Transpiration

A student uses this apparatus to investigate water loss from a plant.

This is the student’s method. step 1 measure the total mass of the flask, water, oil, cotton wool and plant step 2 leave the apparatus for 48 hours step 3 measure the new total mass of the flask, water, oil, cotton wool and plant step 4 calculate the rate of change of mass during the 48 hours Repeat steps 1–4 with the same conditions.

Name the piece of apparatus used in this investigation.

The total mass changes from 207.25 g to 204.50 g in the first 48 hours. Calculate, in grams per hour, the rate of change of mass. Give your answer to two significant figures.

rate of change of mass =_........ grams per hour

Explain why the student repeats the investigation with the same conditions.

The student then changes the conditions surrounding the plant.

Describe how the student could investigate the effect that moving air has on the change in mass.

Explain the effect that faster moving air has on the change in mass.

Describe how the student could modify the investigation to study the role of roots in the loss of water from a plant.

Transpiration (water loss) is affected by several environmental factors.

A student investigates the effect of light intensity on water loss from a plant shoot.

The diagram shows some of the apparatus the student uses.

The student predicts that water loss from a plant shoot will increase as light intensity increases.

Describe how to set up and use the student’s apparatus to test this prediction.

State the dependent variable in this investigation.

Another student uses the apparatus to collect and process the data.

Table 1 shows their results.

Explain why light intensity changes the mean volume of water lost by the shoot.

The student repeats the experiment with a plant that is adapted to live in desert environments.

Table 2 shows the results for this desert plant.

The desert plant has adaptations to survive in desert environments where not much water is available.

Explain why the results for this desert plant, in table 2, are different from the results in table 1.

Read the passage below. Use the information in the passage and your own knowledge to answer the questions that follow. Plants to the rescue Since the early 20th century, the concentration of carbon dioxide in the atmosphere has increased rapidly. This has further increased in recent years due to more cars and the increased demand for electricity for homes and industry. Scientists have found that plants play a critical part in removing this excess carbon dioxide from the atmosphere. Using computer models, the scientists concluded that photosynthesis has increased by 30 per cent. The scientists measured carbonyl sulfide found in ice cores and air samples. In addition to carbon dioxide, plants take in carbonyl sulfide gas during their natural carbon cycle, and this is frequently used as a measure of photosynthesis on a global scale. Terrestrial plants are removing about 29 per cent of carbon dioxide emissions that would otherwise contribute to an increase in the atmospheric carbon dioxide concentration. A carbon sink is an ecosystem, such as a forest, that absorbs more carbon dioxide than it releases. The size of the carbon sink depends on the rate of photosynthesis but also on the levels of deforestation and respiration. The model the scientists used showed that the role of photosynthesis in producing a carbon sink in land plants is larger than estimated in most other models. Other scientists are less confident about using carbonyl sulfide as a measure of photosynthesis. Plant absorption of carbonyl sulfide can vary depending upon the amount of light the plants receive. Therefore, the measure of global photosynthesis could be overestimated. Regardless of the rate at which photosynthesis has increased, scientists agree that excess carbon dioxide is boosting the growth of plants. Trees are becoming leafier, and there is more wood. The wood is where most of the carbon is stored in 25 the plant. In experimental research, scientists exposed plants to double the normal concentration of carbon dioxide found in the atmosphere. Under these increased carbon dioxide conditions, the composition of the leaf tissues changed. This made the leaves tougher for herbivores to eat and made it harder for insect larvae 30 to grow. Scientists have also observed that when plants are exposed to increasing levels of carbon dioxide, the size of the stomatal pores on a leaf increases.

Explain why more cars would result in an increase in atmospheric carbon dioxide. (Lines 1-3)

Explain how increased carbon dioxide can cause climate change.

Explain why increasing carbon dioxide concentration can increase photosynthesis in plants.

Explain why the carbon sink depends upon respiration and deforestation as well as photosynthesis. (Lines 13 to 15)

Light affects gas exchange in leaves. (Lines 19 and 20) Describe an experiment that shows how the net gas exchange from a leaf varies with light intensity.

The photograph shows stomata on a small square of the lower surface of a leaf.

