A student investigated the effect of different growth media on the germination and growth of pollen. The growth media tested are shown in Table 1.1. Table 1.1 component concentration of components in each growth medium :--- :--- :--- :--- medium A medium B medium C sucrose solution / % 10 10 10 boric acid / $\text{mg dm}^{-3}$ 0 100 100 calcium nitrate / $\text{mg dm}^{-3}$ 0 300 300 magnesium sulfate / $\text{mg dm}^{-3}$ 0 0 200 potassium nitrate / $\text{mg dm}^{-3}$ 0 0…

Biology 9700/51 — May/June 2012

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 student investigated the effect of different growth media on the germination and growth of pollen.

The growth media tested are shown in Table 1.1.

Table 1.1

component	concentration of components in each growth medium
	medium A	medium B	medium C
sucrose solution / %	10	10	10
boric acid / $\text{mg dm}^{-3}$	0	100	100
calcium nitrate / $\text{mg dm}^{-3}$	0	300	300
magnesium sulfate / $\text{mg dm}^{-3}$	0	0	200
potassium nitrate / $\text{mg dm}^{-3}$	0	0	100

Fig. 1.1 shows the apparatus used by the student in this investigation.

Fig. 1.1

Fig. 1.1

The student carried out the following procedure when investigating each growth medium, A, B and C:

pollen from a mature anther of a flower was removed with a paint brush
the pollen was mixed with two drops of the growth medium on a slide
the slide was placed in the Petri dish as shown in Fig. 1.1
the apparatus was kept at room temperature
every 10 minutes the slide was removed and the pollen observed using a light microscope
the percentage of the pollen germinated was estimated
the lengths of the 10 longest pollen tubes in a field of view were measured
the mean pollen tube length was calculated.

The results of the three investigations are shown in Table 1.2.

Table 1.2

	mean length of ten longest pollen tubes / $\mu\text{m}$
time / min	10	20	30	40	50	60	70
medium A	23.9	48.5	72.4	97.6	128.2	156.0	180.0
medium B	27.6	58.4	89.9	121.7	154.8	186.8	219.2
medium C	38.2	76.1	112.6	150.6	188.5	226.5	264.7
	percentage germination
medium A	5	18	26	38	51	55	57
medium B	8	12	22	31	46	52	61
medium C	3	8	18	28	36	48	56

(a)

Identify the independent variable and the two dependent variables.

independent ______

dependent ______

(b)

(i)

Describe how the student could have measured the length of the pollen tubes using a microscope.

(ii)

Suggest why the percentage germination was calculated.

(c)

The student carried out $t$ -tests on the results to compare the lengths of the pollen tubes when grown in different media.

To carry out a $t$ -test the number in the sample must be counted. It is also necessary to carry out two calculations from the data.

(i)

State the two calculations that must be carried out in order to complete a $t$ -test.

(ii)

State why the student decided to use 18 degrees of freedom in each of the $t$ -test calculations.

(iii)

The student carried out a number of $t$ -tests to find out if, after 70 minutes, the difference in mean pollen tube length is significant:

between medium A and medium B $t = 2.50$
between medium A and medium C $t = 3.56$
between medium B and medium C $t = 1.94$

Table 1.3 shows the critical values for the $t$ -test.

Table 1.3

degrees of freedom	10	12	14	16	18	20	22	24	26	28	30	40	50	60
probability 0.05	2.23	2.18	2.14	2.12	2.10	2.09	2.07	2.06	2.06	2.05	2.04	2.02	2.01	2.00
probability 0.01	3.17	3.06	2.98	2.92	2.88	2.85	2.82	2.80	2.78	2.76	2.75	2.70	2.68	2.66

State what conclusions can be drawn about the significance of the differences in mean lengths from the three values of $t$ given above.

(d)

From the results in Table 1.2 and the $t$ -tests the student concluded that:

calcium has no effect on the germination of pollen grains but increases the growth rate of pollen tubes
sucrose is needed for pollen tube germination but has no effect on the growth rate of the pollen tube.

(i)

With reference to Table 1.2 state the evidence that supports these conclusions and the evidence that does not support these conclusions.

evidence that supports these conclusions ______

evidence that does not support these conclusions ______

(ii)

Outline a further investigation that the student might carry out to provide more support for the conclusions about the role of sucrose in the germination of pollen and in the growth of pollen tubes.

(iii)

Predict the results of this investigation if the student’s conclusion about the effects of sucrose is correct.

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