+Chapter 36:
Resource
Acquisition and
Transport in
Vascular Plants
Mrs. Valdes
AP Biology
+Overview: Underground Plants
 Success
of plants depends
on their ability to gather &
conserve resources from
environment
 Diffusion, active
transport,
and bulk flow work
together to transfer water,
minerals, and sugars
 “Stone Plants” live mostly
subterranean with two leaf
tips full of clear jelly that
acts as lenses to channel
light underground
Fig. 36-2-1
+
H 2O
H2O
and
minerals
Fig. 36-2-2
+
CO2
O2
H 2O
H2O
and
minerals
O2
CO2
Fig. 36-2-3
+
CO2
O2
Light
H 2O
H2O
and
minerals
Sugar
O2
CO2
+Concept 36.1: Land plants acquire
resources both above and below ground
 Algal
ancestors of land plants absorbed water,
minerals, and CO2 directly from surrounding water
 Evolution of xylem and phloem in land plants 
long-distance transport of water, minerals, and
products of photosynthesis
 Adaptations in each
species represent
compromises between
enhancing photosynthesis
and minimizing water loss
 Stems: conduits
for water
and nutrients; supporting
structures for leaves
 Phyllotaxy: arrangement
of leaves on stem; specific
to each species
 Light absorption
+
 affected
by leaf area index
 total upper leaf surface of plant divided by
surface area of land on which it grows
 affected by light absorption
+Root Architecture and Acquisition of
Water and Minerals
 Soil: resource
mined by root system
 Taproot systems: anchor plants; characteristic of
most trees
 Roots + hyphae of soil fungi form symbiotic
associations called mycorrhizae
 Mutualisms
with fungi helped plants colonize land
Concept 36.2: Transport occurs by short+
distance diffusion or active transport and by
long-distance bulk flow
 Transport
begins with
absorption of resources
by plant cells
 Movement of substances
into/out of cells regulated
by selective permeability
 Diffusion across a membrane
= passive
 Pumping of solutes
across a membrane = active AKA needs energy
 Most solutes pass through transport proteins in cell membrane
 Proton pump: most important transport protein for active
transport
 plant cells create hydrogen ion gradient AKA form of potential
energy that can be harnessed to DO WORK
 contribute to voltage known as membrane potential
 Plant cells use energy
+
stored in proton
gradient and
membrane potential
to drive transport of
many different solutes
 Cotransport:
transport protein
couples diffusion of
one solute to active
transport of another
 “Coattail” effect of
cotransport:
responsible for
uptake of sugar
sucrose by plant cells
+ Diffusion of Water (Osmosis)
 Osmosis:
determines net
uptake/water loss by cell;
affected by solute concentration
and pressure
 Water potential: measurement
that combines effects of solute
concentration and pressure
 Determines
direction of movement
of water (in/out)
 Water flows from regions of
higher water potential to regions
 lower water potential
 Symbol:Ψ
 Units: measure pressure;
megapascals (MPa)
 Ψ = 0 MPa for pure water at sea
level and room temperature
How
Solutes
and
Pressure
+
Affect Water Potential
•
•
REMEMBER: Pressure and
solute concentration affect
water potential
Solute potential (ΨS):
proportional to number of
dissolved molecules
•
•
•
also called osmotic
potential
Pressure potential (ΨP):
physical pressure on a
solution
Turgor pressure: pressure
exerted by plasma
membrane against the cell
wall, and cell wall against
the protoplast
+ Measuring Water Potential
•
•
•
•
Water moves from higher water potential 
lower water potential
Addition of solutes reduces water potential
Physical pressure increases water potential
Negative pressure decreases water potential
Fig. 36-8a
+
(a)
0.1 M
solution
Pure
water
H2O
ψP = 0
ψS = 0
ψ = 0 MPa
ψP = 0
ψS = −0.23
ψ = −0.23 MPa
Fig. 36-8b
+
(b)
Positive
pressure
H2O
ψP = 0
ψS = 0
ψ = 0 MPa
ψP = 0.23
ψS = −0.23
ψ = 0 MPa
Fig. 36-8c
+
(c)
Increased
positive
pressure
H2O
ψP = 0.30
ψP = 0
ψS = −0.23
ψS = 0
ψ = 0 MPa ψ = 0.07 MPa
Fig. 36-8d
+
(d)
Negative
pressure
(tension)
H2O
ψP = 0
ψP = −0.30
ψS = −0.23
ψS = 0
