Download Transport in Plants - St Vincent College

Transport in Plants
Warm up questions-Xylem or Phloem
• Which is nearest the centre of a root?
• Which type of vascular tissue has walls
reinforced with lignin?
• Which transports sucrose from source to sink?
• Which transports sucrose from roots to
leaves?
• Which is involved in the transpiration stream?
A whistle-stop tour of transport in
plants
The leaf
• Photosynthesis takes
place in the chloroplasts
which are mainly in the
palisade mesophyll cells
• Glucose is made by
photosynthesis and can
be converted into sucrose
and starch
• During the hours of
daylight leaves are a
source of carbohydrate
Transport of sucrose (translocation)
• Sucrose enters the
companion cells of the
phloem by active
transport
• Companion cells use ATP
to transport H+ out into
surrounding cells
• This sets up a
concentration gradient of
H+ high concentration
outside the companion
cell, low concentration
inside
Transport of sucrose (translocation)
• The H+ diffuse back into
the companion cells
through co-transport
protein channels in the
plasma membrane.
• Co transport means that
the H+ transport sucrose
molecules through as
they enter the companion
cells.
• This leads to a high
concentration of sucrose
in the companion cells
Transport of sucrose (translocation)
• The sucrose diffuses through
plasmodesmata (small gaps in the
wall between the companion
cells and the phloem sieve tube)
• This lowers the water potential in
the sieve tube so that water
enters the sieve tube by osmosis .
• The hydrostatic pressure in the
sieve tube rises so pushing the
sucrose solution down (or up) to
an area where sucrose is being
used (the sink).
• The movement of the sucrose is
called mass flow
Try these questions
• Which two molecules are needed by the leaf for
photosynthesis?
• How does carbon dioxide get into the leaf?
• Does sucrose move up or down a concentration
gradient when moving from the companion cells
to the sieve tube vessel?
• Which molecule provides an immediate source of
energy for active transport?
• Which organelles would you expect to be most
abundant in the companion cells?
Transport of sucrose
Evidence that sucrose is transported in the phloem
• Supply radioactive CO₂ to the tree, radioactive sucrose
can be detected in the phloem soon afterwards
• Cut away a ring of bark, sucrose will accumulate above
the ring causing a swelling around the trunk.
Evidence that transport in phloem is an active process
When plants are given a metabolic poison that inhibits
respiration, transport of sucrose stops.
Companion cells have many mitochondria
Back to the leaf
• Leaf cells need water
• Water enters plants
through roots
• Water is lost from plants
through leaves by
transpiration.
• Transpiration involves
evaporation followed by
diffusion of water vapour
down a water vapour
potential gradient
Transpiration
• Most water vapour diffuses
through stomata which are
open during daylight hours
to allow carbon dioxide to
enter the leaf.
• Water moves from the
xylem across the mesophyll
cells by the symplast route
(by osmosis from cell to
cell) and by the apoplast
route(along the bundles of
cellulose fibres forming the
cell wall).
Tension
• The diffusion of water
out of the stomata
allows more water to
evaporate from the
surface of the
mesophyll cells. This
draws water along the
symplast and apoplast
routes creating suction
(tension) in the xylem
Cohesion -Tension
• The tension set up in the
leaf draws water up the
xylem vessels in the stem of
the plant.
• Water molecules form
hydrogen bonds causing
cohesion. This produces an
unbroken column up the
xylem.
• A stronger rate of
evaporation leads to more
tension in the water column
and a faster rate of water
movement up the trunk
Measuring the rate of water uptake by
plants
• The potometer
measures the rate of
water uptake by a cut
shoot.
• This can be used as an
estimate of the rate of
transpiration.
• Why is the uptake of
water only an estimate
of transpiration rate?
Structural factors affecting the rate of
transpiration
• Number and size of plant leaves-more and
bigger leaves………………..
• Thickness of waxy cuticle- thicker cuticle
covering leaf………………….
• Number, size and position of stomata - fewer,
smaller stomata on the lower surface of the
leaf………………………………….
Environmental factors affecting the
rate of transpiration
• Temperature- a higher temperature increases
the rate of transpiration because……………
• Relative humidity- high humidity reduces the
rate of transpiration because………………..
• Wind- more air movement increases the rate
of transpiration because………………………
• The rate of transpiration is more rapid in
bright light because……………………………
Xerophytic adaptations
• Plants need water to
remain turgid- when
there is a shortage of
water in the soil plants
will wilt and eventually
die
• Xerophytes are adapted
to survive in dry
conditions
• Like this cactus they have
leaves reduced to needles
to reduce the surface
area for transpiration
Xerophytic adaptations
• Marram grass has rolled leaves
to trap water vapour in the air
inside the roll.
• Hairs on the leaves also help
prevent the water vapour from
being blown away from the
leaf surface. The water vapour
potential gradient becomes
less steep.
• The stomata are sunken into
pits also helping to trap humid
air next to the stomata.
• Thick waxy cuticle.
And finally- the uptake of water from
soil by plant roots
• Root hairs increase the
surface area for the
absorption of water and
dissolved mineral ions.
• Mineral ions move into
root cells by active
transport.
• This lowers the water
potential so….
• Water moves from the
soil into the root cells by
osmosis.
Movement of water across the plant
root
The end