Transport in plants

Write an essay on one of the topics below.

Either

Phosphorus-containing substances and their importance in biological systems.

Or

The mechanisms and importance of transport within organisms.

Chitin is a polysaccharide. The chitin monomer is a β-glucose molecule with one OH group replaced by an NHCOCH₃ group. NHCOCH₃ can be represented by N(Ac).

Figure 1 shows the monomer that forms chitin and the chitin polymer.

Chitin has a similar structure to cellulose.

Use Figure 1 to describe three ways the structure of chitin is similar to the structure of cellulose.

Chitin keeps the tracheae open in the tracheal system of gas exchange in an insect. Gas exchange does not occur in the tracheae.

Explain the importance of one adaptation of the gas exchange surface in the tracheal system of an insect.

Lignin is a polymer found in the walls of xylem vessels in plants. Lignin keeps the xylem vessel open as a continuous tube.

Explain the importance of the xylem being kept open as a continuous tube.

Mangrove trees grow near the sea. Sea water surrounds the lower parts of the trees at high tide.

Scientists investigated the rate of transpiration in a mangrove tree. Figure 9 shows the scientists’ results.

Explain the rate of transpiration between 5 am and midday shown in Figure 9.

Use Figure 9 to calculate the percentage increase in the rate of transpiration from 1 pm to 2 pm.

The higher rate of transpiration at high tide shows that the mangrove tree is absorbing water from the sea water surrounding its roots.

Describe an experiment that you could do to investigate whether the mangrove root cells have a lower water potential than sea water.

You are given:

• a piece of fresh mangrove root

• sea water

• access to laboratory equipment.

Describe the transport of carbohydrate in plants.

Compare and contrast the structure of starch and the structure of cellulose.

Describe the complete digestion of starch by a mammal.

Plants transport sucrose from leaves to other tissues for growth and storage. SUT1 is a sucrose co-transporter protein.

Scientists investigated whether the cells of tobacco plant leaves used SUT1 to transport sucrose to other tissues.

The scientists used a radioactively labelled DNA probe to show that the cells of tobacco plant leaves contained the SUT1 gene.

Describe how they would do this.

Do not include PCR in your answer.

To study the role of SUT1 in tobacco plants, scientists reduced the expression of the SUT1 gene.

When the SUT1 gene is transcribed, the SUT1 mRNA produced is called ‘sense’ SUT1 mRNA. The scientists genetically modified plants by inserting an extra gene so that this also allowed the production of ‘antisense’ SUT1 mRNA.

The scientists had two types of tobacco plants:

• type A – plants that were genetically modified

• type B – plants that were not genetically modified.

Suggest how the production of ‘antisense’ SUT1 mRNA in type A plants would reduce the expression of the SUT1 gene.

The scientists hypothesised that lower rates of sucrose transport from leaves would cause reduced growth.

To test this hypothesis, the scientists provided leaves of type A and type B plants with labelled carbon dioxide (¹⁴CO₂). To estimate sucrose transport out of leaves, they measured the percentage of¹⁴C remaining in the leaves for 16 hours.

Figure 5 shows their results.

Calculate the ratio of percentage of¹⁴C remaining in leaves of type B to type A plants 16 hours after providing¹⁴CO₂

In type B plants, the percentage of¹⁴C remaining in the leaves does not reach zero per cent, as shown in Figure 5.

Suggest two reasons why.

1

2

The scientists measured physiological differences between type A plants and

type B plants.

Table 3 shows the scientists’ results as they presented them.

Sucrose is able to inhibit the production and activity of rubisco in leaves of a plant. Type A plants have decreased dry mass compared with type B plants.

Use all the information to suggest and explain how the physiological factors in Table 3 would contribute to the decreased dry mass observed in type A plants.

Turn over for Section B

Section B

one question.

You are advised to spend no more than 45 minutes on this section.

Scientists investigated stomatal density on leaves of one species of tree.

Figure 9 shows three examples of the square fields of view the scientists used to calculate a mean stomatal density.

Calculate the mean stomatal density in the three fields of view in Figure 9.

Give your answer as number of stomata per mm²

Show your working.

