3.1.3 - transport in plants Flashcards
why do plants need transport systems?
metabolic demands - the underground parts of the plant need oxygen and glucose transported to them
size - some are very large
surface area to volume ratio - cannot rely on diffusion alone
cotyledons
organs that act as food stores for the developing embryo plant and form the first leaves when the seed germinates
dicotyledon
a plant that has two seed leaves or cotyledons
vascular bundle
plant stem structure that contains xylem and phloem tissue
xylem and phloem in the stem
xylem and phloem arranged into vascular bundles with cambium
xylem on inside, phloem on outside
xylem and phloem in the roots
vascular bundles are in the middle to help the plant withstand the tugging strains from wind
cross shaped xylem, and phloem around the xylem
xylem and phloem in the leaves
xylem on top, phloem on the bottom
structure of xylem
long continuous hollow tube
narrow lumen
wall made out of lignin (strong, waterproof and adhesive)
wall contains pits/pores, so water and minerals can leave
structure of the phloem
companion cells (lots of mitochondria for ATP for active loading)
sieve tube elements (little cell contents for ease of flow)
sieve plates (connect the elements together to allow for ease of flow)
purpose of water in plants
turgor pressure provides support
turgor drives cell expansion
loss of water means plants keep cool
mineral ions transported
raw material for photosynthesis
symplast pathway
water moves continuously through the cytoplasm of cells (connected through the plasmodesmata)
apoplast pathway
route through the cell walls and intercellular spaces or plants
Casparian strip
a water impermeable ring of wax in endodermal cells of plants that blocks the passive flow of water and solutes into the stele by way of cell walls
effect of Casparian strip
water can not carry on in the apoplast pathway so it is forced into the cytoplasm of the cell, joining the symplast pathway
significance of the Casparian strip
water now has to pass through the selectively permeable cell membranes, excluding any potentially toxic solutes in the soi; water from reaching living tissues
root pressure
the upward push of xylem sap in the vascular tissue of roots
evidence for the role of active transport in root pressure
- if cyanide (affects mitochondria) is applied, root pressure disappears
- increases with higher temps, decreases with lower temps
- if oxygen falls, root pressure falls
transpiration
evaporation of water from the leaves of a plant, an inevitable consequence of gaseous exchange
transpiration stream
movement of water up through the xylem from the roots to the leaves
process of the transpiration stream
water molecules evaporate from the surface of mesophyll cells into air spaces in the leaf and move out of the stomata into surrounding air by diffusion
this lowers wp, so water moves in by osmosis
repeated across the leaf to the xylem, water moves out of xylem by osmosis into cells of the leaf
adhesion and cohesion cause capillary action and water is moved up.
capillary action
combind force of attraction among the water molecules and with the molecules of surrounding materials, causing water to rise up against the force of gravity.
cohesion-tension theory
the mechanism of water movement from roots to the leaves due tovwater cohesion and water tension
evidence for cohesion tension theory
changes in the diameter of trees - diameter grows at night when there is less transpiration
air is drawn into xylem when its broken
how stomata control the rate of transpiration
when turgor is low, the guard cells close
when the environmental conditions are favourable, guard cells pump in solutes by active transport, increasing the turgor - guard cells become shaped and open pore
how light affects transpiration
light is needed for photosynthesis, so stomata will be open in the day
more light, more stomata open, increasing the evaporation from the surfaces of leaves - more transpiration
how humidity affects transpiration
if the air outside the leaf is humid, the concentration between water vapour inside and outside the leaf is less steep so there is less transpiration
how temperature affects the rate of transpiration
increase in temperature increases the kinetic energy and there fore increases the rate of evaporation
how air movement affects the rate of transpiration
blows away the layer of humid air surface, increasing gradient, meaning more diffusion (more transpiration)
soil water availability and transpiration
if dry plant is under water stress so transpiration is reduced
source (plant)
where assimilates like sucrose are loaded in to
e.g green leaves and stems, storage organs etc
sink (plant)
where assimilates like sucrose are loaded out of
e.g roots that are growing and actively absorbing mineral ions, meristems taht are dividing.
translocation
movement of carbohydrate through a phloem from a source to a sink
assimilates
products of photosynthesis that are transported around a plant
how assimilates are loaded into the phloem
- H+ ions are pumped out of companion cells, using AT
- this creates are diffusion gradient
- therefore h+ ions are diffused back in, with a co transporter
- sucrose is also transported in with this co transport protein
structure of companion cells
many infoldings in their cell membranes to give an increased surface area for the active transport of sucrose in the cell cytoplasm
lots of mitochondria to supply ATP
phloem unloading at sinks
diffusion of the sucrose from the phloem into surrounding cells, the sucrose rapidly moves on into other cells by diffision or is converted into another substance
loss of solutes from the phloem leads to a rise in the wp of the phloem, causing water to move out by osmosis
evidence for translocation
advances in microscopyt allow us to see adaptations of CC
if mitochondria are poisoned, translocation stops
flow of sugar in phloem is 1000 times s=faster than diffusion alone, suggesting active process
use of aphids with translocation
can demonstrate the translocation of organic solutes in the phloem. it has been shown that there is a positive pressure in the phloem that forced sap out the stylet
xerophytes
plants with adaptations that allow them to survive in dry habitats
examples of xerophytes
marram grass and cacti
how waxy cuticle conserves water
minimises water loss due to the waterproof nature of the waxy cuticle
sunken stomata
reduces water loss as the water has to travek a longer distance before it reaches the outside of the leaf
reduced number of stomata
reduces transpiration rate and alos their gas exchange capabilities
reduced leaves
reducing the total number of leaves and size of leaves will reduce the surface area available for water loss
hairy leaves
create a microclimate of still humid air, reducing transpiration
succulents
store water in a specialised parencyma tissue in their stems and toots which are used up in times of drought
hydrophytes
plants taht have special adaptations to allow them to live in areas often saturated by water
examples of hydrophytes
water lillies and water cross
aerenchyma
a soft plant tissue containing air spaces found especially in many aquatic plants
adaptations of hydrophytes
very thin or no waxy cuticle
lots of stomata that are always open
wide flat leaves
air sacs
small roots
