Water and Hydrogen Bonding
Water as a Polar Molecule
Water is a polar molecule because the electrons are not shared equally between the oxygen and hydrogen atoms. Oxygen is more electronegative, so electrons linger closer to oxygen, creating a partial negative region around the oxygen and partial positive regions around the hydrogens. This can be represented with partial charges: ext{O}^{\, ext{δ}^-} ext{- } ext{H}^{\ ext{δ}^+}. The polarity of water gives rise to hydrogen bonding and to many of water’s unique properties, including its behavior as a solvent and its role in biology.
Hydrogen Bonds: Intermolecular vs Intramolecular
Hydrogen bonds are intermolecular bonds, meaning they form between molecules rather than within a single molecule’s covalent or ionic bonds. The bond forms when the hydrogen atom that is covalently bonded to an electronegative atom (such as oxygen or nitrogen) experiences attraction to a lone pair on another electronegative atom in a neighboring molecule. In water, the hydrogen attached to one water molecule (H) is attracted to the lone pairs on the oxygen of a neighboring water molecule, producing an interaction roughly described as ext{O}^{\delta-}- ext{H}^{\delta+} \cdots \text{O}^{\delta-}. Hydrogen bonds are weaker than covalent bonds and ionic bonds, reflecting their nature as relatively weak, noncovalent attractions. They are nonetheless crucial because, collectively, they produce the structural and functional properties of many biological systems. The transcript notes that hydrogen bonds can form between other electronegative atoms as well, such as nitrogen in various molecules. A concrete biological example is the hydrogen bonding between nitrogenous bases in DNA, described next.
Hydrogen Bonds in Biology
Hydrogen bonds are not limited to water-water interactions; they form between hydrogen and electronegative atoms in many contexts. A prominent example is the hydrogen bonding between adenine and thymine in DNA. The nitrogenous bases pair through hydrogen bonds, holding the two DNA strands together. Specifically, the adenine–thymine (A–T) base pair is stabilized by 2 hydrogen bonds. These bonds are essential to the structure of DNA, RNA, and proteins, and they recur throughout biology and on exams like the AP Biology exam.
Consequences of Hydrogen Bonding: Cohesion, Adhesion, Surface Tension
Cohesion
Cohesion is the attraction between like molecules, such as water-water interactions, which arise from hydrogen bonding between water molecules. This cohesion leads to phenomena such as water droplets forming on surfaces and the ability of water to form a continuous, intact column when moving as a liquid. Cohesion is also a key contributor to surface tension, since molecules at the surface are pulled inward by hydrogen bonding with their neighbors.
Adhesion
Adhesion is the attraction between water and unlike substances, such as water sticking to glass or plant cell walls. Adhesion enables water to cling to surfaces and contributes to capillary action when combined with cohesion. For example, water can rise in thin tubes due to the combined effects of cohesive and adhesive forces, a crucial mechanism in plants for moving water from roots to leaves. The balance of cohesion and adhesion also influences the shape of the meniscus (concave or convex) in a tube.
Surface Tension
Surface tension is the energy required to increase the surface area of a liquid, ultimately arising from cohesive forces at the air–water interface. Because water molecules at the surface have fewer neighboring molecules, the remaining hydrogen-bonding interactions pull inward, creating a “film-like” surface. This makes water have a relatively high surface tension, enabling phenomena such as droplets forming on surfaces and certain organisms or objects to interact with the water surface in characteristic ways.
Key Takeaways and Real-World Relevance
Water’s polarity (O δ− and H δ+) drives the formation of hydrogen bonds, which are weaker than covalent or ionic bonds but crucial for the structure of many biomolecules. The hydrogen bonds can form between water molecules and between other electronegative atoms (e.g., in DNA base pairing).
Hydrogen bonding underpins cohesion, adhesion, and surface tension, linking molecular interactions to macroscopic properties like droplet formation, capillary action, and the movement of water in plants.
The A–T base pair in DNA is stabilized by two hydrogen bonds, illustrating how hydrogen bonding contributes to the architecture of genetic material.
These concepts are foundational for biology coursework and standardized exams such as AP Biology, and they illustrate how molecular-scale interactions shape biological structure and function. On