Edexcel IAL Biology 2.3 Core Practical 3 membrane permeability

Standardise tissue dimensions, wash cut surfaces, control treatment volume and time and use colorimetry or conductivity consistently before comparing leakage.

[Maximum number: 9]

Beetroot cells contain a red pigment called betalain. This pigment does not leak out of the cells, unless their membranes are damaged.

A student investigated the effect of detergent on beetroot cell membranes.
Discs of tissue were cut from a beetroot. Any betalain on the outside of the discs was removed.
One disc was placed into each of five test tubes, containing 0.2% detergent solution.
The five tubes were left for 30 minutes at $20^{\circ} \mathrm{C}$ .
The discs were then removed carefully.
Betalain had leaked from the discs and formed a red solution in each test tube.
Light was shone through this solution. The percentage of the light transmitted through the solution was recorded.

The same procedure was repeated using detergent concentrations of 0.0%,0.5%, 1.0% and 2.0%.

3(a)(i)
Concentration of detergent. (1)

3(a)(ii)
Any two of:
- temperature; (1)
- pH; (1)
- volume of detergent solution. (1)

3(a)(iii)
Accept answers from 3(a)(ii), for example:
- temperature: thermostatically controlled water bath / incubator / air-conditioned room / environmental chamber; (1)
- volume of detergent solution: appropriate measuring device; (1)
- pH: use of buffer. (1)

3(a)(iv)
An explanation including:
- to avoid physical/more damage to the discs/membranes/cells; (1)
- which would cause more pigment release; (1)
- causing the results to be invalid. (1)

3(a)(v)
An explanation including:
- to calculate standard deviation / measure reliability / variability; (1)
- to identify anomalies. (1)

3(b)(i)
To allow any pigment leakage that is not due to detergent to be measured / as a control / for comparison / because the reading of 0% detergent is not 100% but only 97%. (1)

3(b)(ii)
A graph showing:
- axes correctly orientated and linear; (1)
- axes correctly labelled with units: x-axis concentration of detergent solution, y-axis mean percentage of light transmitted through the solution; (1)
- correct plotting; (1)
- points joined with straight lines. (1)

3(b)(iii)
Any three of:
- discs that were in detergent will continue to lose pigment because membranes are permanently damaged; (1)
- discs that were in 0.0% detergent solution will have no loss of pigment because membranes were not damaged; (1)
- discs will lose pigment more slowly/stop because there is less/no pigment left in the beetroot; (1)
- discs that were in higher concentrations of detergent will have the slowest rate of loss because they have the least pigment left. (1)
Accept reverse argument.

3(b)(iv)
Any three of:
Similarities:
- both cause pigment leakage. (1)
Differences:
- temperature leads to greater pigment leakage than detergent; (1)
- temperature effect on pigment release is sudden above 40 °C / non-significant below 40 °C, but detergent effect is gradual across the range. (1)
Accept answers in terms of light transmission if the link with pigment release is clear.

3(b)(v)
- Would be 25%.
- From the graph at 60 °C and 70 °C / after 60 °C transmission is the same / has levelled off. (1)
- Because all pigment has leaked out / there is an equilibrium when the graph levels off. (1)

(a)

The table shows the results of this investigation.

Table for Question (a) — Edexcel A-Level Biology AS

[ 9 ]

(i)

Detergent permanently damages beetroot cell membranes.

Explain what will happen to the rate of loss of betalain if these discs are then transferred to test tubes of distilled water.

[ 3 ]

(ii)

In a second investigation, the effect of temperature on beetroot cell membrane permeability was studied.

The graph shows the results of the second investigation.

Compare and contrast the effects of detergent concentration and temperature on the leakage of betalain from beetroot cells.

Figure for Question (ii) — Edexcel A-Level Biology AS

[ 3 ]

(iii)

Estimate the percentage of light transmitted at 80^ C.

Use the information in the graph and your own knowledge to support your answer.

[ 3 ]

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2.3—Core Practical 3 - membrane permeability question 1

Question (a)

Question (i)

Question (ii)

Question (iii)