OCR Biology - Water (Unit 2.1.1.)

Everything you need to know about water for the AS OCR Unit 2 Biology exam.

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  • Created on: 29-05-10 17:13
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Unit 2
Section 1
Water is Essential to Living Organisms
Makes up about 80% of cells contents
Is a reactant in lots of chem. reactions e.g. photosynthesis &
hydrolysis reactions
Is a solvent ­ substances dissolve in it. Most biological reactions take
place in solution.
Transports substances. Liquid + solvent = easily transport materials
e.g. glucose/oxygen around plants/animals.
Helps temp control. Carries away heat energy when evaporated from
surface, therefore cools surface + helps lower temp.
Water Molecules have a Simple Structure
Molecule of water ­ 1 atom of oxygen (O) joined to 2 atoms of
hydrogen (H2) by shared electrons = H2O
Shared negative H electrons are pulled towards O atom. Other side of
H atom left with a slight positive charge.
Unshared negative electrons on the O atom give it a slight negative
Makes water a polar molecule ­ a negative charge on one
side, positive charge on the other.
Negatively charged O atoms of water attract positively charged H
atoms of other water molecules.
The attraction is called hydrogen bonding ­ gives water some of its

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Unit 2
Section 1
Hydrogen Bonds Give Water a High Specific Heat Capacity
Specific heat capacity ­ the energy needed to raise the temperature
of 1g of a substance by 1oC.
H bonds between water molecules can absorb a lot of energy.
So water has a high specific heat capacity ­ needs a lot of energy to
heat it.
Useful for living organisms b/c it stops rapid temp changes ­ keeps
temp fairly stable.…read more


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