Resource
Acquisition
& Transport
Processes
Fig 36.2
I. Membrane Transport
A. Passive Transport
1. Mechanisms
2. Requirements
B. Active Transport
1. Mechanisms
2. Requirements
Fig 36.6
C. Why Transport?
1. Occurs at virtually every level of biological
organization.
2. Enzymes transport electrons, protons, acetyl groups.
3. Membranes transport material across themselves.
4. Cells transport material to and from other cells, and
within themselves.
5. Whole organisms transport water, etc from one
organ to another
II. Plant Transport
A. Background
1. Plants adapted to the division of resources in
the land environment (soil & air) by the differentiation
of the plant body into roots and shoots.
2. But this created a new dilemma, the need to
transport materials between roots and shoots.
3. Sometimes up to 100m away, in all kinds of
environmental conditions.
4. What’s a poor plant to do?
B. Three levels of transport in plants:
1. Cellular – uptake of H2O and solutes by
individual cells.
2. Short-distance - between cells
3. Long-distance – throughout whole plant
(xylem & phloem)
4. Scenarios and Terms
An artificial cell consisting of an aqueous solution enclosed in a
selectively permeable membrane has just been immersed in a
beaker containing a different solution. The membrane is
permeable to water and to the simple sugars glucose and
fructose but completely impermeable to the disaccharide
sucrose.
Which solute(s) will exhibit a net diffusion into the
cell?
a. sucrose
b. glucose
c. fructose
Which solute(s) will exhibit a net diffusion out of the
cell?
a. sucrose
b. glucose
c. fructose
Which solution is hypertonic to the other?
a) the cell contents
b) the environment
In which direction will there be a net osmotic
movement of water?
a) out of the cell
b) into the cell
c) neither
After the cell is placed in the beaker, which of the
following changes will occur?
a) The artificial cell will become more flaccid.
b) The artificial cell will become more turgid.
III. Water Potential
A. Definition
Water potential refers to the free energy of water, its
capacity to do work.
By definition pure free water has a water potential of
zero.
B. Factors
1. Increased
heating, pressure, or elevation
2. Decreased
C. Components of Y
Y = Ys + Yp + Ym
Ys = osmotic potential
Yp = pressure potential
Ym = matric potential
1. Ys = osmotic potential
a. Osmotic potential is a measure of the effect
that solutes have on water potential.
b. Pure water has an osmotic potential of zero.
c. Adding solutes decreases the osmotic
potential because water interacts with solutes. (more
solutes = more negative)
d. Thus Ys is always negative (if solute is
present)
2. Yp = pressure potential
a. A measure of the effect that pressure has on
water potential.
b. Pressure can be positive (when something is
compressed). Pushing
c. Pressure can be negative (when something is
stretched or pulled). Called tension.
d. Water can handle large amounts of tension
because of cohesion – the tendency of water to stick
to itself (H bonding)
Figure 3.2
3. Ym= matric potential
a. Matric potential is a measure of water’s
adhesion to non-dissolved but hydrophilic structures
such as cell walls, membranes, soil particles etc.
b. Adhesion can only decrease water’s free
energy.
c. So matric potential is always negative
Water moves from regions of high Ψ (less negative)
to regions of lower Ψ (more negative).
Why would water move to where it is less free?
– Water acts to dilute, hydrate, decrease tension
– i.e. water acts to stabilize water potentials
– If two Ψ’s are equal, no net movement of
water
Examples
• Ψ = +46MPa
Ψ = -22MPa
• Ψ = -15MPa
Ψ = -300MPa
D. Cells, Water Y, Terms
1. A flaccid cell (Yp = 0). No pressure is being
exerted against the inside of the cell wall. Cell is not
firm.
Why? Solute concentration within the cell is lower
than surroundings. Water leaves the cell.
2. A turgid cell (Yp = +) is filled with water,
exerting pressure against its cell walls. Cell is firm.
Why? Solute concentration within the cell is
higher than surroundings. Water moves into the cell.
E. Terms for cells and water movement
1. Hypotonic solution – low solute concentration
2. Hypertonic solution – high solute concentration
3. A flaccid cell placed in a hypotonic solution will?
4. A turgid cell placed in a hypertonic solution will?
This causes the cell membrane to shrink away from
the cell wall (i.e. plasmolyze).
Fig 36.7a
Fig 36.7b
IV. Cell to Cell Transport
A. Three routes for lateral transport:
1. Trans-membrane – water & solutes move
across the plasma membranes and cell walls of
adjacent cells
2. Symplast – movement through a continuum of
cytoplasm connected by the plasmodesmata of cell
walls.
3. Apoplast – extracellular pathway; movement
through the continuous matrix of cell walls
Fig 36.5
4. Examples of Short Distance Transport
a. Guard Cells
i. Mechanism: Opening
K+ is pumped into GC by active transport. Proton
pump creates membrane potential that drives K+ in.
Thus Ψ inside cell is lower than outside cell.
Water enters into the guard cell by osmosis.
Sunlight, circadian rhythms, & low CO2
concentration in leaf air spaces stimulate the proton
pumps & thus stomatal opening.
ii. Mechanism: Closing
Proton pumps no longer active (darkness)
K+ is lost from the GC, creating lower water
potential outside cell.
Water flows out of GC and cells become flaccid.
Stomatal closure during the day stimulated by water
stress – not enough water to keep GCs turgid.
Fig 36.13
b. Motor Cells
i. Description
Leaves of these plants can flex & fold in response
to stimuli.
