Transport in Plants (OCR A Level Biology)

Sago pondweed has evolved many adaptations to its aquatic environment. Three such adaptations are described below.

Explain the advantage of each adaptation.

Adaptation 1: No waxy cuticle

Adaptation 2: Stem tissue that contains air spaces

Adaptation 3: A thin, flexible stem

Plants lose water by transpiration.

The rate of transpiration varies between different species of plant.

The rate of transpiration can be measured using a potometer.

Plan an investigation into the rate of transpiration in two species of plant that would allow valid data to be collected.

Details of how to set up a potometer are not required.

This question is about the impact of potentially harmful chemicals and microorganisms.

Salts that a plant needs, such as nitrates and phosphates, are taken into root hair cells by active transport.

For which macromolecule does a plant need both nitrogen and phosphorus?

[1]

(ii)

Flooding of fields by seawater can damage crops. Seawater contains dissolved salts, including sodium chloride.

How would flooding affect soil water potential?

[1]

(iii)

Sodium chloride in solution dissociates into Na+ and Cl^-. 

Explain how the Casparian strip prevents these ions from reaching the xylem of the plant by the apoplast pathway.

[2]

Following their formation, assimilates are transported throughout the plant by translocation in phloem.

Phloem sap mainly consists of carbohydrate in the form of sucrose, but also contains other solutes.

(i)

Suggest why it is beneficial to the plant for the carbohydrate to be transferred throughout the plant in the form of sucrose rather than as an alternative carbohydrate.

[2]

(ii)

How is transport in the phloem similar to and different from transport in the xylem?

[2]

Assimilates are loaded into the phloem at the ‘source’ and then transported to the ‘sink’.

(i)

Explain, with a suitable example, how some parts of the plant can act as both a ‘source’ and a ‘sink’.

[2]

(ii)

Fig. 19.1 is a diagram that represents the loading of sucrose into the phloem at the ‘source’.

Fig. 19.1

With reference to Fig. 19.1, explain the process of the loading of sucrose into the phloem and its movement in the phloem.

[6]

Fig. 19.2 is a diagram of a potato plant. Potatoes are tubers which are underground storage organs.

Fig. 19.2

Actively growing tissues have a high demand for carbohydrates. This means that a lot of phloem sap is directed to these tissues and requires sucrose to be unloaded in large amounts. 

In an investigation, potato plants were modified by having a gene for invertase inserted into their DNA so that the gene for invertase would be expressed in the tubers. Invertase is responsible for catalysing the hydrolysis of the disaccharide sucrose. 

A trial experiment was carried out to compare the properties of the modified plants with those that had not been modified. After harvesting, the tubers of three of each type of plant were compared. The results shown in Table 19.1

Table 19.1

Name the bond that is hydrolysed by invertase.

[1]

(ii)

The potato tubers contain monosaccharides.

Compare the concentration of monosaccharides in the modified tubers with those that were not modified.

[2]

(iii)

In the modified plants, the unloading of sucrose is increased in the tubers compared with those that were not modified.

The transport of sucrose to the tubers was also increased in the modified plants.

Using the data and the information given, deduce a possible mechanism to account for the increased unloading and transport of sucrose in the modified plants.

[4]

(iv)

The trial experiment compared the properties of modified potato plants with those that were not modified.

Analyse the data and draw conclusions about the yield of the tubers of modified plants compared with those tubers from plants which had not been modified.

[3]

Fig. 1 below shows a microscope cross section through the centre of a plant root.

Fig. 1

Identify the tissue types labelled M and N in Fig. 1.

Table 1 below shows some results from an investigation into the effect of temperature on root pressure in celery plants.

Table 1

State which statistical test could be used to analyse the results in Table 1. Give a reason for your answer.

The investigation in part (b) also measured the effect of temperature on the activity of celery ATPase enzymes. The results are shown in Table 2 below.

Table 2

Use the data in Table 2 and your own knowledge to explain the effect of temperature on root pressure shown in Table1 in part (b).

Once water enters the vascular tissue in the root, it is drawn up the plant stem in the transpiration stream.

Explain the upward movement of water via the transpiration stream.