CourseCourseQ BankQuestion BankDocsDocuments

1.1 Structure of Water and Hydrogen Bonding

Syllabus
2025
Topic
1.1
Level
—

Learning objectives

1.1A—Explain how the properties of water that result from its polarity and hydrogen bonding affect its biological…Explain how the properties of water that result from its polarity and hydrogen bonding affect its biological function.• Living systems depend on the properties of water to sustain life.- i. Water has polarity, because of the formation of polar covalent bonds between hydrogen and oxygen within water molecules. This polarity contributes to hydrogen bonding between and within biological molecules.- ii. Water has a high specific heat capacity, which allows for the maintenance of homeostatic body temperature within living organisms.- iii. Water has a high heat of vaporization, which allows for the evaporative cooling of the surrounding environment. In living organisms, this property allows for body temperature to be maintained.• The hydrogen bonds between adjacent polar water molecules result in cohesion, adhesion, and surface tension.

From Water's Polarity to Its Biological Functions

Within a water molecule, oxygen and hydrogen share electrons unequally in polar covalent bonds. Oxygen carries a partial negative charge and the hydrogens carry partial positive charges, so water is polar. The partial positive hydrogen of one molecule is attracted to a partial negative region of another, forming a hydrogen bond. Water can also hydrogen-bond with polar regions within and between biological molecules.

Property produced by many hydrogen bonds Molecular explanation Biological function
High specific heat capacity Added energy is used in disrupting hydrogen-bond interactions before molecular motion rises greatly Water resists rapid temperature change and supports stable body temperature
High heat of vaporization Many hydrogen bonds must be overcome for molecules to leave the liquid Evaporation removes heat and cools an organism or its surroundings
Cohesion Water molecules hydrogen-bond to one another Maintains a connected water column, as in xylem
Adhesion Water is attracted to other polar surfaces Helps water remain associated with vessel walls
Surface tension Surface molecules are pulled together by cohesion Produces a resistant surface layer

During evaporative cooling, molecules with enough energy escape from the liquid. The remaining water has lower average kinetic energy, so temperature falls. High heat of vaporization makes this energy removal substantial, helping maintain body temperature.

The O–H bonds inside one water molecule are polar covalent bonds; hydrogen bonds are weaker attractions between molecules or with other polar regions. Cohesion is water-to-water attraction, whereas adhesion is attraction between water and a different polar surface.