Transport in the Xylem of Plants (DP IB Biology: HL)

Define the term transpiration.

Holly is a common type of evergreen plant that can be found in British gardens. The leaves of holly bushes possess particularly thick waxy cuticles. A student investigated the rate of transpiration in holly leaves. They cut 10 leaves for set X and 10 leaves for set Y. The student then covered the leaves in set Y in petroleum jelly. After weighing each set of leaves, they attached the leaves in each set to a separate wire.The student then weighed each set of leaves at 30-minute intervals for a duration of 3 hours.

Their results are seen in the graph below.

Environmental conditions can affect the rate of transpiration in plants. State two environmental variables that should be controlled in this investigation.

As seen in the graph in part b), between 90 minutes and 120 minutes the rate of transpiration begins to slow in both sets of leaves.

Explain why this happens.

The results for the leaves in set X are different from the leaves in set Y.

Suggest an explanation for this.

A potometer can be used to investigate the water uptake of plants under different conditions. The diagram below shows how a student set up a potometer to investigate the rate of water uptake in a plant shoot.

When setting up the potometer one of the precautions the student took to ensure reliable measurements of water uptake was to dry off the leaves before taking any measurements.

Suggest a reason for this.

Identify two other precautions the student should take when setting up the potometer apparatus to ensure they obtain reliable results.

A potometer measures the water uptake of a plant in a given time.

Suggest three reasons why the measurements taken from a potometer do not represent the true rate of transpiration in a plant.

The diagram below shows a transverse section (TS) of a plant stem.

Identify the structure found at J and state its function.

In xylem vessels, hydrogen bonds form between polar water molecules.

Describe the role that hydrogen bonding plays in the cohesion-tension theory of water transport in the xylem of plants.

Marram grass is commonly found on sand dunes, an example of a dry environment where plants have evolved to survive.

Explain how marram grass leaves are adapted to minimise water loss.

Explain how a plant replaces the water it loses via transpiration.

Explain what is meant by the term halophyte.

Give two adaptations that a typical halophyte might have to help it survive in these conditions.

One mark is available for clarity of communication throughout this question.

Describe how a porous pot can be used to model the evaporation of water that occurs from the leaves of a plant.

Angiosperms (commonly known as flowering plants) are a group of plants that have vascular tissue, whereas bryophytes (mosses, liverworts and hornworts) are a group of plants that lack vascular tissue.

Suggest some advantages of possessing vascular tissue.

Explain how minerals are absorbed into plant roots from the soil.

The graph shows the effect of three different environmental factors on the rate of transpiration in a terrestrial plant.

[3]

[3]

Explain why the trend shown by line Y in the graph at part a), could result in decreased growth of a terrestrial plant.

Hydrophytes are aquatic plants which are adapted to living in very wet environments.

Water lilies are an example of a hydrophyte which has evolved to show higher rates of transpiration in order to support their growth in garden ponds.

Some adaptations of water lilies include the following:

• A thinner or absent waxy cuticle
• Stomata on the upper side of the leaf 
• Large flat leaves

Use the image above and your knowledge of plant leaf structure and transpiration to explain how these adaptations may help to maximise the rate of transpiration in hydrophytes.

Terrestrial plants will often not survive if they are surrounded by water in the same way as the hydrophytes described in part c). Flooding of crop fields has become a severe problem in the US with detrimental effects on the growth of crop plants such as potatoes and beans. Crops growing in flooded soil are unable to absorb sufficient amounts of water through their roots, leading to wilting of their leaves.

Explain why, in the event of a flood, the uptake of water into the root cells may be reduced.

Suggest what effect flooding may have on the rate of transpiration in crop plants.

The effect of humidity on the transpiration stream within the xylem of an oak tree was investigated, the results are displayed in the graphs below.

Describe how the trends shown in graph A compare with the trends shown in graph B.

Outline how the changes in humidity lead to the changes in water tension measured in the xylem at the top of the tree.

During the investigation, the scientist also measured the diameter of the tree trunk, the results can be seen in the table below:

Explain how this data and the data in the graphs from part a) support the cohesion-tension theory.

The scientist who carried out the investigation in part a) concluded from his data that water moves through the xylem through the cohesion tension mechanism.

Evaluate his conclusion based on the validity of the evidence provided by the results of this investigation.

The graph shows the relationship between vapour pressure deficit and the rate of transpiration in wheat plants.

Vapour pressure deficit (VPD) is the difference between the amount of moisture in the air and how much moisture the air can hold when its saturated.

Using this information, state whether a high vapour pressure deficit would indicate that air humidity was high or low.

With reference to the data shown in the graph from part a), explain the effect that VPD has on transpiration of wheat between 6:00 and 12:00.

Explain the pattern seen in the results from part a) between 13:00 and 18:00.

Suggest how information about vapour pressure deficit may be useful for growers of wheat plants.

Some students were investigating the effect of capillary tube diameter on the uptake of water by capillary action.

They set up three capillary tubes with diameters of 0.6 mm, 0.8 mm and 1.0 mm and measured the distance moved by water in 30 seconds. Their results can be seen below:

Explain the observations made by the students.

The graph shows the differences in water movement in the xylem of a tree. The measurements were taken in the branches at the top of the tree and in the trunk of the tree.

Using ideas illustrated by the capillary model in part a), explain the results shown in the graph.

Identify a limitation of using capillary tubing as a model to represent the movement of water through the xylem.

The students set up a potometer with a 0.8 mm diameter capillary tube to measure the rate of transpiration in a branch removed from a tree. Over a period of 30 minutes, the students noted that the bubble moved 13.7 cm.

Calculate the rate of transpiration shown by the leaf in mm³hr^-1. Use the equation πr² to calculate the area of a circle.

One mark is available for clarity of communication throughout this question.

Describe the pressure changes which occur in the xylem as a result of water moving through the transpiration stream.

Compare and contrast the different adaptations of xerophytes and halophytes.

Outline the routes that water can take from the soil, through the root cortex to the xylem.