Objectives for Topic 3.1


http://www.patana.ac.th/Secondary/Science/c4b/3/Chem3.1.htm



ELEMENTS of biological importance

  • H, O, C, are the most common.
  • N, S, P are also important as well as the metals Na, Ca, and Fe.
  • H, O, C, N, S and P form covalent bonds (single, double or triple
    e.g. –C-C-, -C=O, –Cexternal image 19050304.jpgN).
  • H, O, C, N, S and P are light.
  • H, O, C, N, S and P very reactive.
  • H, O, C, N, S and P produce diverse compounds which are reduced (readily oxidised

Organic compounds

  • Compounds containing carbon found in living organisms.
  • Not including carbonates, hydrogen carbonates, CO2 or CO.
  • Often based upon a skeleton of carbon.
  • An infinite variety possible.
  • Evolution has chosen a few for use in living organisms.
  • There are four principal groups: sugars, fatty acids, amino acids and nucleotides

WATER

Water is important for living organisms because of it’s unusual properties.

Formula
H2O

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

http://www.saburchill.com/IBbiology/chapters01/006.html

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Weak hydrogen bonds form between the partially positive and negative ends of water molecules. This allows water to remain a liquid, rather than a gas, at room temperatures.


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Thermal properties


The molecules of water can absorb a lot of heat energy. Water has a very high thermal capacity (4.2 J°C-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.

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.