Transpiration: Water Transport in Flowering Plants

2. Think about… 10.1 Transpiration 10.2 Transport in flowering plants 10.3 Support in plants Recall ‘Think about…’ Summary concept map

3. A hollow tree

4. A hollow tree The centre of the trunk has rotted away. But the tree is still growing actively!

5. 1 How can the tree transport water and food between its roots and leaves with the centre of its trunk rotted away

6. 2 What would happen to the tree if the decay occurred in the centre of its roots instead of the trunk Why

7. 3 How can the tree remain upright though its trunk is hollow

8. 10.1 Transpiration water lost from plant surface due to evaporation water absorbed through roots

9. 10.1 Transpiration water lost from plant surface due to evaporation transpiration (蒸騰) water absorbed through roots

10. 10.1 Transpiration 10.1 Demonstration of the occurrence of transpiration 1 Use plastic bags to enclose the lower parts of the plants and the pots. A B plastic bags

11. 10.1 Transpiration 10.1 2 Put the intact potted plant (A) inside a bell jar. This is the experimental set-up. A B plastic bags

12. 10.1 Transpiration 10.1 3 Put the potted plant (B) with the aerial parts, i.e. parts above the ground, removed inside another bell jar. This is the control set-up. A B plastic bags

13. 10.1 Transpiration 10.1 4 Leave both set-ups in bright light for 2 hours. A B plastic bags

14. 10.1 Transpiration 10.1 5 Observe any changes in the bell jars. Test any liquid formed on the walls with dry cobalt(II) chloride paper. A B plastic bags

15. 10.1 Transpiration 10.1 Results and discussion • A layer of moisture and drops of liquid are formed on the wall of bell jar A. The liquid turns dry cobalt(II) chloride paper from blue to pink, indicating the presence of water.

16. 10.1 Transpiration 10.1 Results and discussion • Set-up B is the control. The bell jar remains clear. No liquid is formed inside the bell jar. • The results show that water vapour is released from plant A but not from plant B. This indicates that transpiration takes place in the aerial parts of the plant.

17. 10.1 Transpiration 10.1 Results and discussion • The purpose of the pots enclosed in plastic bags is to prevent the respiration of soil organisms and the evaporation of soil water from affecting the results.

18. 10.1 Transpiration Where does transpiration take place?

19. 10.1 Transpiration leaf stem

20. 10.1 Transpiration stem leaf

21. 10.1 Transpiration leaf 10%of water lost throughcuticle 90%of water lost throughstomata

22. 10.1 Transpiration stem very small proportionof water lost throughlenticelsof woody plants

23. 10.1 Transpiration How does transpiration take place in leaves? Animation 1 Water on the surface of mesophyll cells evaporates into the air space.

24. 10.1 Transpiration How does transpiration take place in leaves? 2 Water vapour in the air space diffuses to the atmosphere through the stoma.

25. 10.1 Transpiration Creation of transpiration pull 1 Water lost from the surface of mesophyll cells is replaced by water in these cells.

26. 10.1 Transpiration Creation of transpiration pull 2 Water is drawn from the neighbouring cells by osmosis.

27. 10.1 Transpiration Creation of transpiration pull 3 Water is finally drawn from the xylem vessels, creating the transpiration pull (蒸騰拉力).

28. 10.1 Transpiration Significance of transpiration 1 During transpiration, evaporation of water absorbs heat from the leaves. cooling effect

29. 10.1 Transpiration Significance of transpiration 2 Transpiration pulls water up through the plants. transport of water and minerals along xylem vessels

30. 10.1 Transpiration Significance of transpiration 3 Water and minerals are drawn into the roots from soil during transpiration. absorption of water and minerals

31. 10.1 Transpiration How do we measure the rate of transpiration? By using a potometer (蒸騰計).

32. 10.1 Transpiration 10.2 Video Measurement of the rate of transpiration using a bubble potometer A bubble potometer can beused to measure the rate of water uptake by a leafy shoot. Since most of the water taken up by plants will eventually be lost through transpiration, it is assumed that the rate of water uptake is the same as the rate of transpiration.

33. 10.1 Transpiration 10.2 1 Cut a leafy shoot from a plant and fit it tightly into the bubble potometer under water.

34. 10.1 Transpiration 10.2 2 Set up the apparatus. leafy shoot reservoir graduated capillary tube tap (closed) bubble water

35. 10.1 Transpiration 10.2 3 Seal off all connections with vaseline to ensure no water leakage. 4 Lift the end of the capillary tube from the beaker of water for 30 seconds and then replace it to introduce an air bubble into the tube.

36. 10.1 Transpiration 10.2 5 Wait for the bubble to move into the horizontal graduated part of the capillary tube. 6 Record the distance travelled by the bubble in a certain period of time (e.g. 5 minutes).

37. 10.1 Transpiration 10.2 Results and discussion • The rate of water uptake can be found out by calculating the rate of movement of the air bubble, i.e. distance travelled by the air bubble per unit time. This is an indirect measurement of the rate of transpiration.

38. 10.1 Transpiration 10.2 Results and discussion • The leafy shoot should be cut and fit into the potometer under water. This prevents air bubbles from entering the xylem vessels of the plant and blocking water uptake.

39. 10.1 Transpiration 10.3 Video Measurement of the amount of water absorbed and lost by a plant using a weight potometer A weight potometer is comprised of two parts: (1) the burette which is used to measure therate of water uptake by a leafy shoot (2) the balance which is used to measure the rate of water loss by the leafy shoot.

40. 10.1 Transpiration 10.3 1 Cut a leafy shoot from a plant and fit it tightly into the weight potometer under water. 2 Set up the apparatus as shown.

41. 10.1 Transpiration 10.3 oil layer burette leafy shoot water top pan balance

42. 10.1 Transpiration 10.3 3 Record the initial water level (Vi) in the burette and the weight (Wi) of the entire set-up. 4 After 24 hours, record the final water level (Vf) in the burette and the weight (Wf) of the entire set-up.

43. 10.1 Transpiration 10.3 Results and discussion The amount of water absorbed by the plant = the change in volume of water in the burette = (Vf –Vi) The amount of water lost by the plant = the change in weight of the entire set-up = (Wf –Wi)

44. 10.1 Transpiration 10.3 Results and discussion The amount of waterabsorbed is slightly greater than the amount of water lost by the plant. This is because some water is used in photosynthesis, growth and other metabolic activities.

45. 10.1 Transpiration Factors affecting the rate of transpiration 1 Light intensity 2 Air movement 3 Relative humidity

46. 10.1 Transpiration 1 Light intensity rate of transpiration light intensity stomata open wider more water vapour diffuses out transpiration rate  light intensity

47. 10.1 Transpiration 2 Air movement rate of transpiration wind blows away water vapour around the stomata steep concentration gradient of water vapourmaintained wind speed

48. 10.1 Transpiration 2 Air movement rate of transpiration diffusion rate transpiration rate  wind speed

49. 10.1 Transpiration 3 Relative humidity rate of transpiration relative humidity of surrounding air concentration gradient of water vapour  relative humidity

50. 10.1 Transpiration 3 Relative humidity rate of transpiration less water vapour diffuses out transpiration rate  relative humidity