Transport in Plants Notes
AP Biology
Mrs. Laux
3 levels of transport occur in plants:
1. Uptake of water and solutes by individual cells
-for photosynthesis and respiration
-ex: absorption of H2O /minerals by root hairs
2. Short distance cell-to-cell transport at level of tissues and organs
3. Long distance transport of sap by xylem and phloem at whole
plant level
A. Transport at the cellular level
-plasma membraneÆselective permeability
1. Passive transport
-when solute travels down a concentration gradient
-no energy
-transport proteins sometimes used
-aid/speed up carrying of materials across
membrane
2. Active transport
-against concentration gradient
-energy requiring process
-proton pumpÆimportant in plants
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Transport in Plants Notes
AP Biology
Mrs. Laux
-like in respiration and photosynthesis in making
of ATP
-here, opposite, proton pump hydrolyzes ATP and
uses energy to drive H+ ions out of the cell
-produces a proton gradient and electrochemical
gradient outside of cell
-can accomplish two things:
a. entry of + ions, such as K+, when concentration
is greater outside the cell than in
-because inside is more (-) with all H+s
pumped out, K+ can easily enter the cell
because of charge with electrochemical
gradient
b. entry of (-) ions, such as nitrate ion-NO3-energy stored by H+ ions on outside of cell
(water behind dam) can be used to
transport solutes against their
concentration gradient
-inside cells is (-), NO3- moving into cell
requires energy
-energy comes from H+ gradient
-energy is used in respiration to
make ATP
c. entry of a neutral solute
-sucrose
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Transport in Plants Notes
AP Biology
Mrs. Laux
Water Potential and Osmosis
-osmosisÆnet uptake or loss of water by the cell; depends on which
component, the cell or extracellular fluid has higher water potential
-water potential, Ψ, psiÆfree energy of water that is a consequence
of solute concentration and applied pressure
-water moves high Ψ Æ low Ψ
-unitÆMPa (megapascals) ~10 atm
-pure H2O-open container 0 MPa
-add solutes, lower Ψ to (-)
-increase pressure, increase Ψ
-decrease pressure, decrease Ψ (-)
-bulk flowÆmovement of H2O due to pressure differences
Ψ = pressure potential and solute concentration potential
Ψ = Ψp – Ψs
0.1 M solution has a solute potential of 0.23 MPa
-in open container Ψp = 0
-therefore, Ψ = Ψp – Ψs
= 0 – 0.23
= -0.23
-plants will gain or lose water to intercellular fluids depending on
their water potential
-if flaccid (limp) cell is placed in a hypertonic solution, cell will
plasmolyze
-protoplast will pull away from cell wall
-if flaccid cell is placed in hypotonic solution, cell will swell and
turgor pressure develops
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Transport in Plants Notes
AP Biology
Mrs. Laux
-when pressure on cell wall is equal to osmotic pressure,
equilibrium is reached
-no more net movement of H2O
Tonoplast
-membrane surrounding large central vacuole in plant cells
-important in regulating intracellular conditions
-contains integral proteins-control movement of solutes between cytosol
and vacuole
-has membrane potential
-proton pumps maintain higher pH in cytosol because pump H+s into
the vacuole
-this proton gradient helps transport solutes into vacuole for storage
B. Short Distance transport at the level of tissues and organs
-can happen via:
1. across plasma membrane and through cell walls
2. symplast routeÆsolutes move from cell to cell through cells
via plasmodesmata
-doesn’t go through membrane but through the thin
streams of cytoplasm
-ex: endodermis
3. the apoplast routeÆwater and solute move past outside of
cell walls
-between cells
-never enters cells
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Transport in Plants Notes
AP Biology
Mrs. Laux
C. Long distance travel through whole plant
-via vascular tissues
-passive transportÆtoo slow
-instead travel via:
-bulk flow through xylem and phloem
-transpiration- pulls sap up tree from roots in xylem
-hydrostatic pressure develops at one end of sieve tubes in
phloem-forces sap along
Absorption of water and minerals by roots
-absorbed via the following pathway:
soilÆepidermisÆroot cortexÆxylem
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Transport in Plants Notes
AP Biology
Mrs. Laux
1. soilÆepidermis:
-most absorption occurs near root tips where epidermis is permeable
to water
-root hairs increase surface area
2. epidermis to root cortex:
-lateral transport is usually a combination of apoplast and symplast
routes
a. apoplast route
-water and minerals flow through hydrophilic cell walls and
pass freely
b. symplast route
-makes mineral absorption possible
