edexcel Biology As Level

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

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

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.

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

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.

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.

Positve ions dissolve in water.

Negative ions dissolve in water.