Xylem Flashcards
What is diffusion?
spontaneous movement of solutes from regions of higher to lower concentration
What is osmosis?
diffusion of water across a selectively permeable membrane
What is the meaning of free energy, chemical potential, and water potential in terms of plant physiology?
free energy represents the potential for performing work (force x distance, in Jmol-1)
chemical potential is a relative quantitative expression of free energy associated with a substance
water potential is the chemical potential of water divided by the volume of 1 mol of water (ie. free energy of water per unit volume), measured in pascals because water potential is essentially pressure
What are the components of the water potential equation?
solute concentration, pressure, gravity
How do solutes affect water potential?
solutes reduce free energy of water by diluting it
increased solute concentration = more negative solute potential = decreased water potential
independent of the nature of the solute
solute potential = (-)(gas constant)x(temperature)x(solute concentration)
How do positive and negative pressure affect water potential?
+ hydrostatic pressure = increased water potential = turgor
- hydrostatic pressure = decreased water potential = tension
hydrostatic pressure is measured as the deviation from standard atmospheric pressure
How does gravity affect water potential?
dependent on height of water above reference state of water, density, and acceleration due to gravity
gravity potential = (density)x(gravity)x(height)
= 0.01 x height
typically omitted in cells and very short plants
water moves in and out of cells in response to a water potential gradient
direction of flow is determined by water potential gradient from high to low and has rate proportional to magnitude
across membranes, rate of movement also depends on hydraulic conductivity
aquaporins provide water-specific channels to facilitate movement of water across membranes
increased solutes in soil, decreased hydration of soil = decreased water potential of soil
water potential of soil divided into same three components
What is the apoplast?
continuous system of cell walls, intercellular spaces, and lumens of non-living cells (xylem and fibres)
when alternated with symplast = transmembrane pathway
What is the symplast?
entire network of cell cytoplasm interconnected by plasmodesmata
when alternated with apoplast = transmembrane pathway
Where is the Casparian strip located and what role does it play?
band of hydrophobic suberin in radial cell walls of the endodermis (inner cell layer of cortex)
allows plant to have selectivity
in at least one cell layer it:
- blocks apoplastic pathway in the endodermis and requires symplastic movement in these cells
- forces water and solutes into transmembrane pathway
What causes differences in root pressure and how does this affect water potential?
selectivity of Casparian strip causes different water potentials between roots and soil
absorption of solutes decreases water potential in root, which causes positive hydrostatic pressure in xylem
happens most when transpiration is low and solute concentration is high
What are the major cell types involved in xylem transport and what are their identifying anatomical features?
tracheary elements
tracheids: found in all plants, long spindle-shaped cells in overlapping vertical files, pits, pit membranes, tori, less than 50 micrometers
vessel elements: found in angiosperms, Gnetales, some ferns, shorter, wider than tracheids with perforated end walls, perforation plate, stacked end to end to form vessel, more efficient, less obstruction at connection point, but wide up to 500 micrometers so cannot overcome gravity, not found in tall trees also due to evolution
What role does surface tension play on water potential throughout a plant?
What is cohesion tension theory?
instead of relying on pressure gradient from roots to top of plant to drive water (since root pressure is less than 0.1 mPa and disappears when transpiration is high), a large tension develops at the top of the plant at the surfaces of cell walls in leaves
water adheres to hydrophilic components (ex. cellulose microfibrils), as water evaporates, surfaces of remaining water are drawn into interstices (forms curved air-water interfaces), curvature induces tension owing to high surface tension of water, as more water is removed, curvature radius increases and water potential becomes more negative
ultimate source of energy is the sun, which increases rate of transpiration from leaves
What is transpiration and how does it affect water movement?
concentration gradient of water vapour controls transpiration, high hydraulic resistance
driving force is difference in water vapour concentration between inside leaf and outside air
main factors affecting rate: leaf temperature, stomatal resistance (number and diameter), boundary layer resistance (wind speed and leaf size)
opening and closing stomata balance water retention and CO2 for photosynthesis
water loss that occurs as a result of allowing sufficient CO2 uptake for photosynthesis, for C3 plants, up to 400 water molecules lost for every CO2 fixed, due to concentration gradient of water 50x greater than CO2, slower CO2 diffusion in air than in water, assimilation of CO2 requires transport across plasma membrane, cytoplasm, and chloroplast envelope, transpiration ratio is reduced in some plants (C4 and CAM) using unique CO2 trapping mechanisms
What is cavitation and how does it affect water movement?
bubble formation from dissolved gases being forced out of solution during freezing, stop water flow
reduce cavitation: pit membranes, interconnectivity, finite lengths of tracheary elements. reduced tension at night, new growth of xylem tissues
What anatomical features of leaves are involved in the movement of water?
waxy cuticle, internal air spaces, stomata
What are guard cells and what role do they play in the function of stomata?
2 main types: dumbbell shaped in grasses, kidney shaped in other plants
increase in turgor pressure of guard cells causes opening of stomata
2 on either side of stomata are connected by cellulose microfibrils, specific alignment of these responsible for opening stomata
in normal cylindrical cells, microfibrils are oriented transverse to the long axis, but in guard cells they fan out radially from the pore
multisensory hydraulic valves: light intensity and quality, temperature, water status, intracellular CO2 concentration
decreased solute potential in guard cells due to ion uptake = lower water potential, which allows water to move into guard cells
volume can increase by 40-100%
What is the difference between macronutrients and micronutrients?
both essential for structure or metabolism, absence causes abnormal growth, development, reproduction
macronutrients: nitrogen, potassium, calcium, magnesium, phosphorous, sulfur, silicon
required tissue levels higher than micronutrients
micronutrients: chlorine, iron, boron, manganese, sodium, zinc, copper, nickel, molybdenum
required tissue levels very small, but needed as cofactors for enzyme synthesis
How is water kept in soil and absorbed by roots?
as soil dries, water recedes into interstices between soil particles
curvature of wair-water surfaces is a balance between minimizing SA/tension and attraction of water to soil particles
water under a curved surface develops a negative water potential
root hairs are outgrowths of epidermal cells that increase surface area (about 60%)
What are the components of soil?
organic matter, water, air
What is guttation?
formation of liquid droplets at edges of leaves through hydrothodes
only happens in small plants because hydrostatic pressure is not enough to overcome gravity potential in larger plants
What is the process by which water moves through xylem?
long distance transport through xylem from soil, independent of solute concentration, extremely sensitive to the radius of the tracheary element, all water in path is moving at same rate at same time
total pressure required to overcome drag and gravity is 0.01 x height for each
