Xylem

Question 1

What is diffusion?

Answer 1

spontaneous movement of solutes from regions of higher to lower concentration

Question 2

What is osmosis?

Answer 2

diffusion of water across a selectively permeable membrane

Question 3

What is the meaning of free energy, chemical potential, and water potential in terms of plant physiology?

Answer 3

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

Question 4

What are the components of the water potential equation?

Answer 4

solute concentration, pressure, gravity

Question 5

How do solutes affect water potential?

Answer 5

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)

Question 6

How do positive and negative pressure affect water potential?

Answer 6

+ hydrostatic pressure = increased water potential = turgor
- hydrostatic pressure = decreased water potential = tension

hydrostatic pressure is measured as the deviation from standard atmospheric pressure

Question 7

How does gravity affect water potential?

Answer 7

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

Answer 8

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

Answer 9

increased solutes in soil, decreased hydration of soil = decreased water potential of soil

water potential of soil divided into same three components

Question 10

What is the apoplast?

Answer 10

continuous system of cell walls, intercellular spaces, and lumens of non-living cells (xylem and fibres)

when alternated with symplast = transmembrane pathway

Question 11

What is the symplast?

Answer 11

entire network of cell cytoplasm interconnected by plasmodesmata

when alternated with apoplast = transmembrane pathway

Question 12

Where is the Casparian strip located and what role does it play?

Answer 12

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

Question 13

What causes differences in root pressure and how does this affect water potential?

Answer 13

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

Question 14

What are the major cell types involved in xylem transport and what are their identifying anatomical features?

Answer 14

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

Question 15

What role does surface tension play on water potential throughout a plant?

Question 16

What is cohesion tension theory?

Answer 16

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

Question 17

What is transpiration and how does it affect water movement?

Answer 17

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

Question 18

What is cavitation and how does it affect water movement?

Answer 18

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

Question 19

What anatomical features of leaves are involved in the movement of water?

Answer 19

waxy cuticle, internal air spaces, stomata

Question 20

What are guard cells and what role do they play in the function of stomata?

Answer 20

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%

Question 21

What is the difference between macronutrients and micronutrients?

Answer 21

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

Question 22

How is water kept in soil and absorbed by roots?

Answer 22

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%)

Question 23

What are the components of soil?

Answer 23

organic matter, water, air

Question 24

What is guttation?

Answer 24

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

Question 25

What is the process by which water moves through xylem?

Answer 25

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