Biological Molecules

• Sugars in complex carbohydrates:

  • Carbohydrate molecules contain the elements carbon, hydrogen and oxygen.
  • In the human body, carbohydrates are broken down by enzymes in the mouth and small intestine.
  • Carbohydrates can be classed into different groups (monosaccharides, disaccharides and polysaccharides) depending on their complexity.
• Monosaccharides:

  Are simple sugars that consists of a single monomer unit e.g. glucose or fructose.

  • Glucose molecules contain lots of energy which can be released in respiration by breaking the bonds between the carbon atoms.
• A disaccharide is made when two monosaccharides join together.

  • Maltose is formed from two glucose molecules.
  • Sucrose is formed from one glucose and one fructose molecule.
• A polysaccharide is formed when lots of monomer units join together in long chains to form a polymer.

  • Starch, glycogen or cellulose are all formed when lots of glucose molecules join together.
  • Polysaccharides are insoluble and therefore useful as storage molecules.
• Animo acids in proteins: (Part 1)

  • Proteins are polymers formed from long chains of monomers called amino acids joined together.
  • Amino acids contain carbon, nitrogen, hydrogen and oxygen atoms.
  • Just 20 different standard amino acids make up all the proteins found in the human body (although many more amino acids occur in nature).
• Animo acids in proteins: (Part 2)

  • Amino acids can be arranged in any order, resulting in hundreds of thousands of different proteins.
  • Examples of proteins include enzymes, haemoglobin, ligaments and keratin.
  • In the human body, proteins that we ingest are broken down into the constituent amino acids.
  • Enzymes in the stomach and small intestine break down the protein.
  • These amino acids can then be used in the synthesis of new proteins.
• Amino acids join together to form proteins:

  Here it is, but in a picture form.

  
• Protein shape:

  • Different proteins have different amino acid sequences resulting in the peptide chain folding into different shapes.
  • The function of a protein is determined by its shape.
  • Even a small difference in the amino acid sequence will result in a completely different protein being formed.
  • In this way, every protein has a unique 3-D shape that enables it to carry out its function.
• Fatty acids and glycerol in lipids:

  • Lipids (fats and oils) are made up of triglycerides.
  • Their basic unit is one glycerol molecule chemically bonded to three fatty acid chains.
  • Lipids contain carbon, hydrogen and oxygen atoms.
  • Unlike carbohydrates and proteins, they don't form polymers (they don't form a long chain of repeating sub-units).
  • Lipids are divided into fats (solids at room temperature)and oils (liquids at room temperature).
  • In the human body lipids are broken down by enzymes called lipases in the small intestine.
• The structure of a triglyceride:

  Here it is, but in a picture.

  
• Benedict's test (reducing sugars):

  A wide range of biological molecules can be tested for using simple chemical tests.

  • This allows the biological molecules within a sample (such as a food substance) to be determined.
• Reducing sugars include simple sugars like glucose that are just made from one unit (monomer), and some sugars made from two units joined together like maltose.

  • The presence of reducing sugars can be tested for with a Benedict's test.
• Test for a reducing sugar:

  • Add Benedict's reagent (which is blue) to the sample solution in a test tube.
  • Heat in a water bath that has been set at 75•C for 5 minutes.
  • Take the test tube out of the water bath and observe the colour.
  • A positive test will show a colour change from blue to orange/dark red.
  • An orange/dark red precipitate (solid particles suspended in the solution) will form.
  • A negative test will remain blue.
  • The amount of colour change is an indication of the amount of reducing sugar present.
  • From: blue → green → yellow → orange → red.
• Diagram of testing for a reducing sugar:

  Here it is, with annotations to.

  
• Iodine test(starch):

  • We can use iodine to test for the presence or absence of starch in a food sample.
  • Add drops of iodine solution to the food sample.
  • A positive test will show a colour change from orange-brown to blue-black.
  • The colour remains orange-brown if there is no starch present.
• Diagram on testing iodine for starch:

  Here it is, with annotations to.

  
• Biuret test (protein):

  • The biuret test can be used to identify if any protein is present in a sample.
  • Add drops of biuret solution (a mixture of sodium hydroxide and copper sulphate) to the food sample.
  • A positive test will show a colour change from blue to violet / purple.
  • If there is no protein present, the solution will stay blue.
• Diagram on testing Biuret for protein:

  Here it is, with annotations to.

  
• Emulsion test (lipids):

  • An emulsion test can be used to identify if any lipids are present in a sample.  
  • Mix the food sample with 4cm3 of ethanol and shake.
  • Allow time for the sample to dissolve in the ethanol.
  • Strain the ethanol solution into another test tube.
  • Add the ethanol solution to an equal volume of cold distilled water (4cm3).
  • A positive test will show a cloudy emulsion forming.
  • The more lipid is present, the stronger the milky colour will be.
• Diagram on testing emulsion for lipids:


  Here it is, with annotations to.

  
• Food test results table:

  Here it is...

  
• Practical investigation safety tips:

  • If you were carrying out these tests you should try to identify the main hazards and think of ways to reduce harm.
  • Biuret solution contains copper (II) sulfate which is dangerous particularly if it gets in the eyes, so always wear goggles.
  • Iodine is also an irritant to the eyes.
  • Sodium hydroxide in biuret solution is corrosive, if any chemicals get onto your skin wash your hands immediately.
  • Ethanol is highly flammable; keep it away from any Bunsen burner.
  • The Bunsen burner itself is a hazard due to the open flame.