Carbon, hydrogen, and oxygen dominate biological macromolecules because their electron structures favor stable covalent bonding, structural diversity, and useful interactions with water, enabling energy storage, information molecules, and cellular architecture.

Why C, H, and O Dominate Biological Molecules

Abundance and Accessibility

• Hydrogen and oxygen are abundant in water, the most common biological solvent and reactant.

• Carbon is widely available in $CO_2$ (air) and dissolved inorganic carbon (aquatic systems), making it a central feedstock for building organic matter.

Covalent Bonding Suits Stable, Complex Structures

Carbon, hydrogen, and oxygen readily form covalent bonds that are stable under typical cellular temperatures and pH, allowing organisms to build large molecules that persist long enough to function but can still be broken down when needed.

Because carbon is tetravalent, it can serve as a versatile backbone while hydrogen and oxygen modify that backbone’s properties (shape, polarity, and reactivity).

Carbon’s four covalent bonds produce predictable 3D geometries (e.g., tetrahedral methane). The figure also contrasts single- versus double-bonded carbon frameworks, showing how bond type changes molecular shape and rotational freedom—key to how carbon skeletons generate structural diversity in biomolecules. Source

Carbon: The Adaptable Backbone Element

Structural Versatility

• Carbon forms single, double, and (less commonly in biology) triple bonds, enabling variation in shape and rigidity.

• Carbon bonds to itself to make chains, branches, and rings, supporting diverse macromolecular architectures.

• Carbon–carbon and carbon–hydrogen bonds are strong enough to be stable but can be rearranged through enzyme-catalysed reactions.

Functional Diversity Through Bonding Partners

By bonding with oxygen and hydrogen, carbon frameworks gain functional groups (for example, hydroxyl-containing regions), creating many distinct molecules without requiring exotic elements.

The figure summarizes major functional groups commonly attached to carbon skeletons and highlights how specific atoms (especially oxygen and hydrogen) alter molecular properties. Seeing these groups side-by-side clarifies why adding O- and H-containing groups can dramatically change polarity, acidity/basicity, and interaction potential in water. Source

Hydrogen: Saturation, Shape, and Energy-Rich Bonds

Completing Carbon Skeletons

• Hydrogen commonly bonds to carbon, “filling” available bonding positions and helping determine 3D shape and flexibility of carbon skeletons.

• The number of hydrogens attached influences whether a region is more reduced (more C–H bonds) or more oxidised (more C–O bonds).

High Potential Energy in C–H Bonds

C–H bonds store substantial chemical energy; when organisms extract energy, electrons from these bonds can be transferred to other molecules in controlled pathways. This helps explain why molecules rich in carbon and hydrogen (especially hydrocarbon regions) are common in energy storage contexts.

Oxygen: Polarity, Solubility, and Controlled Reactivity

Creates Polar Bonds and Hydrogen-Bonding Capacity

Oxygen is highly effective at creating polar covalent bonds (for example, C–O and O–H).

A phospholipid is shown as an amphipathic molecule with a polar, phosphate-containing head and two nonpolar hydrocarbon tails. This visual links oxygen-rich functional groups to hydrophilicity while highlighting how C–H-rich regions remain hydrophobic, explaining lipid behavior in water and membrane formation. Source

These polar regions:

• Increase water solubility of many biomolecules and intermediates

• Provide sites for hydrogen bonding, which stabilises molecular interactions and shapes in aqueous environments

Enables Chemical Reactivity Central to Metabolism

Oxygen-containing groups often form reactive sites where enzymes act, allowing:

• Formation and breakage of bonds during synthesis and breakdown of macromolecules

• Controlled electron transfer in energy-related reactions

How C, H, and O Appear Across Macromolecules (Overview)

Common Patterns that Explain Prevalence

• Carbohydrates: built largely from C, H, and O; many hydroxyl-containing regions make them chemically versatile in water.

• Lipids: contain extensive C–H regions that are energy-rich; comparatively less oxygen often corresponds to lower polarity.

• Proteins and nucleic acids: although they require additional elements, their carbon backbones and many structural/functional features still rely heavily on C–H frameworks and oxygen-containing groups that tune polarity and reactivity.