F4 Biology teaching notes Ch 9: 2nd Term NE 13

9.2  Gas exchange in plants (Book 1B, p. 9-8)

Ø  Plants exchange gases with the surrounding air by

(1) diffusion.

Ø  In terrestrial plants (陸生植物), (2) leaves are the

main sites of gas exchange.

Gas exchange also takes place through (3) stems / roots and (4) roots / stems.

n        Structure of a dicotyledonous leaf: (CE-2009-I-AQ5, CE-2007-I-AQ3, CE-2005-I-BQ9b, CE-2004-I-Q4c)

Structure	Feature and function
(10) Cuticle (角質層)	l   A thin waxy layer covering the epidermis l   Reduces (11) water loss from the leaf surface
Upper and lower (12) epidermis (表皮)	l   Protect the inner layers of cells l   No chloroplasts except the (13) guard cells
Palisade mesophyll (柵狀葉肉)	l   Made up of tightly-packed cylindrical cells that contain many (14) chloroplasts to carry out photosynthesis
(15) Spongy mesophyll (海綿狀葉肉)	l   Made up of irregularly-shaped cells that contain fewer chloroplasts l   Many (16) air spaces among the cells for gas exchange.
(17) Stoma (氣孔)	l   Pore on the epidermis where (18) gas exchange takes place l  Each surrounded by two (19) guard cells l  More on (20) lower epidermis of most terrestrial plants (?)
(21) Guard cells  (保衛細胞) è Fig 4: Guard cells and stoma	l  Specialized lower epidermal cell that contain chloroplasts l   Controls the opening and closing of the stoma: n   allows entry and exit of gases n   prevents excessive water loss
(22) Vascular bundles (維管束)	(23) xylem       (木質部)	n  transports water and (24) minerals
	(25) phloem      (韌皮部)	n  transports food / organic nutrients

n   How gas exchange takes place in leaves:

Gases in	Gases out
1      Gases from the environment diffuse into the air space through the (26) stoma. 2      Gases (27) dissolve in the moist surface of the mesophyll cells and diffuse directly into the cells. 3      Gases diffuse to the neighbouring cells.	i      Gases produced by the cells diffuse to the neighbouring cells. ii     Gases diffuse to the moist surface of the mesophyll cells. iii    Gases diffuse into the (28) air space and diffuse out through the stoma.

n   Structural adaptation of leaves for gas exchange:

Feature	Adaptation
Broad and flat leaves	Provide a large (29) surface area for gas exchange
Thin leaves	Reduce the (30) diffusion distance of gases
Many (31) air space among the spongy mesophyll cells	Allow gases to diffuse freely
(32) Moist surface of the mesophyll cells	Allows gases to (33) dissolve in it and then diffuse into the cells easily
Presence of (34) stomata on the epidermis (especially on lower one)	Allow gases to pass into and out of the leaves freely
Presence of (35) guard cells	Regulate the rate of gas exchange by controlling the opening and closing of stomata

n  Structural adaptation of leaves for reducing water loss

Feature	Adaptation
Epidermis is covered with a layer of water-proof (36) cuticle	Reduce water loss from the leaf surface by transpiration蒸騰.
There are fewer or no stomata on the (37) upper epidermis. (in terrestrial plants)	Direct sunlight on the upper epidermis cause a (35) higher / lower temperature. Fewer stomata help reduce water loss by transpiration.

2      Gas exchange in stems and roots        

n   In herbaceous plants (草本植物), the stems also have (36) stomata for gas exchange. n   In woody plants (木本植物), gas exchange also takes place through small broken parts in the cork layer (木栓) called (37) lenticels (皮孔) on the stem. n   Roots are not covered by (38) cuticle. Gas exchange takes place all over their surfaces.