7.2 Transport Mechanisms

A study investigated the sucrose levels in a courgette leaf during leaf development. The results of the study can be seen in Fig. 1.

Fig. 1

State the days during which the courgette leaf functions as a sink.

[1]

(ii)

State the days during which the courgette leaf could function as a source.

[1]

Use the information in Fig. 1 to explain why the leaf is able to transition from being a sink to a source.

Sucrose is transported from sources to sinks in specialised vascular tissue. Fig. 2 shows the vascular tissue used for the transport of sucrose in plants. 

Fig. 2

Tissue B in Fig. 2 contains many mitochondria.

Explain how this assists in the loading of sucrose into the sieve tube.

Explain how the process in part (c) enables mass flow of sucrose in the sieve tube.

Fig. 1 shows apparatus that can be used to measure the loss of water vapour from leaves.

Fig. 1

The water vapour given out by the area of leaf under the cup increases the pressure inside the tubing, causing the water level in the graduated tube to go down.

Some students used the apparatus in Fig. 1 to test the hypothesis:

The loss of water vapour from the lower surface of the leaves of a plant is greater per unit time than the loss of water vapour from the upper surface of the leaves of the same plant.

State the independent variable and the dependent variable in this investigation.

[2]

(ii)

Describe a method by which the students could use the apparatus shown in Fig. 1 to measure loss of water vapour in order to test the hypothesis.

Your method should be set out in a logical order and be detailed enough to let another person follow it.

[6]

The hypothesis that the students tested was:

The loss of water vapour from the lower surface of the leaves of a plant is greater per unit time than the loss of water vapour from the upper surface of the leaves of the same plant.

The students decided that, to make a valid comparison, they needed to work out the water loss per unit time per unit area of leaf.

Describe how the students obtained the measurement needed to work out the results as per unit area of leaf.

[2]

(ii)

Describe how the students could use this measurement and their results for the loss of water vapour to find out if their results support the hypothesis.

[3]

(iii)

Sketch a bar chart on Fig. 2 to show the results if the hypothesis was supported. You should include the axis labels and the units.

[3]

Fig. 2

The apparatus shown in Fig. 1 was used in another series of experiments to measure water vapour loss from the same leaf in different experimental conditions for the same period of time.

Table 1 shows the results.

Table 1

On Table 1, draw circles around two of the values that may be anomalous.

[2]

(ii)

The aperture of stomata affects the loss of water vapour.

State one conclusion that can be made about the effect of experimental conditions on the aperture of stomata in this plant.

Translocation in the phloem can be regarded as the plant equivalent of the mammalian circulatory system. 

Evaluate this statement. 

Two parameters of the contents of the phloem have an influence on the rate of translocation within that phloem. These are collectively called water potential and given the symbol Ψ.

There are two types of water potential, pressure water potential and solute water potential, which are given the symbols Ψ_p and Ψ_s respectively.

Suggest what gives rise to each type of water potential.

Sugar beet (Beta vulgaris subsp. vulgaris) is the second most important crop grown for table sugar (behind sugar cane), accounting for around 30% of worldwide production. The roots are harvested and processed for their high sugar content. 

Yields of sugar beet can be affected by Beet Jaundice Virus, which is spread by aphids and turn crops yellow (see image below). 

Suggest how Beet Jaundice Virus decreases yields for commercial beet farmers. 

Fig. 1 shows a micrograph image of the lower epidermis of a leaf.

Fig. 1

Use the scale bar in Fig. 1 to calculate the magnification of the micrograph image.

[2]

(ii)

Use the answer to part (i) to calculate the actual length of a guard cell, represented by the line marked X. The length of the line marked X in the image is 4.2 cm.

[2]

The guard cell marked X in Fig. 1 surrounds a single stoma, plural stomata.

Describe the process of water loss that occurs at the stomata.

One way of measuring the rate at which the process described in part (b) is taking place in a tree is to observe the diameter of the tree trunk.

Fig. 2 below shows the results of an experiment in which tree trunk diameter was measured, along with light intensity and temperature, during the course of a 24 hour period.

Fig. 2

Explain the changes in trunk diameter shown in Fig. 2 throughout the 24 hour period.

The concentration of water vapour in the air, known as humidity, can also influence the rate at which the process described in part (b) can occur.

State and explain the impact that increased humidity would have on the process described in part (b).

An investigation was carried out into mass change in two sets of leaves over a 3-hour period. The two sets of leaves, set X and set Y, were removed from two different species of plant and were suspended from a wire throughout the investigation. The mass of the leaves was measured every 30 minutes.

The results of the investigation are shown in Fig. 1 below.

Fig. 1

The starting mean mass of the leaves in Set X in Fig. 1 was 0.1 g. After 90 minutes the mean mass was 0.096 g.

Calculate the percentage change in mass.

Explain the change in mass of the leaves in set X.

Fig. 2 shows images of light micrographs of a leaf from set X and a leaf from set Y.

Fig. 2

(i)

Identify the leaf that comes from set Y.

[1]

(ii)

Explain your choice from part (i).

[2]

Xerophytic plants have adaptations to reduce water loss.

Other than those relevant to part (c), state and explain two structural leaf adaptations that reduce water loss.

Fig. 1 is a diagram of a section through part of a young root.

Fig. 1

Describe the pathways by which water passes from the soil to the cells of the cortex shown in Fig. 1.

Fig. 1 shows the location where mineral ions in the soil enter the plant.

There is a greater density of mitochondria in cell X than in a cell of the root cortex.

With reference to the uptake and transport of mineral ions, suggest why there is a greater density of mitochondria in cell X than in a cell of the root cortex.

Transpiration in plants can be investigated using a potometer, which measures water uptake by plants. Fig. 1 shows a potometer that was used by a student.

Fig. 1

As water is lost by the leaves through transpiration, the air bubble moves along the graduated tube.

The student used this apparatus to investigate the effect of light intensity on the rate of transpiration in plants.

State the independent variable and the dependent variable in this investigation.

[2]

(ii)

List three variables that should be controlled in this investigation and describe how the student could standardise two of these variables.

[3]

Describe how the student could set up and use the potometer shown in Fig. 1 to investigate the rate of transpiration at different light intensities.

Your method should be set out in a logical way and be detailed enough to let another person follow it.

The student carried out further experiments, using the same apparatus, to investigate the effect of two different environmental carbon dioxide concentrations on the rate of transpiration.

These experiments were carried out at a high light intensity and at a low light intensity.

The leafy shoots used in the experiments were taken from the same plant and each shoot had five leaves.

The student calculated the percentage reduction in the transpiration rate from 50 ppm to 730 ppm of carbon dioxide at low light intensities.

The results are shown in Table 1.

ppm = parts per million

Table 1

Complete Table 1 by calculating the percentage reduction in transpiration rate from 50 ppm to 730 ppm of carbon dioxide at the high light intensity.

Show your working.

[2]

(ii)

The student concluded that, as carbon dioxide concentration increased, the transpiration rate in plants decreased at all light intensities.

Explain why this conclusion may not be valid.

[2]