The document discusses the transport systems in plants, explaining that multicellular plants require oxygen, carbon dioxide, water, and nutrients delivered through their vascular tissues. It focuses on the transport of water, describing how water moves from the soil through the roots and xylem tissue up to the leaves, where it is released through transpiration. The transport of water establishes pressure gradients and flow that also facilitates the transport of minerals throughout the plant.
2. Why do multicellular plants
need a transport system?
like animals – need O2 and
nutrients
differ from animals – nature of
the nutrients needed, gases
required, and RATE
3. Particular requirements of plant
cells
CO2 - during daylight-
photosynthesis
6 CO2 + 6 H2O → C6H12O6 + 6 O2
4. Particular requirements of plant cells
Oxygen (O2) – Photosynthesizing
cells make enough O2 for their own
needs, but at a slower rate than
animals
5. Particular requirements of plant cells
Organic nutrients (containing
carbon) – some cells make their
own but other parts must be
supplied
8. Transport of H20, the Process
H20 uptake near root tips (root hairs)
H20 enters xylem
H20 moves up xylem
H20 from xylem to leaf cells
evaporation of H20 into leaf air spaces
transpiration of H20 vapor through open
stoma into the atmosphere
9. Overall
H20 moves from high to low water
potential (ψ)
from soil to root
from root to leaf
10. From Soil to Root Hair
remember Chapter 4, page 61?
11. Root hairs
extensions of epidermis
H20 moves into root hair with the
water potential (ψ) gradient
Soil Root cytoplasm
Low solute High solute (proteins, ions, sugars)
High ψ Low ψ
H20
12. Root hairs
large # of root hairs
– large surface area
- increase rate of osmosis
- root hairs constantly being
replaced
13. Mycorrhizas
fungi which function like root hairs,
form a mutualistic symbiotic
relationship with the root
- useful in poor soil that can’t
grow root hairs
- absorb nutrients, especially
phosphate
16. H2O movement
- H2O moves into the root hair through
epidermal cells
- H2O must move across cortical cells
to the xylem tissue in the center of the
root (Ψ gradient is responsible)
H2O movement can take two
pathways
- which pathway depends on the type of
plant and condition
17. Routes from cell to cell
Moving water & solutes between cells
transmembrane route
repeated crossing of plasma membranes
slowest route but offers more control
symplast route
move from cell to cell within cytosol
apoplast route
move through connected cell wall without crossing cell
membrane
fastest route but never enter cell
18. Apoplast
H2O moves through the cortex by
traveling along criss-cross fibers of
cellulose fibers located in cortical
cell walls
- never enters the cells
20. Symplast
H2O enters the cortical cells
cytoplasm and vacuole
- moves to the next cell by
interconnecting plasmodesmata
channels
- enters the cells
23. Water & mineral uptake by
roots
Mineral uptake by root hairs
dilute solution in soil
active transport pumps
this concentrates solutes (~100x) in root cells
Water uptake by root hairs
flow from high H2O potential to low H2O
potential
creates root pressure
24. Route water takes through
root
Water uptake by root hairs
a lot of flow can be through cell wall
route
apoplasty
25. H2O reaches the stele
Once the H2O reaches the stele
(center), apoplast pathway stops
- endodermal cells have a thick
waterproof waxy band of suberin
in the cell walls – Casparian strip
- can only pass into these cells
by symplast or cytoplasm
movment
26. Controlling the route of
water in root
Endodermis
cell layer surrounding vascular cylinder of root
lined with impervious Casparian strip
forces fluid through
selective cell membrane
& into symplast
filtered &
forced into
xylem vessels
Aaaaah…
Structure-Function
yet again!
27. H2O reaches the stele
in a young root the suberin
deposits form bands in the cell
walls called Caspian strips
The symplast path remains open
28. H2O reaches the stele
in older root, entire cells become
suberised, closing the symplast
path too
Only passage cells are then
permeable to water
29. Suberin deposits
- as cell ages, some cells become
completely blocked and can only
use passage cells
- allows the plant to control
incoming nutrients
- builds root pressure
- H2O crosses the pericycle into
the xylem
31. Xylem
dual function (support & transport)
Made up of four different types of cells
parenchyma cells – standard plant cells, no
chloroplast
fibers – elongated cells with dead lignifies
walls
vessel elements
tracheids
35. Plant cell types in Those would’ve
tissues been great names
for my kids!
