many substances. Molecules that attract water are hydrophilic. Molecules that cannot attract water are hydrophobic. Water dissolves polar nonionic substances, ions, and some nonpolar gases.

Properties of Water (cont.) Water shows cohesion and adhesion. When water molecules cling together with hydrogen bonds it is called cohesion. When water molecules adhere to polar surfaces it is called adhesion.

Properties of Water (cont.)

Properties of Water (cont.) Water also has a high surface tension. The stronger the force between molecules in a liquid, the stronger the surface tension.

Properties of Water (cont.)

Properties of Water (cont.) Water has a high heat capacity, protecting organisms from temperature changes. Water has a high heat of vaporization. These properties of water keep temperatures compatible with those of living organisms.

Properties of Water (cont.) Frozen water (ice) is less dense than liquid water, so ice floats. Unlike other substances, water expands as it freezes.

Properties of Water (cont.)

Acids and Bases When water dissociates, it releases an equal number of ions. Hydrogen ions (H+) Hydroxide ions (OH-) H – O – H H+ + OH-

Acids and Bases (cont.) • Acidic solutions have a high H+ concentration. An acid is a substance that releases H+ when dissolved in water. HCl H+ + Cl- Hydrochloric acid

Acids and Bases (cont.)

Acids and Bases (cont.) • Basic solutions have a low H+ concentration. A base is a substance that releases OH- when dissolved in water. NaOH Na+ + OH- Sodium hydroxide

Acids and Bases (cont.)

pH and the pH scale The pH is a mathematical way of indicating the number of H+ ions in a solution. The pH scale is used to express acidity or basicity (alkalinity).

Buffers and pH Within the body, the pH is kept in a narrow range to maintain health. A buffer is a chemical or combination of chemicals that keeps a pH within a given range. Buffers resist changes in pH by taking up extra H+ or OH- from solution.

Buffers and pH (cont.)