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Summary Plant biology IB HL
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9.1 Transport in the xylem of plants AHLCohesion (H20+H2O) and adhesion (H2O+X): result of the polarity of a water molecule and its
ability to form hydrogen bonds
Although hydrogen bonds are weak the large number of bonds present gives cohesive forces
great strength
Water molecules are strongly cohesive
Transpiration is the loss of water from leaves and stems of
plants.
Cohesion tension theory:
Xylem vessels transport water through the plant.
Water is heated in the mesophyll by sunlight and becomes
vapor.
This vapour transpires out of the stomata - pores in the leaf.
Loss of water generates negative pressure and a
transpiration pull on water molecules in the xylem. More
water is drawn into the leaf.
Higher rates of transpiration lead to a faster transpiration
stream and higher rates of water uptake.
This theory is known as cohesion tension theory.
, Stomata opening caused by:
- sunlight/ high photosynthesis
- reduced CO2 concentration
Stomata closing caused by:
- water shortage: the hormone
absisic acid is produced, forcing
closure to prevent dehydration.
- darkness
Xylem cells contain no cytoplasm this
provides a larger lumen making water
transport more efficient. However
because the cells are are non-living
water transport must be a passive
process.
Cell walls are thickened to make them
stronger
Wall are impregnated with lignin (fibrous
organic polymer)
Lignin may be deposited in different continuous vessels: the reduction of the walls
ways, such as spirals or rings. between cells in a vessel makes it easier for water to
Strengthened xylem walls can move between cells
withstand very low internal pressures
without collapsing.
Tissue Plan Diagram: Dicotyledon Stem
In a cross section (transverse section, TS) of a stem each vascular
bundle consists of large xylem vessels toward the inside and smaller
phloem cells toward the outside. Xylem vessels can be identified by
their large empty lumens and the thickened cell walls.
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