A Level Biological Molecules

A Level Biological Molecules

1. Carbohydrates

   • All Carbohydrates contain the elements Carbon, Hydrogen and Oxygen, in the ratio of 2CH's to 1O.
   • Monosaccharides are glucose, galactose and fructose [galactose and fructose are isomers of glucose] they are single sugars.
     • Isomer: Same molecular formula but different structure. For example, for monosaccharides the formula is C₆H₁₂O₆ and the structures can be shown in the diagram to the left
     • They are hexose sugar as they contain 6 carbons.
     • Alpha Glucose Structure
       • OH is a hydroxyl group.
     • Disaccharides are maltose [glucose+glucose], lactose [glucose and galactose] and sucrose [glucose and fructose]. They are double sugars and joined together by a condensation reaction [loss of water] and a glycosidic bond is formed.
       • The H₂O is removed and a glycosidic bond is formed. [In chemistry this is called an ether bond]
       • EXAMPLE: Glucose [C₆H₁₂O₆] + Fructose [C₆H₁₂O₆] = Sucrose [C₁₂H₂₂O₁₁]
     • Beta Glucose Structure
     • Polysaccharides are starch [alpha glucose], glycogen [alpha glucose] and cellulose [beta glucose]. They are very long chained molecules and are insoluble, therefore they do not affect the water potential of a cell.
       • Starch is large and insoluble, helical in shape so it is compact and stores energy.
       • Glycogen is large and insoluble, helical and branched in shape so it is compact and stores energy.
       • Cellulose is made from Beta glucose [a flip in the hydroxyl group], they are form long straight chains called microfibrils. They are linked by hydrogen bonds which in mass are very strong [cross links] therefore they help provide structural support.
   • Test for Carbohydrates
     • Non-Reducing Sugar: Use Benedict's Solution, heat the mixture in a water bath and add HCL and a black-blue colour will be present.
     • Reducing Sugar: Use Benedict's Solution, heat the mixture in a water bath and a red-brown colour will be present.

2. Proteins

   • What makes each individual amino acid different is the R Group.
     • Amino Group
     • Carboxylic Acid
     • Variable Group [R]
   • Amino Acids are joined together by a condensation reaction.
   • Proteins have 4 structural levels:
     • Primary Structure is the sequence of amino acids which determines how it folds. It contains peptide bonds between amino acids. It looks like a chain essentially.
       • Secondary Structure is the basic level of folding held together by hydrogen bonds, a Beta (β) pleated sheet or an alpha (α) helix is formed.
         • Tertiary Structure is the overall globular structure formed from the polypeptide they contain hydrogen and disulphide bonds. E.g. Enzymes
           • Quarternary structure is more than one tertiary structure attached together. They are also insoluble and have structural or storage roles. E.g. Collagen, Insulin, and Haemoglobin
   • Proteins have a variety of functions, including enzymes, antibodies, transport proteins and structural proteins.
   • You conduct a Biuret Test for proteins. Add a few drops of biuret reagent to the solution, if peptide bonds [protein] the solution will turn purple or lilac.

3. Lipids

   • Triglycerides have one molecule of glycerol and three fatty acids [made from hydrocarbons].
     • Triglycerides are formed by condensation reactions when fatty acids bond with a glycerol molecule water [H₂O] is produced and an ester bond.
       • They are mainly used as energy storage molecules and are insoluble so they don't affect water potential.
     • Fatty acids can be unsaturated and saturated.
       • Unsaturated have at least one double bond between carbon atoms which causes the chain to kink. They are liquid at room temperature.
       • Saturated have no double bonds between carbon atoms, they are saturated with hydrogen atoms. They form straight lines and are solid at room temperature.
   • Phospholipids have a phosphate head, a glycerol molecule and two fatty acids.
     • The phosphate group is hydrophillic and the fatty acid 'tails' are hydrophobic, they make up the phospholipid bi-layer.
       • The centre is hydrophobic so water-soluble substances can't easily pass through it.
   • The emulsion test is used for lipids, grind or crush the substance, adding ethanol to the solution and shaking then adding distilled water and a cloudy white precipitate should be formed.

4. Introduction

   • Monomer - small unit from which larger molecules are made E.g. Monosaccharides, amino acids and nucleotides.
   • Polymer - are molecules made from a large number of monomers joined together.

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