Chemistry of Life
Explain why water, carbon, pH, and chemical bonds matter in living systems. Connect molecular structure to biological function at the cellular level.
A lesson of Biology, in Biology Foundations and Scientific Practice, about 10 minutes.
By the end of this lesson, you will be able to explain the biological significance of water's properties, describe how carbon's bonding creates molecular diversity, explain pH and its significance, and connect atomic structure to the four classes of biomolecules.
The Big Idea
Life is chemistry. Every metabolic reaction, every structural material, every piece of hereditary information is made of atoms bonded together according to chemical rules. Understanding carbon's versatility and water's unusual properties explains why life on Earth works the way it does — and could not work nearly as well with any other chemistry.
Why Water Matters
Water's unique properties arise from hydrogen bonding between its molecules:
Cohesion: water molecules stick to each other → surface tension, water columns in plants.
Adhesion: water sticks to polar surfaces → capillary action in plant xylem.
High heat capacity: water resists temperature change → stabilizes temperature in cells and ecosystems.
High heat of vaporization: evaporation removes large amounts of heat → sweating cools organisms.
Universal solvent: polar molecules and ions dissolve readily → enables biochemical reactions.
Ice floats: frozen water is less dense than liquid water → lakes freeze from the top, protecting aquatic life.
Carbon: The Backbone of Life
Carbon has 4 valence electrons — it forms 4 covalent bonds, creating chains, branches, and rings of virtually unlimited variety.
Functional groups modify carbon skeletons to create different biomolecule properties:
| Group | Formula | Effect |
|---|---|---|
| Hydroxyl | —OH | Polar, hydrophilic; found in alcohols and sugars |
| Carboxyl | —COOH | Acidic; found in amino acids and fatty acids |
| Amino | —NH₂ | Basic; found in amino acids |
| Phosphate | —PO₄³⁻ | Charged; found in DNA, ATP |
| Methyl | —CH₃ | Nonpolar; affects hydrophobicity |
pH and Life
pH = $-\log_{10}[\text{H}^+]$
- pH 7 = neutral; pH < 7 = acidic; pH > 7 = basic
- Cytoplasm ≈ pH 7.2; stomach ≈ pH 2; blood ≈ pH 7.4
Why pH matters: most enzymes have a narrow optimal pH. Outside this range, the enzyme's shape changes and function is lost.