6.2 Transport systems in plants Flashcards
Explain how SA:vol restrictions are overcome in plants
o Whole plants generally have small SA:vol
o This is usually too small to enable the gas exchange demand to be met by simple diffusion (don’t confuse with leaves and hair cells that are adapted to have large SA:vol)
o plants need to move substances to all cells at a quick rate to meet demand
o so they need an efficient transport system
Explain how large flowering plants such as horse chesnut trees depend on a suxccessful transport system
o Large multicellular plants have relatively high metabolic rate
o Need effective transport system to move substances from root tip to growing tip
Decribe how a tranposrt system in a plant enables the metabolic demands to be met
o Only areas with photosynthetic pigments can produce glucose
o All cells need supply of oxygen & glucose
o All cells need to remove waste products of metabolism
o Growth plant regulators (‘hormones’) need to be transported from their site of synthesis to site of use
o Mineral ions need to be taken up & transported to all cells for protein & nucleic acid production
Give 3 reasons a mass transport system is needed in a plant
to meet metabolic demands
to cope with size of plant
to cope with SA:vol
State 6 functions of water within plants
- Turgor pressure (hydrostatic pressure)
o provides hydrostatic skeleton to support stem & leaves
o causes cell expansion -> force enables plant roots to force their way through soil - Transpiration stream acts as cooling mechanism for plant
- Main component of cytosol
- Main component of cell sap
- Enables mineral ions and products of photosynthesis (assimilates) to be transported from where they are obtained to where they are needed (translocation)
- Water is key reactant in photosynthesis
Describe the structure of xylem
No end walls
Elongated, dead cells
Form uninterrupted tubes
Thickened with lignin (woody waterproofing substance)
Cells become more lignified as they age
Has no protoplasm (and hence no metabolic activity)
State the function of bordered pits in xylem tissue
allows lateral movement of water and ions
to keep cells turgid and cool
State the function of xylem tissue
Transport water and dissolved mineral ions up the plant from the roots to the apex
State the function of phloem tissue
to transport dissolved solutes and assimilates e.g. sucrose from source to sink
Describe the structure of phloem tissue
Sieve tube elements:
Lacks nucleus, RER, central vacuole, golgi apparatus - to reduce resistance to the flow of phloem sap and to provide more space for more phloem sap
No tonoplast (and therefore no large central vacuole) and limited cytoplasm which is only located on the periphary – to reduce resistance to flow of assimilates
Few mitochondria
perforated by pores (sieve plates)
Companion cells,
Normal contents i.e. nucleus & all organelles
Linked to protoplasm of PSTE by many plasmodesmata
Many mitochondria – to supply ATP to the PSTE
Explain the significance on the perforated end walls in the sieve tube elements
they form sieve pores
which allow solutes to pass through unimpeded by mass flow
Describe the function of leaves
SA for gas exchange
absorption of light energy for photosynthesis
make organic solutes
what is the function of the stem?
structure/support
lifting leaves to sunlight
transport of substances in vascular tissue
Describe the function of the roots
anchors the plant in the soil
absorbing mineral ions from the soil (by active transport)
taking up water via osmosis in the root hair cells
storage organ (acts as a sink)
State 5 features of a monocotyledon (monocot plant)
single primary leaf (cotyledon)
xylem and phloem in ring in root
vascular bundles scattered in the stem
leaf veins parallel
flowers in multiples of three
fibrous roots (also called adventitious roots)
e.g. cereals/grasses
State 5 features of a dicotyledon (dicot plant)
two primary leaves (cotyledons)
phloem in xylem cross in root
vascular bundles in ring around edge of stem
net veins in leaves
has a main tap root with side branches
flowers in multiples of four and five
State 4 main roles of water within a plant
maintaining turgor pressure
cooling
cytosol and cell sap
transportation of assimilates
key reactant in photosynthesis
Define the term transpiration stream
the movement of water from the roots to the leaves via mass flow and transpiration.
Note: it is also known as the cohesion-tension theory and transpiration pull.
Define the term transpiration
the loss of water vapour (by diffusion) from the underside of the leaf (aerial part of the plant) via the open stomata
State the 3 forces that play a role in transpiration and explain thier importance
Cohesion: water molecules are attracted to each other via hydrogen bonds (as water molecules are polar & have dipoles) hence they are cohesive.
