Biology - Biological molecules: Water

These are complete notes copied out of my textbook completed for homework, on the biological molecule Water. Diagrams included. It is based on the OCR specification for AS Biology. Happy revising! =P

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  • Created on: 13-01-13 00:21
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Reianna Shakil L6EFBiology14/10/12
Biological molecules ­ Water
The human body is made up of 60% water, which had small molecules made of two
atoms of hydrogen combined with one atom of water, formula H2O.
Water is by far the most abundant molecule in our bodies; it is also one of the smallest
and simplest. Water is an amazing substance, with a collection of properties that are
It is difficult to imagine how life of any kind could exit without water.
The search for other life in the universe begins by searching for other planets that may
have liquid water on them.
The structure of a water molecule
Figure 7.1 shows the structure of a water molecule. It is made up of two hydrogen
atoms covalently bonded to one oxygen atom. The bonds are very strong, and it is very
difficult to split the hydrogen and oxygen atoms apart.
A covalent bond is an electron-sharing bond, and in this case the sharing is not equal. The
oxygen atom gets slightly more than its fair share, and this gives it a small negative
charge. This is written (delta minus). The hydrogen atoms have a very small positive
charge, (delta plus).
These tiny charges mean that water molecules are attracted to each other ­ the
positively charged hydrogen atoms on one molecule are attracted to the negatively
charged oxygen atoms on other molecules. The attraction is called a hydrogen bond.
In solid water - ice ­ the hydrogen bonds hold the water molecules in a rigid lattice
formation. As in all solids, the molecules vibrate, but they do not move around.
In liquid water, the molecules have more kinetic energy, moving around past each other,
forming fleeting hydrogen bonds with each other. In water vapour, the molecules are far
apart, scarcely interacting with each other at all.
Water, heat and temperature

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Reianna Shakil L6EFBiology14/10/12
If you heat water, the temperature of the water rises. Temperature relates to the
amount of kinetic energy that the water molecules have.
As heat energy is added to the water, a lot of the energy is used to break the hydrogen
bonds between the water molecules. Because so much heat energy is used for this,
there is less heat energy available to raise the temperature.
Water therefore requires a lot of heating in order to increase its temperature by very
much.…read more

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Reianna Shakil L6EFBiology14/10/12
Solvent properties
Water is an excellent solvent. The tiny charges on its molecules attract other molecules
or ions that have charges on them.
The molecules and ions spread around in between the water molecule. This is called
When an ionic compound such as sodium chloride dissolves in water, the sodium ions
and the chloride ions become separated from each other, making it easy for them to
react with other ions or molecules.…read more

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Reianna Shakil L6EFBiology14/10/12
Density and viscosity
Water molecules are pulled closely together by the hydrogen bonds between them, and
this makes water a relatively dense liquid. The density of pure water is 1.0g cm -3. Ethanol
has a density of only 0.79g cm-3.
Most living organisms, containing a lot of water, have a density which is close to water's
own density, making it easy for them to swim.
Aquatic organisms often have methods of slightly changing their average density ­ e.g.…read more


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