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IB Biology: Plant Biology Summary
Summary of Plant Biology IB HL Content
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9.1 XYLEM TRANSPORTTranspiration is the loss of water vapour from the stems and leaves of plants
★ Light energy converts water in the leaves to vapour, which evaporates from the
leaf via stomata
★ New water is absorbed from the soil by the roots, creating a difference in pressure
between the leaves (low) and roots (high)
★ Water will flow, via the xylem, along the pressure gradient to replace the water lost
from leaves (transpiration stream)
Stomata are pores on the underside of the leaf which facilitate gas exchange
(needed for photosynthesis)
★ As photosynthetic gas exchange requires stomata to be open, transpiration will be
affected by the level of photosynthesis
★ Hence, transpiration is an inevitable consequence of gas exchange in the leaf
Water is lost from the leaves of the plant when it is converted into vapour
(evaporation) and diffuses from the stomata. Some of the light energy absorbed by
leaves is converted into heat, which evaporates water within the spongy mesophyll.
This vapour diffuses out of the leaf via stomata, creating a negative pressure
gradient within the leaf. This negative pressure creates a tension force in leaf cell
walls which draws
water from the xylem
(transpiration pull).
The water is pulled
from the xylem
under tension due to
the adhesive
attraction between
water and the leaf cell walls.
The amount of water lost from the leaves (transpiration rate) is regulated by the
opening and closing of stomata
★ Guard cells flank the stomata and can occlude the opening by becoming
increasingly flaccid in response to cellular signals
★ When a plant begins to wilt from water stress, dehydrated mesophyll cells release
the plant hormone abscisic acid (ABA)
★ Abscisic acid triggers the efflux of potassium from guard cells, decreasing water
pressure within the cells (lose turgor)
★ A loss of turgor makes the stomatal pore close, as the guard cells become flaccid
and block the opening
, Transpiration rates will be higher when stomatal pores are open than when they are
closed
★ Stomatal pores are responsible for gas exchange in the leaf and hence levels of
photosynthesis will affect transpiration
★ Other factors that will affect transpiration rates include humidity, temperature, light
intensity and wind
Cohesion is the force of attraction between two particles of the same substance (e.g.
between two water molecules). Water molecules are polar and can form a type of
intermolecular association called a hydrogen bond. This cohesive property causes
water molecules to be dragged up the xylem towards the leaves in a continuous
stream
Adhesion is the force of attraction between two particles of different substances (e.g.
water molecule and xylem wall). The xylem wall is also polar and hence can form
intermolecular associations with water molecules. As water molecules move up the
xylem via capillary action, they pull inward on the xylem walls to generate further
tension
The xylem is a specialised
structure that functions to
facilitate the movement of water
throughout the plant
★ It is a tube composed of dead
cells that are hollow (no
protoplasm) to allow for the
free movement of water
★ Because the cells are dead,
the movement of water is an
entirely passive process and
occurs in one direction only
★ The cell wall contains numerous pores (called pits), which enables water to be
transferred between cells
★ Walls have thickened cellulose and are reinforced by lignin, so as to provide
strength as water is transported under tension
9.2 PHLOEM TRANSPORT
Translocation is the movement of organic compounds (e.g. sugars, amino acids)
from sources to sinks
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