Ch. 38 Water & Sugar Transport in Plants Flashcards
transpiration
loss of water via evaporation from the aerial parts of plant
conditions for transpiration to occur
1) stomata are open
2) air surrounding leaves is drier than air inside leaves
water potential
the potential energy of water in a certain environment compared with the potential energy of pure water at room T and atmospheric pressure
- living orgs: water potential = solute potential + pressure potential
water flow based on water potential
water flows from areas of HIGHER water potential to areas of LOWER water potential
high –> low
solution
a homogenous, liquid mixture containing several substances
solute
any substance that is dissolved in a liquid
isotonic
solute concentrations in the cell and the surrounding solution are the same
- no net movement of water
hypotonic
solution has lower solute concentration than the solution on the other side of the membrane
- results in the loss of water & shrinkage of the membrane-bound structure
osmosis
diffusion of water across a selectively permeable membrane from a region of low solute concentration (high water concentration) to a region of high solute concentration (low water concentration)
solute potential
a component of potential energy of water caused by difference in solute concentrations at two locations
- total solute concentration relative to pure water
low solute potential
high concentration of solutes
wall pressure
inward pressure exerted by a cell wall against the fluid contents of a living plant cell
turgor pressure
outward pressure exerted by the fluid contents of a living plant cell against its cell wall
- pressure inside the cell
- counteracts movement of water due to osmosis
turid
swollen & firm
- result of high internal pressure
pressure potential
any kind of physical pressure on water
- can be positive or negative
megapascal (MPa)
a unit of pressure (force per unit area) equivalent to 1 million pascals (Pa)
pascal (Pa)
a unit of measurement commonly applied to pressures (force per unit area)
flaccid
limp as a result of low internal (turgid) pressure
- no wall pressure
ie. wilted plant leaf
wilt
to lose turgor pressure in plant tissue
factors that influence movement of water
1) osmosis
2) solute potential
3) pressure potential
when solute potential is negative
1) compare solute potential to that of pure water
2) solute potentials are always negative b/c they are compared to water
3) there area always some salts in the cell–water potential is lower than that of water –> pure water will move into the cell
when solute potential is positive
1) potential pressure from the turgor pressure is (+) inside living cells
2) effects of equilibirium result in no net movement, no water movment
salt-adapted species
respond to low water potentials by accumulating solutes in root cells
- lowers solute potential of these plants
dry-adapted species
cope by tolerating low solute potentials
- lose water to the atmosphere
dry air
few water molecules present exert low pressure
warm air
water molecules move farther part & exert lower pressure
water-potential graident
a difference in water potential in one region compared with that in another region
- determines direction that water moves (always higher to lower)
major hypotheses for how water could be transported to shoots
1) root pressure
2) capillary action
3) cohesion-tension
vascular tissue
tissue that transports water, nutrients, and sugars
- contains xylem & phloem
tissues in the root
1) epidermis
2) root hairs
3) cortex
4) endodermis
5) pericycle
epidermis
outermost layer of cells
“outside skin”
root hair
a long, thin outgrowth of the epidermal cells of plant roots
- provide increased surface area fro water and nutrient absorption
cortex
(in plants) a layer of ground tissue found outside the vascular bundles of roots and outside the pith of a stem
- stores carbohydrates
endodermis
a cylindrical layer of cells that separates the cortex from the vascular tissue and location of the Casparian strip
- controls ion uptake
- prevents ion leakage from the vascular tissue
“inside skin”
pericycle
a layer of cells that forms the outer boundary of the vascular tissue
“around circle”
water pathways
1) transmembrane route
2) apoplastic pathway
3) symplastic pathway
Casparian strip
a waxy later containing suberin
- prevents water movement through the walls of endodermal cells
- blocks apoplastic pathways of water and ion movement
suberin
waxy substance found in the cell walls of cork tissue and in the Casparian strip of endodermal cells
- forms water repellent cylinder
root pressure
positive pressure of xylem sap in the vascular tissue of roots
- generated during night as result of ion accumulation from soil & osmotic water movement into the xylem
- pressure potential that develops in roots
- cannot push water all the way up a tall tree
drives water up against the force of gravity
guttation
excretion of water droplets from plant leaves
- visible in the morning (dew)
- caused by root pressure
capillarity
tendency of water to move up a narrow tube due to adhesion, cohesion, and surface tension
- draws water up xylem cells
- result of adhesion creating an upward pull at the water-container interface, surface tension creating upward pull all across the surface, & cohesion transmitting both forces to the water below
- cannot pull water up a tall tree
aka capillary action
adhesion
molecular attraction among UNLIKE molecules
ie. water interacts with glass walls of capillary tube through hydrogen bonding
surface tention
a cohesive force that causes molecules at the surface of a liquid to stick together, thereby resisting deformation of the liquid’s surface & minimizing its surface area
