Zusammenfassung der Ressource
Plant biology - unit 1
- Why do plants need a transport
system
- Need to take substances from environment
- E.g. Nitrates, water
- Epithelial cells in large multicellular plants can
receive supply by simple diffusion
- What substances need to be moved?
- Water and soluble minerals
- Move upwards
- Transported in the
xylem
- Sugars (most commonly sucrose)
- Move up or down the plant
- Transported in
the phloem
- Need to return waste to the environment
- E.g. Carbon dioxide
- Particular problem in
plants
- Roots can take up water but
not sugars
- Leaves can produce sugars,
but can't take up water from
the air.
- So transport system is required
to exchange substances all over
the plant
- Vascular tissue
- Xylem and phloem are found in vascular bundles
- In the root
- Xylem in the core in an X shape
- Arrangement provides strength against pulling and crushing
forces which roots experience
- Phloem is dotted around each arm
of xylem vessel
- In the stem
- Xylem in the inside
- Phloem on the outside of the ring
- Cambium in the middle
separating the xylem and
phloem
- Cambium are meristem
cells that can
differentiate to xylem or
phloem
- Arrangement provides strength and flexibility
against bending that stems and branches
experience
- Complete ring of vascular bundles just under the bark
- In the leaves
- Form the midrib and veins of a plant
- Two major groups of flowering plants
- Monocotyledons
- Depends on how many first leaves they have
- Two groups have different patterns of veins
- Dicotyledons
- Have a branching network of veins
- Veins get smaller as they move
further from the midrib
- Xylem on the inside of the phloem
- In a slightly curved V shape in centre of leaf
- XYLEM
- Consists of tubes to carry water and
dissolved minerals
- Fibres help support the plant and
living parenchyma cells
- Long thick cell walls of xylem vessel element
- Impregnated with lignin
- Waterproofs the cells and causes
them to decay and die
- Leaves a long column With no
end plates or contents
- Lignin
- Strengthens the wall and prevents it from collapse
- Keeps vessel open when water supply is low
- Can be spiral, annular,
reticulate patterns
- Prevents vessel from being too rigid
- Allows flexibility of stem or branch
- Can be gaps in lignin called PITS
- Allow lateral movement
of water to other vessels
- Allow water movement to
living parts of the plant
- Adaptations of XYLEM
- Tubes are end to end to form a
continuous column
- Tubes are narrow so water
column doesn't break easily
- Flow of water is not impeded because:
- No end walls
- No cell contents e.g. RER
- No nucleus or cytoplasm
- Lignin prevents wall
collapsing
- Pits allow lateral water
movement
- Lignin patterns allow
flexibility when growing
- Phloem
- Transport sugars and contain two cells:
sieve tube elements and companion cells
- Sieve tube element
- Contain very little
cytoplasm and no
nucleus
- Tubes lined up end to end
- Transport sucrose mainly-
dissolved in water to firm sap
- Has perforated sieve end
plates to allow sap to flow
through it
- Companion cell
- Have a large nucleus and dense
cytoplasm
- Numerous mitochondria to
produce ATP for active
processes
- Companion cell carries out metabolic
processes required by sieve tube
- ATP used for loading at the source
- Plasmodesmata link the
cytoplasm of the
companion cells and
sieve tbe
- Water uptake and movement
- Water uptake from the root
- Epidermis of root cell is
covered by root hair
cells
- Increase surface area
- Absorb minerals from the soil
by active transport
- Uses ATP
- Minerals reduce water potential of cell cytoplasm
- Makes water potential of cell lower than
the water potential of soil
- Water potential gradient rule
- Water is taken up across the plasma membrane by osmosis
- Movement across the root
- Driven by an active process at
the endodermis
- Endodermis is a layer of cells
surrounding the xylem
- Also known as the starch sheath
- Due to grains of starch being present - indicates energy use
- Casparian strip in the endodermis
- Special cells with a waterproof strip
- Blocks the apoplast
Pathway forcing the
symplast pathway
- Endodermis cells move minerals by osmosis
- From cortex to xylem by
active transport
- Decreases water
potential in xylem
- Water moves into the xylem by osmosis
- Reduces water potential in
cells outside of the
endodermis
- Creates a gradient
across whole cortex
- Enter text here
- Role of the casparian Strip
- Blocks apoplast
pathway between xylem
and cortex
- Water and dissolved nitrate ions
must move through cell membrane
- Transporter proteins in
cell membranes
- Nitrates can be actively
transported into xylem
- Lowers water potential in xylem,
creating a gradient
- Blocks water from moving back from the
xylem to the cortex again
- How does way move up the stem?
