transport in plants Flashcards
what substances do plants need to transport over long distances?
why?
water and minerals from roots to leaves for turgor, p/s, cooling, and nutrients
sucrose and amino acids from source to sink
diffusion is not sufficient bc distances too long
what substances do plants need to transport over short distances?
O2 and CO2
can rely on diffusion alone bc leaves are thin so short diffusion distance and plants have a lower metabolic rate than animals so lower demand for O2 (not v active). in general, leaves and roots are adapted for gas exchange
what are the 4 types of cells in xylem tissue
xylem vessels
xylem tracheids
fibres
parenchyma
are the 4 types of cells in xylem tissue dead or living?
all dead (xylem vessels, xylem tracheids, fibres) except parenchyma, which is living
xylem vessels structure and function
long, tubular structures formed by water conducting cells end to end, who’s transverse cell wall has broken down to form a continuous tube
transport water and minerals from roots to leaves
thick walls have lignin so have structural support function (impermeable to water and solutes)
mature xylem vessels are dead and the protoplasm has disintegrated leaving hollow tubes
xylem tracheids structure and function
water conducting cells
transport water and minerals from roots to leaves
have lignin so have structural support function
fibres in xylem structure and function
elongated cells
lignified
support function only
parenchyma structure and function
packing tissue
support function only
xylem vessels vs xylem tracheids differences
vessels are shorter and wider, tracheids are longer and thinner
vessels are continuous tubes with no end walls, tracheids have tapered ends with perforations in end walls
vessels are more efficient at water conduction, tracheids are less efficient at water conduction (used for water storage)
xylem vessels and xylem tracheids similarities in structure
both dead
both have pits
both water conducting cells
both have lignin
functions of xylem
transport water and minerals
provide mechanical support (lignification of cellulose cell wall)
why is the lumen of xylem vessels hollow
less/no resistance to flow of water
why are xylem vessels fairly narrow
the column of water doesn’t break easily
why are the walls of xylem vessels lignified
lignin adds strength and rigidity so prevents collapse under the large tension/negative pressure/transpiration pull
lignin is waterproof so impermeable to water so that it doesn’t leak out of the xylem
what part of a xylem vessel doesn’t contain lignin
pits
why don’t pits in xylem vessels contain lignin
allow lateral flow of water between xylem vessels
allows water to leave xylem or bypass blockage
types of lignified cell wall thickenings
spiral
annular
reticulate
pitted
why is lignin arranged in spirals around the lumen of the xylem
more flexibility
prevents stem breakage during growth/movement
components of phloem
sieve tube elements
companion cells
components of companion cells in phloem
small vacuoles
Golgi
cellulose cell wall
lots of ribosomes
large nucleus
RER
plasmodesma
mitochondria
function of plasmodesmata in companion cells of phloem
connect companion cells and sieve tube elements
facilitate movement of substances between the cells e.g. sucrose
enables cell signalling
why do companion cells have many mitochondria
because they are very metabolically active
companion cells brief function
service and maintain sieve tube element
components of sieve tube element
few small mitochondria
endoplasmic reticulum
amyloplasts (starch grains)
cytoplasm
sieve plate with sieve pores
NO NUCELUS RIBOSOMES GOLGI OR VACUOLE
location of cytoplasm in sieve tube element
why
pushed up to the sides so there is only a thin peripheral layer
less resistance to flow of assimilates (sucrose and amino acids)
why do sieve tube elements not have a nucleus, ribosomes, Golgi or vacuole
would take up too much space and impede the flow
function of sieve plate
may be to help keep STE open
allows for blocking with callose as a defence mechanism
sieve pores function
sap can easily pass from cell to cell (STE to STE)
parenchyma cells in phloem function
support and storage
act as packing tissue
(living cells)
evidence that transport of organic material occurs in the phloem?
