Practicals for AS1

Question 1

what colour does DCPIP turn when Vitamin C present

Answer 1

blue dye that turns colourless

Question 2

Method for the detection of vitamin C

Answer 2

Transfer 1cm of ​DCPIP solution​ into a test tube with a pipette.
2. Add ​Vitamin C solution dropwise​ to the DCPIP solution. Shake after each drop.
3. Record the ​volume​ of Vitamin C that is required to ​change the colour​ of the
DCPIP.
4. Repeat the experiment and replace the Vitamin C solution with the ​fruit juices​.

Question 3

Method for daphnia experiment

Answer 3

1. Dilute the caffeine solution with ​distilled water ​to produce several ​different concentrations​.
2. Place some cotton wool (to ​restrict movement​) on a cavity slide. Add ​one​ large water flea.
3. Use filter paper to absorb the water around the flea.
4. Then use a dropping pipette to add a ​few drops of distilled water​ to the slide.
   Do not ​use a coverslip to prevent conditions from becoming ​anoxic​.
5. Use a stop clock to time a ​minute ​and record the ​number of heartbeats​.
6. Repeat the experiment, replacing the distilled water with a ​caffeine solution​.

Question 4

Conclusion of daphnia experiment

Answer 4

Caffeine ​increases​ the heart rate of Daphnia by increasing the release of stimulatory neurotransmitters​. As concentration of caffeine increases, heart rate also increases.

Question 5

Method for membrane permeability experiment

Answer 5

Cut beetroot into five equal sized pieces. Wash these pieces afterwards to remove any pigment that leaked out while cutting.

Pipette 5 cm3 of water into five different test tubes.

Prepare water baths at five different temperatures. E.g. 20°C, 30°C, 40°C, 50°C, and 60°C.

Place each test tube in the different water baths. They all need to be incubated for the same duration.

Remove the test tubes from the water baths. Carefully remove the beetroot pieces, leaving the remaining coloured liquid in the test tubes.

Using a colorimeter (as discussed in tutorial 17), measure how much light is being absorbed by the liquid in each test tube.

The greater the absorbance, the more coloured, This means that there has been a greater degree of permeability within the beetroot’s cell membranes, resulting in a large amount of pigment leaking out.

Question 6

Conclusion of membrane permeability experiment

Answer 6

As the temperature​ increases​, the permeability of the cell-surface membrane also ​increases​. This is because the proteins in the membrane ​denature​ as the heat damages the bonds​ in their ​tertiary structure​. This ​creates gaps​ in the membrane so it is easier for molecules to pass through it.

At low temperatures, phospholipids have ​little energy​ and are ​packed closely together​ to make the membrane​ rigid​. This causes a ​decrease​ in permeability and ​restricts molecules​ from crossing the membrane.

Question 7

Method for the effect of enzyme and substrate concentrations on the initial rates of reactions.

Answer 7

Dilute​ stock solution of trypsin with distilled water to produce solutions with concentrations of 0.2%, 0.4%, 0.6% and 0.8%.

1. Make a ​control​ by adding 2cm3​ ​ of trypsin solution and 2cm3​ ​ of distilled water. Use this to set the colorimeter absorbance to zero.
2. To another cuvette, add 2cm​3​ of milk suspension and 2cm3​ ​ of the stock trypsin solution. Mix, place in the colorimeter and ​measure absorbance at 15 second intervals for 5 minutes​.
3. Rinse the cuvette with distilled water.
4. Repeat step 3 at ​all trypsin concentrations​

Question 8

Conclusion for effect of enzyme and substrate concentrations on the initial rates of reaction experiment.

Answer 8

Milk contains a white protein called ​casein​ which, when broken down, causes the milk to turn ​colourless​. ​Trypsin​ is a​ protease enzyme​ which ​hydrolyses the casein.
● As concentration of trypsin ​increases​, the ​number of enzyme-substrate complexes forming also increases​ because enzymes and substrates are more likely to collide. This means that the ​rate of reaction increases​ up to the optimum ​enzyme concentration.
www.pmt.education

● The ​rate plateaus​ at the point where ​all substrates​ occupy an active site. Increasing the enzyme concentration won’t increase rate as ​substrate concentration ​is limiting the rate.

Question 9

explain the relationship between increasing the concentration of ethanol to the relative colour of the ethanol solution

Answer 9

permeability of membrane increases as ethanol increases in concentration (1)

ethanol disrupts membrane (structure) (1)

• (therefore) more {betalain / pigment} diffuses through membrane (1)
• therefore {increases the absorbance / decreases transmission} of light (in the colorimeter) (1)

Question 10

method for investigating the effect of solvent concentration on Cell Membrane Structure

Answer 10

Cut and wash five equal sized pieces of beetroot.

Prepare 5 different test tubes with 5cm3 of the following concentration of solvents:

Water
10% Ethanol
30% Ethanol
50% Ethanol
70% Ethanol

Put each piece of beetroot into the different test tubes. Seal the test tube openings with a rubber bung and shake the tubes.

Leave the beetroot pieces submerged for an hour. Periodically shake the tubes during this duration.

After an hour, carefully remove the beetroot pieces from the solution. Use a glass rod to help you fish out the pieces. Leave the remaining coloured liquid behind.

Use a colorimeter to measure light absorbance by the liquid in each test tube.