The Chemical Context of Life

Introduction

Understanding the chemical basis of life is essential for studying biology. All living organisms are composed of matter, which is organized into elements, atoms, and molecules. The unique properties of these chemical components underlie the structure and function of biological systems.

Concept 2.1: Matter, Elements, and Compounds

Definitions and Key Concepts

• Matter: Anything that takes up space and has mass. All organisms are composed of matter.

• Element: A substance that cannot be broken down into other substances by chemical reactions. Each element is defined by its unique type of atom.

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

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

Example: Sodium (Na) is a highly reactive metal, and chlorine (Cl) is a poisonous gas. When combined, they form sodium chloride (NaCl), or table salt, which is safe to eat. This demonstrates the emergent properties of compounds.

Elements in the Human Body

Most of the human body is composed of a small number of elements. The following table summarizes the major elements and their relative abundance:

Note: Trace elements such as boron (B), chromium (Cr), cobalt (Co), copper (Cu), and others are required in very small amounts (less than 0.01% of body mass) but are essential for life.

Concept 2.2: Atomic Structure and Element Properties

Atoms and Subatomic Particles

Each element consists of unique atoms, which are the smallest units of matter that retain the properties of the element.

• Neutrons: Subatomic particles with no electrical charge.

• Protons: Subatomic particles with a positive charge.

• Electrons: Subatomic particles with a negative charge.

Atoms have a dense nucleus containing protons and neutrons, surrounded by a cloud of electrons.

Atomic Number, Mass Number, and Atomic Mass

• Atomic Number (Z): The number of protons in the nucleus of an atom. This defines the element.

• Mass Number: The sum of protons and neutrons in the nucleus.

• Atomic Mass: The total mass of an atom, approximately equal to the mass number (measured in atomic mass units, amu).

Example: Helium (He) has an atomic number of 2 and a mass number of 4.

Isotopes

• All atoms of an element have the same number of protons but may differ in the number of neutrons.

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

• Radioactive Isotopes: Unstable isotopes that decay spontaneously, emitting particles and energy.

Applications: Radioactive isotopes are used in medicine as tracers and in radiometric dating to determine the age of fossils and rocks.

Concept 2.3: Chemical Bonds and Molecular Formation

Types of Chemical Bonds

• Covalent Bonds: Atoms share pairs of valence electrons. Can be single, double, or triple bonds depending on the number of shared electron pairs.

• Electronegativity: The tendency of an atom to attract electrons in a covalent bond. Differences in electronegativity lead to:

  • Nonpolar Covalent Bonds: Electrons are shared equally.

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

• Ionic Bonds: Formed when one atom transfers electrons to another, resulting in oppositely charged ions (cations and anions) that attract each other.

Example: Sodium (Na) donates an electron to chlorine (Cl), forming Na+ and Cl-, which combine to form NaCl (table salt).

Weak Chemical Interactions

• Hydrogen Bonds: Weak attractions between a hydrogen atom covalently bonded to an electronegative atom (like oxygen or nitrogen) and another electronegative atom.

• Van der Waals Interactions: Weak attractions due to transient local partial charges when electrons are distributed asymmetrically in molecules.

Weak interactions are crucial for the three-dimensional structure and function of large biological molecules, such as proteins and DNA.

Molecular Shape and Function

• The shape of a molecule is determined by the arrangement of its atoms and the chemical bonds between them.

• Molecular shape is critical for biological recognition and function, such as enzyme-substrate interactions and hormone-receptor binding.

Concept 2.4: Chemical Reactions

Making and Breaking Bonds

• Chemical Reactions: Processes that make and break chemical bonds, transforming reactants into products.

• Reactants: Starting materials in a chemical reaction.

• Products: Substances formed as a result of a chemical reaction.

Example: The formation of water from hydrogen and oxygen:

Chemical Equilibrium

• Most chemical reactions are reversible; products can revert to reactants.

• Chemical Equilibrium: The point at which the forward and reverse reactions occur at the same rate, and the concentrations of reactants and products remain stable (not necessarily equal).

Additional info: At equilibrium, the system is dynamic, with continuous formation and breakdown of products and reactants, but no net change in their concentrations.