removal of all the tissues external to the xylem: when the phloem is cut away the sieve elements respond by rapidly blocking the sieve pores and sucrose accumulates above the ring of bark cut away
aphids can be used to collect sap from plants. if the aphid is cut away, the exuding sap is collected in a capillary tube. when the stem is sectioned, the send of the aphids stylet is found in the phloem
features which distinguish sieve tubes from xylem vessels
sieve plates
no pits
xylem vessels are hollow
xylem vessels have lignin
components in TS section through leaf
waxy cuticle
upper epidermis
palisade mesophyll
spongy mesophyll
lateral vein
xylem (above)
phloem (below)
vascular bundle/midrib
air spaces
lower epidermis
function of waxy cuticle
waterproof to prevent water loss/ fungal disease
lower epidermis function
has stomata/guard cells for gas exchange
upper epidermis function
transparent to allow light to pass through
air spaces in leaf function
maintain concentration gradient
label diagram
A- from outside to inside: cuticle, epidermis, collenchyma
B-pith of parenchyma
C-sclerenchyma tissues (bundle cap)
D-phloem
E-xylem
LINE=CAMBIUM
function of collenchyma in stem
provides some support
function of cortex of parenchyma in stem
storage and supporting function
function of pith of parenchyma in stem
storage and supporting
function of sclerenchyma tissue in stem
support
label diagram
A-endodermis
B-conjunctive tissue
C/D/F- xylem
E-phloem
pericycle inside endodermis
cortex outside endodermis
meristem definition
area of undifferentiated cells which can divide and differentiate into other cell types
where is meristem found
root and shoot tips
in cambium of vascular bundles
pericycle cells in root
features of meristematic cells
have thin cell walls containing little cellulose
do not have a vacuole
do not have chloroplasts
where are new cells formed in plants
what do some differentiate into
in cambium region by mitosis
xylem vessels or phloem sieve tubes/companion cells
how do cells differentiate into xylem vessels
lignin deposited in walls (cells die and lose cell contents)
end walls break down (xylem forms continuous column)
how do cells differentiate into sieve tube elements
lose most of their organelles and develop sieve plates
how do cells differentiate into companion cells
retain organelles
increase number of mitochondria
compare cell walls of xylem vessels and phloem sieve tubes
xylem: cellulose and lignin (impermeable)
phloem: cellulose, fully permeable, plasmodesmata
compare cells of xylem vessels and phloem sieve tubes
xylem: stacked end to end, dead
phloem:stacked end to end, living
compare end walls of xylem vessels and phloem sieve tubes
xylem: absent
phloem: present. sieve plates and sieve pores
compare diameter of xylem vessels and phloem sieve tubes
xylem: larger
phloem: smaller
compare cell contents of xylem vessels and phloem sieve tubes
xylem: no cell contents, hollow
phloem: peripheral cytoplasm, no nucleus, golgi, vacuole, ribosomes
compare transported substances of xylem vessels and phloem sieve tubes
xylem:mineral ions and water
phloem: sucrose and amino acids (assimilates)
compare loading and unloading of xylem vessels and phloem sieve tubes
xylem: water is absorbed in the roots and unloaded via pits into leaves
phloem: active loading at the source and passive unloading at the sink
compare direction of transport in xylem vessels and phloem sieve tubes
xylem: upwards (single direction)
phloem: bidirectional
compare method of transport in xylem vessels and phloem sieve tubes
xylem: mass flow (passive), driven by transpiration pull, cohesion and adhesion. active= root pressure
phloem: mass flow and active loading
importance of water in plants
evaporative water loss cools plants bc high latent heat of vaporisation
mineral ions and photosynthetic products transported in aqueous solution
water is a raw material for photosynthesis
osmosis in plant cells results in turgor pressure, providing a hydrostatic skeleton to support stems and leaves
turgor drives cell expansion
root hair cells: structure of hairs
large surface area
thin surface layer (just cell plasma membrane and cell wall) so short diffusion distance
why do root hair cells contain many mitochondria
provide ATP for active transport of minerals
root hair cells solute concentration
high bc maintains water potential gradient between soil water and the cell
cytoplasm and vacuole sap of root hair cell contain many diff solutes, lowering water potential so water moves in by osmosis
(soil water has a v low conc of dissolved minerals so v high water potential)
2 pathways that water molecules take between cells in roots
apoplast pathway
symplast pathway
apoplast pathway:
cells
movement method
cell walls and intercellular spaces
mass flow
symplast pathway:
cells
movement method
cell membrane, cytoplasm, plasmodesmata
osmosis
what is the endodermis
a layer of cells surrounding vascular tissue
what is the Casparian strip
a band of waxy material called Suberin
runs around each of the endodermal cells, forming a waterproof layer
what does the casparian strip do
prevents water moving across the root any further in the apoplast pathway, so it transfers to the symplast pathway, when it is forced into the cytoplasm
diversion form apoplast to symplast pathway: what happens
significant diversion: water passes through a selectively permeable cell surface membrane, excluding toxic soil solutes from living tissue, as carrier proteins for toxins are lackign
solute concentration in cytoplasm of endodermal cells vs xylem cells
dilute in cytoplasm compares to xylem cells so endodermal cells move mineral ions into the xylem by active transport
what does AT of mineral ions into xylem cause
water potential of xylem cells is much lower than that of endodermal cells, so rate of water movement into xylem by osmosis down the water potential gradient increases from endodermis through symplast
can water return to the apoplast pathway inside vascular bundle?