Tension: the evaporation of water molecules from the walls of the PMC to the mesophyll air spaces generates a suction (tension) that draws more water into the leaf
Adhesion: the water molecules are attracted to the cellulose walls of the xylem vessels and the hydrophilic parts of lignin which aids the mass flow of water up the xylem (do not just say adhesion between water and the cell wall or adhesion between water and lignin)
Describe the events that occur at the surface of the leaf that play a role in transpiration
- Water evaporates from the cellulose cell walls of the mesophyll cells into mesophyll air spaces
- Water vapour then diffuses through the mesophyll air spaces to the sub-stomatal air spaces
- Water vapour then diffuses down the water potential gradient through the open stomata to the surrounding external air
Describe how water moves through the xylem during transpiration
- Xylem vessels transport water up the stem of the plant from roots to the leaves due to a difference in water potential at the top of the stem compared to the bottom
- Water is drawn up the xylem by capillary action due to hydrogen bonds between the adjacent water molecules (which are polar i.e. possess dipoles)
- Water moves up the xylem in a continuous column due to cohesion forces
- At the leaves the water moves out of the xylem into the cells by the apoplast pathway
- this is described as the cohesion-tension theory
- i.e. the transpiration pull
Suggest the meaning behind the statement “transpiration is the necessary evil in a plant”
water is lost through stomata, which must be open for gas exchange for photosynthesis
Describe how stomata are closed
abscisic acid (ABA) binds to specific receptors on membranes of guard cells
Ca2+ ion channels open and Ca2+ enter
proton pumps activated
influx of H+ generates a PMF
PMF opens voltage-gated K+ channels
K+ leave via facilitated diffusion
w.p. of cytosol increases
∴ water leaves via osmosis down w.p.g.
cells become flaccid –> closed stoma
Outline the apoplast pathway
o Water goes through cellulose cell walls
o Walls are very absorbent so water diffuses along the cellulose fibres as well as moving by osmosis through the spaces between the fibres
o As cell walls are freely permeable this route offers little resistance and hence most of the water travels via this route
NB when water reaches the endodermis the pathway is blocked by the Casparian strip (continuous band of cells that have waxy suberin layer in their cell walls). At this point the water has to cross the endodermis via the symplast pathway – this is useful as the csm is partially permeable and allows the cell to control which substances in the water pass through to the xylem vessels via the parenchyma cells.
Outline the symplast pathway
o Water travels through the csm to the protoplasm of the cell via osmosis from the RHC through the cells of the root cortex
o water moves down the w.p.g.
o cytoplasm of neighbouring cells are connected via plasmodesmata and water also moves between these intercellular connections
Outline the vacuolar pathway
water moves vacuole to vacuole via neighbouring cells, crossing the symplast and apoplast pathways
Explain why traspiration occurs
- Main reason = as a consequence of gas exchange
- Plants must open their stomata to enable the uptake of CO2 for PHS and for the release of O2 to the atmosphere.
- Insufficient CO2 will result in the rate of PHS reducing
- Hence this reduces glucose production which is potentially fatal to theplant
- However, it also helps regulate the temperature of the plant and ensures that mineral ions are transported through the plant (as a consequence of mass flow of the water which carries dissolved ions within in it).
Explain the effect of windspeed/air movement on the rate of transpiration
blows water vapour away from leaf
reduces humidity
increases w.p.g.
∴ faster rate of diffusion of water vapour
Explain the effect of light intensity on the rate of transpiration
increases rate of PHS
∴ more stomata open
∴ water vapour can diffuse out faster
Explain the effect of temperature on the rate of transpiration
Higher temp = water molecules possess more kinetic energy = greater evaporation from the spongy mesophyll cells into air spaces inside the leaf = increases the diffusion gradient between the air inside of the leaf and the external atmosphere = faster rate of diffusion of water molecules from sub-stomatal air space to external atmosphere
ALSO
Increase in temperature increases the concentration of water vapour that can be held by the external air before it becomes saturated (so decreases its humidity and water potential maintaining the diffusion gradient for longer)
Explain the effect of varying number of leaves has on the rate of transpiration
more stomata
larger SA for gas exchange/loss of water vapour
Explain the effect of varying cuticle thickness has on the rate of transpiration
thicker = harder for water vapour to diffuse out
Define translocation
the movement of nutrients around the plant from source to sink
State 3 examples of a source
green leaves/stems
storage organs
food stores in germinating plants
State 3 examples of a sink
roots
dividing meristem
food stores
Describe how is sucrose loaded into phloem via the symplast route
S into cytoplasm of mesophyll
S into STE via diffusion through plasmodesmata
H2O follows by osmosis from xylem
hydrostatic pressure generated
S moves through phloem by mass flow
Describe how is sucrose loaded into phloem via the apoplast route
S into companion cells/STEs by diffusion
H+ pumped out of companion cells and return down concentration gradient via co-transpiration proteins with S
S builds up in companion cells/STEs
H2O follows by osmosis
turgor pressure increases
water + assimilates move into STEs, reducing pressure in companion cells –> movement by mass flow
Describe how companion cells are adapted for their function
infolding membrane –> incr. SA
many mitochondria –> incr. ATP synthesis
plasmodesmata for incr. mass flow
Describe how assimilates are unloaded from phloem
diffusion into tissues - v. rapid
loss of S from phloem –> incr. water potential in phloem
water drawn out into surrounding cells
Describe how water moves throug hthe xylem during transpiration
- Xylem vessels transport water up the stem of the plant from roots to the leaves due to a difference in water potential at the top of the stem compared to the bottom
- Water is drawn up the xylem by capillary action due to hydrogen bonds between the adjacent water molecules (which are polar i.e. possess dipoles)
- Water moves up the xylem in a continuous column due to cohesion forces
- At the leaves the water moves out of the xylem into the cells by the apoplast pathway
- i.e. by cohesion-tension