-active transport into epidermal and cortex cells allows
sufficient supply of minerals to pass through
-diffusion will not allow enough into cells-concentration too
low
-ex: transport proteins of tonoplast and plasma membrane
actively pump K+ into root cells and Na+ out
3. root cortexÆxylem
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Transport in Plants Notes
AP Biology
Mrs. Laux
-only minerals using symplast route move directly into vascular
tissues
-minerals and water passing through apoplasts are blocked by the
Casparian strip
-water and minerals enter stele through the cells of the endodermis
-Casparian strip only allows certain ions to pass into stele
-endodermal cells discharge contents that it is carrying via symplast,
into the apoplast of xylem
-tracheids and vessels are part of apoplast
-as water and minerals enter apoplast (via diffusion and active
transport) they are free to enter tracheids and vessels
Transpiration of sap through xylem
-sap=water + minerals
-water transported up from roots must replace that lost by
transpiration (evaporation of water from leaves)
-also provides nutrients (minerals) to shoot system
-How water (sap) climbs xylem
1. pushing xylem sapÆroot pressure
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Transport in Plants Notes
AP Biology
Mrs. Laux
-as water enters the stele, it enters via osmosis
-brings with it minerals/ions from the soil
-as water moves up the column, the concentration decreases
in roots, even though mineral concentration remains high, this
causes water flow because of the higher concentration of
solutes inside the stele
-during the day, this forces water upward (root pressure) to an
extent and water evaporates through air spaces via
transpiration
-at night, transpiration is decreased, stomata are closed, and
root pressure continues
-this causes water/sap droplets to escape through
leaves
-called guttation
-this can be seen as small droplets of sap on grass and
leaves in the morning
-Root pressure, although it does have an effect on the movement of sap
through the xylem, it is not the leading mechanism driving the ascent of
xylem sap
a. cannot keep up with rate of transpiration
-water evaporates more quickly
b. can only force water up a few meters
2. Pulling xylem sapÆcohesion-tension theory
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Transport in Plants Notes
AP Biology
Mrs. Laux
-deals with two aspects:
-transpiration pulls sap upward
-cohesion of water transmits upward pull along entire length of the
xylem
a. transpirational pull
-depends on negative pressure
-gaseous water in air spaces diffuses into drier atmosphere
through stomata
-lost water vapor is replaced by evaporation from mesophylls
bordering the air spaces
-as water evaporates, this causes a negative pressure, tension
in xylem (syringe)
-this tension causes water to be pulled from xylem
through mesophyll, towards surface film on cells
bordering the stomata
-bulk flow of water through xylem cells occurs as molecules
evaporate from leaf; therefore, as one water molecule
evaporates from leaf, it pulls all the other water molecules up
along behind it
-this is because of:
b. cohesion and adhesion of water
-allows for transpirational pull to occur
-cohesion between water molecules due to H-bonding allows
one cell to pull “chain” of cells behind it
-adhesion of water (via H-bonding) to hydrophilic cell walls of
xylem cells also helps pull against gravity
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Transport in Plants Notes
AP Biology
Mrs. Laux
-small diameter of tracheids and vessels is important to
adhesion
-results in capillary action
-movement of liquids in narrow tubes
-capillary action makes small contribution unless
coupled with transpirational pull
-upward pull of sap causes tension in xylem, decreases Ψ, and
allows passive flow soilÆstele
-flowing water via capillary action forms a meniscus in xylem
Water vapor diffuses from the moist air spaces of the leaf to the drier air outside via
stomata. Evaporation from the water film coating the mesophyll cells maintains the high
humidity of the air spaces. This loss of water causes the water film to form menisci that
become more and more concave as the rate of transpiration increases. A meniscus has a
tension that is inversely proportional to the radius of the curved water surface. Thus, as
the water film recedes and its menisci become more concave, the tension of the water film
increase. Tension is a negative pressure- a force that pulls water from locations where
hydrostatic pressure is greater. The tension of water lining the air spaces of the leaf is the
physical basis of transpirational pull, which draws water out of xylem.