The scientists used leaves from individual trees that had grown in different areas of

the world in different years. Each tree had grown in an area and year with known carbon dioxide concentration.

Their results are shown in Figure 10.

Give a null hypothesis for this investigation and name a statistical test that would be appropriate to test your null hypothesis.

From 1910 to 2000, the carbon dioxide concentration in the atmosphere increased from 300 parts per million to 365 parts per million.

Use Figure 10 to calculate the mean rate of change in stomatal density from 1910 to 2000.

Give your answer as number of stomata per mm² per 10-year period.

Show your working.

A journalist saw Figure 10 and suggested that future increases in atmospheric carbon dioxide concentration could result in less transpiration.

Evaluate his suggestion.

Scientists investigated the effect of a heat treatment on mass transport in barley plants.

• They applied steam to one short section of a leaf of the heat-treated plants. This area is shown by the arrows in Figure 9.

• They did not apply steam to the leaves of control plants.

• They then supplied carbon dioxide containing radioactively-labelled carbon to each plant in the area shown by the rectangular boxes in Figure 9.

• After 4 hours, they:

• found the position of the radioactively-labelled carbon in each plant. These results are shown in Figure 9.

• recorded the water content of the parts of the leaf that were supplied with radioactively-labelled carbon dioxide. These results are shown in Table 4.

Figure 9

A – Heat-treated Plant
B – Control Plant, not heat treated

The scientists concluded that this heat treatment damaged the phloem.

Explain how the results in Figure 9 support this conclusion.

The scientists also concluded that this heat treatment did not affect the xylem.

Explain how the results in Table 4 support this conclusion.

The scientists then investigated the movement of iron ions (Fe³⁺) from the soil to old and young leaves of heat-treated barley plants and to leaves of plants that were not heat treated. Heat treatment was applied half way up the leaves. The scientists determined the concentration of Fe³⁺ in the top and lower halves of the leaves of each plant.

Their results are shown in Figure 10.

What can you conclude about the movement of Fe³⁺ in barley plants? Use all the information provided.

A student used a potometer to measure the movement of water through the shoot of a plant. The potometer is shown in Figure 5. As water is lost from the shoot, it is replaced by water from the capillary tube.

In one experiment, the air bubble moved 7.5 mm in 15 minutes. The diameter of the capillary tube was 1.0 mm.

Calculate the rate of water uptake by the shoot in this experiment.

Give your answer in mm³ per hour. Show your working. (The area of a circle is found using the formula, area = πr²)

The student wanted to determine the rate of water loss per mm² of surface area of the leaves of the shoot in Figure 5.

Outline a method she could have used to find this rate. You should assume that all water loss from the shoot is from the leaves.

The rate of water movement through a shoot in a potometer may not be the same as the rate of water movement through the shoot of a whole plant.

Suggest one reason why.

Aquaporins are channel proteins that allow the diffusion of water across membranes. One type of aquaporin, called PIP1, can also transport carbon dioxide molecules across membranes.

Figure 6 shows the structure of a water molecule and of a carbon dioxide molecule. They are drawn to the same scale.

Suggest two reasons why water molecules and carbon dioxide molecules can both pass through PIP1.

The scientists first produced transgenic poplar trees. These trees all had a length of foreign DNA inserted into them. This DNA led to the production of single-stranded RNA that specifically inhibited expression of the gene for PIP1.

The scientists then measured the difference in the amount of PIP1 in leaves of transgenic poplars and in leaves of wild type poplars without the foreign DNA. The amount of PIP1 in the transgenic poplars was approximately 15% of that in the wild type poplars.

Using this information, what can you conclude about the effect of the foreign DNA in the transgenic poplar trees?

The transgenic poplars still produced some PIP1. Suggest why.

The scientists investigated the importance of PIP1 in the movement of water and carbon dioxide through the tissues of leaves of poplar trees.

They measured the mean rates of movement of carbon dioxide and water through the tissues of leaves of transgenic poplars and through the tissues of leaves of wild type poplars.

Their results are shown in Figure 7.

Figure 7

Using only Figure 7, evaluate the importance of PIP1 in the movement of carbon dioxide and water through leaves of poplar trees.

Section B

one question.