transport in plants (9) Flashcards
what are dicotyledonous plants
these are plants that make 2 cotyledons (food stores) in their seeds.
what are the 3 reasons for a transport system in plants/ why do multicellular plants need transport systems
high metabolic demands - underground parts of plant don’t photosynthesise so they need waste substances removed and o2 and glucose transported to them. mineral ions need to be transported to all cells to make proteins.
size - bigger, taller plants need efficient transport systems to move substances up and down plant.
SA - plants have a large SA: volume ratio so they cant solely rely on diffusion to supply cells w everything
what are cotyledons
food stores
what are the 2 types of dicotyledonous plants. and give an example of each
herbaceous - soft tissues, short life cycle (little plants)
woody - lignified tissues, long lived (trees)
how are the xylem and phloem arranged in the stems, roots, leaves
vascular bundles
where in the stem are the vascular bundles located and why are they there
around the edge to give support and strength
where in the roots are the vascular bundles located
in the middle to help plant withstand tugging strains from the wind blowing the stem and leaves
where are the vascular bundles in the leaves and why there
in the midrib of a dicot leaf. helps support structure of leaves.
in a vascular bundle where are the phloem and xylem located
phloem on outside and xylem on inside
function of xylem tissue
structural support and transports water + minerals up plant.
what is the function of the spiral band of lignin in the xylem vessel
spiral bands allow vessels to elongate, thus allowing tissues to expand w/o breaking the vessel walls
what is the function of the bordered pits in the vessel
they act as channels for the transport of h20 +minerals between the adjacent cells
is their lignin in the phloem
no
are their bordered pits in the phloem
no
what is plasmodesmata
gaps in cell wall between companion cells + sieve tube elements
what feature would not be found in mature xylem vessels
cytoplasm
xylem vessels contain lignin carbohydrate. what is this for
allows water to pass into adjacent vessels
a feature of the sieve tubes
contains perforated cross walls
what roles does water have within plants
turgor pressure - provides a hydrostatic skeleton to support stems + leaves.
evaporation- keeps plants cool.
turgor drives cell expansion -
the force that enables plant roots to force their way thru tarmac + concrete.
transport - mineral ions + products of photosynthesis are transported in aqueous solutions
raw material - h20 is raw material for plants
adaptations of the root hair cell
they are microscopic so can penetrate easily between soil and branches.
large SA: V ratio
thin surface layer for fats diffusion =osmosis
the concentration of solutes in the cytoplasm of root hair cells maintains a water potential gradient between the soil water and cell
talk about soil water
has a very low concentration of dissolved minerals so has a high water potential. while RHCs have a high conc of solvents so the water potential in rhc is lower this drives more water into the RHCs by osmosis
what is another reason for the lignin in the xylem
lignin reinforces the xylem so it doesn’t collapse under the transpiration pull
how are sieve plates formed and what are they for
in the areas between the cells the cell walls become perforated = sieve plates.
they let the phloem contents flow through
as large pores appear in the phloem what happens
the tonoplast, nucleus and other organelles break down. so mature phloem cells have no nucleus
after the water is in the RHC how does it travel to the xylem. what are the 2 pathways water moves thru the plant to the xylem
apoplast + symplast pathways
explain symplast pathway
symplast is the continuous cytoplasm of the living plant cells that is connected thru the plasmodesmata - by osmosis.
the RHC has a higher water potential than the next cell along cos pf water diffusing in from the soil making the cytoplasm more dilute, so water moves from the RHC into the next cell by osmosis.
this process repeats cell to cell until the water has reached the xylem.
as water leaves a RHC the water potential of cytoplasm drops again, this maintains a a steep water potential gradient
explain apoplast pathway
this is the movement of water through cell walls + intercellular spaces. water fills the spaces between the open network of fibres in the cellulose cell wall. as water molecules move into the apoplast more are pulled behind due to cohesive forces between the water molecules. the pull from water moving up into the xylem and from the cohesive forces creates a tension that makes a continuous flow of water through the xylem w little resistance
describe pathway of water after reaching the root
the water moves across the route via symplast or apoplast pathway until it reaches endodermis.
the water runs around the apoplast strip, the apoplast strip is made of suberin so the water is forced into the cytoplasm (symplast pathway). to get into cytoplasm the water crosses the selectively permeable cell surface membranes. the endodermal cells move mineral ions into the xylem by active transport, this lowers the water potential of the xylem , lower than of the endodermal cells. creates root pressure. this increases the rate of water moving into the xylem by osmosis down a conc gradient from the endodermis down the symplast pathway
what is the endodermis
a layer of cells that surrounds xylem + phloem
what is suberin and what does it prevent
a strip of waxy material that runs around each of the endodermal cells - the apoplast strip. it prevents the movement of water thru the cell walls (apoplast pathway)
1st evidence for active transport
poisons (cyanide) affect the mitochondria, so the root pressure disappears. root pressure needs energy, mitochondria produce ATP . no ATP no energy.