-actively flowing water in xylem tissues never forms
meniscus because there is continuously flowing water
If water vapor or another gas is allowed to enter xylem tissue
-cavitationÆformation of a water vapor pocket in xylem
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Transport in Plants Notes
AP Biology
Mrs. Laux
-vessels cannot function again unless refilled by root pressure
-this can only happen in small plants, root pressure can only
raise water a few meters
-pits between xylem vessels allow for detours around cavitated area
-secondary growth adds new xylem vessels each year
-ascent of xylem sap is solar powered
-sun causes evaporation; therefore, negative pressure
-bulk flow in xylem depends only on this pressure
-while osmosis in roots and leaves are due to small gradients in
water potential caused by both solute and pressure gradients
Control of the Stomata
-opening and closing of the stomata influence gas exchange, transpiration,
ascent of sap, and photosynthesis
-when stomata are open, CO2 can enter: therefore, photosynthesis can
occur
-H2O can escape
-good, because water and nutrients can be carried up xylem to
leaf cells
-also evaporative cooling-allows cooling
-bad, because too much water can escape and cause
desiccation
-When stomata are closed
-no water loss, therefore, escape wilting (too much evaporation, not
enough delivery
-No CO2 enters; therefore, no photosynthesis can occur
-also as concentration of CO2 decreases in air spaces and
concentration of O2 increases (product of photosynthesis) in
air spaces, run risk of photorespiration occurring
-each stoma is surrounded by 2 guard cells
-cell walls are not uniform, the cell wall bordering stoma is thicker
than the rest of cell wall
-composed of cellulose microfibrils arranged radially, from outside
to stoma side
-when turgid (full of water) cells “buckle” due to microfibrils
and stomata open
-when flaccid (water leaving) guard cells sag and openings
close
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Transport in Plants Notes
AP Biology
Mrs. Laux
-Guard cells control the following:
1. stomata close when temps are high
-reduces water loss
-shuts down photosynthesis
2. stomata open when CO2 concentrations are low inside the leaf
-allows photosynthesis
3. stomata close at night and open during the day
-because in leaf, CO2 low during the day-being used for
photosynthesis, at night, no photosynthesis- CO2
concentration remains high (because of respiration)
How guard cells control stomata:
-guard cells open when turgid (filled with water)
-closed when no water (flaccid)
-this uptake and loss of water is controlled mainly by the uptake and loss
of K+ ions by guard cells
-when K+ diffuses into cell and then vacuole by plasma membrane
and then the tonoplast, this decrease water potential (Ψ) in guard
cells
-water flows into guard cell
-high Ψ to low Ψ
-most of water is stored in vacuole; therefore, tonoplast also
plays a major role
-this increase in positive charge is balanced by:
-uptake of Cl- ions
-uptake of other negative ions, especially organic acids
-loss of H+, from organic substances/acids
-closing of guard cells results when K+ exits the cell and
creates osmotic loss of water
Guard cells open and close because of internal and external cues
-stomata open at dawn in response to:
1. light induces guard cells to take up K+ by
-beginning photosynthesis, making ATP for H+ pumps
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Transport in Plants Notes
AP Biology
Mrs. Laux
2. decrease of CO2 in air spaces due to onset of
photosynthesis
3. internal clock will cause guard cells to open even if kept in
dark (circadian rhythm~24 hours)
-guard cells may close stomata if:
1. water deficiency results in flaccid guard cells
2. a hormone is produced due to water deficiency
3. high temps increase CO2 in air spaces due to increased
respiration, closing guard cells
Evolutionary aspects that reduce transpiration:
1. Plants have structural adaptations to hot, dry climates
2. CAM photosynthesis
Transport of sugars
ÆtranslocationÆtransport of products of photosynthesis by phloem
to the rest of the plant
-in angiosperms, sieve tube members are specialized cells that
function in translocation
-pores in sieve plates allow phloem sap to move freely along sieve
tube
-phloem sap=sucrose, minerals, amino acids, and hormones
-Considered “source-to-sink” transport
-sourceÆorgan where sugar is produced (usually leaves) or where
starch is broken down (leaves, stems, roots, etc.)
-sinkÆorgan that consumes or stores sugar
-ex: growing parts of plants, fruits, non-green parts of plants,
stems, trunks, etc.
-sugar flows sourceÆsink
-can depend on season
-ex: tuberÆsink when storing in summer, source in
spring
-minerals are also transported to sinks
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Transport in Plants Notes
AP Biology
Mrs. Laux
-sink is usually supplied by nearest source
-direction of phloem can change, depending on location of source
and sink
-whereas, xylem, only up plant
Phloem Loading and Unloading
-sugar from source must be loaded into sieve member before being
translocated to a sink
-process follows:
1. Sugars enter sieve tube members
-soluble carbohydrates, sucrose and fructose, move from
sight of production to sieve tube members by active transport
-this develops a higher concentration of solutes in the sieve
tube than in the sink
2. Water enters sieve tube members
- Ψ is higher outside sieve tube than in sieve tube; therefore,
water diffuses down concentration gradient and into sieve
tube member
3. Pressure in source sieve tube member move water and sugars to
sieve tube member at sink through sieve tubes
-move via bulk flow (because of pressure)
4. Pressure is reduced at the sink as sugars are removed for use by
nearby cells
-by active and passive transport
-water diffused out of sieve tube members, relieving pressure
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