mass transport in plants Flashcards
root structure
- water enters root hair cells via epidermis
- passes cells of cortex, across endodermis into xylem
- xylem passes water up the plant to the leaves
root adaptations
- extensions to increase SA to increase uptake of water
- thin cell wall so shorter diffusion distance
- RHC actively tranport ions from soil across membranes via carrier proteins
- water potential becomes more negative
xylem
transports water and mineral ions
phloem
transports sugars and organic substances from where formed via PS to where needed (eg: shoots, flowers, fruits)
explain how water enters the xylem from tge endodermis in the root and is then transported to the leaves
ROOT
- casparian strip blocks apoplast pathway
- active transport by endodermis
- of ions into xylem
- lower water potential in xylem
XYLEM TO LEAF
- transpiration from leaves
- creates cohesion between water molecules
- adhesion between water molecule and wall of xylem
- creates a continuous column of water
symplastic pathway
- slower, resistance to movement of water
- via cytoplasm and membranes by OSMOSIS
apoplastic pathway
- faster, larger gaps between cellulose molecules so less resistance
- along cellulose cell walls by DIFFUSION until reaches casparian strip (in endodermis)
vascular bundle
zylem and phloem tissue together
xylem always inside
xylem adaptations
- dead hollow tubes so allows easier water flow
- cell walls strengthened with lignin so rigid, withstands tension and waterproof
- end walls break down to form a continuous tube with no end walls so continous column
- thick walls to withstand hydrostatic pressure
- narrow to enable capillary action
- pits enable water to move laterally between xylem vessels
root pressure moves water through the xylem. describe what causes root pressure.
- active transport by endodermis
- of ions into xylem
- lowers water potential of xylem
- water enters by osmosis
transpiration
loss of water from the leaf due to evaporation
cohesion-tension theory and transpiration
- transpiration from leaves
- lowers water potential of mesophyll cells, so water moves by osmosis into these cells
- creates a water potential gradient across leaf between mesophyll cells and air spaces
- water from xylem enters leaf
- continuous column of water formed
- cohesion of water molecules
- adhesion between water and xylem
why does the diameter of a tree trunk decrease?
- movement of water makes xylem more narrower
- negative tension
describe how a high pressure is produced in the leaves
- water potential becomes more negative
- as sugar enters phloem
- water enters phloem by osmosis
- increased volume of water = increased pressure
name a factor that can affect the rate of transpiration
- light intensity
- temperature = rate of diffusion increases, increases KE
- humidity = greater difference = greater rate of diffusion down a water potential graident
- air movement = increases water potetial gradient
a leafy shoot is cut diagonally under water
stops xylem sealing
care is taken to prevent water getting on leaves. wy?
prevents stomata being blocked by water
the potometer is filled completely with water making sure there are no air bubbles. why?
might get into xylem
and disrupt the continuous column of water
potometer is removed from under the water and all joints are sealed with waterproof jelly. why?
stops air from getting in and water escaping potometer
limitations of potometer:
- vol of water taken up doesnt always equal vol of water lost via transpiration. may also be used for: respiration, photosynthesis, support
- potometer measures uptake via stem and not roots of a normal plant
give 2 precaustions taken when setting up a potometer to get the most accurate results possible
- seal joints/ ensure airtight
- cut shoot under water
- cut shoot at a slant
- dry off leaves
- shut tap
- insert into apparatus under water
- ensure no air bubles
xerophytic plant adaptations to reduce transpiration
- reduced no of stomata
- stomata in pits
- hairs to trap water
- rolled leaves
- leaves reducied into spines
- thick waxy cuticle
solutes
dissolved substances
phloem structure
- transport organic solutes (eg: sucrose) around plants
- have sieve tubes. no nucleus, living cells
- have companion cells, carry out living functions for sieve cells
- companion cells have mitochondria to synthesise ATP via aerobic respiration for active transport of solutes
translocation
- movement of solutes/ assimilates (eg: amino acids, sucrose) to where needed.
- requires energy
- from sources (where produced) to sinks (where used)
- enzymes at sink convert solutes into storage molecules (eg: starch) to maintain a concentration gradient
- always a lower concentration at sink than source
describe the mass flow hypothesis for the mechanism of translocation in plants
i- n source, sugars are actively transported into phloem
- by companion cells
- lowers water potential of sieve tube
- water enters by osmosis (increase in volume = increase in pressure)
- increase in prsssure causes mass movement towards sink
- sugars in sink converted for respiration for storage
evidence FOR mass flow hypothesis
- ring of bark removed from stem then bulge above ring
- fluid of bulge higher conc of sugars than fluid below: evidence for DOWNWARD flow
- aphids pierce phloem and sap flows quicker near leaves than stem: evience of PRESSURE GRADIENT
- autoradiography where radioactive tracer can be tracked during translocation from source to sink
evidence AGAINST mass flow
- sugar transported to many different sinks, not just to the one with highest water potential as model suggests
- sieve plates would be abarrier for mass flow
- a lot of pressure required to get solutes through at a reasonable rate
