Pearson Edexcel Advanced Level Biology Unit 3 notes.
Describe how membrane structure can be investigated practically, e.g. by the effect of alcohol concentration or temperature on membrane permeability.
Monai Elnaasani
Describe how to investigate the vitamin C content of food and drink.
Past paper - January 2012, January 2017
1. Crush the fruit/vegetable in water
2. Filter and prepare an extract
3. Prepare vitamin C solutions of known concentration e.g.1%
4. Titrate the DCPIP solution against known concentration of vitamin C, note the volume of
DCPIP used
5. Titrate the extract against DCPIP drop by drop
6. Shake gently after each drop, until blue color disappears
7. Record the volume of DCPIP extract
8. Find the concentration of vitamin C using a calibration curve
9. Repeat the experiment and draw a graph
Making an extract;
• Take equal mass of fruit
• Mix with equal volume of water
• Stir for an equal amount of time
• Filter
• Collect an equal volume of each extract
Dependent variable;
• Volume of juice required to decolourise 1cm3 of DCPIP
Independent variable;
• Fruit juice or heating time or storage time… depends on experiment
Controlled variables;
• Same fruit/vegetable or from the same plant if doing the investigation with more than one
method
• age/ripeness of fruit or vegetable
• Temperature
• DCPIP concentration
• Same number of shaking
• Same end point colour
• Same volume of extract or DCPIP
Safety;
• Use goggles
• Lab coat
• Gloves
Errors;
• Difficult to control temperature
• Difficult to judge the color change
• Loss of solution while transferring
• Too much shaking changes colour back to blue
• Accuracy of measuring equipment
Vitamin C acts as a reducing agent to DCPIP
Heating affects vitamin C content because cell membrane is damaged increasing permeability of
membranes, Vitamin C moves out by diffusion since it’s water soluble
Pearson Edexcel Advanced Level Biology Unit 3 - notes for success
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