Parenchyma
“typical” plant cells = least specialized
photosynthetic cells, storage cells
tissue of leaves, stem, fruit, storage roots
Collenchyma
unevenly thickened primary walls = support
Sclerenchyma
very thick, “woody” secondary walls =
support
rigid cells that can’t elongate
dead at functional maturity
37. Parenchyma
Parenchyma cells are relatively unspecialized, thin,
flexible & carry out many metabolic functions
all types of cells develop from parenchyma
38. Xylem - vessel elements
make up the xylem vessels
began as normal cell, but lignin in
cells
walls impermeable to water
- as it builds up, cells die,
empty lumen left
- no lignin at plasmodesmata
– kept open = pits
39. Xylem - tracheids
- non living lignified walls
- not open ended, not tubes
- H2O moves through pits
- primitive plants, ferns &
conifers
43. H2O movement in the
xylem
H2O crossed cortex, epidermis,
pericycle into xylem vessels
- moves up vessels to the
leaves
- in root - xylem in center
- in stem – near the outer
region
44. How does H20 get from the
xylem to the leaf?
- H20 evaporates from cell walls of
mesophyll
- H20 replaced from xylem
46. Rise of water in a tree by bulk
flow
Transpiration pull
adhesion & cohesion
H bonding
brings water &
minerals to shoot
Water potential
high in soil →
low in leaves
Root pressure push
due to flow of H2O
from soil to root cells
upward push of
xylem sap
47.
48.
49. From leaf to
atmosphere –
transpiration
- inside leaf mesophyll – air spaces
that are saturated with H20 vapor
- H20 evaporates fro mesophyll cell
walls
- internal leafs spaces are in direct
contact with the atmosphere through
the stomata
- diffuse down the gradient to the
atmosphere
50. transpiration
loss of H20
- low humidity – transpiration
HIGH, large gradient
- high humidity – transpiration
LOW
- high temperature/wind –
transpiration rate HIGH
51. Control of transpiration
Stomata function
always a compromise between
photosynthesis & transpiration
leaf may transpire more than its weight in
water in a day…this loss must be balanced
with plant’s need for CO2 for photosynthesis
a corn plant transpires 125 L of water in a
growing season
52. Regulation of stomata
Microfibril mechanism
guard cells attached at tips
microfibrils in cell walls
elongate causing cells to
arch open = open stomata
shorten = close when water
is lost
Ion mechanism
uptake of K+ ions by guard
cells
proton pumps
water enters by osmosis
guard cells become turgid
loss of K+ ions by guard cells
water leaves by osmosis
guard cells become flaccid
53. Other cues
light trigger
blue-light receptor in plasma membrane of guard cells
triggers ATP-powered proton pumps causing K+ uptake
stomates open
depletion of CO2
CO2 is depleted during photosynthesis (Calvin cycle)
circadian rhythm = internal “clock”
automatic 24-hour cycle
54. stomata
stomata open/close – control rate of
transpiration
- CO2 must come in, so H20 will leaves
(HIGH transpiration)
- bright day, CO2 more needed for
photosynthesis
- but then more water loss through
transpiration
- plant will have to compromise
55. Hydrostatic pressure
pressure exerted by liquid
- H20 removed from the xylem
reduces pressure
- creates a high to low gradient
- (e.g. suck on a straw, reduces
pressure at the top, liquid rises)
- H20 flows up the xylem
56. Hydrostatic pressure
- H20 moves by mass flow (moves as
one body of liquid)
-H20 molecules attracted to each
other (H bonds) – cohesion
-H20 molecules attracted to lignin (H
bonds) – adhesion
- air bubbles stop movement (break
water column) but pits allow H20 to
move around
57. Root pressure
transpiration reduces hydrostatic
pressure at top of xylem
- plants also increases root
pressure to increase pressure
differences
- active secretion of solutes,
mineral ions, into the water of the
xylem vessels in the root
58. Root pressure
cells surrounding the xylem use active
transport to pump solutes across their
membranes and into the xylem
- the presence of solutes lowers the
water potential drawing water from the
surrounding root cells
- increases pressure at the base of
the xylem vessels
- root pressure is not essential
59. Comparing rates of
transpiration
- hard to measure H2O vapor
leaving the plant
- since most H2O lost by a plant
was taken in by the roots
- a measurement of transpiration
could be approximated by
measuring the amount of water
uptake
60. potometer
apparatus used to measure uptake
- must be airtight
- plant stem in water with rest of
plant above the airtight seal
- cut the stem with a slanting cut
- as water evaporates it is drawn
into the xylem from the capillary tubing
and can be measured
- an experiment can expose the
plant to varying conditions to compare
the rate of transpiration
61.
