Transport In Plants Flashcards
Define translocation
movement of assimilates through the plant phloem sieve tubes from source to sink
what are the 4 main processes involved in translocation?
- active loading of sucrose into phloem @ source
- water follows in by osmosis so hydrostatic pressure increases @ source end
- solutes removed @ sink so water follows by osmosis; reduces sink-end hydrostatic pressure
- pressure gradient pulls sap down sieve tube by MASS FLOW
outline the process of active loading into phloem sieve tube
- H+ actively pumped out companion cell using ATP; conc. outside increases
- H+ accompanied by sucrose diffuses back into companion cell facilitated by co-transporter proteins in companion cell membrane
- increase sucrose conc. in companion cell; diffuses through plasmodesmata into sieve tube element
Explain how sap is able to move down the sieve tube
as sucrose enters @ source, water (from xylem & companion cells) follows in by osmosis increasing the hydrostatic pressure
as solutes are removed @ sink, water potential is increased so water leaves by osmosis
this sets up a hydrostatic pressure gradient, drawing the water from source to sink
distinguish between source & sink, using examples
source = areas that produce/store the nutrients
eg. roots store sucrose as starch & leaves photosynthesise it
sink= area that needs nutrients
eg. meristems (growing tissue) need amino acids
roots as it is transported there for storage
technically anywhere as all cells need it for respiration
what is transpiration a consequence of?
consequence of gas exchange; when stomata open to allow CO2 to enter for photosynthesis, water diffuses out, as WP is higher inside leaf than out, pulling the whole column up the xylem
define transpiration
water movement through a plant and its evaporation from aerial parts;
a consequence of gas exchange
outline how water from soil enters root hair cell
mineral ions mineral ions actively absorbed from soil, lowering WP in RHC
water follows in by osmosis down WP gradient
how does water from root hair cell reach the xylem? (6 mark)
now the first root cortex cells have more negative WP than the RHC so water follows by osmosis; like a domino effect the next cell will have more negative WP so osmosis continues until xylem reached
there are 2 DIFFERENT pathways the water is able to travel vis osmosis:
1. Symplast; through cytoplasm & plasmodesmata
2. Apoplast; non-living parts; through cell walls & spaces between them as the walls are v absorbent so water can diffuse through
BUT once casparian strip in root endodermis cells is reached, cell walls blocked so must all be symplast pathway
when xylem reached: mineral ions actively transported into xylem, lowering WP so water follows in by osmosis through cell membrane
how does water move up the xylem vessel? (6 mark)
- Cohesion-Tension Theory & 2. Adhesion
- Cohesion-tension: H20 form H-bonds with each other so are attracted together; they move as 1 column, so as H20 evaporates from top of column out of stomata, this creates tension (suction),pulling the whole column up
- Adhesion capillary action occurs as xylem is so narrow & H20 molecules are attracted to the walls of xylem; pulls the H20 up the sides of the column against gravity
what is the casparian strip? why is it useful?
a waxy strip in cell walls of root endodermis cells that blocks the apoplast (cell wall&gap) pathway
forces water in symplast pathway to go through cell membrane to get into xylem; controls which substances enter
structure of xylem
continuous column of dead cells joined end-to-end to form a tube;
- no end walls; uninterrupted water column so effective capillary action
- no cytoplasm/organelles; continuous columnists again
- lignified walls; woody substance to support the xylem vessels & prevent collapsing inwards during suction action of cohesion-tension theory
- small bordered pits in walls = where theres no lignin so not waterproof; water can move in/out into adjacent xylem/phloem via osmosis
structure of phloem
SIEVE TUBE ELEMENTS:
- no nucleus & few organelles, little cytoplasm = sap can flow through by mass flow
= living cells joined end-to-end to form sieve tubes; cytoplasm of adjacent cells connected through perforations in sieve end plates
COMPANION CELLS
- linked to sieve tube elements via plasmodesmata
- carries out living processes for the sieve elements since they have no nuclues&few organelles
PHLOEM PARENCHYMA & PHLOEM FIBRES = supportive tissue
xerophytes adaptions to water availability
to reduce water loss in dry conditions:
- stomata in sunken pits; sheltered from wind so reduces WP gradient; slows transp. rate
-hairs on epidermis trap moist air around stomata to reduce WP gradient
-rolled leaves also traps moist air
-thick waxy cuticle on epidermis = waterproof swatter can’t evaporate through
CACTI: leaves reduced to spines so less SA for water loss, green stem for photosynthesis, widespread roots, succulent (stores water in stem)
hydrophytes adaptions to water availability
to survive in water/moist (where o2 level is low):
- air spaces in roots/stems allow o2 from leaves down to root for aerobic respiration
- air spaces in leaves allow the plant to float as it is less dense than water, so it can absorb sunlight and be in contact with air for gaseous exchange
- stomata are on upper epidermis (surface) of floating leaves to maximise gas exchange; contact with air
- flexible leaves & stems prevent water current damage; they don’t need rigidity as water supports them
