6.2 Transport systems in plants Flashcards
what is the role of a mass transport system in a plant?
to meet metabolic demands
to cope with size of plant
to cope with SA:vol
what is the structure of xylem?
elongated walls of dead cells impregnated with lignin (lignification)
fibres
xylem parenchyma - packing cells
what is the function of bordered pits?
allows lateral movement of water and ions
to keep cells turgid and cool
what is the function of xylem?
to transport water from roots to the rest of the plant
what is the function of phloem?
to transport organic material from source to sink
what is the structure of phloem?
sieve tube elements, perforated by pores (sieve plates)
- no nucleus, few organelles, little cytoplasm
- bidirectional
companion cells, with plasmodesmata for exchange of materials with sieve tube elements
what is 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
what is the function of roots?
anchorage
absorbing nutrients from the soil
taking up water via osmosis
storage organ
what are the features of a monocotyledonous plant?
single primary leaf in seed
xylem and phloem in ring in root
vascular bundles scattered in the stem
leaf veins parallel
flowers in multiples of three
e.g. cereals/grasses
what are the features of a dicotyledonous plant?
two primary leaves in seed
phloem in xylem cross in root
vascular bundles in ring around edge of stem
net veins in leaves
flowers in multiples of four and five
what is the role of water in a plant?
maintaining turgor pressure
cooling
cytosol and cell sap
transportation of assimilates
key reactant in photosynthesis
what is transpiration?
the movement of water through a plant and the evaporation of water through aerial parts of a plant
why is transpiration a ‘necessary evil’?
water is lost through stomata, which must be open for gas exchange for photosynthesis
how are stomata 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
water moves into space outside CSM and through cell wall/extra cellular spaces
casparian strip (waxy suberin layer) prevents water moving across cell walls
outline the symplast pathway
water moves through plasma membrane into cytoplasm via plasmodesmata down w.p.g.
outline the vacuolar pathway
water moves vacuole to vacuole via neighbouring cells, crossing the symplast and apoplast pathways
how does windspeed/air movement affect transpiration?
blows water vapour away from leaf
reduces humidity
increases w.p.g.
∴ faster rate of diffusion of water vapour
how does light affect transpiration?
increases rate of PHS
∴ more stomata open
∴ water vapour can diffuse out faster
how does temperature affect transpiration?
increases Ek of water vapour
∴ more evaporation
∴ more movement down w.p.g.
how does the number of leaves affect transpiration?
more stomata
larger SA for gas exchange/loss of water vapour
how does the thickness of the cuticle affect transpiration?
thicker = harder for water vapour to diffuse out
what is translocation?
the movement of nutrients around the plant from source to sink
give three examples of a source
green leaves/stems
storage organs
food stores in germinating plants
give three examples of a sink
roots
dividing meristem
food stores
how is sucrose (S) loaded into phloem - symplast?
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
how is sucrose (S) loaded into phloem - apoplast?
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
what are the adaptations of companion cells?
infolding membrane –> incr. SA
many mitochondria –> incr. ATP synthesis
plasmodesmata for incr. mass flow
how are assimilates unloaded from phloem?
diffusion into tissues - v. rapid
loss of S from phloem –> incr. water potential in phloem
water drawn out into surrounding cells