62. Xerohytes
plants that live in places where water
is scarce
- adaptations include
-dune grass - eaves can roll up
exposing a tough waterproof cuticle
to the outside air while the stomata
remain open in the enclosed middle of
the roll
63. Xerohytes
hairs help trap a layer of moist air close to
the leaf surface
- cactus – flattened photosynthetic
stem, stores water, leaves are reduced to
spines which reduces surface area for
transpiration
- waxy coating cuts down on water loss
66. Left and right Epidermis of the cactus Rhipsalis
dissimilis.
Left: View of the epidermis surface. The crater-shaped
depressions with a guard cell each at their base can be
seen.
Right: X-section through the epidermis & underlying
tissues. The guard cells are countersunk, the cuticle is
thickened. These are classic xerophyte adaptations.
67. Marram grass possesses:
rolled leaves, leaf hairs and
sunken stomata. These
adaptations make it resistant
to dry conditions and of
course sand-dunes which
drain very quickly retain very
little water.
73. Translocation
- term used to describe the
transport of soluble organic
substances (assimilates) – such as
sugars
- assimilates are transported in sieve tube
elements in the phloem tissue
74. Phloem tissue
types of cells
sieve (tube) elements
companion cells
parenchyma
fibers
75. Sieve elements
sieve tube – made up of many sieve
elements joined end to end vertically
to form a continuous column
a living cell with a cellulose cell wall, a
plasma membrane, cytoplasm
containing ER and mitochondria
76. Sieve elements
amount of cytoplasm is very small,
only providing a thin layer inside the
cell wall,
no nucleus or ribosomes
sieve plate – end walls made of
perforated sieve plate
pores allow free movement of liquids
77. Companion cells
each sieve elements has at least on
companion cell lying close beside it
same organelles of other plant cells,
including small vacuole and nucleus
78. Companion cells
contain more mitochondria and
ribosomes, are metabolically very
active
numerous plasmodesmata pass
through their cell walls making direct
contact between the cytoplasm of the
companion cell and sieve element
79. The contents of phloem
sieve tubes
liquid called phloem sap or sap
not easy to collect phloem sap
when phloem tissue is cut, the sieve
elements respond by rapidly blocking
the sieve pores in a process called
clotting
80. phloem sieve tubes
pores become blocked first by plugs
of phloem protein strands and then
the carbohydrate callose
callose – molecular structure similar
to cellulose, long chains of glucose
units linked by β 1-3 glycosidic bonds
83. Aaaaah…
Phloem
Structure-Function
again!
Living cells at functional maturity
lack nucleus, ribosomes & vacuole
more room: specialized for
liquid food (sucrose) transport
Cells
sieve tubes
end walls, sieve plates, have pores to
facilitate flow of fluid between cells
companion cells
nucleated cells connected to the sieve-
tube
help sieve tubes
89. How are aphids used to
sample sap?
