AS Level Practical Questions

• Explain how to carry out an investigation to test these predictions

  • Cut beetroot into the same size pieces using the same beetroot

  • Rinse the beetroot thoroughly to remove the excess betalain

  • Use a range of detergent concentrations from 0% to 2%

  • Use the same volume of detergent

  • Beetroot should have the same time in solution

  • Control variables like age and source of beetroot

  • Use a colorimeter to measure absorbance in AU

  • Repeat the experiment three times to get an average

  
• Name a piece of apparatus that would be used to measure the light absorbance of this mixture.

  colorimeter

  
• Explain why the light absorbance of the mixture changed over time.

  • light absorbance decreased

  • because starch hydrolysed by amylase

  • therefore iodine solution stays yellow-brown colour
• State and justify a suitable control for this investigation.

  • plain water

  • to show resting heart rate

  
• Variables relating to the caffeine solution and the ghost shrimps should have been controlled.

  
  

  State and justify two variables that should have been controlled in this investigation.

  Variable relating to the caffeine solution

  • temperature of solution

  • a cold temperature would decrease the heart rate

  
  

  Variable relating to the ghost shrimps

  • same age of ghost shrimp

  • so caffeine would affect the ghost shrimp equally
• Explain one advantage of using ghost shrimps for this investigation.

  • transparent body

  • therefore heart is visible
• Comment on the effects of dopamine alone and dopamine with caffeine on the heart rate of Daphnia in this investigation.

  • adding caffeine to dopamine increases heart rate

  • increasing the concentration of dopamine increases heart rate

  • larger increase when both are used together at higher

  concentration

  • overlap between error bars indicates no significant difference between dopamine concentrations

  
• State and justify a suitable control for these investigations.

  • distilled water

  • to show the solvent did not have an effect

  
• Devise a procedure that could have been used to produce the results shown in the graph.

  • Daphnia immobilised on slide

  • acclimatisation time in dopamine and caffeine solution

  • use of concentrations from graph

  • suitable method for counting and recording heart rate

  • use Daphnia of same age

  • repeats and calculation of mean

  
• Milk at the start of a feed does not contain any vitamin C. Milk at the end of a feed can contain 10 mg of vitamin C per 100cm³ of milk.

  
  

  Devise a method to measure how the concentration of vitamin C changes during the course of a feed.

  • five samples of breast milk tested from different points of feed

  • description of titration of breast milk using DCPIP

  • standardisation of titration technique

  • use of a calibration curve to determine the vitamin C content in each breast milk sample

  • repeat titration to achieve concordant results
• A method for pickling red onions is to immerse them in wine vinegar. This vinegar contains ethanoic acid. The vinegar causes anthocyanin pigments to leave the onion cells, entering the solution.

  
  

  Explain why pickling in vinegar would result in anthocyanin pigments leaving the onion cells.

  • increased permeability of cell surface membrane

  • the low pH would denature proteins in the cell surface membrane

  • as vinegar affects bonds in protein
• A cell surface membrane is partially permeable.

  
  

  The phospholipid bilayer is important in controlling the movement of molecules through the membrane.

  
  

  Explain how the structure of a phospholipid molecule contributes to the partial permeability of a cell surface membrane.

  • phospholipid molecule contains a hydrophilic phosphate head and hydrophobic fatty acid chains

  • allows non-polar molecules to pass through the membrane

  • polar molecules cannot pass through the phospholipid bilayer
• Calculate the mass of orange juice required to provide the same mass of vitamin C that is in 100g of blackcurrant juice.

  52, 180

  (180/52)*100 = 346 / 346.2 / 346.15g
• Devise an investigation that can be used to collect these data.

  • titration using DCPIP

  • using the same volume of fruit juice

  • keep the concentration of DCPIP the same

  • DCPIP changing from blue to colourless as a suitable end point

  • determine the concentration of Vitamin C by using a calibration curve

  • calculation of vitamin C per 100g from concentration

  determined

  
• State one variable that should have been controlled in this investigation.

  • size of zebrafish

  
• Analyse the data to justify a conclusion for this investigation.

  • caffeine increases the heart rate of the zebrafish

  • the results show a calculated increase

  
• Explain the advantage of using young zebrafish for this investigation

  • transparent body

  • therefore heart is visible

  
• Give one reason why a different ethical issue has to be considered when using zebrafish instead of Daphnia in this investigation

  • zebrafish are vertebrates therefore they are more likely to feel pain

  
• Comment on the validity of this conclusion

  • the conclusion is not valid

  • as pH would reduce faster at higher substrate concentrations

  • however the rate of reaction for A did not decrease immediately

  • lower pH would result in the denaturation of the enzyme and

  reduction in the rate of reaction