Biology

• All life on Earth shares a common chemistry. This provides indirect evidence for evolution.
• Despite their great variety, the cells of all living organisms contain only a few groups of carbon based (organic) compounds that interact in similar ways.
• Carbohydrates


  Commonly used by cells as respiratory substrates. They also form structural components in plasma membranes and cell walls.
• Monomers


  Small identical or similar molecules which can be condensed (joined/linked together) to make larger molecules called polymers.
• Polymers


  Large molecules made from joining many (3 or more) identical or similar monomers together.
• Monomer linking into polymers

  1. Condensation reaction

  2. Removal of one water molecule

  3. Forms a bond

  
• Polymer breakdown into monomers

  1. Hydrolysis reaction

  2. Addition of one water molecule

  3. Breaks the bond between two monomers

  
• Carbohydrate structure


  Elements: Carbon, Hydrogen, Oxygen

  Ratio of H:O is 2:1

  Generic formula: (CH2O)n when n=3 to 7

  
• Monosaccharides


  Carbohydrate monomers that can join together in condensation reactions to create disaccharides and polysaccharides

  
• Monosaccharides


  • Glucose, galactose, fructose

  
• Alpha glucose


  Chemical formula: C6H12O6

  Structure shown with carbon atoms numbered

  
• Bonds between glucose molecules often refer to the carbon atom the bond attaches to, e.g. 1-4 glycosidic bond.

  
• Condensation reaction between monosaccharides

  1. Two monosaccharides (glucose) join

  2. Forms a disaccharide (maltose)

  3. Releases one water molecule

  
• Glycosidic bond


  Bond that forms between monosaccharides in a condensation reaction

  
• Disaccharides


  • Maltose, lactose, sucrose
• Hydrolysis of disaccharides


  1. Disaccharide (maltose) + water

  2. Yields two monosaccharides (glucose)

  3. Glycosidic bond is broken

  
• Carbohydrates


  • Monosaccharides
  • Disaccharides
  • Polysaccharides (starch, glycogen, cellulose)
• Starch


  • Long, linear chains of alpha glucose

  Coils into a helix

  Compact structure good for storage

  Insoluble so doesn't affect water potential

  Large so can't diffuse out of cells

  
• Amylopectin


  • Branched chain of alpha glucose

  1. 4 and 1-6 glycosidic bonds

  Large surface area for rapid hydrolysis by enzymes

  Insoluble so doesn't affect water potential

  Large so can't diffuse out of cells

  
• Glycogen


  • Similar to starch but with shorter chains and more branching

  Larger surface area for rapid hydrolysis into glucose

  Stored in muscles and liver

  Insoluble so doesn't affect water potential
• Alpha glucose is the monomer found in starch and glycogen. Beta glucose is the monomer found in cellulose.

  
• Cellulose


  • Long, straight, unbranched chains of beta glucose

  Every other beta glucose rotates 180 degrees

  Chains held together by hydrogen bonds to form microfibrils/macrofibrils

  Provides strength and support to plant cell walls

  
• The structure of cellulose


  Is related to its role in providing strength and support to plant cell walls
• Hydrogen bonds are important in cellulose molecules as they hold the chains/cellulose molecules together, providing strength and rigidity.
• Benedict's test for reducing sugars


  Add Benedict's solution, heat to 95°C

  Colour change to green/yellow/orange/red precipitate indicates reducing sugars
• If no colour change in Benedict's test, could indicate presence of non-reducing sugars.
• Benedict's test for non-reducing sugars


  If no change in Benedict's test for reducing sugars, then heat the solution further
• Two reducing sugar solutions (A and B) have an enzyme added

  After 20 minutes, solution B had twice as much precipitate as A
• Precipitate


  Shows reducing sugar present
• Benedict's test for reducing sugars

  1. 2 different reducing sugar solutions (A & B) of the same concentration have an enzyme added

  2. Solution B had twice as much precipitate than A after 20 minutes
• Suggest why solution B had twice as much precipitate than A after 20 minutes
• Benedict's test for Non-Reducing Sugar


  Sucrose is the only NR sugar you need to know however there are others
• Steps to show a non-reducing sugar is present

  1. Heat a sample with acid for a few minutes to hydrolyse the glycosidic bonds

  2. Neutralise the solution with an alkali

  3. Heat again with Benedict's reagent

  4. Brick red precipitate shows a positive result
• The Benedict's test is only a semi-quantitative test for sugars
• Colorimeter


  Quantitative test that measures the intensity of light transmitted through a solution/sample
• Standardising the method for colorimeter

  1. Shake samples before testing

  2. Zero the colorimeter before use

  3. Use the same filter throughout

  4. Use same volume for each reading
• Arbitrary unit (AU)


  Unit used to measure absorbance in a colorimeter
• Producing a calibration curve for an unknown reducing sugar

  1. Make up known concentrations of the sugar

  2. Carry out Benedict's test on each sample

  3. Measure absorbance/transmission with colorimeter

  4. Plot calibration curve with concentration on X axis and absorbance/transmission on Y

  5. Find unknown concentration from calibration curve
• Testing for starch


  1. Add potassium iodide (KI) solution

  2. Blue-black colour indicates presence of starch
• Triglycerides


  Storage molecules made from glycerol and 3 fatty acids