Motor cells are the “joints” where this flexing occurs.
Accumulate or expel potassium to adjust their Ψ &
thus turgidity.
ii. Examples: Venus’ flytrap
Oxalis – leaves fold in sunlight to minimize
transpiration; open in shade.
Transpiration = loss of water vapor from the stomata
http://www.uccs.edu/~ppbotany/Colo_family/Oxalid/oxalis_stricta_P.htm
c. Transfer Cells
i. Description
Cell walls have many finger–like projections on the
inner surface.
The plasma membrane is pressed firmly against
these convolutions, creating an increase in surface area
Greater surface area means more molecular pumps & thus
high – volume solute transport
Found in areas of rapid, high volume transport: saltexcreting glands or sugar loading into phloem
d. Root Cells
i. Description
Soil particles coated with water, minerals; adhere to
hydrophyllic epidermal cells of root hair
Soil solution moves freely through epidermal cells &
cortex via symplast and apoplast pathways
Endodermis – selective barrier to soil solution between
cortex & stele. Sealed together by the waxy Casparian
strip (Suberin) – forces soil solution in apoplast to pass
through the selectively permeable membrane of the
endodermis.
Once through the endodermis, soil solution freely
enters the xylem
Fig 36.8
Suberin
V. Long Distance Transport
Xylem: Transpiration (evaporation from
leaves) creates a tension which pulls sap up
from the roots, in direction of lower Ψ.
Phloem: Hydrostatic pressure at one end of
the sieve tube forces sap to the other end of
the tube (= bulk flow).
A. Xylem Transport = sap
1. Forces for Xylem sap both pushed & pulled
up the stem WHY?
a. Root Pressure
Stele has high concentration of minerals, decreasing
Ψ. Water flows in, creating pushing pressure
b. Guttation – exudation of water droplets by
leaves during the night when transpiration is low.
Caused by root pressure
Fig 36.9
c. Cohesion/Adhesion
i. Transpiration – cohesion – tension mechanism
ii. Transpirational pull: Ψ of air typically << than Ψ
of leaf, thus evaporation.
iii. Water remaining in leaf is tightly adhered to cell
walls in the mesophyll
iv. This adhesion & surface tension of the water
creates negative pressure – the pulling force
Figure 36.10
d. Aided by:
i. Cohesiveness of water – allows water to be pulled
up in a continuous column without breaking
ii. Adhesion of water to hydrophyllic cell walls of
the xylem, creates tension (negative pressure/ pull)
iii. Diameters of tracheids & vessel elements are
small, so lots of surface area for adhesion
iv. Since movement is by bulk flow (i.e. no
membranes to pass through), Ψs is not involved in
the overall process
Fig 36.11
2. Speed of Xylem Sap Translocation
•
•
•
•
•
Examples
Conifers
Angiosperm Trees
Herbs
Vines
Max. Speed (cm/hr)
120
600 - 4400
6,000
15,000 (0.6 mi/hr)
Why is xylem transport in trees much slower than
in herbs?
3. Forces that act against transpirational pull:
A. Gravity
B. Hydraulic resistance
4. Control of Transpiration
a. Guard cells! – balance two contrasting needs of the
plant:
i. Conserve water
ii. CO2 for photosynthesis
b. Water Use Efficiency (WUE) = g H2O lost / g CO2
assimilated by photosynthesis average = 600/1
c. Adaptations by Desert plants to increase
their WUE:
i. Small thick leaves (less SA for water loss)
ii. Thick cuticle
iii. Stomata only on bottom of leaves
iv. High-volume water storage (cacti)
v. Crassulacean Acid Metabolism (CAM) –
plants take in CO2 only at night, so that stomata only
have to be open at night.
5. Wilting
a. Why?
Transpirational pull is greater than the delivery of
water by the xylem. Cells lose turgor pressure &
stomata close
b. Trans-stomatal and Trans-cuticular Transpiration
(gH20/dm2/hr)
Trans-stomatal Trans-cuticular
Herbs:
Coronilla varia
Stachys recta
Oxytropis pilosa
Trees:
Pinus sylvestris (pine)
Picea abies (spruce)
Fagus sylvaticus (birch)
1810
1620
1600
190
180
100
527
465
330
13
15
90
B. Phloem Transport
1. Direction and Material
a. Phloem sap = sucrose, amino acids, hormones
b. Sieve tubes carry sap from sugar source (e.g.
leaves) to sugar sink (e.g. growing roots, shoot tips,
stems, flowers, fruits)
c. Thus not unidirectional e.g. tubers can be source in
spring and sink in fall
2. Phloem loading
a. Sap moves into sieve tubes via companion cells by
symplast and apoplast pathways
b. Loading is by active transport only – why?
Sap in sieve tubes is highly concentrated with solutes,
thus phloem unloading is passive (source to sink)
Fig 36.15
3. Mechanism
a. Pressure-flow Hypothesis:
i. Phloem loading creates high sugar
concentration at source, decreasing Ψ.
ii. Thus water flows into sieve tubes, creating
hydrostatic pressure (pushing pressure: positive).
iii. Less pressure at sink end, where sugar is
leaving sieve tube for consumption
iv. Thus movement from source to sink
Fig 36.16
4. Speed of Phloem Sap Translocation
Examples
conifer stem
angiosperm tree
vine
grass
sunflower
corn
Maximum Speed (cm/hr)
13-48
48
72
168
240
660
We learn to move on, but we never stop learning..