- Root pressure
- Active transport in the endodermis
drives water into the xylem
- Forces the water up the xylem
due to the constant water potential
gradient
- Theory would only force water a few
metres up a stem
- Wouldn't work with tall trees
- Transpiration pull
- Loss of water by evaporation
must be replaced by water in the
xylem
- Water molecules stick together due to cohesion
- Pulls water in a continuous column
- The pull creates tension in the
walls, which is why lignin is
required for extra strengh
- Cohesion-tension theory
- If transpiration stream is broken in one
xylem vessel it can be controlled by another
vessel through pits
- Capillary action
- Forces attract water molecules
to the side of the vessel
- Force - adhesion
- As xylem vessels are very narrow, pulls water up
vessels this way
- Movement between cells
- Water potential
- Total potential energy of the water molecules
in a system. Measure of how likely water will
be lost down a water potential gradient
- Water potential of pure water
is a,ways zero
- Water potential of plant cytoplasm is
always negative
- Due to solutes reducing water potential
- Cellulose cell wall prevents
bursting and provides flexibility
- Full - turgid
- Pressure exerted on
walls, creates a
pressure potential
- Reduces influx of water
- Plasmolysis
- Plasma membrane pulls
away from cell wall
- Occurs when plant cells
lose water by osmosis
- Because it is in a solution
with a more negative water
potential than the cell
- Apoplast pathway
- Cellulose walls have many water filled
spaces between cellulose molecules
- Water moves through these gaps
- Water does not pass through
any plasma membranes
- Means minerals and ions can be
transported
- Symplast pathway
- Waters enters the cytoplasm
- Travels through plasmodesmata from one cell to another
- Plasmodesmata link
cytoplasm s of adjacent
cells
- Vacuolar Pathway
- Similar to symplast pathway
- Can also travel through vacuoles of cells
- Translocation
- Movement of sucrose along
the phloem
- At the sink
- Sucrose may be converted to
starch for storage, or used for
metabolic processes e.g.
Respiration
- Reduces sucrose conc. in cells
- Sucrose moves to surrounding cells by
diffusion or active transport
- Increases water potential in phloem
- Water moves into
surrounding cells by
osmosis
- Reduces hydrostatic
pressure at sink
- At the source
- Sucrose eating sieve tube reduces water potential
- Water moves into the sieve tube by osmosis
- Creates a high hydrostatic pressure
- Hydrostatic pressure gradient creates from source to sink
- The movement of assimilates (mainly sucrose)
throughout the plant in the phloem tissue, from
the source to sink
- Mass flow
- Movement of assimilates in water
- Down a gradient
- Up or down a plant
- Occurs in different tubes in
different directions at the same
time
- Loading into the phloem
- ATP is used to actively
pump H* ions out of the
companion cell to
surrounding tissue
- Creates a diffusion gradient for ions to return
- Co transporter proteins allow hydrogen
ions to bring sucrose into cell
- Conc. of sucrose builds up in companion cell
and diffuses through plasmodesmata into sieve
tube
- Evidence for translocation
- Radioactively labelled CO2 can be
traced in the phloem
- Ringing a tree, causes
sugars to collect above
the ring
- Fluid collected from
aphids shows sucrose
- When sectioning the
stem, the styles is found in
the phloem
- Evidence for metabolic processes
- Companion cells have
many mitochondria
- Translocation can be stopped
- By a metabolic poison
inhibiting ATP production
- Rate of flow of sugars is so high that
energy must be required to drive it
- pH of companion cells
is higher than
surrounding cells
- Higher conc of sucrose at source
- Evidence against transolcation
- Not all the solutes in the phloem sap move at the same rate
- Sieve plate role is unclear
- Sucrose moves at the same rate around the whole plant
- Doesn't move quicker to areas of a lower concentration