ψ = −0.30 MPa ψ = −0.23 MPa
•Water potential affects uptake and loss of water by plant
cells
•Flaccid cell: placed in environment with higher solute
concentration
•cell will lose water and undergo plasmolysis
•If same flaccid cell placed in solution with lower solute
concentration
•cell will gain water and become turgid
•Turgor loss in plants causes wilting
•can be reversed when the plant is watered
+Aquaporins: Facilitating Diffusion of Water
 Aquaporins: transport
proteins in cell membrane
that allow passage of water
 rate
of water movement likely regulated by
phosphorylation of aquaporin proteins
+Three Major Pathways of Transport
 Transport
also regulated by compartmental structure of
plant cells
1: Plasma membrane directly controls traffic of molecules
into/out of the protoplast
2: Plasma membrane is barrier between two major
compartments, the cell wall and the cytosol
3: Vacuole, large organelle that occupies as much as 90% or
more of protoplast’s volume
 Vacuolar membrane regulates transport between cytosol and
vacuole
 In most plant tissues, cell wall and
cytosol continuous from cell to cell
 Symplast: cytoplasmic continuum
 Plasmodesmata: cytoplasm of
neighboring cells connected by
channels
 Apoplast: continuum of cell walls
and extracellular spaces
 Water
and minerals can travel through plant by
+three routes:



Transmembrane route: out of one cell, across a cell wall, and into
another cell
Symplastic route: via continuum of cytosol
Apoplastic route: via cell walls and extracellular spaces
+Bulk Flow in Long-Distance Transport
 Bulk
flow: movement of a fluid driven by
pressure; required for efficient long distance
transport of fluid
 Water and solutes move
together through:
 tracheids
and vessel elements
of xylem
 sieve-tube elements of phloem
 Efficient
movement possible
because mature tracheids and
vessel elements have no
cytoplasm, and sieve-tube
elements have few organelles
in their cytoplasm
+ Concept 36.3: Water and minerals are
transported from roots to shoots
 Plants
can move large
volume of water from their
roots to shoots
 Most water and mineral
absorption occurs near root
tips, where epidermis is
permeable to water and root
hairs are located
 Root
hairs account for much
of surface area of roots
 After
soil solution enters
roots, extensive surface area
of cortical cell membranes
enhances uptake of water
and selected minerals
Transport
of Water and Minerals into
+
•Xylem
Endodermis: innermost layer of cells in root cortex
•
•
•
•
Surrounds vascular cylinder
Last checkpoint for selective passage of minerals from
cortex into vascular tissue
Water can cross the
cortex via the
symplast or apoplast
Casparian strip:
waxy endodermal
wall blocks apoplastic
transfer of minerals
from cortex to
vascular cylinder
+ Bulk Flow Driven by Negative Pressure
in the Xylem
 Transpiration: evaporation
of water from a
plant’s surface; Plants lose large volume of
water
 Xylem sap: Water replaced
by bulk flow of water and
minerals; from steles of
roots to stems and leaves
 Is sap mainly pushed up
from the roots, or pulled
up by the leaves?
+ Pushing Xylem Sap: Root Pressure
 At
night, transpiration very low, root cells continue
pumping mineral ions into xylem of vascular cylinder,
THUS! Lowering water potential
 Positive root pressure relatively weak and is minor
mechanism of xylem bulk flow
 Water flows in from the root cortex, generating root
pressure
 sometimes
results in guttation
 exudation of water droplets on tips or edges of leaves
+Pulling Xylem Sap: The TranspirationCohesion-Tension Mechanism
 Water
pulled upward by negative pressure in xylem
 Transpiration Pull:
 Water vapor in airspaces of leaf diffuses down water potential
gradient 
exits leaf via
stomata
 Transpiration
produces
negative
pressure
(tension) in
leaf exerts
pulling force
on water in
xylem
RESULT:
pulling water
into leaf
Cohesion
and
Adhesion
in
+
Ascent of Xylem Sap
•
Transpirational pull on xylem
sap transmitted from leaves
to root tips and even into soil
solution!
