Biological Molecules: Water
AS Biology: BY1
Structure of water
Water (H2O) consists of two hydrogen atoms covalently
bonded to one oxygen atom.
Each hydrogen shares a pair of electrons with
the oxygen. The oxygen has a greater affinity
for electrons than the hydrogens, so it ‘pulls’
the electrons closer.
δ–
δ+
104.5 °
δ+
This makes the oxygen slightly negative (indicated by δ–) and the
hydrogens slightly positive (indicated by δ+).
This creates different charged regions, making water a polar molecule.
Because it has two charged regions it is dipolar.
Hydrogen bonds
Many of the properties of water are due to its ability to form hydrogen
bonds.
The slight negative charge on the oxygen atom makes it attract the
slightly positive hydrogen atom of another water molecule.
hydrogen bond
The numerous hydrogen bonds in water make it a very stable structure.
Inorganic Compounds: Water
 Polar molecule
 Makes a good solvent
(polar substances dissociate
forming solutes) form of key
elements for many organisms
 Good for transport-not
viscous
 Good reaction medium
 Adheres to surfaces (to aid
transport in xylem)
Figure 2.4a
Water as a solvent
Properties and biological roles of water
Inorganic Compounds: Water
 Hydrogen bonding between water
molecules (weak bonds)
 Creates cohesion (sticking to each
other)
 Water high surface tension
 High Specific Heat Capacity (temperature
buffer-good for organisms and enzymes)
 High Latent Heat of Evaporation (for
cooling by sweating)
Figure 2.4b
Water
 H+ and OH participate in chemical
reactions
 Maximum density at 4oC
 Allows ice to form on top thus
insulating lakes/ponds
 Prevents all of lake from freezing and
killing organisms
 Transparent (light can get through
for photosynthesis)
Multiple-choice quiz
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What’s the carbohydrate?
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