Core Practical 5

Question 1

Method for investigating effect of temp on membrane permeability

Answer 1

1. Using cork borer and knife, cut pieces of beetroot into 1cm length cylinders
2. Place in distilled water overnight to remove any dye released on preparation, wash and blot dry
3. Place 8 boiling tubes of distilled water into 8 water baths of different temperatures, add a piece of beetroot to each and leave for 30 mins
4. Remove beetroot and shake tubes to disperse dye
5. Set colorimeter to % absorbance on blue/green filter, calibrate using distilled water in a cuvette first then add 2cm3 of beetroot solution from first temp to a new cuvette
6. Place into colorimeter to read % absorbance
7. Repeat

Question 2

independent variable

Answer 2

temp of water

Question 3

dependent variable

Answer 3

% transmission of light through resulting solution

Question 4

Evaluation issues

Answer 4

-beetroot may have skin on, affecting surface area
-difficulty in maintaining temp
-accurate reading of colorimeter
-accurate size of beetroot
-from different parts of root
-ensuring same amount of time a different temps

Question 5

equipment

Answer 5

-raw beetroot
-size 4 cork borer
-white tile
-knife
-ruler
-beaker
-forceps
-water baths
-boiling tubes
-thermometer
-colorimeter and cuvettes
-stop clock
-distilled water
-syringe

Question 6

calculations and outcome

Answer 6

-to calculate % transmission: 100 - %absorbance
-as temp increased, % transmission slightly increased to point at which it greatly decreased due to membrane molecules gaining more heat energy, vibrating more to a point where vibrations caused large gaps in membrane enabling release of dye
-proteins also denatured leaving large pores

Question 7

Controlled variables

Answer 7

-vol of distilled water
-time left in water
-size of beetroot piece (surface area)
-source of beetroot
-vol in the curvette
- wavelength (blue/green) colorimeter

Question 8

Safety

Answer 8

H: water baths temp above 50
R: burns
S: take care, allow steam to escape away from face

H: sharp items such as cork borer and knife
R: cuts
S: be careful

Question 9

List the variables that were controlled during the experiment and state how they were controlled.

Answer 9

● the volume of bathing water in each tube (10 cm3)
● the surface area and volume of the beetroot cylinders (dependent on size of cork borer; 1 or 2 cm in length)
● the equilibration time (5 minutes)
● the soaking time for the cylinders (15 minutes)
● the volume of coloured liquid in the cuvettes (e.g. 4 cm3)
● the colorimeter filter/wavelength used (blue/green)
● the part of the beetroot the core was taken from (e.g. the centre)
● the age, variety and storage time of the beetroot (the same beetroot or beetroots from the same batch may have been used).

Question 10

Suggest why the tubes were placed in the water baths for 5 minutes before the cylinders were added.

Answer 10

The temperature must be equilibrated to ensure the tubes contain water at the correct temperature. This allows confidence that the effect of the correct temperature is being assessed

Question 11

Why were the beetroot cylinders washed with distilled water and dried before the experiment began?

Answer 11

to remove excess surface pigment from the cut cells at the edge. This excess pigment would distort the transmission readings, giving inaccurate results.

Question 12

Use the trend line of your graph to describe the effect of temperature on the percentage transmission between 0 °C and 70 °C.

Answer 12

The percentage transmission decreases as the temperature rises. Initially there is little increase, but at around 40-60 °C the percentage transmission decreases sharply. At higher temperatures the rate of decrease usually levels out.

Question 13

Explain your results in detail in terms of what is happening to the beetroot membrane. low temp

Answer 13

lower temp: the tonoplast and plasmalemma are intact and betalain molecules are too large to pass through these membranes so light transmission remains high.

Question 14

Explain your results in detail in terms of what is happening to the beetroot membrane. high

Answer 14

greater the kinetic energy, faster movement and diffusion of pigment molecules. Greater kinetic energy also causes phospholipids of the membrane to become more fluid and bonds between the fatty acid tails can begin to separate so that some pigment molecules can pass through. Therefore more pigment passes through the membrane, decreasing the amount of light that can pass through the liquid (percentage transmission).

Question 15

Explain your results in detail in terms of what is happening to the beetroot membrane. very high

Answer 15

The point of sudden increase in percentage transmission occurs when proteins in the membrane begin to lose their tertiary structure. At higher temperatures, the protein molecules in the membrane become completely denatured and the membrane develops gaps through which the pigment can flood out.
Eventually, the change in transmission levels out as the concentration of pigment is the same inside and outside the cells