•
•
facilitated by:
• cohesion of water molecules
to each other
• adhesion of water molecules
to cell walls
Drought stress OR freezing
can cause cavitation
•
formation of water vapor
pocket by a break in chain
of water molecules
Fig. 36-15a
+
Water
molecul
e
Root
hair
Soil
particle
Water
Water uptake
from soil
Fig. 36-15b
+
Xylem
cells
Cohesion
and
adhesion in
the xylem
Adhesion
by hydrogen
bonding
Cell
wall
Cohesion
by hydrogen
bonding
Fig. 36-15c
+
Xylem
sap
Mesophyll
cells
Stoma
Transpiration
Water
molecule
Atmosphere
+ Xylem Sap Ascent by Bulk Flow
Know…
1- Movement of xylem
sap against gravity is
maintained by
transpiration-cohesiontension mechanism
2- Transpiration lowers
water potential leaves
which generates
negative pressure
(tension) that pulls
water UP through
xylem
3- NO energy cost to
bulk flow of xylem sap
+ Concept 36.4: Stomata help
regulate rate of transpiration
 Leaves
generally have broad
surface areas and high
surface-to-volume ratios
 THIS increases
photosynthesis and
increases water loss through
stomata
 About 95% of water a plant
loses escapes through
stomata
 Each stoma is flanked by a
pair of guard cells, which
control diameter of stoma
by changing shape
+ Mechanisms of Stomatal Opening and
Closing
 Changes
in turgor
pressure open and
close stomata
 Result
from reversible
uptake/loss of
potassium ions by
guard cells
Fig. 36-17a
+Guard cells turgid/Stoma open
closed
Guard cells flaccid/Stoma
Radially oriented
cellulose microfibrils
Cell
wall
Vacuole
Guard
cell
(a) Changes in guard cell shape and stomatal opening and
closing (surface view)
Fig. 36-17b
+Guard cells turgid/Stoma open
closed
H2O
Guard cells flaccid/Stoma
H2O
H2O
H2O
H2O
K+
H2O
H2O
H2O
H2O
(b) Role of potassium in stomatal opening and closing
H2O
+Stimuli for Stomatal Opening and Closing
 Generally, stomata
open during day and close at
night to minimize water loss
 Stomatal opening at dawn triggered by:
 light,
 CO2
depletion
 Internal “clock” in guard cells
 All
eukaryotic organisms have internal clocks;
circadian rhythms are 24-hour cycles
+Effects of Transpiration on Wilting and
Leaf Temperature
 Plants
lose A LOT of water
by transpiration
 Lose
H2O and H2O not
replace by H2O transport
 plant wilt
 Transpiration
also results
in evaporative cooling:

can lower temperature of
leaf  prevent
denaturation of various
enzymes involved in
photosynthesis and other
metabolic processes
+Adaptations That Reduce Evaporative
Water Loss
 Xerophytes:
plants
adapted to arid
climates
 have
leaf modifications
that reduce rate of
transpiration
 crassulacean
acid
metabolism (CAM):
form of photosynthesis
where stomatal gas
exchange occurs at
night
Fig. 36-18
Oleander leaf cross section and
flowers
Cuticl
Upper epidermal tissue
e
100 µm
+
Ocotillo (leafless)
Trichome Crypt Stomata Lower epidermal
s
tissue
(“hairs”)
Ocotillo
leaves
Ocotillo after heavy rain
Old man cactus
Concept
36.5:
Sugars
transported
from
leaves
+
and other sources to sites of use or storage
 Translocation: process
of transporting products of
photosynthesis through phloem
 Phloem sap : aqueous solution high in sucrose
 travels from sugar source to sugar sink
 sugar source: organ that is a net producer of sugar
 Ex: mature leaves
 sugar sink: organ that is a net consumer
or storer of sugar, such as a tuber or bulb
 Storage organ can be both sugar sink in
summer and sugar source in winter
 Sugar MUST BE loaded into sieve-tube
elements before being exposed to sinks
 Depending on species, sugar move by:
 symplastic
 both symplastic and apoplastic pathways
 Transfer cells: modified companion cells
that enhance solute movement between
apoplast and symplast
Fig. 36-19a
+
Mesophyll cell
Cell walls (apoplast)
Companion
(transfer) cell
Plasma membrane
Plasmodesmata
Key
Apoplast
Symplast
Mesophyll
cell
BundlePhloem
sheath cell parenchyma cell
Sieve-tube
element
+
 In
many plants,
phloem loading
requires active
transport
 Proton
pumping
and cotransport of
sucrose and H+
enable cells to
accumulate sucrose
 At
sink, sugar
molecules diffuse
from phloem to sink
tissues and followed
by water
Bulk
Flow
by
Positive
Pressure:
The
Mechanism
+
of Translocation in Angiosperms
 In
studying
angiosperms,
researchers
concluded sap moves
through sieve tube
by bulk flow driven
by positive pressure
 Pressure
flow
hypothesis explains
why phloem sap
always flows from
source to sink
Concept
36.6:
Symplast
is
highly
dynamic
+
• Symplast: living tissue
responsible for dynamic
changes in plant transport
processes
• Plasmodesmata can change in
permeability in response to:
• turgor pressure
• cytoplasmic calcium levels
• cytoplasmic pH
• Plant viruses can cause
plasmodesmata to dilate
• Mutations that change
communication within
symplast can lead to changes
in development
+Electrical Signaling in the Phloem
 Phloem
allows for rapid electrical
communication between VERY separated
organs
 Phloem is “superhighway” for systemic
transport of
macromolecules
and viruses
 Systemic
communication
helps integrate
functions of whole
plant
+ You should now be able to:
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
Describe how proton pumps function in transport of materials
across membranes
Define the following terms: osmosis, water potential, flaccid,
turgor pressure, turgid
Explain how aquaporins affect the rate of water transport across
membranes
Describe three routes available for short-distance transport in
plants
Relate structure to function in sieve-tube cells, vessel cells, and
tracheid cells
Explain how the endodermis functions as a selective barrier
between the root cortex and vascular cylinder
Define and explain guttation
Explain this statement: “The ascent of xylem sap is ultimately
solar powered”
Describe the role of stomata and discuss factors that might affect
their density and behavior
Trace the path of phloem sap from sugar source to sugar sink;
describe sugar loading and unloading