Homeostasis in Plants (CIE A Level Biology)

Fig. 1 shows a cross section of a leaf as viewed through a light microscope at x250 magnification.

Fig. 1

Fig. 1 indicates that the image size of a palisade mesophyll cell is 2 cm. 

Calculate the actual length of a palisade mesophyll cell.

Identify structures P, Q, and R in Fig. 1

(i)

Give two features of structure R that help it to carry out its function;

[2]

Suggest how the features given in part i) help structure R to carry out its function;

[2]

Structure R surrounds a stoma.

State the role of the stomata in plant homeostasis.

Fig. 1 shows an electron microscope image of two guard cells.

Fig. 1

Give two features other than the stomatal pore that help to identify the cells in Fig. 1 as guard cells.

If the guard cells in Fig. 1 were living, suggest the environmental conditions that might be present.

Explain the process that would cause the size of the stomatal pore in Fig. 1 to increase.

State the conditions in which abscisic acid (ABA) plays a role in plant physiology.

Name and outline the roles of the two inorganic ions that play a role in the action of abscisic acid in leaf function. 

A leaf has a surface area of 10.5 cm² on its lower side. 

Stomata are found at a density of 112 per mm² in the species of leaf in question.

Calculate an estimate of how many stomatal guard cells this leaf will have on its lower surface. 

In many plant species, the guard cells on the underside of the leaf contain chloroplasts, whereas the surrounding cells of the lower epidermis do not.

This is a surprising finding given the low light intensity usually found on the underside of leaves. 

Suggest and explain a role for the chloroplasts that are found in the guard cells on the underside of leaves. 

A student who wanted to investigate stomata looked at a lower leaf surface under a microscope. Their field of view is shown in Fig. 1

Fig. 1

The diameter of the microscope’s field of view was 1.4 mm and the lower leaf had a total surface area of 220 mm². The area of a circle can be calculated using the following formula:

πr²

Calculate the total number of stomata on the lower surface of the student’s leaf. Give your answer to the nearest whole number.

The student read about some research that had been carried out into the effect of atmospheric carbon dioxide on plant growth. Three different carbon dioxide levels were tested; high, medium, and low. The results relating to stomatal aperture are shown in Fig. 2. Note that the bars show standard deviation and that the term aperture refers to the opening between the guard cells. 

Fig. 2

Describe what the student could conclude from Fig. 2 about the effect of atmospheric carbon dioxide on stomata.

Suggest an explanation for the results shown in Fig. 2

The study done by the student in part b) also looked at the effect of carbon dioxide concentration on plant biomass. Fig. 3 shows the results.

Fig. 3

The student concluded from Fig. 3 that the currently increasing carbon dioxide levels in the earth's atmosphere could have a positive effect on plant growth around the world.

Use Fig. 3 and your own knowledge to evaluate the student's conclusion.

Fig. 1 shows the percentage of stomata that are open over a 96 hour period.

Fig. 1

Describe the opening and closing of stomata in Fig. 1.

[1]

Other than the changing light intensity shown in Fig. 1, give two factors that could be contributing to the pattern described in part i).

[2]

Outline why the pattern shown in Fig. 1 is advantageous to the plant.

Research has shown that increased light intensity decreases the activity of the chemical abscisic acid (ABA) in plants.

Explain how each of the following circumstances would cause a decrease in ABA activity at increased light intensity:

Electrons from chlorophyll cause an ABA precursor molecule to be converted into a chemical called zeaxanthin.

[1]

Glucose deactivates ABA.

[1]

Enzymes that break down ABA are activated at high concentrations of oxygen.

[1]

At low light intensity the level of ABA inside a plant increases.

Explain how this affects stomata.

Scientists investigated the stomatal pore size (µm) and the concentrations of potassium and starch (µmol) within guard cells in basil leaves over 24 hours.

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

Fig. 1

Describe the results of the experiment. 

The scientists calculated the mean stomatal pore diameters and the standard deviation. 

Describe what the standard deviation shows in relation to the data in Fig. 1.

Use the data in Fig. 1 to explain the changes in stomatal pore diameter and potassium content after midday. 

The scientists suggested that starch molecules act as a sink for osmotic ions leading to stomatal closing.

Describe the evidence in Fig. 1 that supports this.