(Source: © Kallayanee Naloka/Shutterstock) The total area of the lower surface of the leaf is 150 cm². Using the photograph and the total lower surface area of the leaf, estimate the number of stomata on the lower surface of the leaf.

number of stomata =_........

Stomata also have a role in water transport in the plant. Explain the role of stomata in water transport.

A student uses this method to investigate water uptake and water loss by a plant shoot.

• pour 100 cm³ of water into a measuring cylinder

• place a plant shoot into the measuring cylinder

• cover the surface of the water with oil

• place the measuring cylinder and plant shoot on a balance and record the total mass

• shine light on the plant shoot using a lamp

• record the volume of the water in the measuring cylinder after four days, and after eight days

• record the total mass of the measuring cylinder and plant shoot after four days, and after eight days

The diagram shows the student’s apparatus.

State the reason for using the oil.

The table shows the student’s results.

The volume of water taken up by the plant shoot is equal to the change in volume of water in the measuring cylinder. This is called the water uptake. Calculate, in cm³ per day, the mean rate of water uptake by the plant shoot during the eight days.

mean rate =_........ cm³ per day

Comment on the changes in total mass of the measuring cylinder and plant shoot, compared with the changes in volume of water in the measuring cylinder. [1 cm³ of water has a mass of 1 g]

Explain why the rate of water loss would be different if a working fan is placed in front of the plant shoot.

Read the passage below. Use the information in the passage and your own knowledge to answer the questions that follow.

Antitranspirants

Antitranspirants are substances applied to plants to reduce water loss. There are three types of antitranspirant, film‑forming compounds, stomatal regulators and reflective compounds.

The first type of antitranspirant contains film‑forming compounds. These compounds are oils or waxes that are mixed with water to form an emulsion. This emulsion is then sprayed onto the leaves of plants. The emulsion forms a physical barrier that covers the stomata. The film is designed to prevent water loss but allow gas exchange. Scientists have found that that the film‑forming antitranspirant is effective in increasing yield and increasing photosynthesis in drought conditions. The increased leaf turgor in plants sprayed with film, compared to control plants, shows that the film decreases water loss and maintains turgor in drought conditions.

Film‑forming antitranspirants are used in horticulture to reduce water stress. This can occur during storage, transport and planting of container grown plants. Application of the antitranspirant before transplanting, helps the plant survive until the root system is established. The film‑forming compounds are used to reduce drying out in rolled turf and in transporting flowering plants and trees with root balls.

The second type of antitranspirants are stomatal regulators that reduce the size of the stomatal pore. Stomatal pores affect diffusion as well as transpiration. Therefore, photosynthesis and growth may change when stomatal regulators are applied to leaf surfaces. If the stomatal pores close completely this will stop photosynthesis. However, if stomatal openings are only reduced in size, transpiration will be reduced but some photosynthesis will continue.

The third type of antitranspirants are reflective compounds. These work by adding a white coating to leaves. This changes the amount of light that is reflected by the leaf surface. The coating is only applied to the upper surface of leaves so does not block the stomatal pores. Coating a leaf with a reflective chemical can reduce leaf temperature, transpiration rate and also increase the efficiency of water use. Experiments show that transpiration rate can be reduced by up to 30% and leaf temperature reduced from 40C to 30C after a coating of reflective compound is applied to leaf surfaces.

Crops depend on photosynthesis for growth and final yield. Therefore, it is unlikely that currently available antitranspirants would increase the yield of a crop unless the crop suffers from a low water supply or very high rates of evaporation. Spraying film‑forming or stomata regulating antitranspirants on a field‑grown cereal crop under drought conditions, produced a grain yield increase of 17%. Experiments also show that any effect that antitranspirants have on the minerals supplied to the leaves has little effect on plant growth.

Explain why plants that have been dug up and transported are at most risk of drying out (Lines 13 to 18).

The photograph shows some stomata on a leaf surface.

Use the photograph to determine the number of stomata per mm² on this leaf surface.

[1 mm = 1000 µ m]

number of stomata per mm² =_........

Explain why stomatal regulators reduce photosynthesis (Lines 19 to 24).

Explain why reflective compounds should only be applied to the upper surface of a leaf (Lines 27 and 28).

Explain why reducing leaf temperature reduces transpiration rate (Lines 28 to 32).