yes: to enter xylem and move up plant
what results in root pressure?
active pumping. of minerals into xylem produces water movement by osmosis
(independent of transpiration effects)
what does root pressure do?
gives water a push up the xylem, but isn’t a major factor in vertical movement
why is the transpiration stream important
carries water for p/s to the palisade cells in the leaves
the water carries essential mineral salts in solution
evaporation from the leaves has a cooling effect
what is transpiration a consequence of
gaseous exchange
mechanism of stomata opening
stomata allow gaseous exchange: surrounded by guard cells (contain chloroplasts for p/s and to produce ATP)
ATP is used to drive AT ion pumps
to open guard cells pump ions into cell, lowering WP
water enters by osmosis, causing cells to become turgid and bend apart, thus opening the stoma
3 processes involved in moving water up the stem
transpiration pull
capillary action
root pressure
how does transpiration pull aid in moving water up the stem
air flow around leaf takes humid air away, maintaining a diffusion gradient
water evaporates & diffuses out of leaf mesophyll cells into air spaces, lowering WP in spongy mesophyll cells so water moves into them from xylem down WP gradient
as H2O leaves the whole column of water behind is pulled up due to cohesive forces
this is called transpiration pull/shoot tension/negative pressure
how does capillary action aid movement water up stem
(COHESION TENSION THEORY)
columns of water in xylem are held together by cohesion (H2O molecules hydrogen bond with each other)
also adhesion (attraction between water molecules and sides of xylem)
collectively these forces are known as capillarity/capillary action
water is moving by mass flow
evidence that water is carried in dead cells (xylem)
a tree trunk with a steam jacket still transpires (steam kills living cells but plant still transpires so water must be carried in dead cells: the xylem)
cut stem placed in picnic acid poison will still transpire: so transport is in dead cells
evidence that water is carried in xylem
cut stem placed in dye & later sectioning reveals dye in the xylem
if plants allowed to draw up fatty substances then the lumen of the xylem vessels becomes blocked (suggests xylem can take up substances but not designed to transport fat)
ringing experiments which involved removing the phloem do not affect water transport: if xylem removed, plant would wilt bc no transport to leaves
evidence for role of root pressure in water transport in xylem
plants poisoned with cyanide cannot respire so no ATP produced so no AT can take place so not root pressure. this is proven to affect transpiration
if oxygen level falls, root pressure falls
if temp increases then root pressure increases, and if decrease in temp then root pressure decreases (proves respiration reactions are important for supplying ATP for root pressure)
evidence for cohesion tension theory
changes in tree diameter can be detected: it is smallest when rate of transp is greater (tension created narrows vessels, and lignin prevents vessel from collapsing under tension created)
at night, transp rates fall and tension is at lowest, so diameter increases
if air gets into xylem transpiration stops (column of water broken so no forces of cohesion)
coloured water will rise up narrow tube to a height greater than the level of the liquid it is stood in (capillarity evidence)
how does number of leaves affect rate of transpiration
more stomata so faster rate
how does number and position of stomata affect rate of transpiration
leaves with more stomata lose more water vapour than those w fewer
open stomata increase rate of transpiration
larger stomata means water vapour lost more quickly
more stomata on lower epidermis than upper decreases rate bc shaded
how does the presence of a cuticle affect rate of transpiration
thicker layer= less water vapour lost (decreased rate)
young leaves and shade plants have thinner cuticles
how does light intensity affect rate of transpiration
in light, stomata open up to allow gaseous exchange for p/s
higher light intensity increases rate of transp
how does temperature affect rate of transpiration
higher temp increases rate of transpiration bc:
increased rate of evaporation from cell surfaces (water vapour potential in leaf increases)
increased rate of diffusion through stomata bc water molecules have more KE
lower relative water vapour potential in air so more rapid diffusion of molecules out of leaf (steeper conc grad)
how does relative humidity affect rate of transpiration
increased relative humidity in air decreases rate of water loss bc smaller water vapour potential gradient between the air spaces in the leaf and the air outside
how does air movement/wind affect rate of transpiration
air moving outside the leaf will carry away water vapour that has just diffused out of the leaf
this maintains high water vapour potential gradient
how does water availability affect rate of transpiration
if there is little water in the soil, plant cannot replace the water that is lost
if there is insufficient water in the soil, then the stomata will close and the leaves wilt (cells surrounding stomata lose turgidity)
what does a photometer measure
the rate of uptake of water
assumed that this is equal to the water lost by transpiration
why is rate of uptake of water not always equal to water lost by transpiration?
