biology core practical pdf

• Benedict's test


  Tests for reducing sugars
• Benedict's test


  • Can reveal if a sugar is reducing or non-reducing
  • Can estimate the concentration of reducing sugar
• Equipment for Benedict's test

  • Benedict's reagent
  • Test tubes
  • Water bath set to 90 °C
  • Pipette(s)
  • Test tube rack
  • Stop watch
• Benedict's test method


  1. Pipette 5 cm³ of the solution being tested into the test tube

  2. Add 2 cm³ of Benedict's reagent

  3. Place the test tube into a water bath at 90 °C for 2 minutes

  4. Remove the test tube and observe the colour
• Interpretation of Benedict's test results

  • Blue: negative (no reducing sugar)
  • Colours closer to brick-red (green → yellow → orange → red): increasing concentrations of reducing sugar
• Iodine test


  Tests for starch
• Iodine test method


  1. Pipette 2 cm³ of the test solution into a test tube

  2. Add 2 drops of potassium iodide solution

  3. A colour change from brown to blue-black indicates the presence of starch
• Colour standards


  Known concentrations of starch and reducing sugars are tested to produce colour standards, which can then be used to estimate the concentrations of unknown solutions
• Benedict's reagent tests for reducing sugars
• The iodine test turns blue-black in the presence of starch
• Procedure to find the concentration of an unknown reducing sugar solution

  1. Carry out Benedict's tests for a range of standard reducing sugar solutions to produce colour standards

  2. Carry out a Benedict's test on the unknown solution (using the same volume of solution and length of time in the water bath)

  3. Compare the resulting colour to the colour standards

  
• Quantitative results


  The absorbance of the samples could have been measured using a colorimeter
• Benedict's test is not suitable for measuring the total sugar content of fruit juice, as it only tests for reducing sugars and not non-reducing sugars like sucrose

  
• Ways to improve the reliability of the Benedict's reagent investigation

  • Larger quantities of each sample concentration could have been made, so repeat readings could be taken
  • Results from the whole class could be collated to give an average estimate
• Ways to evaluate the accuracy of the experimental value for the concentration of glucose in fruit juice


  • Compare with the reference values on the nutrition label
  • Calculate the concentration of glucose in the juice and then compare with the estimate generated experimentally

  
• Variables controlled in the iodine investigation

  • Concentration of iodine
  • Volumes of solutions
  • Time in water bath
  • Benedict's reagent (or other reasonable suggestion)
• Qualitative test


  Tells you whether something is present or absent
• Quantitative test


  Enables you to determine exactly how much of the substance is present
• Semi-quantitative test


  Enables you to estimate roughly how much of the substance is present
• Wavelength of 650 nm


  Red light, which is absorbed by the blue/black starch-iodine complex
• Blank


  A cuvette containing distilled water and dilute iodine/KI solution, used to set the absorbance of the colorimeter to zero
• A scratched cuvette should not be used as it would absorb some light and interfere with absorption
• Reducing sugar


  A sugar that has a free aldehyde or ketone group in its molecule that can reduce (donate electrons to) Benedict's reagent, changing copper(I) ions to copper(I) ions
• DCPIP


  An oxidising agent that is blue when oxidized and colourless when reduced
• Procedure to investigate vitamin C content of fruit juices

  1. Add 1 cm³ of DCPIP solution to a test tube

  2. Fill a 5 cm³ syringe with 5 cm³ of vitamin C solution

  3. Add the vitamin C solution from the syringe one drop at a time to the test tube, shaking gently after each addition

  4. Stop adding the vitamin C once the blue DCPIP has turned colourless

  5. Record the volume of the solution that has been added to cause the colour change

  6. Repeat the process with each of the 4 fruit juices

  
• Sources of error in the vitamin C investigation

  • Difficulty determining the end-point colour change, especially in coloured fruit juices

  
• Variables in the vitamin C investigation

  • Independent variable: type of fruit juice
  • Dependent variable: volume of juice added to decolourise DCPIP

  
• Calculating a mean value makes it easier to spot anomalies and reduces the effect of errors
• Reason for using syringes instead of burettes

  Syringes are easier to control and allow smaller volumes to be added precisely
• The fruit juice that contained the highest concentration of vitamin C was the one that required the least drops to decolourise the DCPIP

  
• Direction of electron movement in DCPIP-vitamin C reaction

  DCPIP gains electrons from the ascorbic acid (vitamin C)
• Vitamin C as an antioxidant

  Vitamin C readily loses electrons/becomes oxidised, preventing other cellular components/chemicals from becoming oxidised
• Reason for gentle shaking when adding vitamin C to DCPIP

  Vigorous shaking would introduce oxygen and reoxidise the DCPIP, making it difficult to determine the true end-point
• Proposed investigation to determine if vitamin C supplements benefit people

  1. Use a large sample of volunteers

  2. Match volunteers by age, gender, mass, lifestyle, diet

  3. Collect baseline urine samples to test vitamin C levels

  4. Divide volunteers into groups - no supplement, small supplement, large supplement

  5. Make it blind/double blind and give placebo to some

  6. Control diet and collect more urine samples to test vitamin C levels

  7. Compare vitamin C levels between groups
• Core Practical 3: Investigating membrane properties

  Beetroot can be used to investigate cell membrane permeability, as a damaged membrane allows the purple pigment to leak out
• Equipment for membrane permeability investigation

  • Beetroot
  • Scalpel
  • 4 test tubes
  • Colorimeter
  • Cuvettes
  • Stopwatch
  • Water baths
  • Pipette
• Method for investigating effect of temperature on membrane permeability

  1. Place beetroot cubes in test tubes

  2. Expose to different temperatures in water baths

  3. Measure absorbance of the liquid using a colorimeter to determine pigment release
• Permeability


  The ability of a substance to pass through a membrane
• Beetroot can be used to investigate the permeability of cell membranes, since when its cell membranes are damaged a coloured pigment, that gives beetroot its purple colour, leaks out
• Membrane permeability


  The higher the permeability, the more pigment is released outside of cells