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topic 3
mass transport in plants
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Cards (15)
Describe the function of
xylem tissue
Transports water (and
mineral ions
) through the stem, up the plant to leaves of plants
Suggest how
xylem tissue
is adapted for its function
cells
joined with no end walls forming a long continuous tube -
water
flows as a continuous column
cells contain no
cytoplasm
/ nucleus - easier water flow / no obstructions
thick cells wall with
lignin
- provides support / withstand tension / prevents water loss
pits in side walls - allows lateral water movements
Explain the
cohesion-tension theory
of water transport in the
xylem
Leaf
Water lost from leaf by
transpiration
- water evaporates from
mesophyll cells
into air spaces and water vapour diffuses through stomata
Reducing
water potential
of mesophyll cells
So water drawn out of xylem down a water potential gradient
Xylem
Creating tension ('negative pressure' or pull) in xylem
Hydrogen bonds result in cohesion between water molecules so water is pulled up as a continuous column
Water also adheres to walls of xylem
Root
Water enters root via
osmosis
Describe how to set up a
potometer
Cut a shoot underwater at a slant - prevent air entering
xylem
Assemble potometer with
capillary tube
end submerged in a beaker of water
Insert shoot underwater
Ensure apparatus is watertight / airtight
Dry leaves and allow time for shoot to
acclimatise
Shut tap to
reservoir
Form an air bubble - quickly remove end of capillary tube from water
Describe how a
potometer
can be used to measure the rate of
transpiration
Potometer estimates transpiration rate by measuring water uptake:
record position of air bubble
record distance moved in a certain amount of time
calculate volume of water uptake in a given time:
use radius of
capillary tube
to calculate
cross-sectional area
of water
multiply this by distance moved by bubble
calculate
rate of water uptake
- divide volume by time taken
Describe how a
potometer
can be used to investigate the effect of a named
environmental
variable on the rate of
transpiration
carry out the practical, change one variable at a time (e.g.
wind
,
humidity
,
light
or
temperature
)
keep all other variables constant
Suggest limitations in using a
potometer
to measure rate of
transpiration
rate of water uptake might not be same as rate of transpiration
- water used for support / turgidity
- water used in
photosynthesis
and produced during
respiration
rate of movement through shoot in potometer may not be same as rate of movement through shoot of whole plant
- shoot in potometer has no roots whereas a plant does
-
xylem
cells very narrow
Suggest how
light intensity
and temperature affect
transpiration rate
Light intensity - increases rate of transpiration
stomata
open in light to let in
CO2
for
photosynthesis
allowing more water to evaporate faster
stomata close when it's dark so there is a low transpiration rate
Temperature - increases rate of transpiration
water molecules gain
kinetic energy
as temperature increases
so water evaporates faster
Suggest how different
wind intensity
and
humidity
affect
transpiration rate
Wind intensity - increases rate of transpiration
wind blows away water molecules from around
stomata
decreasing
water potential
of air around stomata
increasing water potential gradient so water evaporates faster
Humidity - decreases rate of transpiration
more water in air so it has a higher water potential
decreasing water potential gradient from leaf to air
water evaporates slower
Describe the function of
phloem tissue
transports
organic substances
e.g.
sucrose
in plants
Suggest how
phloem tissue
is adapted for its function
sieve tube elements
no nucleus / few
organelles
- maximise space for / easier flow of organic substances
end walls between cells perforated (sieve plate)
Companion cells
many
mitochondria
- high rate of respiration to make ATP for active transport of solutes
What is
translocation
?
movement of
assimilates
/ solutes such as
sucrose
from
source cells
to
sink cells
by mass flow
Explain the
mass flow hypothesis
for translocation in plants
At source, sucrose is actively transported into
phloem sieve tubes
/ cells
By
companion cells
This lowers water potential in sieve tubes so water enters from xylem by
osmosis
This increases
hydrostatic pressure
in sieve tubes (at source) / creates a hydrostatic pressure gradient
So mass flow occurs - movement from source to sink
At sink, sucrose is removed by active transport to be used by
respiring cells
or stored in storage organs
Describe the use of
tracer experiments
to investigate transport in plants
Leaf supplied with a
radioactive
tracer eg. CO2 containing radioactive
isotope
14C
Radioactive carbon incorporated into organic substances during
photosynthesis
These move around plant by
translocation
Movement tracked using
autoradiography
or a
Geiger counter
Describe the use of
ringing experiments
to investigate transport in plants
remove / kill
phloem
e.g. remove a ring of bark
Bulge forms on source side of ring
Fluid from bulge has higher conc. of sugars than below - shows sugar is transported in phloem
Tissues belong ring die as cannot get
organic substances
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