water issued in p/s and support (turgor pressure)
at night stomata are closed so decreased rate
some plants e.g. xerophytes have adaptations to decrease rate of transpiration
examples of xerophytes
cacti
marram grass
cacti adaptations to reduce water loss
thick waxy cuticle on epidermis reduces water loss by evap. some have waxy cuticle on upper and lower leaves
spines instead of leaves reduces SA for water loss. p/s occurs in stem
stomata closed at hottest times of day when transp rates would be highest
crassulacean acid metabolism (CAM) ensures CO2 taken in at night can be sued for p/s during day
succulents store water in stem/leaves
shallow, extensive root system or long tap root
marram grass adaptations to reduce transpiration rate
curled/rolled leaves to minimise SA of moist tissue exposed to air & protects leaves from wind & funnels water to roots
stomata buried in pits, shattered from wind. layer of hairs on epidermis. both cause moist air to be trapped in pits, slowing transp by lowering WP gradient
thick waxy layer on epidermis reduces water loss by evap
example of hydrophyte
waterlilies
waterlilies adaptations
found in aquatic habitats so don’t need to reduce water loss, but need to cope with low O2
air spaces in tissues helps float, also O2 storage for respiration.
floating increases light received, whilst root/stem air spaces allow O2 movement from leaf to underwater parts
stomata only present on upper surface to maximise gas exchange
flexible leaves and stems, supported by water around them.
no need for rigid stem: this reduces damage by water currents
why do waterlilies have reduced structure
why do they have more/larger air spaces
water provides support
makes stem and leave buoyant
why do substances move from source to sink in translocation?
sugars are produced in the leaves during p/s. respiration in plants depends upon these products of p/s, which are frequently some distance away for those areas of the plant needing to release the chemical energy in these products.
at highest conc at source, lower at sink
what is translocation
the movement of dissolved solutes like sucrose and amino acids to where they are needed in the plant.
what are assimilates
dissolved substances that are transported and become incorporated into plant tissue
what is the main transported assimilate
why is it not used up during transport?
sucrose
it is soluble and metabolically inactive, so not used up during transport
examples of translocation sources
green leaves
stems
food stores in roots, seeds, tubers (begin unloading stores at beginning of growing period)
main sinks in translocation
actively dividing meristems
parts laying down food stores e.g. developing seeds and fruits
metabolically active tissues e.g. those actively absorbing mineral ions in roots
3 stages of translocation and are they active or passive
phloem loading (active)
pressure flow/ mass flow (passive: bulk movement due to a pressure gradient)
phloem unloading (passive: facilitated diffusion)
what is active loading in translocation
AT is used to load sucrose against concentration gradient at the source
stages of active loading in translocation
ATP hydrolysis supplies energy to pump H+ ions out of companion cell into leaf mesophyll cells
this sets up an electrochemical gradient for H+ ions
H+ ions and sucrose bind to co-transporter protein
both are co-transported by facilitated diffusion (secondary AT) into the companion cell
sucrose then diffuses via plasmodesmata into the sieve tube element at the source
what is pressure flow in translocation?
mass flow: sucrose is moved from source to sink
stages of pressure flow in translocation
sucrose conc has increased in the sieve tube element at the source
this decreases WP so water moves in down WP gradient by osmosis from xylem
hydrostatic pressure increases in sieve tube element near the source
assimilates move from high to low HP
what is phloem unloading in translocation?
sucrose is unloaded at any point where it is needed (sink)
stages of phloem unloading in translocation
sucrose moves into sink cells via facilitated diffusion (passive)
WP increases in sieve tube element near sink so water moves out of sieve tube element into xylem by osmosis
HP decreases at sink so gradient for mass flow is maintained
sucrose used by source cells to maintain concentration gradients e.g. converted to glucose and used in respiration
evidence for translocation
cyanide poisons mitochondria, stopping translocation as active processes require energy as ATP from respiration, needed for translocation
sucrose flow 10,000x faster than diffusion alone, providing evidence for mass flow (can be proven using aphids to collect sap and measure rate)
what do you call parts of the cellulose cell wall of a xylem vessel element where no lignin has been deposited
pits
does water enter the cytoplasm through the cell surface membrane in the apoplast or symplast pathway?
symplast
does water enter vacuoles in the apoplast or symplast pathway?
symplast
does water move from cell to cell through plasmodesmata in the symplast or apoplast pathway?
symplast
does water move from cell to cell through intercellular spaces in the apoplast or symplast pathway?
apoplast
how does water move from the xylem in the root to the leaf (one word)
transpiration pull
how does water move from mesophyll cell walls to intercellular air spaces (one word)
evaporation
how does water vapour move from intercellular air spaces to the atmosphere outside the leaf (one word)
diffusion
