Chapter 2:

The Chemical Basis of Life

All living things are made up of chemical elements that interact to form molecules essential for life. The properties of atoms, the nature of chemical bonds, and the role of water in biological systems are fundamental to understanding how life functions at a molecular level.

Atoms and Elements

Biology is the study of life and living organisms.

Organisms share seven key characteristics of life:

Cellular Organization – All living things are made of one or more cells.
Ordered Complexity – Life is highly organized and structured.
Sensitivity – Living things respond to stimuli (e.g., plants grow toward light).
Growth, Development, and Reproduction – Organisms grow and pass genetic material to offspring.
Energy Utilization – Life requires energy (from food, sunlight, or chemicals).
Homeostasis – Organisms maintain stable internal conditions.
Evolutionary Adaptation – Life evolves to survive in changing environments.

Biological Importance: Organization of Life

Atoms combine to form elements, and only 12 elements are found in significant amounts in living systems, including

oxygen (O), carbon (C), hydrogen (H), nitrogen (N), phosphorus (P), and sulfur (S).

Chemical Bonds and Molecules

Ionic Bonds: Electrons are transferred from one atom to another, creating charged ions (e.g., NaCl – table salt).

Covalent Bonds: Atoms share electrons to form molecules (e.g., H₂O – water).

Nonpolar covalent bonds: Equal sharing of electrons (e.g., O₂).
Polar covalent bonds: Unequal sharing of electrons, creating partial charges (e.g., H₂O).

Hydrogen Bonds: Weak attractions between molecules, important in DNA and water properties.

Water: The Essential Molecule of Life

Water is a polar molecule, meaning it has a partial positive charge (H side) and a partial negative charge (O side). This allows for hydrogen bonding, leading to water’s unique properties:

Cohesion & Adhesion: Water molecules stick to each other (cohesion) and to other surfaces (adhesion), allowing capillary action in plants.
High Specific Heat: Water absorbs heat slowly and retains heat longer, helping regulate body temperature and climate stability.
High Heat of Vaporization: It takes a lot of energy to evaporate water, allowing for cooling effects like sweating.
Density of Ice: Ice is less dense than liquid water, meaning it floats and insulates aquatic ecosystems.
Water as a Solvent: Water dissolves polar and ionic substances, making it the "universal solvent" in biological systems.

Acids, Bases, and pH

source: https://www.sciencelearn.org.nz/images/4557-ph-scale

Acids donate H⁺ ions, increasing hydrogen ion concentration (pH < 7).

Bases remove H⁺ ions or add OH⁻ ions, decreasing hydrogen ion concentration (pH > 7).

Buffers help maintain a stable pH in organisms by neutralizing acids or bases.

Why This Matters?

Understanding chemical interactions helps explain how life works at the molecular level, from DNA replication to digestion.
Water’s unique properties make it essential for all biological processes.

Biological Importance : Cells must maintain a pH balance to prevent damage to proteins and enzymes. Blood uses bicarbonate buffers to maintain a stable pH (~7.4).

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