Chapter 2: The Chemical Context of Life

Overview: A Chemical Connection to Biology

Biology is deeply intertwined with chemistry, as living organisms are governed by the fundamental laws of physics and chemistry. Understanding the chemical basis of life is essential for comprehending biological processes.

• Biology as a multidisciplinary science: Integrates principles from physics and chemistry.

• Living organisms: Composed of matter and subject to chemical laws.

Concept 2.1: Matter Consists of Chemical Elements in Pure Form and in Combinations Called Compounds

Matter is anything that occupies space and has mass. All living organisms are composed of matter, which is made up of elements and compounds.

• Element: A substance that cannot be broken down to other substances by chemical reactions.

• Compound: A substance consisting of two or more elements combined in a fixed ratio.

• Example: Table salt (sodium chloride, NaCl) is a compound made from sodium and chlorine elements.

Elements and Compounds

Elements are the building blocks of matter, while compounds are combinations of elements with unique properties.

• Elements: 92 naturally occurring; each has unique properties.

• Compounds: Exhibit characteristics different from their constituent elements.

The Elements of Life

Only a small subset of elements are essential for life. These elements are found in all living organisms and are critical for biological processes.

• About 20–25% of the 92 elements are essential to life.

• Main elements: Carbon (C), oxygen (O), nitrogen (N), and hydrogen (H) make up 96% of living matter.

• Other important elements: Calcium (Ca), phosphorus (P), potassium (K), sulfur (S).

• Trace elements: Required in minute quantities (e.g., iron, iodine).

Concept 2.2: An Element’s Properties Depend on the Structure of Its Atoms

Atoms are the smallest units of an element that retain its properties. The structure of atoms determines the chemical behavior of elements.

• Each element consists of unique atoms.

• Atoms are composed of subatomic particles: protons, neutrons, and electrons.

Subatomic Particles

Atoms are made up of three main subatomic particles, each with distinct properties.

• Protons: Positive charge, found in the atomic nucleus.

• Neutrons: No charge, found in the atomic nucleus.

• Electrons: Negative charge, orbit the nucleus.

Atomic Number and Atomic Mass

The atomic number and atomic mass are fundamental properties of atoms, determining their identity and mass.

• Atomic number: Number of protons in the nucleus.

• Mass number: Sum of protons and neutrons.

• Atomic mass: Approximate total mass of an atom.

Formula:

Isotopes

Isotopes are variants of elements with different numbers of neutrons. Some isotopes are radioactive and have important biological applications.

• Isotopes: Atoms of the same element with different numbers of neutrons.

• Radioactive isotopes: Decay spontaneously, emitting particles and energy.

• Applications: Biological research, medical diagnostics (e.g., PET scans).

The Energy Levels of Electrons

Electrons occupy energy levels or shells around the nucleus. The arrangement of electrons influences chemical reactivity.

• Energy: Capacity to cause change.

• Potential energy: Energy due to location or structure.

• Electron shells: Electrons are found in shells with different energy levels.

• Valence shell: Outermost shell, determines chemical properties.

Electron Distribution and Chemical Properties

The distribution of electrons in an atom’s electron shells determines its chemical behavior and reactivity.

• Periodic table arrangement reflects electron distribution.

• Elements with full valence shells are chemically inert.

Electron Orbitals

Orbitals are three-dimensional spaces where electrons are found most of the time. Each shell contains a specific number of orbitals.

• First shell: 1 s orbital (holds 2 electrons).

• Second shell: 1 s orbital and 3 p orbitals (holds 8 electrons).

• Electron configuration: Determines chemical bonding.

Concept 2.3: The Formation and Function of Molecules Depend on Chemical Bonding Between Atoms

Atoms with incomplete valence shells can interact with other atoms to form chemical bonds, resulting in molecules with specific properties.

• Chemical bonds: Attractions that hold atoms together.

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

Covalent Bonds

Covalent bonds involve the sharing of electron pairs between atoms. They are the strongest bonds in biological molecules.

• Single bond: Sharing of one pair of electrons.

• Double bond: Sharing of two pairs of electrons.

• Structural formula: Represents bonding (e.g., H—H for H2).

• Molecular formula: Shows number of atoms (e.g., H2).

Electronegativity and Polar Covalent Bonds

Electronegativity is an atom’s ability to attract electrons in a covalent bond. Differences in electronegativity lead to polar and nonpolar covalent bonds.

• Nonpolar covalent bond: Electrons shared equally.

• Polar covalent bond: Electrons shared unequally, creating partial charges.

• Electronegativity: Higher electronegativity means stronger pull on electrons.

Ionic Bonds

Ionic bonds form when electrons are transferred from one atom to another, resulting in charged ions that attract each other.

• Cation: Positively charged ion.

• Anion: Negatively charged ion.

• Ionic compound: Formed by ionic bonds (e.g., NaCl).

• Crystals: Ionic compounds often form crystalline structures.

Weak Chemical Bonds

Weak bonds, such as hydrogen bonds and van der Waals interactions, play crucial roles in the structure and function of biological molecules.

• Hydrogen bonds: Attraction between a hydrogen atom and an electronegative atom.

• Van der Waals interactions: Weak attractions due to transient charge differences.

• Help maintain shapes of large molecules and facilitate interactions.

Molecular Shape and Function

The shape of a molecule is critical to its function in biological systems. Molecular shape is determined by the positions of atoms’ orbitals and the types of bonds formed.

• Shape: Determines how molecules interact with each other.

• Hybridization: Orbitals may hybridize, creating specific shapes.

• Example: The shape of water (H2O) is bent due to its polar covalent bonds.

Additional info: These notes expand upon the original slides by providing definitions, examples, and context for key concepts in the chemical basis of life, suitable for introductory college biology students.