Biology [Module 3:Transport in plants]

why do plants need a transport system?

-large in size

-so have a small SA:V, making diffusion insufficient for nutrient and water distribution.

-a transport system enables efficient transportation of water and minerals from the soil through their root hair cells and transport these to there parts of the plant(xylem)

-transport sucrose from photosynthesis to various parts of the plant (phloem)

root hair cells

-epidermal cells

-absorb mineral ions(lower the water potential)by active transport water by osmosis and increase the surface area for absorption.

Apoplastic pathway

-in the cortex water moves in through the spaces between the cell wall as water is cohesive(h2o molecules stick together because of h-bonds)

-it stops at the endodermis where the Casparian strip is located

symplastic pathway

-water moves through cytoplasm from one cell to another by osmosis via plasmodesmata

vacuolar pathway

-water is transported through the vacuole

how is a water potential gradient set up

-when water enters the cells cytoplasm the water potential gradient becomes higher than the next cell setting up a water potential gradient towards endodermis(h20moving by symplasts pathway)

-endodermal cells actively secrete ions into the xylem which lower the water potential of the xylem,so water moves in by osmosis

Transpiration

-evaporation of water from the plant through the leaf to the atmosphere(loss of water vapour)

Evaporation steps

-h2o potential in the cell is higher than in the atmosphere so water vapour moves out through the stomata

-mesophyll cells now have a lower h2o potential than neighbouring cells, so water enters the cell by osmosis

-h2o potential is set up yay pulls water(capillary action)from the xylem(transpiration pull) across the mesophyll cells and out the atmosphere

-movement of water though the xylem creates low hydrostatic pressure and thus tension

-cohesion of h2o molecules enable h2o to move by mass flow ,pulled by tension from above(via cell wall)

Cohesion-Tension theory

hydrostatic pressure in the xylem is lower than the atmosphere pressure than puts the xylem under tension

why can’t transpiration be avoided?

-stomata need to open so O2&CO2 can diffuse in/out of the cell

-CO2 for photosynthesis

-O2 fro aerobic reationspiration

-plants have to transport H2O from the roots t their leaves for photosynthesis

-so plants lose a lot of water through transpiration

-as leaves have large SA for photosynthesis ,lots of H2O vapour is lost by transpiration

why is the bag sealed to reduce transpirational loss of water?

-no root hair cells=no active transport of mineral ions ,H2O cannot enter plant by osmosis

-but plant cells are still losing water by transpiration

-by sealing the bag h2o vapour cannot escape from the atmosphere,thsi increases the water potential in the bag ,less steeper water potential gradient is set up wit the mesophyll cells of the leaf

-less/no h2o is lost by mesophyll cells

benefit of transpiration

-moves mineral ions and hormones around the plant

-mainains cell turgidity

-h2o enters the plant by osmosis

-cytoplasm pushes against cell wall exerting turgor pressure

-many turgid cells that support the plant to stay upright

-supplies h2o for growth, cell elongation-photosynthesis

-evaporation has a cooling effect on the plant

xerophytes

-plants adapted to a dry habitat e.g cactus

adaptations of xerophytes

-thick cuticle=stops uncontrolled evaporation through leaf cells

-small leaf SA=less SA for evaporation and transpiration; spines protects from predators

-low stomata density=stomata closes during times with low h2o availability which reduces transpiration through stomata

-sunken stomata(fund in pits)=traps h2o vapor reducing h2o loss

-rolled leaves=maintainshumid air around stomata, reduces water vapour potential gradient

hydrophytes adaptations

-thin cuticle

-reduced root system=less developed vascular tissues so plenty of h2o availability

-stomata on the upper surface=gaseous exchange can occur

-large air spaces=keeps afloat ro absorb sunlight + allowso2 to diffuse quickly in order to reach the roots

factors that affect rate of transpiration

-light intensity=makes stomata to be always open so increased rate of transpiration

-temperature =gives more KE to water particles so increased rate of transpiration

-air movement=wind removes water vapour around the stomata so there is a steeper water pot.grad. with leaves ,so increases rate of transpiration

potomter

estimates rate of transpiration

precautions using a potometer

-cut the stem under water to prevent any air from entering the xylem

-set up the potometer under water to make sure no air bubbles present

steps potometer

-apply petroleum jelly/vaseline to make all the joints air tight

-dry the leaves

-close the tap so water can no longer go to the reservoir as this could affect the volume of water being measured

-allow the plant to acclamaties for 30 min

-record the start point and start timer

translocation

-phloem

- movement of assimilates from the leaf of the plant to the sink

active loading

-at the source

-sucrose loaded into the sieve tube(ATP required).H+ ions actively pumped out of the companion cell

-high conc of h+ ions causes facilitated diffusion back into companion cell.sucrose is caries with the h+ ions through transport protein in the plasma membrane

-increasing the conc of sucrose in companion cell causes it to diffuse through the plasmodesmata into the sieve tube element

unloading

-at the sink

-sucrose is actively loaded into the site tube and lower the water pot.

-water enters the sieve tube by osmosis

-sap so transported down the hydrostatic pressure gradient towards the sink by mass flow

-sucrose is removed by surrounding cells increasing the water pot

-water moves out and lowers the hydrostatic pressure