transport in plants Flashcards
what are the 3 main reasons why plants need transport systems
- metabolic demands
- size (bigger plants need transport systems)
- SA:V ratio ( varies in different parts of the plant- stem, trunks and roots means they have a small SA:V ratio- so cant rely on diffusion alone)
what is a dicotyledonous plant
make seeds that contain 2 cotyledons (organs that act as food stores for the developing embryo plant and form the first leaves when seed germinates)
what is the vascular bundle in herbaceous dicots
xylem and phloem
where are the vascular bundles in the stem
around the edge to give strength and support
where are the vascular bundles in roots
in the middle to help the plant withstand the tugging strains that result from wind
where are the vascular bundles in leaves
the midrib of a dicot leaf
xylem
- non living tissue
- transport of water and mineral ions, support
- flow is from roots to shoots/leaves
- long, hollow structures made by cells fusing together end to end
- lignin spirals in lumen walls reinforce xylem vessels so they dont collapse
- parenchyma packs around the xylem vessels, storing food and tannin deposits
phloem
- living tissue
- transports organic solutes from leaves (up and down plant)
- sieve tube elements contain sieve plates (perforated end walls)
- companion cells linked to sieve tube elements by plasmodesmata
- companion cells are active, carrying out the living functions for the sieve tube elements which have lost their organelles
how does water diffuse into root hair cells
- soil water has low conc. of dissolved minerals and high water potential
- RHC contains high solute conc. so water potential is lower
- water potential gradient maintained
the symplastic pathway
- water moves by osmosis through a continuous cytoplasm of plant cells connected through plasmodesmatas
- RHC has higher water potential than the next cell so water diffuses by osmosis
- when water leaves RHC, its water potential falls again, maintaining steep water potential gradient between cell and soil
apoplastic pathway
- movement of water through cell walls
- as water moves towards xylem, more takes the apoplastic pathway due to cohesive forces
- tension created by the pull of water and its cohesive properties- so continuous flow of water through cellulose cell wall
- reaches endodermis containing Casparian strip (waxy material suberin) which forms waterproof layer
- apoplastic pathway water is forced into the symplastic pathway
how does water move into xylem
- solute conc in endodermal cells is lower than in xylem
- so w.p. of xylem is lower than endodermal cells
- in addition, endodermal cells move mineral ions into xylem by active transport
- increases rate of osmosis into xylem down a w.p. gradient
(root pressure)
what is transpiration
the loss of water vapour through stomata
transpiration stream
- water enters roots by osmosis and transported up xylem
- reaches leaves where it diffuses through to the mesophyll cells and into the air spaces
- diffuses into external air through stomata down a concentration gradient
- loss of water in mesophyll cells lowers w.p. so water moves into cell
- adhesion and cohesion results in capillary action where water can rise up a long narrow tube against gravity
- transpiration pull of the continuous stream of water to xylem to replace water lost by evaporation results in tension in the xylem
cohesion-tension theory
- model of water moving from soil in a continuous stream up the xylem and across the leaf
light intensity affecting transpiration rate
- required for photosynthesis
- increasing light intensity increases no. of stomata open
- increasing rate of water vapour diffusing out
humidity affecting transpiration rate
- high relative humidity outside plant reduces water vapour concentration gradient
- lowers rate of transpiration
temperature affecting transpiration rate
- increase in temp means increase in K.E. of water molecules
- increases rate of evaporation from mesophyll cells into air
air movement affecting transpiration rate
- higher wind speed means higher concentration gradient between internal air and external air
- as there is a thinner layer of moist air outside the stomata
what is translocation
- transporting organic compounds in the phloem from source to sink
- the products of photosynthesis transported are assimilates
phloem loading
- soluble products of photosynthesis(assimilates) are loaded into phloem by active processes
- sucrose is the main one transported
the symplast route of phloem loading
- diffusion through plasmodemsta from mesophyll cells to sieve tubes
- largely passive
- water follows by osmosis
- creates a pressure of water that moves sucrose through phloem by mass flow
the apoplast route of phloem loading
- in companion cells, sucrose moves into cytoplasm across cell membrane in an active process
- H+ ions actively mumped out of companion cell into surrounding tissue using ATP
- H+ ions return to companion cell down conc. gradient via co-transport protein
- sucrose is co-transported
- sucrose conc. in companion cell/phloem increases
- water follows and moves by mass flow
- results in increase of turgor pressure