ie. pulls water column up to minimize air-water interface
cohesion
a molecular attraction among LIKE molecules
ie. holds water molecules in the water column together
meniscus
the concave boundary layer formed at most air-water interfaces due to adhesion and surface tension
cohesion-tension theory
water is pulled up to the tops of trees along a water-potential gradient, via forces generated by transpiration at leaf surfaces
- leading hypothesis
- does not require energy
because of hydrogen bonding between water molecules, water is pulled up through xylem in continuous columns
cohesion-tension theory possible due to
1) a continuous column of water throughout the plant
2) hydrogen bonding between water molecules
bulk flow
a mass movement of a fluid/molecules along a pressure gradient
(ie) water movement in through plant xylem and phloem
process of the cohesion-tension theory
1) water vapor diffuses out of leaf
2) water evaporates inside leaf
3) water is pulled out of xylem
4) water is pulled up xylem
5) water is pulled out of root cortex
6) water moves from soil into root
evidence of cohesion-tension theory
cut a actively transpiring leaf at its petiole, watery fluid in xylem withdraws from the edge toward inside of leaf
- xylem sap is under tension
- little to no xylem sap exits leaf
crassulacean acid metabolism (CAM)
a type of photosynthesis
CO2 is fixed and stored in organic acids at night
- day: stomata open
- night CO2 released to feed Calvin cycle
temporally different than C3 photosynthesis
reduces water and CO2 loss by photorespiration
(ie) pineapple
C4 photosynthesis
a type of photosynthesis
CO2 is fixed into 4-C sugars rather than 3-C like in C3 photosynthesis
- spatially different than C3 photosynthesis
enhances photosynthetic efficiency in hot, dry environments by reducing loss of oxygen due to photorespiration
(ie) cactus in the desert; sugarcane
bundle-sheath cell
type of cell found around the vascular tissue (veins) of plant leaves
- Calvin cycle for C4 plants occurs here
- rubisco abundant
rubisco
enzyme that initiates the 1st step of Calvin cycle during photosynthesis: addition of a molecule of CO2 to ribulose biphosphate
translocation
movement of sugars through phloem by bulk flow
- specifically from sources to sinks
source
a tissue where sugar ENTERS the phloem
- high sugar concentrations
(ie) stem
sink
tissue where sugar EXITS the phloem
- low sugar concentrations
(ie) flowers & roots
sieve-tube element
an elongated sugar-conducting cell in phloem that lacks nuclei
- has sieve plates at both ends
- allows sap to flow to adjacent cells
alive at maturity
specialiezed parenchyma cell types in phloem
1) sieve-tube element
2) companion cell
companion cell
a cell in the phloem connected to adjacent sieve-tube elements via plasmodesmata
- provide materials to maintain sieve-tube elements & function in loading and unloading of sugars into sieve-tube elements
alive at maturity
pressure-flow hypothesis
hypothesis that sugar movement through phloem tissue is due to differences in the turgor pressure of phloem sap
phloem loading
(pressure-flow hypothesis)
sucrose is moved by active transport from source cells through companion cells to sieve-tube members
may depend on a proton pump and a cotransporter
phloem unloading
companion cells remove sucrose from the sieve-tube members into the sink root cells
- creates phloem sap w/ a high water potential
- water then moves back into the xylem
passive transport
ions or molecules DIFFUSE across a plasma membrane (along their electrochemical gradient)
energy not required
facilitated by channels and carriers (membrane protein)
examples of passive transport
1) channel proteins
2) carrier proteins
channel protein
membrane protein that forms a pore
- admits one or a few types of ions or molecules
- passive transport
carrier protein
membrane protein that facilitates diffusion of small molecule (ie. glucose) across a membrane by a process involving a reversible change in the shape of the protein
- passive transport
- conformational change
large molecules
facilitated diffusion
passive movement of a substance across a membrane with the assistance of transmembrane carrier proteins or channel proteins
pay
active transport
movement of ions or molecules across a membrane against an electrochemcial gradient
- requires energy (ATP) & assistance of a transport protein (pump)
examples of active transport
1) pump
2) symporter
3) antiporter
pump
membrane protein that can hydrolize ATP & change shape to power active transport of a specific ion/molecule across a plasma membrane against its electrochemical gradient
cotransporter
transmembrane protein that facilitates diffusion of an ion down its previously established electrochemical gradient
- uses the energy of that process to transport some other substance against its concentration gradient
types of cotransporters
1) symporter
2) antiporter
symporter
cotransport protein that transport solutes AGAINST a concentration gradient
- uses energy released when a different solute moves in the same direction down its electrochemcial gradient
1D
antiporter
carrier protein that allows an ion to diffuse down an electrochemcial gradient
- uses the energy of that process to transport a different substance in the opposite direction against its concentration gradient
2D
secondary activ etransport
transport of an ion/molecule in a defined direction against and with its electrochemical gradient
proton pump
a membrane protein that can hydrolyze ATP to power active transport of protons (H+ ions) across a membrane against an electrochemical gradient
aka H+ ATPase
tonoplast
membrane surrounding a plant vacuole