2nd evidence for active transport
root pressure increases with a rise in temp, this suggest chemical reactions are involved cos increasing temp increase chemical reactions (partucles move faster and collide more often, so chemcal reaction rate increases)
3rd evidence for active transport
if levels of oxygen or respiratory substrate fall root pressure falls. oxygen is needed for respiration
4th evidence for active transport
root pressure and transpiration are independent of eo - when water transpiration is low xylem sap can still be seen exuded from the tips of leaves.
what is ttranspiration
loss of water. when the stomata open for gas exchange, water moves out of the leaf by diffusion
why does stomata close at night (dont forget some stomata stay open allthetime)
reduces water loss. no photosynthesis at night so demand for o2 is low
what do guard cells do
when conditions are optimal they pump in solutes by active transport which increases the turgor.
2 features of guard cells
guard cells have cellulose rings around them, so they cannot swell but can lengthen.
inner wall not as flexible so the cells curve becoming bean shaped, creating a pore.
when water is scarce what happens with the guard cells
a hormonal signal is sent from the roots and trigger turgor loss in the guard cells thus closing the stomata
what are the 5 stages of the transpiration stream
1 - water enters root by osmosis
2 - water transported up xylem up to leaves
3 - water moves by osmosis and diffusion across membranes along the apoplast pathway from the xylem
4 - water evaporates from freely permeable cellulose cell walls of the mesophyll cells into the air spaces
5 - water vapour moves out of the leaves thru the stomata down a concentration gradient/diffusion gradient
what is cohesion tension theory
driving force for water movement up from the roots to the leaves
adhesion + cohesion = ?
water exhibiting capillary action, the process by whuch h20 can rise up a narrow tube agianst gravity
what is the transpiration pull
where water is drawn up xylem in a continuos stream to replace h20 lost by evaporation
evidence for cohesion tension theory
changes in tree diameter - when transpiration as at its height during the day tension in xylem is at a highso the tree shrinks in diameter. at night are the lowest rates of transpiration so diameter increases.
breaking a xylem vessel - cut a flower stem, put in water and air will be drawn into the xylem rather than water as the continuous stream of water molecules held tg by cohesive forces has been broken
limitations to transpiration?
in high intensity light, rate of photosynthesis is fast so stomata must beopen to supply the co2 but then the plant will lose alot of water, in some case too much water is lost
water availability is often limited
we measure transpiration with a …
potometer
rate of water uptake calculation (potometer experiment)
rate of water uptake = distance moved by air bubble/time taken for air bubble to move that distance
for the potometer experiment where should the stem be cut
underwater so no air bubbles go into the stem
a control of this experiment?
all joints must be sealed w waterproof jelly to ensure any water loss measured is as a result of transpiration from the stem/leaves
5 factors affecting transpiration?
light, humidity, temp, air movement, soil water availability
how does light affect transpiration
increased light intensity=increase photosynthesis. increased light intensity= more stomata open for gas exchange, this increases the rate of water vapour diffusing out + therefore increasing the evaporation from the surfaces of the leaf
how does humidity affect transpiration
high humidity lowers rate of transpiration cos of reduced water vapour potential gradient between the inside of the leaf + outside air. dry air has opposite effect + increases rate of transpiration.
how does temperature affect transpiration
increase temp increases KE of water molecules, so increases rate of evaporation from spongy mesophyll cells into air spaces of the leaf. higher temp increases conc of water vapour that the external air can hold before becoming saturated (so decreases its relative humidty and its water potential)
how does air movement affect transpiration
each leaf has a layer of still air around it trapped and little hairs on the leaf to decrease air movement close to the leaf. water vapor that diffuses out of the leaf accumulates here, so the water potential around the stomata increases in turn reducing the diffusion gradient. lomg period of still air will reduce transpiration
hoe does soil water availability affect transpiration
if soil is dry the plant will be under water stress and so raate of evaporation will be reduced
when diffusion gradient is increased what else increases
rate of transpiration
what is translocation
the transport of organic compounds ususally in the form of sucrose (produced in leaves in paliisade mesophyll))
why is sucrose transported instead of glucose
increases energy storage. its an efficient energy transfer
transport occurs from a source to a sink, true or false
true
what is a source
regions of plant that produce assimilates by photosynthesis or form storage materials (leaves)
what is a sink
regions of a plant that require assimilates to supply their metabolic needs (roots, fruits)
what is the phloem loading
the movement of sucrose into the phloem to be transported. its an active process, requires ATP
what are the 2 main ways that sucrose is loaded
symplast and apoplast route
how are companion cells adapted for this loading
- many infoldings
- many mitochondria
see kerboodle for further detail
what is phloem unloading
kerboodle pg 209
evidence for mass flow in phloem
kerboodle pg 209 for further detail
phloem sap can be exuded from hole usig aphids.
flow of sugars 10000x faster than if just diffusion was responsible
if mitochondria of companion clells are poisones w cyanide translocation stops
microoscopy allows us to see adaptations of the companion cells
what is apoplast route
kerboodle pg 208
2 adaptations of sieve tubes that enable mass flow
theyre stacked and their end walls (sieve plates) have lrg pores thru which their cytoplasm can flow
how are assimilates loaded into the phloem
