Topic 2: Molecular Biology

2.2 Water

Romy Friedman

DP Biology

Syllabus

Essential idea:

Water is the medium of life

Water (H2O)

• There is a ratio of 2:1 for Hydrogen: Oxygen

• Water is a compound formed from two elements:

oxygen and hydrogen

Forming a water molecule

covalent bonding

• Hydrogen has 1 valence electron

• An oxygen shares an electron with two hydrogen

atoms, forming two covalent bonds

• Oxygen has 6 valence electrons

Polarity of water

• Water is a unique substance earth, and

therefore essential to life, because of

its polar nature

Partial Charge• Oxygen exerts a great pull over the shared

electrons. They move a little closer to the oxygen

• That means that the oxygen part of the molecule is

slightly negative

• The hydrogen end of the molecule is slightly

positive

Hydrogen bonding in water

Polarity relates to

solubility in waterHydrophillic

• A substance that is attracted to water and forms

intermolecular bonds with water molecules

• Ionic compounds and substances with polar molecules

Hydrophobic

• Means that waters molecules are more strongly attracted to

each other than to the substances

• Hydrophobic substances are insoluble in water

Water as the Universal Solvent

The Nature of Science

• Theories are used to explain natural

phenomena

• Hydrogen bonding explains the unique

behaviour of water that allows life on

earth

4 Property types of Water

• Cohesive

• Adhesive

• Thermal

• Solvent

Water is Cohesive

Explanation

• Water molecules cohere because hydrogen

bonds form between them

Example of benefits to organisms

• Transpiration stream

Water is Adhesive

Explanation

• The polarity of water molecules makes them

adhere to surfaces that are polar (and therefore

hydrophillic)

Example of benefits to organisms

• Transpiration stream & gas exchange in plants

Water has Thermal

PropertiesExplanation

• High melting point & boiling point

• High latent heat of vaporization

• High specific heat capacity

Example of benefits to organisms

• Liquid (aquatic) habitats that are stable

• Effective coolant in leaves or sweat

Water as a habitat

Water as a Coolant

Water is a Solvent

Explanation

• Many molecules dissolve in water due to its

polarity including ions or polar molecules

Example of benefits to organisms

• Medium for metabolic reactions

Water as the medium of life