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A Level Biology
3.3- Exchange and Transport
Mass transport- plants
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Cards (45)
What do xylem vessels transport in plants?
Water and
mineral ions
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What are xylem vessels made up of?
Dead, hollow
cells
with no
end walls
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How do xylem cells form a continuous tube?
They
are
stacked
on top of
each
other
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Why do xylem cells have no organelles or cytoplasm?
To
create
more
space
for transporting
water
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What substance strengthens the xylem vessel?
Lignin
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What forces allow water to move upwards in the xylem?
Tension
and
cohesion
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What creates tension in the xylem?
Water evaporation from leaves (
transpiration
)
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How does cohesion affect water movement in the xylem?
Water molecules stick together, forming a
column
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What is adhesion in the context of xylem function?
Attraction of water to
non-water
molecules
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What is transpiration?
Loss of water vapor through
evaporation
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Where does transpiration mainly occur in plants?
Through gaps in the leaf called
stomata
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Why do plants close their stomata at night?
To minimize
transpiration
when not
photosynthesizing
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How does light intensity affect transpiration rate?
Increased light intensity
raises
transpiration rate
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How does temperature influence transpiration?
Higher temperature increases
transpiration rate
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What effect does wind have on transpiration?
Wind
increases
the rate of
transpiration
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How does humidity affect transpiration rate?
Higher
humidity
decreases
the rate of transpiration
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What does a potometer measure?
The amount of
water uptake
by a plant
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Why is the assumption made that water uptake equals transpiration?
Because some water is used for
turgidity
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What is the first step in using a potometer?
Cut a
shoot
underwater to prevent air entry
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Why should the shoot be cut at a slant?
To increase the
surface area
for water uptake
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What should be done after inserting the shoot into the potometer?
Dry the leaves and leave for
30
minutes
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What is the purpose of shutting the tap on the potometer?
To create a closed
capillary
tube system
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How is the speed of the air bubble calculated?
By measuring
distance moved
per
unit time
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What do phloem vessels transport?
Dissolved substances like
sucrose
and amino acids
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What are the two types of cells in phloem vessels?
Sieve tube elements
and
companion cells
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What is the function of sieve tube elements?
To transport solutes through the
phloem
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How do companion cells support sieve tube elements?
By providing energy for
active loading
of sucrose
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What is translocation in plants?
Movement of dissolved substances from
sources
to sinks
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What happens to sucrose when it reaches a sink?
It is converted into
starch
for storage
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What is the mass flow hypothesis?
A theory explaining
solute transport
in
phloem
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What occurs during the first stage of the mass flow hypothesis?
Sucrose
moves from
companion cells
into
sieve tubes
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How does water enter the phloem according to the mass flow hypothesis?
By
osmosis
, increasing
hydrostatic pressure
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What maintains the hydrostatic pressure gradient in phloem?
Pressure difference between
source and sink cells
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What happens to water at the sink end of the phloem?
Water moves out by
osmosis
, increasing
water potential
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How is sucrose actively loaded into sieve tube elements?
Through
co-transport
with
hydrogen ions
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What evidence supports the mass flow hypothesis?
Radioactive
tracers track sucrose movement
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What evidence challenges the mass flow hypothesis?
Sieve plates
may
impede
mass flow
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How can translocation be investigated using radioactive tracers?
By exposing leaves to
carbon-14
gas
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What is autoradiography used for in plant studies?
To track
radioactive
sugars in plants
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What are the factors affecting the rate of transpiration?
Light intensity
: More light increases transpiration
Temperature
: Higher temperature increases transpiration
Wind
: Windier conditions increase transpiration
Humidity
: More humidity decreases transpiration
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