Transport in Plants Flashcards
why do plants need transport systems?
- meet their metabolic demands
- size+continous growth=down SA:V ratio=x just rely on diffusion
xylem: function+structure
- function
- transport water+minerals
- structure
- xylem vessels=end walls dissapear during development=continous tube & lignin is deposited in rings/spirals/pits/reticulate
- xylem parenchyma=storage+may contain tannin
- xylem fibres=lignified secondary walls=mechanical strength
- non-living=oraganelles disappear as it develops=needs to be strengthened
phloem: function+structure
- function
- transports solutes eg. sucrose/amino acids
- structure
- sieve tube elements=end walls perforated=sieve plate & make up a sieve tube=cylindrical column
- living cell=has cytoplasm but other organelles disappear as it differentiates
- companion cells=associated w 1 or 2 sieve plates+contain many mitochondria=produces ATP=active transport+plasmodesmata fund bw these+sieve tube elements=ATP can permeate into all phloem
Transpiration definition
Loss of water through evaporation from the leaves
Why does transpiration occur?
- Water evaporates from the moist surfaces of mesophyll cells
- stomata open=absorb CO2 for photosynthesis=provides a pathway for water vapour loss through the open stomata
- Water vapour moves down a water potential gradient from the air spaces in the leaf into the atmosphere
- side effect of gas exchange
Factors that affecting transpiration rate
- light intensity=high light intensity=stomata open=max CO2 absorption=water vapour diffuses out=up rate
- temp=high temp=water molecules have more EK=move faster=up rate
- humidity=high humidity=smaller conc gradient=down rate
- wind speed=high wind speed=replaces humid air with dry air=up conc gradient=up rate
Potometer practical
- Cut the shoot underwater at slant=up SA for water uptake
- Assemble the potometer with the shoot submerged in water
- Keep the capillary tube end of the potometer submerged throughout the experiment
- Check that the apparatus is airtight (seal with vaseline)
- Dry the leaves+let shoot time to acclimatise
- Shut the tap+form an air bubble and record its position.
- Measure the distance the air bubble moves and the time taken.
Change one variable at a time and keep everything else constant.
Rate of transpiration equation
Rate=volume of water (area of a cylinder)/time
How does water move in a plant?
- Water enters a plant’s root hair cells via osmosis —> cytoplasm/walls towards the xylem=transports water from the roots up to the leaves
- to reach xylem
apoplast pathway=water moves through cell walls+intracellular spaces due to cohesion (bw water molecules)+adhesion (bw water and other substances)
symplast pathway=water moves through cytoplasm+plasmodesmata (due to a water potential gradient) - at Casparian strip=made of Suberin=waterproof=blocks apoplast pathway at root endodermis+forces water into symplast=to protect plant from harmful substances
- water reaches xylem=takes it up to leaves=water leaves xylem into leaf cells via apoplast pathway
- rot pressure=pushes water up xylem up to 1 m
- caused by active mineral pumping into xylem (energy from starch sheath=layer of endodermal starch grains)
Evidence that root pressure is an active process
- more oxygen=more respiration=more ATP=more root pressure
- cyanide affects mitochondria=less ATP=less root pressure
- root pressure follows the rise+fall of temp
Cohesion-Tension theory
- water=dipolar=forms H bonds with water molecules (cohesion)+surfaces (adhesion)
- cohesion=water forms a continuous column of water in the stem
- transpirational pull=tension created when water evaporates out of stomata=water column pulled up through xylem towards stomata=more water pulled up=tension on xylem=narrower+longer=changes in diameter
Adaptations of xerophytes (desert plants)
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Thick waxy cuticle=longer diffusion distance=less transpiration
-rolled/folded leaves=creates a microclimate of humid air=lower water potential gradient=less transpiration - hairs on leaves=trap moist air on leaf’s surface=lower water potential gradient
- Sunken stomata= creates a microclimate of humid air=lower water potential gradient=less transpiration
