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Powerpoint Presentation: Water

Biochemistry Index

Biochemistry
Enzymes
Enzyme Mechanisms

Factors Affecting Enzymes: Substrate concentration
Factors Affecting Enzymes: pH
Factors Affecting Enzymes: Temperature
Inhibitors
Enzymes and Homeostasis
Enzymes and Biotechnology
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Haemoglobin
The Steroids

Topic Chapters Index

 

Solvent properties

The polar properties of water make it a good solvent for polar molecules (e.g. sugars and alcohols), which form hydrogen bonds with the water molecules, and ionic compounds (e.g. salts, acids and bases) , which dissociate into their component ions.

Water is a very important transport medium for living organisms because of its solvent properties and because it remains a liquid over a large range of temperatures.

Cohesion

The cohesion (stickiness) between water molecules means that a very tall thin column of water can be supported before it breaks. The tallest trees are at the physical limits of water cohesion (sequoia and eucalyptus are 100m tall).

Surface tension

Water molecules hold together forming a skin at the surface. This is strong enough for some organisms to be supported.

Density

Water is densest at 4°C whilst it is still a liquid. So ice floats on the surface of water. Organisms which live in water do not risk freezing solid so easily. Freezing is usually fatal. Water forms a good habitat for living organisms.

Transparency

Water is a transparent liquid, light passes though it. Blue light, with the most energy, penetrates furthest, red light is the weakest and penetrates least. Plants can photosynthesise under water and animals can use their visual systems.

 

BIOCHEMISTRY

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Water

Formula

H2O

Structure

Slightly negative at the oxygen end and slightly positive at the hydrogen end

polar water molecule

The association between the polar water molecules

The association between the polar water molecules

Thermal properties

The molecules of water can absorb a lot of heat energy. Water has a very high thermal capacity (4.2 J°1C-1 g-1).The hydrogen bonding forms a lattice which does not easily fall apart as the temperature rises.

Compared to molecules of similar size and properties water has a very high melting point and boiling point.

Molecule

Formula

Molecular
mass

Melting point
/ °C

Boiling point
/°C

Methane

CH4

16

-184

-161

Ammonia

NH3

17

-78

-33

Water

H2O

18

0

+100

Hydrogen fluoride

HF

20

-92

+19

Hydrogen sulphide

H2S

34

-86

-61

 

Water is a very, thermally stable medium which helps living organisms resist changes in their environment.

To make water change from a liquid to a vapour requires a lot of energy. Evaporation of water on a the surface of a body cools it down significantly.

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