8.2 Water Uptake & 8.3 Transpiration (IGCSE)

The diagram below shows a potted plant and the same plant 48 hours later.

What has caused the plant’s appearance to change?

• Water loss by translocation is greater than water uptake.

• Water evaporating from the leaves by translocation.

• Water loss by transpiration is greater than water uptake.

• Water loss from the leaves by osmosis.

Answer explanation

The correct answer is C because:

• After 48 hours the leaves have wilted, you can see them drooping in the image

• Wilting is a sign that a plant has insufficient water – it has lost more water by transpiration than it has taken up and therefore water has moved out of cells, causing them to become flaccid.

A & B are incorrect because translocation is the term used to describe the movement of sucrose and amino acids in the phloem.

D is incorrect because water loss occurs by evaporation of water from mesophyll cells inside the leaf and then diffusion of this water vapour through the stomata. Osmosis describes the movement of water between solutions separated by partially permeable membranes.

Reorder the phrases below to describe the pathway of water as it travels from the roots through a plant.

• mesophyll cells

• root hair cell

• air spaces

• stomata

• xylem

The diagram shows a simplified water cycle.

Which of the processes listed below would be involved in the loss of water from the tree?

• Respiration

• Translocation

• Photosynthesis

• Evaporation

Answer explanation

The correct answer is D because:

Plants lose water through transpiration. Water vapour can diffuse out of the leaves through the stomata. This water vapour comes from the spongy cells within the leaf; water will evaporate from the surface of these cells.

A is incorrect because respiration produces water, but it is not the process by which water is lost from the plant.

B is incorrect because translocation is the movement of nutrients and amino acids through the phloem.

C is incorrect because photosynthesis requires water as a source of hydrogen to produce carbohydrates.

The diagram shows a simplified water cycle.

Which of the processes listed below would be involved in the loss of water from the tree?

• Respiration

• Translocation

• Photosynthesis

• Evaporation

Answer explanation

The correct answer is D because:

Plants lose water through transpiration. Water vapour can diffuse out of the leaves through the stomata. This water vapour comes from the spongy cells within the leaf; water will evaporate from the surface of these cells.

A is incorrect because respiration produces water, but it is not the process by which water is lost from the plant.

B is incorrect because translocation is the movement of nutrients and amino acids through the phloem.

C is incorrect because photosynthesis requires water as a source of hydrogen to produce carbohydrates.

A potometer can be used to measure the rate that a piece of plant transpires under various conditions.

Part of a potometer is shown in the diagram below.

Which statement, relating to how a potometer works, is false?

• A potometer uses the transpiration stream to produce movement of an air bubble that can be measured.

• In a potometer, an air bubble moves away from the piece of plant tissue that is under investigation.

• In order to determine the rate of transpiration, the movement of the bubble must be measured alongside the time taken.

• The leaves of the plant in the potometer must be dry during the experiment.

Answer explanation

The correct (false) statement is B. (in a potometer, an air bubble moves away from the piece of plant tissue that is under investigation) because:

The air bubble moves towards the plant. the transpiration stream pulls water into the xylem at the bottom of the piece of plant. The bubble moves with the water, therefore towards the plant. 

A, C and D are all correct statements.

For A, the transpiration of water pulls water up from the potometer, provided there is a tight seal between the end of the cut stem of the plant and the water supply beneath. 

For C, the rate of reaction (or any process) always requires a measurement of time so that, in this case, the speed of uptake of water per minute/hour can be measured and stated quantitatively.

For D, water on the leaves may block stomata and prevent water vapour from diffusing out, hence affecting the rate of transpiration. 

A dicot plant is placed into a bowl of water coloured with red dye.

After 1 hour, a cross section of one of the roots is cut.

After 4 hours, a cross section is cut from the stem.

Which labelled regions would be stained red in each case?

• 1 and 3

• 1 and 4

• 2 and 4

• 2 and 3

Answer explanation

The correct answer is D because:

2 is the xylem as seen in a cross-section of a root in a dicotyledonous plant, and 3 is the xylem in a cross-section taken of the stem. The dye molecules are dissolved in water which moves up the xylem through the plant.

A is incorrect because 1 is the phloem. The phloem tissue is located in the outer region of the vascular bundle.

B and C are incorrect because 4 is the phloem in the root, which is found in smaller bundles surrounding the larger xylem vessels.

Which TWO of the following processes are involved in the loss of water from the leaves of plants?

• diffusion

• osmosis

• condensation

• evaporation

Answer explanation

Water molecules evaporate from mesophyll cells, forming water vapour (a gas) which diffuses from an area of higher concentration in the air spaces inside the leaf to a lower concentration in the air outside the leaf (unless it’s very humid and the air is saturated with water!)

A decrease in which of the following factors would normally cause the transpiration rate to increase?

• Stomatal density

• Light intensity

• Temperature

• Humidity

Answer explanation

The correct answer is D because:

The less humid (drier) the atmosphere, the higher the diffusion gradient between the inside of the leaf and the outside (through the stomatal pore).

A is incorrect because a decrease in stomatal density means less transpiration (as the leaves have fewer stomata).

B is incorrect because a reduction in light intensity causes stomata to close, reducing the rate of transpiration.

C is incorrect because a reduction in temperature means the water vapour molecules move more slowly, so they would diffuse out of the stomata at a lower rate.

A decrease in which of the following factors would normally cause the transpiration rate to increase?

• Stomatal density

• Light intensity

• Temperature

• Humidity

Answer explanation

The correct answer is D because:

The less humid (drier) the atmosphere, the higher the diffusion gradient between the inside of the leaf and the outside (through the stomatal pore).

A is incorrect because a decrease in stomatal density means less transpiration (as the leaves have fewer stomata).

B is incorrect because a reduction in light intensity causes stomata to close, reducing the rate of transpiration.

C is incorrect because a reduction in temperature means the water vapour molecules move more slowly, so they would diffuse out of the stomata at a lower rate.

Which environmental factor is most likely to be responsible for a plant to wilt?

• A decrease in light intensity.

• An increase in humidity.

• A decrease in the amount of available water.

• A decrease in wind speed.

Answer explanation

The correct answer is C because:

If a plant does not get enough water, it becomes limp and wilts. This is because water has left the cells and they have become flaccid, providing less support to the overall structure of the leaf. If a cell does not contain enough water there is no pressure pushing outwards against the cell wall and the cells will all shrink a little.

A is incorrect because a decrease in light intensity would just reduce the rate of photosynthesis, not cause the plant to wilt.

B is incorrect because an increase in humidity would reduce transpiration from the plant, meaning it would retain more water.

D is incorrect because a decrease in wind speed would reduce transpiration from the plant, meaning it would retain more water.