Download plants Ch. 36

Chpt 36: Transport In Plants
Transport Overview
1- uptake and loss of water and solutes by
individual cells (root cells)
2- short-distance transport from cell to cell
(sugar loading from leaves to phloem)
3- long-distance transport of sap within xylem
and phloem in whole plant
Whole Plant Transport
1- Roots absorb water and
dissolved minerals from
soil
2- Water and minerals are
transported upward from
roots to shoots as xylem
sap
3- Transpiration, the loss
of water from leaves,
creates a force that pulls
xylem sap upwards
4- Leaves exchange CO2 and O2 through
stomata
5- Sugar is produced by photosynthesis in
leaves
6- Sugar is transported as phloem sap to roots
and other parts of plant
7- Roots exchange gases with air spaces of
soil (supports cellular respiration in roots)
Proton pumps (chpt 8)
Passive transport--Solutes diffuse down their
gradients – requires no E (proteins in plants
that passively transport water = aquaporins)
Cross slowly unless they can pass through
transport proteins
Active transport – pumping solids across
membrane against gradient – requires E—
Carried out by special proteins
Proton pump -most impt active
transporter in plants p.150
Membrane proteins store E by generating
voltage across (separating charges)
membranes.
Uses ATP for power
Moves pos charges (H+) so creating a
voltage difference & a chemical difference
that can be used to drive other processes
You expend a little E & get more in return
Water potential (ψ) differences
drive water transport
Plant survival depends on balancing water uptake with
water loss!
Hypotonic –lower solute concentration
Hypertonic – higher solute concentration
Water will move from hypo to hyper
Since plant cells have a cell wall, pressure must also be
taken into consideration
Water potential includes concentration & pressure
Water potential & concentration
Water moves from higher water potential to
lower water potential
This moving water can do work so it has the
potential to do work!
Measured in megapascals (Mpa) 1MPa = 10 atm
Adding solutes lowers water potential so, if [↑]
then ψ↓
Water potential & pressure
If pressure goes ↑ then ψ↑
So, ψ = ψpressure + ψ[solute]
Look at p.751
Cellular Transport
Water transport
√ Osmosis; hyper-more
solute; hypo-less solute;
iso- same solute
Cell wall creates physical
pressure to go along with
[solute]
Water moves from high to low water potential
Flaccid (limp, isotonic –plants need to be in
hypotonic envr.);
Plasmolysis (cell loses water in a hypertonic
environment; plasma membrane pulls away);
Turgor pressure (influx of water due to osmosis;
hypotonic environment)

flaccid
Cells with vacuoles (vacuolated)
have 3 compartments
Cell wall
Cytosol
Vacuole –membrane around vac. called
tonoplast
Transport within tissues/organs
Tonoplast - vacuole membrane– controls traffic
between vacuole & cytosol
Plasmodesmata – connections between dif. Cells cytosolic connection
Symplast route (lateral) – all of the plant cells are
connected & this is called a cytoplasmic continuum
or symplast route –materials move from inside 1 cell
to inside the next cell
Apoplast route (lateral) – walls of adjacent cells
are also in contact = continuum of cell walls –
water etc… travel across tissue via cell walls &
extracellular spaces
Diffusion process is fine for cell to cell, but to
slow for long distances
Bulk flow (long distance) -movement of a fluid
drive by pressure (in xylem)
Transport of xylem
Capillary action (rise of liquids in a narrow
tube) –due to adhesion (water sticks to walls) is
minimal
Osmosis creates Root pressure: at night (low
transpiration), roots cells continue to pump minerals
into xylem; this generates pressure, pushing sap
upwards; usually doesn’t create a great amt of
pressure
Root pressure causes guttation – exuding water
droplets on tips of grass blades in morn.
Transport of Xylem Sap –
cohesion-tension theory
Transpiration: loss of water vapor from leaves pulls
water from roots (transpirational pull or tension);
cohesion (water sticks to water) produces a single polymerlike column of water from roots to leaves
Bulk Flow occurs when water evaps from leaves– as
molecule evaps it pulls up a column behind it (sun
causes transpiration so sun provides E for the
process!)
Remember – water moves from
greater water potential to lesser!
Transpiration control – balance between
opening for CO2 & closing to maintain water
Stomata close when Temp is high- reduces H2O
loss, but stops photosyn
Stomata open when CO2 levels low – CO2 required
for photosyn.
Stomata close at night ([CO2] high due to
respiration so no need to open….)
Guard cells control the size of the stomata
Scanning electron micrograph of Equisetum (horsetail
or scouring rush) epidermis. Note the oval stomatal
apparatuses in the center of the stem.
Pea
Leaf
Stoma,
Vicea
sp.
Transpirational Control
 Stomata opening- ↑K+ ions create a gradient that’s used to move
H2O into guard cells causing g-cells to enlarge & open stomata
(K+ charge gradient in dif plant species balanced by H+ out or
Cl- ions in)
 Photosynthesis-Transpiration compromise…
 Xerophytes (plants adapted to arid environments)~ thick cuticle;
small spines for leaves
Translocation of Phloem Sap
Translocation: movement of
food through phloem to a
sink
Sugar source: sugar
production organ (mature
leaves)
Sugar sink: sugar storage
organ (growing roots, tips,
stems, fruit)
Pressure-flow hypothesis
1-sugar enters sieve-tube members via active
transport – this creates a concentration gradient
between source (higher) & sink (lower)
2-water enters sieve-tube members by diffusion –
passive transport
3- pressure gradient in tube moves water & sugars to
sieve-tube members at sink (bulk flow due to
pressure)
4 – pressure at sink reduced as sugars are
removed to by used –removing sugars decreases
conc of solutes which lowers water pressure at
sink & helps keep process going
5- in plants the sugar can also be “removed” by
changing it to starch – it doesn’t have to be
immediately used
6- xylem then recycles water from sink to source
Driving force for
water movement in
plants is
transpiration &
cohesion!
Driving force for
sugar movement in
plants is a
concentration
gradient!