Biology 9700/53 — October/November 2016

Cambridge A-Level · Planning, Analysis and Evaluation · worked solutions for every part, with the mark scheme

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Topics Planning · Analysis, Conclusions and Evaluation

Q1PlanningAnalysis, Conclusions and EvaluationFree sample

(a)

The opening and closing of stomata involves the movement of potassium ions into and out of guard cells in response to environmental factors, for example light and water stress.

This movement of potassium ions is influenced by the concentration of calcium ions in the cytoplasm of guard cells. When plants are water stressed, abscisic acid (ABA) release causes the concentration of calcium ions in the cytoplasm to increase. The increase in calcium ions causes a series of changes resulting in the loss of potassium ions, so the stomata close in light.

A student investigated the effect of calcium chloride in overcoming the effect of light on stomata.

The student was provided with:

$500\ \text{cm}^3$ of $1.0\ \text{mmol dm}^{-3}$ buffered calcium chloride ( $\text{CaCl}_2$ ) solution
freshly picked leaves from a plant that had been kept in light for 2 hours so that all the stomata were open.

The student soaked three epidermal strips of leaves in each of a range of calcium chloride solutions for 3 hours and recorded the number of closed stomata. An epidermal strip is made by peeling the epidermis from a leaf as a single layer.

(i)

Identify the independent and dependent variables in this investigation.

independent variable ______

dependent variable ______

(ii)

The student used the $1.0\ \text{mmol dm}^{-3}$ $\text{CaCl}_2$ solution provided to prepare $50\ \text{cm}^3$ of four other concentrations by reducing the concentration by half each time.

State the method that the student used to make these four concentrations.

(b)

(i)

Suggest a hypothesis that the student could test about the effect of $\text{CaCl}_2$ on the opening and closing of stomata.

(ii)

Describe a method the student could use to investigate the effect of different concentrations of $\text{CaCl}_2$ solution on the closing of stomata.

Your method should be detailed enough for another person to follow and should not include the details of how to dilute the $1.0\ \text{mmol dm}^{-3}$ solution of $\text{CaCl}_2$ .

(c)

The student also tested the hypothesis:

The stomata on the upper epidermis of a leaf and the stomata on the lower epidermis respond differently to ABA.

Young leaves that had just expanded were removed from seedling plants and cut into strips $5\ \text{mm}$ wide and $10\ \text{mm}$ long.
The upper and lower epidermis were removed from five leaf strips and put into the same concentration of buffered ABA solution.
This process was repeated for each different concentration of ABA solution.
These were left in a constant environment and supplied with carbon dioxide-free air for two hours.
The diameter of each stomatal aperture was measured using an eyepiece graticule.
The eyepiece graticule was calibrated using a stage micrometer at $\times 400$ and the stomata measured at the same magnification using the same microscope.
10 randomly selected stomata on each of 5 epidermal strips from the lower and upper epidermis were measured at each ABA concentration.

(i)

State two ways in which the method of measuring the diameter of the stomata has been standardised.

______
______

(ii)

Fig. 1.1

Fig. 1.1 shows stomata at different stages of opening.

Fig. 1.2

Fig. 1.2 shows the eyepiece graticule and stage micrometer used by the student.

The diameter of a stomatal aperture was 3 eyepiece units.

Calculate the actual size of the aperture.

actual size = ______ $\text{µm}$

(d)

Fig. 1.3

Fig. 1.3 shows the results of the experiment. The 95% confidence limits are shown for each mean stomatal aperture.

(i)

State one piece of evidence in Fig. 1.3 that supports the student’s hypothesis that:

The stomata on the upper epidermis of a leaf and the stomata on the lower epidermis respond differently to ABA.

(ii)

The confidence limit of each of the mean values plotted on Fig. 1.3 is 95%.
State what this tells you about each mean value.

(iii)

State a statistical test that could be used to find out if the difference in the stomatal aperture at different concentrations of ABA is significant.

Give a reason for your choice.

test ______

reason ______

(e)

The experimental procedure carried out by the student was considered to have given data that could be trusted to support the hypothesis:

The stomata on the upper epidermis of a leaf and the stomata on the lower epidermis respond differently to ABA.

Describe the aspects of the experimental procedure which ensure that the data can be trusted.

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