Lecture 3

Unity and Diversity of Life

Evolution as a Unifying Theme

Biology is the study of life, which is characterized by both remarkable unity and extraordinary diversity. The theory of evolution provides a framework for understanding how all living organisms are related and how they have diversified over time.

• Unity of Life: All living things share fundamental characteristics, such as the use of DNA as genetic material, cellular structure, and similar biochemical pathways.

• Diversity of Life: Life on Earth is incredibly diverse, with millions of species adapted to different environments and lifestyles.

• Evolution: The process by which populations of organisms change over generations. Evolution explains both the similarities (unity) and differences (diversity) among living organisms.

Example: The finches of the Galápagos Islands, studied by Charles Darwin, exhibit a variety of beak shapes and sizes adapted to different food sources, illustrating both unity (common ancestry) and diversity (adaptation to different environments).

Darwin's Theory of Evolution by Natural Selection

Charles Darwin proposed that evolution occurs through the mechanism of natural selection, which explains how species adapt and diversify.

• Three Main Points of Darwin's Theory:

  1. Individuals in a population vary in their traits.

  2. More offspring are produced than can survive, leading to competition for resources.

  3. Individuals with advantageous traits are more likely to survive and reproduce, passing those traits to the next generation.

• Speciation: The process by which new species arise, often as a result of populations becoming isolated and evolving independently.

Example: The evolution of different finch species on the Galápagos Islands from a common ancestor, each adapted to specific ecological niches.

The Chemical Basis of Life

Atoms, Elements, Compounds, and Molecules

All matter, including living organisms, is composed of atoms, which combine to form elements, compounds, and molecules.

• Atom: The smallest unit of an element that retains the properties of that element.

• Element: A substance that cannot be broken down into other substances by chemical means. Each element is defined by its number of protons.

• Compound: A substance formed when two or more elements are chemically combined in fixed ratios.

• Molecule: Two or more atoms held together by covalent bonds.

Example: Water (H2O) is a compound and a molecule, consisting of two hydrogen atoms and one oxygen atom.

Elements Essential to Life

Living organisms are primarily composed of a few key elements.

• Four Most Common Elements in Living Matter: Carbon (C), Hydrogen (H), Oxygen (O), and Nitrogen (N).

• Essential Elements: Elements required for an organism to survive, grow, and reproduce.

• Trace Elements: Elements required by an organism in minute quantities (e.g., iron, iodine).

Subatomic Particles and Atomic Structure

Atoms are composed of three main subatomic particles:

• Protons: Positively charged particles found in the nucleus. The number of protons defines the atomic number and the element.

• Neutrons: Neutral particles found in the nucleus. The number of neutrons can vary, resulting in different isotopes of an element.

• Electrons: Negatively charged particles that orbit the nucleus in electron shells.

Atomic Nucleus: The central core of the atom, containing protons and neutrons.

Atomic Number, Mass Number, and Isotopes

• Atomic Number (Z): The number of protons in an atom's nucleus. Determines the element.

• Mass Number (A): The total number of protons and neutrons in the nucleus.

• Isotopes: Atoms of the same element with different numbers of neutrons. Some isotopes are stable, while others are radioactive.

Example: Carbon-12 and Carbon-14 are isotopes of carbon, with 6 protons but 6 and 8 neutrons, respectively.

Electron Arrangement and Chemical Properties

The arrangement of electrons in shells around the nucleus determines an atom's chemical behavior.

• Electron Shells: Energy levels where electrons are found. The first shell holds up to 2 electrons, the second up to 8, and so on.

• Valence Electrons: Electrons in the outermost shell. They determine how an atom interacts with other atoms.

• Octet Rule: Atoms tend to gain, lose, or share electrons to achieve a full outer shell, usually 8 electrons.

Example: Oxygen has 6 valence electrons and needs 2 more to fill its outer shell, so it tends to form two bonds.

Periodic Table and Chemical Bonds

The periodic table organizes elements by increasing atomic number and similar chemical properties.

• Groups: Columns in the periodic table; elements in the same group have similar valence electron configurations.

• Periods: Rows in the periodic table; elements in the same period have the same number of electron shells.

• Chemical Bonds: Atoms form bonds to achieve stable electron configurations. The main types are ionic, covalent, and hydrogen bonds.

Table: Comparison of Subatomic Particles

Particle	Charge	Location	Relative Mass
Proton	+1	Nucleus	1
Neutron	0	Nucleus	1
Electron	-1	Electron shells	1/1840

Key Formulas and Equations

• Atomic Number: $Z = \text{number of protons}$

• Mass Number: $A = \text{number of protons} + \text{number of neutrons}$

Valence Electrons and Bonding

The number of valence electrons determines how many bonds an atom can form.

• Hydrogen: 1 valence electron; forms 1 bond

• Oxygen: 6 valence electrons; forms 2 bonds

• Nitrogen: 5 valence electrons; forms 3 bonds

• Carbon: 4 valence electrons; forms 4 bonds

Summary Table: Common Elements in Living Matter

Element	Symbol	Atomic Number	Valence Electrons	Typical Number of Bonds
Hydrogen	H	1	1	1
Oxygen	O	8	6	2
Nitrogen	N	7	5	3
Carbon	C	6	4	4

Additional info: Radioactive isotopes are used in medicine (e.g., cancer treatment, medical imaging) and in dating fossils. The only subatomic particle directly involved in chemical reactions is the electron, specifically the valence electrons.