- aphids have a tubular mouthpart
– stylets
- stylet is inserted through the
surface of the plant’s stem/leaf
into the phloem
- phoem sap flows through the
stylet
90. Why doesn’t the stylet clot?
- the diameter of the stylet is so
small it does not allow the sap to
flow out rapidly enough to switch
on the plant’s phloem clotting
mechanism
stimulus-response
91. How translocation occurs
- like water movement in the xylem,
sap moves by mass flow
- mass flow in xylem resulted from
pressure difference created by the
water potential gradient between the
soil and air, this required no energy
from the plant
- to create the pressure differences
needed for mass flow in the phloem, the
plant has to use energy, an active
process
92. How translocation occurs
- active loading – sucrose is actively
loaded into the sieve elements , usually
where photosynthesis takes place
- as sucrose is loaded, this decreases
the water potential in the sap
- through osmosis water enter the sap,
moving down the water potential
gradient
- in the root, (or other area along the
way), sucrose may be removed, again
water follows by osmosis
93. How translocation occurs
- this creates a pressure difference:
hydrostatic pressure is high in the part
of the sieve tube in the leaf and lower in
the part of the root
- water flows from high to low taking
dissolved solutes with it
source – area of the plant where
sucrose is loaded
sink - area of the plant where sucrose
is taken out
94. Loading sucrose into phloem
photosynthesis produces trioses
which are converted into sucrose
sucrose, in solution, moves from
the mesophyll cells to the phloem
- moves by the symplast pathway
(cell-to-cell via the plamodesmata)
or the apoplast pathway traveling
along the cell walls
95. Loading sucrose into phloem
- sucrose is loaded into companion cells
by active transport
- ATP is used to pump H+ ions outside
the companion cell
- H+ reenter the cell traveling down
their concentration gradient traveling
with sucrose (against its through carrier
proteins (co-transporter proteins) in
the plasma membrane
96. Unloading sucrose from
phloem
little is known about this process
- believed sucrose moves from the
phloem though diffusion down a
concentration gradient
- this gradient is maintained through
cells converting sucrose to something
else to maintain the gradient
- invertase – enzyme that hydrolyzes
sucrose to glucose and fructose
97. Evidence for the mechanism
of phloem transport
- the role of sieve pores and phloem proteins
were topics of considerable arguments in the
70’s and 80’s
- now known phloem proteins are not present
in living active phloem tissue
- evidence that sap moves by mass flow is
considerable
- rate of phloem transport is 10,000 X faster
than diffusion
- actual rates of transport measured match
closely with those calculated from measured
pressure differences at source and sink
98. Evidence for the mechanism
of phloem transport
- evidence of ‘active’ loading of sucrose into
sieve elements in sources such as leaves
- - phloem sap always has a relatively high pH
about 8, expected if H+ ions are being
transported out of the cells
- - electrical potential of -150mV, consistent
with an excess of H+ ions outside the cell
compared to inside
- - ATP is present in phloem sieve elements in
large amounts, expected it is required for
active transport
99. Comparison of sieve elements
and xylem vessels
Similarities
- liquid moves in mass flow along
a pressure gradient
- liquid moves in tubes formed
from cells stacked end to end
100. Comparison of sieve
elements and xylem vessels
Differences
- water transported through dead xylem
vessels
- translocation through live phloem sieve tubes
requires active loading of sucrose at sources
thus requiring living cells
- xylem has lignified walls, phloem does not
- lignified, dead xylem vessels are entirely
empty providing a tube through which water
can flow unimpeded
101. Comparison of sieve elements
and xylem vessels
- lignified, dead xylem vessels have
strong walls for plant support
- end walls of elements disappear where
phloem sieve elements form sieve
plates which prevent the phloem sieve
elements from collapsing
- sieve plates allow the phloem to seal
itself if damaged, i.e. part of a blade of
grass is eaten by a herbivore
102. Comparison of sieve
elements and xylem vessels
- phloem sap has a high turgor
pressure because of its high solute
content, it would leak out quickly if the
sieve elements did not seal
- phloem sap contains valuable
substances such as sucrose, which
clotting protects from loss
- clotting may also prevent the entry of
microorganisms which might feed on
the nutritious sap or cause disease
Notas do Editor
The hyphae of mycorrhizal fungi extend into soil, where their large surface area and efficient absorption enable them to obtain mineral nutrients, even if these are in short supply or are relatively immobile. Mycorrhizal fungi seem to be particularly important for absorption of phosphorus, a poorly mobile element, and a proportion of the phosphate that they absorb has been shown to be passed to the plant.
