Biological Molecules

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- A water molecule ­ H2O is covalently bonded (share electrons)
- Because shared electrons are pulled towards the oxygen
the hydrogen atoms are slightly positive (+ ) and the
oxygen atom is slightly negative (- ) which makes the
molecule polar
- The negatively charged oxygen atoms on one water
molecule attract the positively charged hydrogen atoms of
other water molecules ­ called hydrogen bonding
- These bonds give water a higher than would be expected specific heat capacity (the
energy needed to raise the temp of 1gram by 1°C) as H-bonds can absorb a lot of energy.
This stops rapid temperature changes in organisms (80% of cells are water).
- A high latent heat of evaporation means that a lot of energy is needed to break H-bonds
- Water molecules are very cohesive (attract each other) because they are polar which

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Release energy, cell wall in plants (cellulose) and energy store (starch in
plants, glycogen in animals)
Are large complex molecules made of long chains of monosaccharide's
Glucose ­ monosaccharide with six carbon atoms = alpha () and beta ()
2…read more

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Monosaccharide's are joined together by glycosidic
During synthesis a H atom on one monosaccharide
bonds with an OH group on the other and a molecule of
H20 is produced (condensation)
In reverse a molecule of water reacts with the glycosidic
bond causing it to break (Hydrolysis)
A disaccharide is where two monosaccharide's join
together ­ two -glucose molecules = maltose
3…read more

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A polysaccharide is where more than two monosaccharides join together ­ many
-glucose molecules = amylose
Starch ­ main energy storage material in plants
- cells get energy from glucose, plants store energy as starch
- starch is a mixture of two polysaccharide -glucose chains
- Amylose ­ long, unbranched chain, coiled structure, good for
food storage as more can fit in a small space.…read more

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Glycogen ­ main energy storage material in animals
- cells get energy from glucose but store with glycogen.…read more

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H-Bonds to form strong microfibrils = structural
Tests for the presence of carbohydrates:
Starch test ­ add a solution of iodine in potassium iodide to the sample. Result if starch is present =
colour change from yellow-brown to Blue-Black.
Reducing sugars ­ all monosaccharide and disaccharide sugars are known as reducing sugars ­ a
molecule can react with other molecules by giving electrons to them. Heat the reducing sugar to
80°C with Benedict's solution (alkaline copper sulphate).…read more

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Carry out the reducing sugar test again ­ if sucrose is present the result will now be positive because
glucose and fructose monosaccharide's are present
Using calorimetry to determine glucose concentration
A calorimeter measures the amount of light absorbed by a solution. The more concentrated
the colour of the solution, the higher the absorbance. It works by shining a beam of light
through a sample. A photoelectric cell picks up this light and gives a reading on how much
light has passed through.…read more

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results are then plotted on a graph.
Unknown solutions are then tested in the same way
Triglyceride ­
- One glycerol molecule
+ three fatty acids
- Tails are hydrophobic
and so lipids are
insoluble in water.…read more

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Phospholipids ­ lipids in cell membranes
- same structure as triglycerides except that one fatty acid chain is replaced by a phosphate group
- The phosphate group is ionised = hydrophilic while the tail is still hydrophobic.
Cholesterol ­ often found in cell membranes = hydrocarbon ring attached to HC tail. Also has a
polar hydroxyl group making it soluble.…read more

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