Ch 36 Transport in Plants
I. Plant Transport
A. Move onto land required specialized tissues
1. Roots- take up water & minerals
2. Shoots- take up CO2 & collect light energy
3. Transport tissues carry things where they are needed.
B. Transport generally
1. Transport involves 3 processes
a. Absorption & loss of water & solutes by individual cells
b. Short distance transfer from cell to cell
c. Long distance transfer by xylem (for water) and phloem (for sugar)
2. Membranes are selectively permeable- have 2 kinds of transport- active & passive
a. Passive transport- no energy required
1)
diffusion across a membrane moving down the concentration gradient
2)
diffusion assisted by transport proteins
a) carrier proteins- specifically bind & transport a solute
b) selective channels- selectively allow specific solutes to flow through
 some are gated- open & close on signal
b. Active transport- requires energy to be used
1)
Diffusion is against the concentration gradient
2)
Plant proton pump- uses ATP, pumps H+ ions out of cell
a) Creates membrane potential, used to
 Push positively charged ions into negatively charged cell
 Cotransport solutes that enter w/ H+ ions going back into the cell
 Chemiosmosis where energy releasing & energy consuming processes move H+
C. Water Potential
1. Osmosis – passive transport of water across a membrane
a. Moves hypotonic (low salt) solution into hypertonic (high salt) solution
b. Moves from high water potential into lower water potential
c. Moves from high water concentration into low water concentration
d. Aquaporins- water specific transport proteins that speed up osmosis
2. Water potential- measurement used to predict the direction of water movement
3. Ψ (psi) is the symbol for water potential
a. Measured in megapascals (MPa); 1 MPa = 10 atmospheres of pressure
4. Ψ = 0 MPa is the potential for pure water under no pressure
a. Water potential is affected by solute concentration and any pressure put on the solution
b. Pressure
1)
Pressure added increases Ψ because makes water move
2)
Tension- negative pressure, lowers Ψ
3)
Pressure potential can be positive or negative
4)
Pressure potential is represented as ΨP
c. Solute concentration
1)
Adding solutes lowers Ψ because it decreases the purity of the water
2)
Solute concentration is called solute potential
3)
Solute potential is always negative
4)
Solute potential is represented as ΨS
5. Water potential can be calculated by taking into account both solute & pressure potentials
a. Ψ = ΨP + ΨS
6. If a cell is in a hypertonic solution (lower Ψ), then it will lose water
7. If a cell is in a hypotonic solution (higher Ψ), then it will gain water
II. Moving Water- Moving the xylem sap
A. Influence of plant structures
1. Plant structure is divided into 3 compartments- cell wall, cytosol, & vacuole
a. Plasma membrane- regulates water movement for the cell
b. Tonoplast- membrane around vacuole, regulates water movement into & out of vacuole
1)
Proton pump maintains high [H+] in vacuole, low [H+] in cytosol
2. Symplast- continuum of cytoplasm from one cell to the next & the next by means of the plasmodesmata
3. Apoplast- continuum of cell wall connect to cell wall throughout the plant
B. Short distance transfer- movement of water
1. Transmembrane- across plasma membrane into the cell
2. Symplastic movement- from cell to cell through plasomodesmata
3. Apoplastic movement- along the cell walls outside the cells
C. Long distance transfer- called bulk flow
1. Caused by tension in xylem vessels due to transpiration
2. Caused by hydrostatic pressure in phloem sieve tubes
D. Uptake of water
1. Roots- take in water especially at root tips where the root hairs are
2. Mycorrhizae- symbiotic fungi associated with roots
3. Both root hairs & Mycorrhizae increase surface area to increase absorption
4. Water also moves along apoplast, getting more access to plasma membranes for greater osmosis
(Minerals are taken in by selective transport)
5. Endodermis- innermost layer of cortex cells surrounding the stele
6. Casparin strip- a ring of suberin (waterproof) surrounds each endodermis cella. Water can’t enter xylem in root core (stele) unless it gets across cell membrane into the symplast
7. Water & minerals in apoplast pass through endodermal cells to xylem
E. Transport of Xylem sap
1. Root pressure- water can flow into xylem at roots, but can’t flow out once there-endodermis stops it
a. Forces fluid up xylem
b. Guttation- excess water may ooze from leaves when transpiration happen fast enough
2. Xylem Sap is pulled up the stem- Transpiration-Cohesion-Tension Mechanism
3. Water is a polar molecule & forms hydrogen bonds between molecules
a. Cohesion- hydrogen bonds formed with neighboring water molecules
b. Adhesion- hydrogen bonds formed with hydrophilic elements in narrow xylem elements
4. Transpiration- water evaporates off the surface of spongy mesophyll cells & out the stomates
a. Water moves by osmosis from cells having more water into cells that just lost water
 i.e. from higher water potential to lower water potential in a neighboring cell
b. Creates tension in xylem tube, causing water to move up from the roots, pulling other water molecules,
stuck together by cohesion, along, too.
