Zusammenfassung der Ressource
Transport in Plants
- Xylem
- Transports water and
mineral ions up a plant's
stem to leaves
- Root - xylem found
in centre as a x
- Provides support
for root
- Stem - xylem found
toward the outside
- Reduces bending
- Leaf - xylem makes
up network of veins
- Supports leaves
- Adaptations
- Xylem vessels are long
tube like structures
- Formed from vessel
elements joined end to end
- No end walls
- Allows water to pass up
through middle of cell easily
- Dead cells,
containing no
cytoplasm
- Woody substance
lignin thickens cell walls
- Stops walls
collapsing inwards
- Lignin in spiral patterns
- Flexibility
- Prevents stem from
breaking
- Amount of lignin
increases as cell gets
older
- Supports xylem walls
- Small pits in walls
where there is no lignin
- Allows water and mineral
ions to move in/out of
vessels
- Phloem
- Transports dissolved
substances - sugars (like
sucrose) up and down a plant
- Root - phloem found in
centre outside of xylem
- Supports root
- Stem - phloem on
outside
- Reduces bending
- Leaf - phloem on inside
making up network of
veins
- Supports leaves
- Adaptations
- Formed of cells
arranged in tubes
- Purely transport
tissue
- Contains phloem fibres, phloem
parenchyma, sieve tube elements
and companion cells
- Not used for support
- Sieve tube elements
- Living cells forming tube
for transporting sugars
through plant
- Joined end to end
to form sieve tubes
- Sieve parts are
end walls
- With holes to allow
sugars to pass
through them
- Sieve plates
- No nucleus, thin layer
of cytoplasm and few
organelles
- Sieve tube elements
cannot survive on
their own
- Cytoplasm of adjacent cells
is connected through holes in
sieve plates
- Companion cells
- Companion cell for
every sieve tube element
- Helps it survive
- Carries out living functions
for both itself and sieve tube
elements
- Provides energy to
actively transport sugars
- Multicellular organisms
- Small surface are to
volume ratio
- Uptake water,
minerals and sugars
- Get rid of waste
substances
- Diffusion would be too
slow without a transport
system
- Water transport
- Entering plant
- 1) Water enters
through root hair cells
- 2) Passes through
root cortex and endodermis
- 3) Reaches xylem
- Water moves down a
water potential gradient
- Soil around roots has high
water potential
- Leaves have lower
water potential
- Water constantly
evaporates from them
- Water potential
difference creates
a gradient
- Keeps water moving
through plant from roots
to leaves
- Through roots
- Water travels by osmosis
through roots via root cortex,
into xylem by pathways
- Symplast pathway
- Goes through living
parts (cytoplasm) of
cells
- Cytoplasm of
neighbouring cells connect
through plasmodesmata
- Small channels in
cell walls
- Apoplast pathway
- Goes through
non-living parts of
cells (cell walls)
- Walls absorbant
= easy diffusion
- When reaching endodermis
cells in root, path blocked by
casparian strip
- Waxy, waterproof
strip in cell walls
- Now takes
symplast pathway
- After this water
moves into xylem
- Useful as water has to go
through cell membrane first
- Control if
substances in
water get through
- Through leaves
- Water leaves xylem and
moves into cells by
apoplast pathway
- Water evaporates from
cell walls into spaces
between cells in leaf
- Stomata open = water
moves out of leaf as water
vapour into air
- Down water
potential gradient
- Transpiration
- Up plant
- Water movement from roots
to leaves = transpiration
stream
- Cohesion
- Water molecules
stick together
- Causes column of
water in xylem to
move upwards as one
- Tension
- As water evaporates
from leaves, this creates
suction at top of xylem
- Pulls more water up
into leaf
- Movement of water causes
more water to enter stem
through root cortex cells
- Adhesion
- Water molecules attracted
to walls of xylem vessels
- Helps water rise up
through vessels
- Air bubbles in xylem can
block column of water from
reaching cells
- Transpiration
- Evaporation of water from
a plant's surface
- Happens as a result
of gas exchange
- Plant need to open stomata to let
in CO2 for photosynthesis, to
produce glucose
- Opening stomata
lets water out
- Higher concentration of
water inside leaf than in
surrounding air
- Water moves out
down water potential
gradient
- Factors affecting
transpiration rate
- Light
- More light = faster
transpiration rate
- Stomata open wider
the lighter it gets
- Allowing more water to
evaporate
- Dark = stomata
closed
- Little transpiration
- Temperature
- Higher temp = faster
transpiration rate
- Warmer water molecules have
more energy = evaporate from
cells inside leaf faster
- Increases water potential
gradient between inside and
outside of leaf
