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Class notes Biology Oxford IB Diploma Programme: Biology Course Companion
IBDP HL Biology - 2.2 Water Notes
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9.1 Transport in the Xylem of Plants
Essential idea
Structure and function are correlated in the xylem of plants.
Understandings
Transpiration is the inevitable consequence of gas exchange in the leaf.
Plants transport water from the roots to the leaves to replace losses from transpiration.
The cohesive property of water and the structure of the xylem vessels allow transport under tension.
The adhesive property of water and evaporation generate tension forces in leaf cell walls.
Active uptake of mineral ions in the roots causes absorption of water by osmosis.
Applications
Adaptations of plants in deserts and in saline soils for water conservation.
Models of water transport in xylem using simple apparatus including blotting or filter paper, porous pots
and capillary tubing.
Skills
Drawing the structure of primary xylem vessels in sections of stems based on microscope images.
Measurement of transpiration rates using potometers.
Design of an experiment to test hypotheses about the effect of temperature or humidity on transpiration
rates.
LEAF STRUCTURE
,TRANSPIRATION
Transpiration is the inevitable consequence of gas exchange in the leaf.
Plants transport water from the roots to the leaves to replace losses from transpiration.
Transpiration is the loss of water vapour from the stems and leaves of plants.
water in the leaves converted to vapour by light energy, then evaporates via stomata.
tiny openings or pores in plant tissue that allow for gas exchange.
new water absorbed from soil by roots; create pressure gradient between leaves (low) and roots (high).
water flows via xylem along the pressure gradient to replace the water lost (transpiration stream).
Stomata are pores on the underside of the leaf which facilitate gas exchange for photosynthesis.
photosynthesis level affects transpiration rates.
photosynthetic gas exchange requires stomata to be open.
hence, transpiration is an inevitable consequence of gas exchange in the leaf.
, EVAPORATION
The adhesive property of water and evaporation generate tension forces in leaf cell walls.
Water is lost from the leaves when it is converted into vapour (evaporation) and diffuses from the stomata.
some light energy absorbed by leaves converts to heat; evaporates water in the spongy mesophyll.
this vapour diffuses out via stomata, creating a negative pressure gradient within the leaf.
creates a tension force in leaf cell walls which draws water from the xylem (transpiration pull).
water pulled from the xylem under tension due to adhesion between water and the leaf cell walls.
Regulating water loss
Amount of water lost from the leaves (transpiration rate) is regulated by the opening and closing of stomata.
guard cells can block the opening by becoming increasingly flaccid in response to cellular signals.
when a plant begins to wilt from water loss, dehydrated mesophyll cells release the plant hormone abscisic acid
(ABA).
triggers flow of potassium from guard cells, decreasing their water pressure; they lose turgor.
loss of turgor makes the stomatal pore close, as the guard cells loosen and block the opening.
Transpiration rates will be higher when stomatal pores are open.
pores facilitate gas exchange in the leaf, hence photosynthesis levels will affect transpiration.
other factors affecting transpiration rates: humidity, temperature, light intensity and wind.
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