Chapter 2: The Chemical Context of Life

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

All living organisms are composed of matter, which is anything that takes up space and has mass. Matter is made up of elements, and these elements can combine to form compounds with unique properties.

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

• Compound: A substance consisting of two or more elements in a fixed ratio. Compounds have emergent properties different from their constituent elements.

• Emergent Properties: New characteristics that arise when elements combine to form compounds.

The Elements of Life

Of the 92 naturally occurring elements, only a small fraction are essential for life. The majority of living matter is composed of just a few elements.

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

• Major Elements: Carbon (C), Hydrogen (H), Oxygen (O), and Nitrogen (N) make up about 96% of living matter.

• Other Important Elements: Calcium (Ca), Phosphorus (P), Potassium (K), and Sulfur (S) make up most of the remaining 4%.

• Trace Elements: Required in minute quantities (e.g., iron, zinc).

Case Study: Evolution of Tolerance to Toxic Elements

Some elements can be toxic, but certain species have adapted to survive in environments containing these elements. For example, some plants thrive in serpentine soils, which are rich in heavy metals.

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

Each element consists of unique atoms, which are the smallest units of matter that retain the properties of the element. Atoms are composed of subatomic particles: protons, neutrons, and electrons.

• Proton: Positively charged particle found in the nucleus; determines the atomic number.

• Neutron: Electrically neutral particle found in the nucleus; contributes to atomic mass.

• Electron: Negatively charged particle orbiting the nucleus; determines chemical behavior.

Atomic Number and Atomic Mass

• Atomic Number (Z): Number of protons in the nucleus; unique to each element.

• Mass Number (A): Sum of protons and neutrons in the nucleus.

• Atomic Mass: Approximate total mass of an atom, measured in daltons.

Isotopes and Radioactivity

Atoms of the same element can have different numbers of neutrons, resulting in isotopes. Some isotopes are unstable (radioactive) and decay over time, releasing energy.

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

• Radioactive Isotope: An isotope that decays spontaneously, emitting radiation.

• Applications: Radioactive tracers in medicine, radiometric dating of fossils.

Radiometric Dating

Radiometric dating uses the decay of radioactive isotopes to estimate the age of rocks and fossils. The rate of decay is expressed as the isotope's half-life.

• Half-life: The time required for half of the radioactive atoms in a sample to decay.

The Energy Levels of Electrons

Electrons have potential energy due to their position relative to the nucleus. They occupy specific energy levels, or shells, and can move between shells by absorbing or releasing energy.

• Potential Energy: Energy that matter possesses due to its location or structure.

• Electron Shells: Levels where electrons are found, each with a characteristic energy.

Electron Distribution and Chemical Properties

The arrangement of electrons in shells determines an atom's chemical behavior. The periodic table reflects the electron configurations of elements.

Valence Electrons and Chemical Reactivity

• Valence Electrons: Electrons in the outermost shell; determine chemical reactivity.

• Inert Elements: Elements with full valence shells are chemically unreactive (e.g., noble gases).

Electron Orbitals

Orbitals are three-dimensional spaces where electrons are likely to be found. Each shell contains a specific number of orbitals, and each orbital holds up to two electrons.

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

Atoms with incomplete valence shells can share or transfer electrons, forming chemical bonds. These bonds hold atoms together in molecules and compounds.

Covalent Bonds

Covalent bonds involve the sharing of valence electrons between atoms. Molecules are formed when two or more atoms are held together by covalent bonds.

• Single Bond: Sharing of one pair of electrons.

• Double Bond: Sharing of two pairs of electrons.

• Valence: Bonding capacity of an atom, usually equal to the number of unpaired electrons in the valence shell.

• Electronegativity: The attraction of an atom for electrons in a covalent bond.

• Nonpolar Covalent Bond: Electrons are shared equally.

• Polar Covalent Bond: Electrons are shared unequally, resulting in partial charges.

Ionic Bonds

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

• Cation: Positively charged ion (lost electrons).

• Anion: Negatively charged ion (gained electrons).

• Ionic Compound (Salt): Compound formed by ionic bonds, often found as crystals (e.g., NaCl).

Weak Chemical Interactions

In addition to strong covalent and ionic bonds, weak interactions such as hydrogen bonds and van der Waals interactions play crucial roles in biological systems.

• Hydrogen Bond: Attraction between a hydrogen atom covalently bonded to an electronegative atom and another electronegative atom.

• Van der Waals Interactions: Weak attractions between molecules due to transient local charges.

Molecular Shape and Function

The shape of a molecule is determined by the positions of its atoms' orbitals and is critical for its function. Molecular shape influences how molecules interact and recognize each other in biological systems.

• Hybridization of Orbitals: s and p orbitals may combine to form specific shapes (e.g., tetrahedral).

• Molecular Mimicry: Molecules with similar shapes can bind to the same biological receptors (e.g., morphine and endorphins).

Concept 2.4: Chemical Reactions Make and Break Chemical Bonds

Chemical reactions involve the making and breaking of chemical bonds, transforming reactants into products. All chemical reactions are reversible, and equilibrium is reached when the forward and reverse reactions occur at the same rate.

• Reactants: Starting materials in a chemical reaction.

• Products: Substances formed from a chemical reaction.

• Chemical Equilibrium: State where the concentrations of reactants and products remain constant.

Photosynthesis: An Example of a Chemical Reaction

Photosynthesis is a key chemical reaction in biology, converting carbon dioxide and water into glucose and oxygen using sunlight.

• Equation:

Summary of Key Concepts

• Atomic structure determines the properties of elements and their interactions.

• Chemical bonds (covalent, ionic, hydrogen, van der Waals) are essential for the structure and function of biological molecules.

• Chemical reactions rearrange matter and are fundamental to life processes.

Additional info: This chapter provides foundational chemistry concepts essential for understanding biological molecules and processes, forming the basis for subsequent topics in general biology.