Describe the role of one named mineral ion in plant growth.

Describe how water is transported from the soil to the leaves in plants.

The diagram shows a cross‑section through a leaf.

Which layer contains palisade mesophyll cells?

Which set of environmental conditions would produce the fastest rate of transpiration from this leaf?

Scientists investigate the effect of changing carbon dioxide concentration on the density of stomata of wheat plants. They grow wheat plants from seed in different concentrations of carbon dioxide. After three weeks, they take a leaf from each plant and calculate the mean density of stomata.

State the independent variable in this investigation.

Give two abiotic variables that the scientists could control.

To calculate the mean density of stomata, leaf sections are viewed with a microscope. The number of stomata within six circular areas of the leaf are counted. The results for one leaf are shown in the table.

The radius of each circular area is 0.40 mm. area of circle = πr² [ π = 3.14] Calculate the mean density of stomata on the leaf surface.

mean density =_........ stomata per mm²

The investigation shows that in increased carbon dioxide concentrations there is a lower mean density of stomata. The scientist concludes that in hot dry areas, with increased carbon dioxide concentrations, it would be an advantage for wheat to have a lower mean density of stomata. Discuss the scientist’s conclusion.

A student investigates the rate of evaporation from a clay pot and the rate of transpiration by a plant. The clay pot is porous and has small holes that allow water to evaporate.

The student keeps the clay pot and the plant in the same conditions next to a closed window. He measures the rates at different times of the day.

The table shows the student’s results.

The student uses this apparatus to measure the rate of evaporation from the clay pot.

water evaporates from the

Explain the changes in the rate of evaporation from the clay pot.

Suggest how the student could measure the rate of evaporation from the clay pot.

Explain one factor that affects transpiration from the plant that does not affect evaporation from the clay pot.

Draw a labelled diagram of the apparatus the student could use to determine the rate of transpiration by a plant.

The diagram shows a transverse section through a leaf.

Explain how layer A is adapted for its role.

_........

Explain how layers B and C are adapted for photosynthesis and gas exchange.

Explain how layer D is able to regulate gas exchange.

Gardeners sometimes apply a spray called an anti-transpirant to plant leaves.

The spray is impermeable to water vapour but allows other gases to pass through.

Discuss whether an anti-transpirant spray will promote plant growth.

Plant roots absorb water from soil.

This water is transported to the leaves and then moves into the air.

Which of these processes is used to absorb water from the soil?

Name the tissue that transports water to the leaves.

Name the process that moves water vapour into the air.

Which of these reduces the movement of water from the leaves into the air?

Give two uses of water in a plant.

The diagram shows the path that the water takes when it is absorbed from the soil into the xylem of a plant.

Explain how root hair cells are adapted for efficient absorption of water.

Explain how water is transported from the soil to the leaves.

Describe how you could determine the rate of water loss from a leafy shoot.

Transpiration is the evaporation of water from the surface of a plant.

Explain why transpiration is useful for plants.

A student uses this apparatus to measure the rate of transpiration in still air and the rate of transpiration in windy conditions.

The student calculates the total loss of water after one hour in mg per cm² of leaf area10 for the plant in still air.

He then repeats this calculation for the plant in windy conditions.

The graph shows the student’s results.

The total leaf surface area from which transpiration occurs is 250 cm².

Calculate the loss of water from this plant in still air in mg per hour.

loss of water =_........ mg per hour

Explain the results shown by the graph.

Justify two factors that the student needs to control to make a valid comparison of transpiration in still air and in windy conditions.

A student uses this apparatus to investigate the effect of windy conditions on transpiration.

This is the student’s method.

● measure water uptake for 30 minutes without a fan blowing air at the shoot

● measure water uptake for 30 minutes with a fan blowing air at the shoot

● measure the total surface area of the leaves

The student divides the water uptake by the total surface area of the leaves.

The table shows the student’s results.

Explain how the student sets up and uses this apparatus to measure the water loss.

Give two abiotic factors that the student should control in his investigation.

The total surface area of the plant leaves is 200 cm².

Calculate the time in minutes for the plant to take up 1.0 cm³ of water without a fan.

The student concludes that plants will grow better in windy conditions.

Evaluate this conclusion.