Five Fundamental Characteristics of Living Things

Introduction to Biology

Biology is the scientific study of life. To distinguish living organisms from non-living forms, biologists identify several key characteristics shared by all living things.

• Cells: All organisms are composed of membrane-bound cells, which are the basic units of life.

• Energy: Living organisms acquire and utilize energy to carry out essential functions.

• Information: Organisms process hereditary information encoded in genes and respond to environmental signals.

• Replication: All organisms are capable of reproduction, ensuring the continuation of their species.

• Evolution: Populations of organisms undergo evolutionary changes over time.

Cells and Molecules

Key Molecules and Molecular Structures in Cells

Understanding cellular function requires knowledge of the molecules and molecular structures that compose cells. The major classes of biological molecules include:

• Atoms & Bonds to Molecules: The basic building blocks of matter, forming molecules through chemical bonds.

• Water: Essential for life, with unique properties due to its molecular structure.

• Proteins and Amino Acids: Serve as enzymes, structural components, and signaling molecules.

• Nucleic Acids (RNA & DNA): Store and transmit genetic information.

• Lipids & Membranes: Form cellular membranes and store energy.

• Sugars & Carbohydrates: Provide energy and structural support.

Example: An average human is made up of about 50 trillion cells, each containing proteins, lipids, carbohydrates, and nucleic acids.

Living Organisms and Matter

Elements and Atoms

All living organisms are composed of matter, which consists of elements. Elements are pure substances made up of only one type of atom.

• Matter: Anything that has mass and occupies space.

• Elements: Fundamental units of matter, each represented by a unique symbol (e.g., O for oxygen, C for carbon).

• Atoms: The smallest identifiable unit of an element.

Of the 118 known elements, only a few are essential for life. Approximately 99% of body weight consists of six elements: oxygen, carbon, hydrogen, nitrogen, calcium, and phosphorus.

Atomic Structure

Components of the Atom

Atoms consist of a nucleus containing protons and neutrons, surrounded by electrons in energy shells.

• Protons: Positively charged particles in the nucleus; the number of protons defines the atomic number.

• Neutrons: Neutral particles in the nucleus; together with protons, they determine the atomic mass.

• Electrons: Negatively charged particles that orbit the nucleus in energy levels (shells).

Bohr Model: Electrons occupy specific energy shells around the nucleus, forming an electron cloud.

Atomic Number: Number of protons in the nucleus.

Mass Number: Sum of protons and neutrons.

Isotopes: Atoms of the same element with different numbers of neutrons, resulting in different masses.

Periodic Table of Elements

Organization and Classification

The periodic table organizes elements based on their atomic structure and properties. Dmitri Mendeleev developed the first periodic table, arranging elements by increasing atomic number and recurring chemical properties.

• Rows (Periods): Indicate energy levels of electrons.

• Columns (Groups): Elements with similar chemical properties.

Example: Elements essential for life (C, H, N, O, P, S) are found in specific regions of the periodic table.

Electron Arrangement and Chemical Bonds

Valence Electrons and Bond Formation

The arrangement of electrons, especially in the outermost shell (valence shell), determines how atoms interact and bond to form molecules.

• Valency: The number of unpaired electrons in the valence shell, indicating bonding capacity.

• Atoms are most stable when their valence shells are full.

Example: Nitrogen has three unpaired electrons (valency 3); oxygen has two (valency 2).

Types of Chemical Bonds

Covalent, Ionic, and Hydrogen Bonds

Atoms bond together to achieve stable electron configurations, forming molecules through different types of chemical bonds:

• Covalent Bonds: Atoms share unpaired electrons in their valence shells. These are the strongest bonds, forming stable molecules. Covalent bonds can be single, double, or triple, depending on the number of shared electron pairs.

• Nonpolar Covalent Bonds: Electrons are shared equally between atoms (e.g., H2).

• Polar Covalent Bonds: Electrons are shared unequally, resulting in partial charges (e.g., H2O).

• Ionic Bonds: Atoms transfer electrons, resulting in charged ions (cations and anions) that attract each other.

• Hydrogen Bonds: Weak attractions between molecules or parts of molecules with partial charges, important in biological systems (e.g., between water molecules, DNA strands).

• Van der Waals Interactions: Weak electrical bonds formed by momentary attractions due to electron movement.

Electronegativity and Bond Polarity

Determining Electron Sharing

Electronegativity is the tendency of an atom to attract electrons in a chemical bond. It is influenced by:

• Number of protons in the nucleus (more protons = higher electronegativity)

• Distance of valence shell from the nucleus (closer = higher electronegativity)

Differences in electronegativity between atoms lead to polar covalent bonds, where electrons are not shared equally.

Atoms, Ions, and Molecules

Formation and Properties

Atoms can gain or lose electrons to form ions:

• Anion: Atom that has gained electrons, resulting in a net negative charge.

• Cation: Atom that has lost electrons, resulting in a net positive charge.

Molecules are formed when two or more atoms bond together chemically.

Summary Table: Types of Chemical Bonds

Key Equations and Concepts

• Atomic Number:

• Mass Number:

• Electronegativity:

Applications and Examples

• Water: The polar covalent bonds in water molecules result in hydrogen bonding, which gives water its unique properties essential for life.

• DNA: Hydrogen bonds hold together the two strands of the DNA double helix.

• Proteins: Covalent and hydrogen bonds determine protein structure and function.

Additional info: These notes expand on the brief points in the provided materials, offering definitions, examples, and academic context suitable for college-level General Biology students.