1.
Chapter 10
Transport in multicellular
plants

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

6.
Particular requirements of plant cells
 Inorganic ions & H2O → NH4+,
NO3-, H20, get from roots

7.
Transport of H20
 H20 is transported in xylem tissue

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

14.
Mycorrhizae

15.
Root Hair to Xylem

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

19.
Apoplast

20.
Symplast
 H2O enters the cortical cells
cytoplasm and vacuole
 - moves to the next cell by
interconnecting plasmodesmata
channels
 - enters the cells

21.
Symplast

22.
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

23.
Route water takes through
root
 Water uptake by root hairs
 a lot of flow can be through cell wall
route
 apoplasty

24.
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

25.
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!

26.
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

27.
H2O reaches the stele
 in older root, entire cells become
suberised, closing the symplast
path too
 Only passage cells are then
permeable to water

28.
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

29.
H2O reaches the stele

30.
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

31.
Xylem

32.
Plant tissues
 Dermal
 “skin” of plant
 single layer of tightly
packed cells that
covers & protects plant
 Vascular
 transport materials
between roots & shoots
 xylem & phloem
 Ground
 everything else:
storage, photosynthetic
 bulk of plant tissue

33.
Plant cell types in tissues

34.
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

35.
Plant cell types in tissues

36.
Parenchyma
Parenchyma cells are relatively unspecialized, thin,
flexible & carry out many metabolic functions
all types of cells develop from parenchyma

37.
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

38.
Xylem - tracheids
 - non living lignified walls
 - not open ended, not tubes
 - H2O moves through pits
 - primitive plants, ferns &
conifers

39.
Xylem
dead cells →
water-conducting
cells of xylem

40.
Xylem

41.
Xylem

42.
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

43.
How does H20 get from the
xylem to the leaf?
 - H20 evaporates from cell walls of
mesophyll
 - H20 replaced from xylem

44.
Ascent of xylem “sap”
Transpiration pull generated by leaf

45.
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

46.
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

47.
transpiration
 loss of H20
 - low humidity – transpiration
HIGH, large gradient
 - high humidity – transpiration
LOW
 - high temperature/wind –
transpiration rate HIGH

48.
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

49.
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

50.
 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

51.
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

52.
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

53.
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

54.
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

55.
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

56.
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

57.
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

58.
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

59.
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

60.
All Cacti are xerophytes

61.
Transverse Section Through Leaf of Xerophytic Plant

62.
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.

63.
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.

64.
BYB3 June 2001 Question 8 part c

65.
BYB3 June 2001 Question 8 part c
ANSWERS

66.
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

67.
Phloem tissue
 types of cells
 sieve (tube) elements
 companion cells
 parenchyma
 fibers

68.
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

69.
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

70.
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

71.
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

72.
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

73.
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

74.
Phloem: food-conducting
cells
 sieve tube elements & companion cells

75.
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

76.
sieve plate
Phloem
sieve
tubes

77.
Phloem: food-conducting cells
 sieve tube elements & companion cells

78.
Vascular tissue in herbaceous
dicot stems monocot
trees & shrubs grasses & lilies

79.
Root structure: monocot

80.
Root structure: dicot
phloem xylem

81.
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

82.
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

83.
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

84.
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

85.
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

86.
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

87.
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

88.
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

89.
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

90.
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

91.
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

92.
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

93.
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

94.
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