1.1Chemistry of Life

Structure of Water and Hydrogen Bonding — AP BIOLOGY Revision Notes & Topical Questions

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1.1 Structure of Water and Hydrogen Bonding — Notes — AP Biology notes: Explain how the properties of water that result from its polarity and hydrogen bonding affect its biological function.Structure — a water molecule has two hydrogen atoms covalently bonded to one oxygen atomOx…

Bond types
Plant transport
Surface tension
High specific heat capacity
High heat of vaporization
Hydrophilic

Section 1 of 9Polarity and hydrogen bonding

Polarity and hydrogen bonding

Structure

Structure a water molecule has two hydrogen atoms covalently bonded to one oxygen atom
Oxygen is more electronegative, so the shared electrons are pulled towards it.
Oxygen carries a partial negative charge (δ−) and each hydrogen a partial positive charge (δ+).
The molecule is bent (about 104.5°), so it is polar.

Hydrogen bond

Hydrogen bond a weak attraction between the δ+ hydrogen of one molecule and the δ− oxygen of another

Each water molecule can form up to four hydrogen bonds.
Individually they are weak but collectively strong, and they constantly break and reform in liquid water.

Bond types

Bond types hydrogen bonds form between molecules, whereas the O–H bonds inside a molecule are covalent

Key idea

Key idea water's polarity allows hydrogen bonding, which gives water the properties that sustain living systems

Section 2 of 9Cohesion, adhesion and surface tension

Cohesion

Cohesion forces of attraction between molecules of the same type, here water to water

Adhesion

Adhesion attractive forces between molecules of a different type, here water to polar surfaces such as xylem walls

Plant transport

Plant transport water molecules are drawn upwards behind other water molecules in the xylem because of cohesion
Adhesion between water and the xylem walls draws water upwards by capillary action.
This supplies the plant with water and transports dissolved mineral ions.

Surface tension

Surface tension cohesion pulls surface molecules towards the molecules below the surface
This lets small organisms such as pond skaters walk on the surface of ponds and lakes.

Section 3 of 9Specific heat capacity and heat of vaporization

Specific heat capacity and heat of vaporization

High specific heat capacity

High specific heat capacity hydrogen bonds reduce the movement of water molecules, so a lot of energy is needed to raise the temperature of water
The hydrogen bonds must be disrupted before kinetic energy can increase.
Water's specific heat is 4.18 J g⁻¹ °C⁻¹, so water is a thermal buffer and changes temperature slowly.

Benefit of thermal stability

Aquatic birds and mammals use less energy on thermoregulation.
Enzyme activity in aquatic organisms stays close to optimal despite external temperature change.

High heat of vaporization

High heat of vaporization a lot of energy is needed to convert liquid water to a gas, because hydrogen bonds must be broken before molecules escape
Evaporating water absorbs heat from its surroundings, producing a cooling effect.
Sweating and panting cool animals, and transpiration keeps leaves cooler than the surrounding air.

Section 4 of 9Density of ice and water as a solvent

Property	Description	Importance for living organisms
Low density when frozen	Hydrogen bonds form an open lattice when water freezes, so ice floats on water	Polar ice provides a habitat for some animals and acts as an insulating layer over water
Solvent	Polar water molecules are attracted to charged particles, so ionic compounds such as NaCl dissolve	Water transports substances in solution and is a medium for chemical reactions

Hydrophilic

Hydrophilic polar and charged substances dissolve in water

Hydrophobic

Hydrophobic nonpolar substances such as oils do not dissolve and cluster together, which drives membrane formation and protein folding

Section 5 of 9Water in cells and chemical reactions

Medium

Medium water is the solvent for cell reactions and a transport medium in blood and sap

Hydrolysis

Hydrolysis water is added to break a bond, as when a polymer is digested

Dehydration synthesis

Dehydration synthesis water is removed to join monomers

Photosynthesis

Photosynthesis water is split to supply electrons and protons

Section 6 of 9Ionisation of water and pH

Ionisation

Ionisation water ionises slightly to H⁺ and OH⁻, and in pure water at 25 °C [H⁺] is 10⁻⁷ mol/L, giving pH 7

pH scale

pH scale pH = −log[H⁺], so a change of one pH unit is a tenfold change in hydrogen ion concentration

Acids and bases

Acids and bases acids raise [H⁺] and bases lower it

Buffers

Buffers systems such as hydrogencarbonate in blood resist pH change, which matters because enzymes have an optimum pH

Section 7 of 9More from the specification

1High specific heat capacity of water

High specific heat capacity of water extensive intermolecular hydrogen bonds buffer aquatic temperature swings, providing a stabilising means for aquatic organisms.

2Carbohydrate

A molecule of carbon, hydrogen and oxygen, including sugars, starch, glycogen and cellulose.

3Lipids

Concentrated energy, insulation, cell membranes.

4Amino acid

The unit from which proteins are made; the sequence decides the protein's shape and function.

Section 8 of 9Key terms

Term	Meaning
Hydrogen bond	A weak attraction between a partially positive hydrogen and a partially negative atom on another molecule.
Polar molecule	A molecule with an uneven charge distribution, such as water.
Cohesion	Attraction between molecules of the same type.
Adhesion	Attraction between molecules of a different type.
Surface tension	Resistance of a liquid surface to breaking, caused by cohesion.
Specific heat capacity	Energy needed to raise 1 g of a substance by 1 °C; high for water.
Heat of vaporization	Energy needed to convert a liquid to a gas; high for water.
Hydrophilic	Polar or charged and able to dissolve in water.

Section 9 of 9Exam technique

Confusing hydrogen bonds (between molecules) with covalent bonds (within a molecule).

Saying water is polar because it has a charge — it is overall neutral with partial charges.

Mixing up cohesion (water to water) and adhesion (water to another surface).

Stating that ice is denser than liquid water — it is less dense because of the open lattice.

Forgetting that each pH unit is a tenfold change in H⁺ concentration.

Last updated: 15 October 2026