AS & A Level Biology - 9700 Paper 3 2014 Summer Zone 4

(a) (i) Water is added to the beaker in Fig. 1.1. Describe the expected trend in the concentration of ascorbic acid in the water over a period of 15 minutes.
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(iv) A plant tissue contains 0.0001 mol dm^-3 ascorbic acid and starch. Suggest how you would make sure that the iodine test showed the presence of all the starch (100%).
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(ii) Record the volume of I needed to reach the end-point.
volume ............. [1]

You are required to:
• set up Visking tubing containing P as in Fig. 1.3
• decide the level of water to put into the beaker
• remove samples of the water surrounding the Visking tubing at 5 minute intervals for 15 minutes
• compare the ascorbic acid concentrations in the samples.

(iii) Draw on Fig. 1.3 the level of the water
• before you remove any samples (label 'before'),
• after the total volume of water needed for all the tests has been removed (label 'after'). [2]

(iv) In order to compare the ascorbic acid concentrations, state one variable which you will need to standardise when finding the volume of I added to each sample.
Describe how you will standardise this variable.
variable ...............................................................
description ..........................................................................................................................
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(v) Prepare the space below and record your results. [5]

(vi) Describe how the results support your expected trend as stated in (a)(i).
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(vii) If you had been provided with 1.0% ascorbic acid solution, suggest how you would modify this investigation to find the percentage concentration of ascorbic acid in the water after 15 minutes.
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(viii) A systematic error occurs when apparatus with scales are used, since the scales may be slightly different.
For example, when measuring the same line, two rulers may give different lengths. However, as long as the same ruler is used for all the measurements, the trend is not affected because the error is consistent.

State one piece of apparatus used in this investigation that may have a systematic error. Suggest whether this affected your results and give a reason for your answer.
apparatus ..........................................................................................................................
reason .................................................................................................................................
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(b) Iodine solution (iodine in potassium iodide solution) turns blue-black when starch is present in plant tissues.
However, as ascorbic acid is also found in plant tissues, some scientists investigated the effect of testing for starch with iodine solution when there was ascorbic acid present.

The concentration of ascorbic acid was 0.0001 mol dm^-3 and the concentration of starch solution was standardised.

The percentage of starch which reacted with the iodine solution was measured.

The results are shown in Table 1.1.

Table 1.1

| volume of iodine solution /cm³ | percentage of starch which reacted with iodine solution |
|-----------------------------------------|-----------------------------------------------------|
| 0.0 | 0.0 |
| 0.5 | 2.0 |
| 1.5 | 5.0 |
| 2.0 | 36.0 |
| 2.5 | 68.0 |

(i) Plot a graph of the data shown in Table 1.1.
You will need to consider the answer to (b)(ii) before you plot your graph. [4]

(ii) Estimate the volume of iodine solution needed for 100% of the starch to be reacted. Show on your graph how you obtained the volume of iodine solution.
volume of iodine solution ......................................................... cm³ [1]

(iii) Explain how the presence of ascorbic acid may affect the use of iodine solution as a test for the presence of starch in different plant tissues.
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(a) M1 is a stained transverse section showing the tissues where there is gas exchange in a mammal.

(i) Make a large drawing to show the details of three whole structures which are specialised for gas exchange (alveoli). The walls of one alveolus must be touching the walls of the other two alveoli. On your drawing, use a ruled label line and label to show one nucleus. [4]

(ii) On your drawing, use a ruled label line and label to show where gas exchange takes place. [1]

(iii) State one observable feature, shown in your drawing, that supports the conclusion that gas exchange is a function of this tissue. Explain how this feature enables gas exchange to take place.

feature ....................................................................................................................................

explanation ..........................................................................................................................
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(b) Fig. 2.1 is a photomicrograph of a stained transverse section through the tissues in a leaf where gas exchange takes place. This plant species is native to South America.



(i) Use a ruled label line and the label Z on Fig. 2.1 to show one observable feature which has reduced transpiration. Explain how this feature has decreased the rate of transpiration.

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(ii) Make a large drawing of the whole cells shown inside the area shown by the line on Fig. 2.1. On your drawing, use a ruled label line and label to one guard cell. [5]

(iii) Calculate the magnification of Fig. 2.1 using the scale bar. You may lose marks if you do not show your working or if you do not use appropriate units.

magnification × .......................................................... [4]