Transpiration is the loss of water vapour from the leaves of a plant.
(a) Complete the sentence describing transpiration.
Water evaporates from the surfaces of the …… cells into the air spaces and then …… out of the leaves, through the stomata, as water vapour.
(b) Explain how water moves upwards in the xylem.
(c) A student investigated the effect of wind speed on the rate of transpiration in a small Pachira aquatica tree.
They measured the rate of transpiration when the plant was placed in different wind speeds in both the light and the dark. The tree was given an adequate supply of water.
The results of this investigation are shown in Fig. 2.1.
(i) Describe and explain the results shown in Fig. 2.1.
(ii) The investigation was repeated in the light with the same species of tree in an environment where the humidity was higher.
Draw a line on Fig. 2.1 to predict the results in the light with increased humidity.
(iii) The investigation described in 2(c) was repeated (different wind speeds in both the light and the dark).
However, the tree was not given an adequate supply of water during the investigation.
Predict the result and explain your prediction.
▶️ Answer/Explanation
(a) mesophyll; diffuses;
Explanation: Transpiration begins when water evaporates from the moist surfaces of mesophyll cells (the photosynthetic cells in leaves) into the air spaces within the leaf. This water vapor then diffuses out through the stomata, which are small pores on the leaf surface that allow for gas exchange.
(b)
Water moves upwards in the xylem due to three main factors:
- Transpiration pull: As water evaporates from the leaves (transpiration), it creates a negative pressure that pulls water up through the xylem vessels.
- Cohesion-tension theory: Water molecules stick together (cohesion) due to hydrogen bonding, forming a continuous column of water from roots to leaves.
- Adhesion: Water molecules also adhere to the walls of the xylem vessels, helping to counter gravity.
This combined action creates a continuous flow of water from roots to leaves, known as the transpiration stream.
(c)(i)
Description:
- As wind speed increases, the rate of transpiration increases in both light and dark conditions.
- Transpiration rates are consistently higher in light conditions compared to dark conditions at all wind speeds.
- The rate of increase is more pronounced in light conditions, especially at higher wind speeds (above 4 m/s).
Explanation:
- Increased wind speed removes water vapor from around the leaf faster, maintaining a steeper concentration gradient for diffusion.
- In light, stomata are open for photosynthesis, allowing more water vapor to escape, while in dark conditions many stomata close.
- Light also provides energy that increases the kinetic energy of water molecules, enhancing evaporation.
(c)(ii)
A line should be drawn that shows transpiration increasing with wind speed but remaining below the original light condition line at all points.
Explanation: Higher humidity means the air already contains more water vapor, reducing the concentration gradient between the leaf interior and exterior. This decreases the rate of transpiration at all wind speeds compared to normal humidity conditions.
(c)(iii)
Prediction: The transpiration rate would be lower at all wind speeds, particularly in light conditions, with little change in dark conditions.
Explanation:
- With inadequate water supply, the plant conserves water by closing stomata (especially in light when they would normally be open).
- Closed stomata dramatically reduce water loss, lowering transpiration rates.
- In dark conditions, where stomata are already mostly closed, the effect would be less noticeable.
- The plant may wilt as cells lose turgor pressure due to water shortage.
Fig. 5.1 is a graph showing the effect of temperature on the rate of transpiration from the upper and lower surfaces of a leaf that is provided with a constant supply of water
(a) Describe the results shown in Fig. 5.1.
(b) Explain reasons for the shape of the graph for the upper surface of the leaf at X and at Y in Fig. 5.1.
(c) Suggest how the structure of the lower surface differs from the upper surface of the leaf used in this investigation.
▶️ Answer/Explanation
Ans:
(a) The graph shows that the transpiration rate increases with temperature before plateauing. The lower surface has a higher transpiration rate than the upper surface, and both stabilize at nearly the same temperature.
(b) At X, the rate increases because higher temperature boosts evaporation and diffusion. At Y, the rate stabilizes as factors like humidity or stomatal limits restrict further increase.
(c) The lower surface likely has more stomata or a thinner cuticle, facilitating higher transpiration compared to the upper surface.