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Revision
Mass transport
Transport of water in the xylem
Plants have xylem vessels are hollow thick-walled tubes where the vast majority of water is
transported
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The humidity of the atmosphere is typically less than air spaces next to stomata
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This requires the
stomata to be open so that water vapour molecules diffuse out of the air spaces into the
surrounding air
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So, by altering the size of the stomatal pores, plants can control their
rate of transpiration
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Another factor involved in the movement of water up the xylem, from the roots to the leaves, is
cohesion-tension
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Transpiration pull is when a column of water is pulled up the xylem as result of
transpiration
Transpiration pull puts xylem under tension, so negative pressure within xylem is
created, hence the name cohesion-tension theory
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These include:
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During the day, transpiration is at its greatest so there is more tension in the xylem
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At night, transpiration is at its lowest, so there is less tension in the xylem and so
diameter of the trunk increases
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This is because the
continuous column of water is broken and so water molecules can no longer stick together
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Instead air is drawn in, which is
consistent with it being under tension
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Xylem vessels have no end walls means that xylem forms a series of continuous,
unbroken tubes from root to leaves
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You can use it to estimate
how different factors affect the transpiration rate
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g
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g
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It move solutes
from ‘sources’ to ‘sinks’
- Enzymes maintain concentration gradient from
source to sink by changing the solutes at sink
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The mass flow hypothesis is best supported theory
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g
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This lowers the water potential inside sieve tubes, so water
enters tubes by osmosis from xylem and companion cells
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(Sink) At sink end, solutes removed from phloem
which increases water potential inside sieve tubes, so water leaves tubes by osmosis
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(Flow) Result is a pressure gradient from source end
to sink end
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When they reach
sink the solutes will be used up (e
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in respiration) or stored (e
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as starch)
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There is evidence both for and against mass flow
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The liquid found in this swollen
region are found to have a higher concentration of sugars than fluid below the region
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Another evidence is that pressure in the phloem can be investigated using aphids which
have needle-like mouthparts which penetrate the phloem
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Radioactive tracer e
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radioactive carbon (14C) can be used to track the movement of
organic substances in a plant
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The objections to this is that sugar travels to many different sinks, not just one with the
highest water potential
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A lot of
pressure needed for solutes to get through
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This can be done by
supplying part of plant with organic substance that has a radioactive label, tracking its
movement
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The plant is killed and
then the plant is placed onto photographic film- wherever film turns black, the radioactive
substance is present
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