- Water evaporates
out of leaf faster
- Humidity
- Lower humidity =
faster transpiration rate
- Water potential gradient
increases between leaf and air if
air surrounding plant is dry
- Lower in air,
higher in leaf
- Moves down water
potential gradient
- Increase in water potential
gradient = increases
transpiration
- Wind
- Increase in wind = faster
transpiration rate
- Air movement blows
away water molecules
from around stomata
- Water potential in air
is now more lower than leaf
- Increases water
potential gradient
- Increases rate
of transpiration
- Potometers
- Estimates
transpiration rates
- Measures water
uptake by a plant
- Use to estimate how different
factors affect transpiration rates
- Process
- 1) Cut shoot underwater
at a slant
- Prevents air entering xylem
which can block water column
- Increases surface area
for water uptake
- 2) Assemble potometer in
water and insert shoot
underwater
- 3) Remove potometer from
water, but keeping end of
capillary tube in beaker of water
- 4) Check potometer is
watertight and airtight
- 5) Dry leaves
- Make sure water potential
around leaves isn't higher
than it should be
- 6) Allow time for shoot to
acclimatise, then shut
tap
- 7) Remove end of capillary
tube from beaker until one air
bubble has formed
- Then put end of tube
back into water
- 8) Record starting
position of air bubble
- 9) Start stopwatch and record
distance the bubble moves per
unit of time
- Keep conditions
constant throughout
experiment
- Factors etc
- Xerophytic plants
- Examples
- Cacti
- Pine trees
- Prickly pears
- Stomata are
sunk in pits
- Trapping water vapour
- Reduce transpiration by
lowering water potential
gradient
- Hair layer on
epidermis
- Traps moist air
round stomata
- Reduces water potential
gradient between leaf and air
- Curled leaves
- Traps moist air
- Lowers exposed surface
area for losing water
- Protects stomata
from wind
- Less stomata
- Fewer places for water
vapour to diffuse out of
leaf
- Thick waxy layer
on epidermis
- Layer is waterproof
- Water cannot pass through
to evaporate
- Spines for leaves
- Reduces surface area
for water loss
- Translocation
- Movement of
assimilates in plant
- Sugars
- Requires energy
- Happens in phloem
- Moves assimilates
from source to sink
- Source = where
assimilates are produced
- High
concentration
- Sink = where assimilates
are used up
- Low concentration
- Enzymes maintain concentration
gradient from source-sink
- Changing dissolves substances
at sink into something else/by
breaking them down
- = lower concentration at
sink than source
- Examples
- Sucrose
- Source = leaves
- Sink = food storage
organs and meristmems in
roots/stems/leaves
- Potatoes
- Sucrose converted to
starch in sink
- Lower concentration
of sucrose at sink
than in phloem
- Mass flow
- Hypothesis
- 1) Source
- Active transport loads
assimilates into sieve
tubes of phloem
- Lowering water potential
inside sieve tubes
- Water enters
tubes by osmosis
- Creates high pressure
inside sieve tubes at
source end of phloem
- 2) Sink
- Assimilates removed from
phloem to be used up
- Increases water
potential inside
sieve tubes
- Lowers pressure
inside sieve tubes
- 3) Flow
- Result of pressure gradient
from source end to sink end
- Gradient pushes assimilates along
sieve tubes to where they are needed
- Evidence
- Support
- Removing ring of bark from
woody stem = bulge forms
above ring
- Includes phloem but not xylem
- Fluid in bulge has higher
concentration of sugars than
below the ring
- Sugars cannot move past
the area where bark has
been removed
- Evidence downward
flow of sugars
- Investigating pressures in
phloem using aphids
- Aphids pierce phloem, bodies
removed, mouthparts left behind
allowing sap to flow out
- Sap flows out quicker nearer
leaves than further down stem
- Evidence for
pressure gradient
- Metabolic inhibitors in
phloem = translocation stops
- Inhibitors stop ATP
production
- Evidence active
transport is involved
- Against
- Sugar travels to many
different sinks
- Not just to one with
highest water potential
- As model suggests
- Sieve plates would create
barrier to mass flow
- A lot of pressure would be
needed for assimilates to get
through at reasonable rate
- Experiment
- Showing mass flow
hypothesis
- Two containers A and B
- Lined with selectively
permeable membrane like cells
- Top tube connecting
A and B represents
phloem
- Bottom tube connecting A
and B represents xylem
- A = source end
- Contains concentrated
sugar solution
- B = sink end
- Contains weak
sugar solution
- Water enters A by
osmosis
- Increases pressure = sugar
solution flows along top tube
- Pressure
increases in B
- Forcing water out and
back through bottom tube