- Small, needle-like leaves= low SA for transpiration to occur
- water storage organs=store water in specialised parenchyma tissue in stem/root=appear swollen
- root adaptations=long+narrow (reach below surface) or wide+shallow (more SA to absorb as much water as possible)
- avoiding problems=lose leaves+become dormant=leave seeds to germinate+grow rapidly when rain falls or survive as storage organs eg bulbs, tubers, etc or survive dehydration=appear dead but els recover when falls
Adaptations of hydrophytes
- thin waxy cuticle=short diffusion distance=faster transpiration
- lots of open stomata=more water loss
- broad+flat leaves=more SA for diffusion=faster transpiration
- aerenchyma=lots of air spaces=faster diffusion+maintains buoyancy=close to water surface=more light for photosynthesis=stomata open=more water loss (formed by apoptosis)
- hydathodes=pores on side of leaf=more eater loss
- reduced root system=only small roots needed to extract nutrients from water
- reduced veins in leaf=xylem reduced=x need to transport water throughout plant
Translocation definition
Transport of assimilates in the phloem from source to sinks using ATP
Loading/unloading of assimilates (mass flow hypothesis)
loading
- assimilates move through symplast pathway=cytoplasm+plasmodesmata=passive (diffusion) or apoplast pathway=cell walls+intracellular spaces=active=involves movement into companion cells by active transport
- apoplast pathway=modified companion cells/transfer cells pump H+ out of the cytoplasm via a proton pump and into their cell walls=active=needs ATP
- large conc H+ in the cell wall of the companion cell=H+ move down the conc gradient back to the cytoplasm of the companion cell
- H+ move through a cotransporter protein+carry sucrose with them=sucrose then moves into the sieve tubes via the plasmodesmata from the companion cells
- Companion cells have infoldings in their plasma membrane= more SA for active transport of solutes+lots of mitochondria to provide the energy for the proton pump
- allows some plants to build up the sucrose in the phloem
unloading
- sucrose diffuses into sieve tubes from companion cells through plasmodesmata=low water potential=water drawn in from xylem by osmosis=more hydrostatic pressure n sieve tubes at source
- solute actively removed from sieve tube=higher water potential=water leaves phloem by osmosis=less hydrostatic pressure at sink=creates pressure gradient=solutes go from source to sinks
- assimilates can move u/down due to a high turgor pressure in phloem
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Loading/unloading of assimilates (mass flow hypothesis)
loading
- assimilates move through symplast pathway=cytoplasm+plasmodesmata=passive (diffusion) or apoplast pathway=cell walls+intracellular spaces=active=involves movement into companion cells by active transport
- apoplast pathway=modified companion cells/transfer cells pump H+ out of the cytoplasm via a proton pump and into their cell walls=active=needs ATP
- large conc H+ in the cell wall of the companion cell=H+ move down the conc gradient back to the cytoplasm of the companion cell
- H+ move through a cotransporter protein+carry sucrose with them=sucrose then moves into the sieve tubes via the plasmodesmata from the companion cells
- Companion cells have infoldings in their plasma membrane= more SA for active transport of solutes+lots of mitochondria to provide the energy for the proton pump
- allows some plants to build up the sucrose in the phloem
unloading
- sucrose diffuses into sieve tubes from companion cells through plasmodesmata=low water potential=water drawn in from xylem by osmosis=more hydrostatic pressure n sieve tubes at source
- solute actively removed from sieve tube=higher water potential=water leaves phloem by osmosis=less hydrostatic pressure at sink=creates pressure gradient=solutes go from source to sinks
- assimilates can move u/down due to a high turgor pressure in phloem
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Evidence to show mass flow requires energy
- cyanide poisons mitochondria=x in companion cells=x translocation
- fast movement in phloem due to active transport for mass flow
- aphid stylet pressure+sucrose con higher in source than sink