Functions of the Symplast and Apoplast in Transport How do water and solutes move from one location to another within plant tissues and organs? For example, what mechanisms transport water and minerals from the root hairs to the vascular cylinder of the root? Such short–distance transport is sometimes called lateral transport because its usual direction is along the radial axis of plant organs, rather than up and down along the length of the plant. Three routes are available for this transport. By the first route, substances move out of one cell, across the cell wall, and into the neighboring cell, which may then pass the substances along to the next cell in the pathway by the same mechanism. This transmembrane route requires repeated crossings of plasma membranes as the solutes exit one cell and enter the next. The second route, via the symplast, the continuum of cytosol within a plant tissue, requires only one crossing of a plasma membrane. After entering one cell, solutes and water can then move from cell to cell via plasmodesmata. The third route for short–distance transport within a plant tissue or organ is along the apoplast, the pathway consisting of cell walls and extracellular spaces. Without entering a protoplast, water and solutes can move from one location to another within a root or other organ along the byways provided by the continuum of cell walls.
The endodermis, with its Casparian strip, ensures that no minerals can reach the vascular tissue of the root without crossing a selectively permeable plasma membrane. If minerals do not enter the symplast of cells in the epidermis or cortex, they must enter endodermal cells or be excluded from the vascular tissue. The endodermis also prevents solutes that have been accumulated in the xylem sap from leaking back into the soil solution. The structure of the endodermis and its strategic location in the root fit its function as sentry of the border between the cortex and the vascular cylinder, a function that contributes to the ability of roots to transport certain minerals preferentially from the soil into the xylem.
The endodermis, with its Casparian strip, ensures that no minerals can reach the vascular tissue of the root without crossing a selectively permeable plasma membrane. If minerals do not enter the symplast of cells in the epidermis or cortex, they must enter endodermal cells or be excluded from the vascular tissue. The endodermis also prevents solutes that have been accumulated in the xylem sap from leaking back into the soil solution. The structure of the endodermis and its strategic location in the root fit its function as sentry of the border between the cortex and the vascular cylinder, a function that contributes to the ability of roots to transport certain minerals preferentially from the soil into the xylem.
The transpiration–cohesion–tension mechanism that transports xylem sap against gravity is an excellent example of how physical principles apply to biological processes. In the long–distance transport of water from roots to leaves by bulk flow, the movement of fluid is driven by a water potential difference at opposite ends of a conduit. In a plant, the conduits are vessels or chains of tracheids. The water potential difference is generated at the leaf end by transpirational pull, which lowers the water potential (increases tension) at the “upstream” end of the xylem. On a smaller scale, water potential gradients drive the osmotic movement of water from cell to cell within root and leaf tissue. Differences in both solute concentration and turgor pressure contribute to this short–distance transport. In contrast, bulk flow depends only on pressure. Another contrast with osmosis, which moves only water, is that bulk flow moves the whole solution, water plus minerals and any other solutes dissolved in the water. The plant expends no energy to lift xylem sap by bulk flow. Instead, the absorption of sunlight drives transpiration by causing water to evaporate from the moist walls of mesophyll cells and by lowering the water potential in the air spaces within a leaf. Thus, the ascent of xylem sap is ultimately solar powered.
Leaves generally have broad surface areas and high surface area–to–volume ratios. The broad surface area is a morphological adaptation that enhances the absorption of light needed to drive photosynthesis. The high surface area–to–volume ratio aids in the uptake of carbon dioxide during photosynthesis as well as in the release of oxygen produced as a by–product of photosynthesis. Upon diffusing through the stomata, CO2 enters a honeycomb of air spaces formed by the spongy parenchyma cells. Because of the irregular shape of these cells, the internal surface area of the leaf may be 10 to 30 times greater than the external surface area we see when we look at the leaf. Although broad surface areas and high surface area–to–volume ratios increase photosynthesis, they also have the serious drawback of increasing water loss by way of the stomata. Thus, a plant’s tremendous requirement for water is part of the cost of making food by photosynthesis. By opening and closing the stomata, guard cells help balance the plant’s requirement to conserve water with its requirement for photosynthesis
Guard cells arbitrate the photosynthesis–transpiration compromise on a moment–to–moment basis by integrating a variety of internal and external stimuli. Even the passage of a cloud or a transient shaft of sunlight through a forest canopy can affect the rate of transpiration.