F4 Biology teaching notes Ch 10: 2nd Term NE 15

10 Transpiration, transport and support in plants

10.1  Transpiration (蒸騰) in plants (Book 1B, p. 10-3)        

Ø  is the loss of water in the form of water vapour from its surface into the atmosphere.

Ø  takes place:

n mainly through the (2) stomata of the leaves and young stems

n also through:

l the (3) stomata of the leaves and young stems

l  and (4) lenticels on the woody stems. 

A

How does transpiration take place through stomata? (Book 1B, p. 10-4)

Œ     Water on the surfaces of the mesophyll cells (5) diffuses into the air space. The air space becomes almost (6) saturated (飽和) with water vapour.

     The water vapour concentration in the air space is now

(7) higher than that in the atmosphere outside the leaf.

      Water vapour (8) diffuses through the stomata into the atmosphere.

B

Creation of transpiration pull (Book 1B, p. 10-5)

Œ     During (9) transpiration, mesophyll cells near the air space lose water continuously into the air

space. The water potentials of these mesophyll cells therefore (10) decreases.

v     Water is then drawn from the adjacent cells to these mesophyll cells

by (11) osmosis.

w     After losing water, the adjacent cells, in turn, draw water from their

neighbouring cells.

x     Eventually, water is drawn from the (12) xylem vessels.

y     (13) Transpiration pull

(蒸騰拉力) is created to pull water up the xylem vessels from the roots.

C

Significance of transpiration (Book 1B, p. 10-5)

Ø   Transpiration creates transpiration pull:

n   for the transport of (14) water and (15) minerals in the xylem vessels.

n   also helps the (16) absorption of water by the roots.

Ø   During transpiration, the evaporation of water from the mesophyll cells removes (17) heat from the leaves. This produces a (18) cooling effect to prevent plants from being overheated under hot conditions.

D

How are stomata distributed on leaves? (Book 1B, p. 10-6)

Ø   Different types of plants have different numbers and distribution of stomata on their leaves.

Ø  Comparison of the distribution of stomata and other features between different types of plant:

Terrestrial dicotyledonous plants (陸生雙子葉植物)

Submerged plants

Floating plants

n Stomatal density on the upper epidermis is             (19) higher / lower than that on the lower epidermis n reduces water loss by transpiration through upper epidermis which is directly illuminated by sunlight.

n  Few or no stomata on both the upper and lower epidermis n  Not covered with (20) cuticle (?) n  Dissolved gases, water and minerals (21) diffuse directly into the leaves through all surfaces

n Stomata on the (22) upper epidermis only n Gas exchange takes place mainly through the stomata on the upper epidermis n (23) Lower epidermis not covered with cuticle n Water and minerals diffuse directly into the leaves through the lower epidermis n Presence of large (24) air space between cells that provides buoyancy.

E

Structural adaptations of leaves to prevent excessive loss of water through transpiration (Book 1B, p. 10-7)

Feature	Adaptation
Epidermis covered with         (25) cuticle	which is almost impermeable to water. This reduces water loss through leaf surfaces.
Few or no stomata on the          (26) upper epidermis of dicotyledonous leaves	The upper epidermis is directly illuminated by the sun and its temperature is (27) higher. Few or no stomata on it helps reduce water loss due to transpiration.
(28) Guard cells control the opening and closing of stomata	(29) Closing of the stomata reduces water loss through transpiration when plants are deficient in water.