edexcel Biology As Level

Edexcel Biology AS Level Water

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  • Created by: Summer
  • Created on: 27-03-11 08:15


Water=Vital Living Organism =Water up to 80% of cells contents

Functions in and out of a cell:

·        Water is a solvent (some substances dissolve in it) most biological reactions take place in solutions= Important

·        Water transport substances. Can be transported more easily if dissolved in a solvent and liquid (water)- easily transport (glucose, oxygen) around plants and animal cells

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Water Molecules

Water Molecules have a simple structure:

1.     A molecule of water (H20) is 1 atom of oxygen (O) joined to 2 atoms of hydrogen (H2) by shared electrons.

2.     Shared negative hydrogen electrons are pulled towards the oxygen atom, other side of hydrogen ion left with a slight + charge.

Unshared negative electron on the oxygen atom gives it a slight negative charge

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4. Makes a water dipolar molecule- it has a - charge on one side and a + charge on the other.


5. The - charged oxygen atoms of water attract the + charged hydrogen atoms of other water molecules.


6. This attraction is called Hydrogen Bonding.

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Diagram of Water Molecule Structure: (handrawn)

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Structure of water molecule—properties---function


Water’s Dipole Nature makes it very cohesive:


1.     Cohesive=attraction molecules of the same type (e.g. 2 water molecules) very cohesive, stick together because there Dipolar


2.     Helps water to flow= great for transporting substances

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Water's Nature

Water’s Dipole nature makes it a good Solvent:

 1.     A molecule is Dipolar if it have a – charged bit and a + charged bit.

2.     Lots of important substances in bio reactions are ionic- one – charged ion and one + charged ion.

3.     Water- the + end of water molecule will be attracted to the – ion.

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4.     The – end of the water molecule will be attracted to the +ion.

 5. Meaning the ions will get completely surrounded by water molecules--- they are dissolved

6. Therefore water’s dipole nature makes it useful as a solvent for other polar molecules.


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Positve ions dissolve in water.

Negative ions dissolve in water.

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