c. This creates low water potential and tension in the xylem & cells of the roots, so water moves into the
roots from the soil by passive transport.
F. Guard cells control transpiration
1. Transpiration/Photosynthesis Ratio- Plants lose ~ 600g of H2O for 1g of CO2 used up making glucose
2. Stomates must be open & cells must expose a lot of surface area to get CO2, but water evaporates more easily due
to large surface area and open stomates!
3. Pairs of guard cells control whether stomate is open or closed
a. Depends on turgor (water pressure) in guard cell
1)
Turgid guard cell has high pressure, causes cell to buckle outward, creating a pore or
stomate
2)
Flacid guard cells sag against each other- shuts stomate
3)
Water moves in or out of guard cells based on water potential
b. Cells pump in K+ to decrease potential so water moves in
 Loss of K+ means increase of water potential causing water to move out of cell
 H+ may be pumped in or out to compensate for movement of K+
c. Aquaporins also control water movement into & out of guard cells
G. Regulating Guard cells
1. Stomates usually open at dawn. Why?
a. Light signals guard cells to pump in K+, water potential drops & water lows in
b. Depleted CO2 in air spaces signals guard cells to open
c. Circadian rhythms – usual opening time occurs
2. Lack of water
a. Abcisic acid- a plant hormone- is produced by mesophyll cells when water is not available
b. Signals guard cells to close
H. Leaf Adaptations
1. Xerophytes- pants that live in dry, arid climates
a. Small thick leaves, few stomates, heavy cuticle
b. CAM (crassulacean acid metabolism) plants
1)plants store up CO2 as the organic crassulacean acid at night when stomates are open
2)Close stomates during the day to minimize water loss
3)Still have a carbon supply from the stored CO2. Photosynthesis can occur without losing much
water.
c. C4 plants
III. Moving Food- Translocation of (moving) the Phloem Sap
A. Phloem vessels- have a sieve plate w large plasmodesmata perforating it to allow phloem sap to move thru
1. Phloem sap- ~30% sucrose, minerals, amino acids. Plant hormones, & water
2. Direction of transport changes according to the plant’s needs
B. Sugar moves from source to sink
1. Source- where sugar is made
2. Sink- where sugar is used or stored as starch
C. Loading sugar into sieve tube members (phloem cells): 2 strategies
1. Moves thru symplast of mesophyll cells in leaves directly into sieve tube members via plasmodesmata
2. Moves thru symplast of mesophyll cell into apoplast space near a sieve tube member
a. Transport cells- specialized companion cells in the phloem
b. Transport cells load phloem into sugar from apoplast into sieve tube members
c. Requires energy to load sugar, also proton pumps & possibly cotransport to get it across a cell membrane
against the sugar concentration gradient. This is how sugar can become so highly concentrated in the
phloem
D. Unloading sugar- various strategies are used
1. Concentration gradient favors unloading because sugar is being used or made into starch; lowers sugar
concentration!
2. Loading sugar in= ↑hydrostatic pressure in phloem; unloading sugar = ↓hydrostatic pressure
3. Result: sugars move from sugar source to sugar sink
4. Water problem- [water] increases as sugar is pulled out of solution
a. Xylem recycles water in opposite direction; from sugar sink to sugar source