9.1 transport in the xylem of plants Flashcards
almost all plants are …. p- a-
photosynthetic autotrophs
What 2 types of transport tissue do vascular plants have
xylem and phloem
what is transpiration?
the loss of water vapour thru leaves, stems, and other (above ground) parts of plant
transpiration is the consequence of…
gas exchange in the leaf
where does transpiration occur through
mainly through open stomata
how is transpirtation connected to photosynthesis
A tension is created in the plant due to transpiration and this acts as a driving force for the uptake of water from the soil and the movement of water to the shoots.
water required by photosynthesis
adaptations of leaves for gas exchange (2)
- stomata
tiny pores, opening and closing controlled by 2 guard cells - lower tissue layer (spongy mesophyll)
provides large surface area and moist surface neccesary for gas exchange
describe the exchange of gases in the leaf leading to transpiration 4
- co2 conc drops = co2 from air spaces dissolve, diffuse into cells
- air conc drops = net movement of co2 molecules into leaf thru stomata
- O2 diffuses out leaf cells into int. air spaces (then into atmos thru stomata)
- transpiration: water vapour diffuses out of leaf into atmos
function of xylem vessels
transports water and dissolved minerals
- from roots to all parts of plant
structure of xylem vessels (that allow transport under tension) 3
- long continuous tubules (from roots thru stems)
- hollow: dead at maturity, cell mem. + int. structures + horizontal cell walls break down
- walls strengthened with lignin (binds with cellulose – provides great strength and rigidity)
what to note when drawing structure of pri xylem vessels
- continuous tubule
- xylem wall should have gaps (pits) – enable exchange of water molc
- lignin represented via spiral/rings
cohesive property of water 2 factors, briefly
- cohesion
- water molc form weak hydrogen bonds - adhesion
- polarity of water (interacts with hydrophillic cellulose in cell walls)
cohesive property of water: cohesion
water molecules form weak hydrogen bonds (bc of polarity)
= transpirational pull can extend thru long columns of water
cohesive property of water: adhesion
polarity interacts with hydrophilic cellulose in cell walls = create pull to draw water out xylem into cells
transpirational pull 4 steps `
- cellulose in mesophyll (leaf) cell walls is hydrophilic – water adheres = film of water on cell surface
- water vapour diffuses out stomata = internal air spaces less humid = water evaps from moist mesophyll cell walls into air spaces
- when evaporates = pulling force on water molecules within cell (cohesion)
- tension caused by pull of evap – draws water from xylem into leaf cells
adaptations of root hairs for mineral ion uptake (2, related to energy demands)
active uptake = high demand for atp
- plasma membrane of root hairs – many protein pumps –> AT of mineral ions from water into cytoplasm of cell
- high rate of cellular respiration, many mitochondria, high o2 gas demand
how does uptake of mineral ions cause absorption of water
high conc of mineral ions in cytoplasm = low water potential = osmosis of water in
water potential of pure water
0.0MPa
what are Xerophytes
plants that adapted to live in conditions where liquid water is hard to obtain (eg deserts)
- some have special tissues for water storage, some adapt to reduce water loss thru transpiration
10 adaptations of desert plants for water conservation + xerophytes too
- thick waxy cuticle on leaf/stem: reduces non-stomatal transpiration rate
- fewer stomata: reduces transpiration rate
- stomata in sunken pits: maintains humidity – decr. transp.
- fine hairs on leaf underside: maintains layer of moisture near stomata – decr transp.
- CAM physiology – stomata close during day
- reduced air spaces in leaf mesophyll: decr surface area
- few/small leaves/ photosynthesis in stem: red. surface area
- curled/rolled leaves: reduced surface area + incr humidity
- deep, highly branched roots: incr ability to take up water
adaptations of saguaro cactus 6
- thick waxy cuticle on stem
- one tap root + highly branched system
- stores water in stem
- reduced S.A. (p.sis in stem)
- leaves reduced to spines
- CAM photosynthesis (stomata closed in day)
adaptations of Marram grass 4
- thick outer cuticle
- rolled shape
- few stomata
- hair like structures
what are halophytes
plants that have adapted to areas with high salinity (Eg ocean shoreline)
– makes establishment of higher conc of ions in roots more difficult
- some have adaptations to secrete salt, some establish high concs of other solutes, some conserve water in the plant
adaptations of halophytes 6
- salt storage in vacuoles: compartmentalizes, protects organelles and enzymes from damage
- high conc of organic solutes: incr osmolarity = water can still enter
- salt storage glands in leaf: accumulates salt in limited area, release thru crystals
- leaf abscission: breaking off leaves with toxic levels of salt
- selectively permeable membrane (in roots): excluses salt by having no ion channels to allow passage of Na/Cl / has AT pumps to remove
- xerophytic adaptations: few stomata, water storage, thick cuticle etc
adaptations of mangrove trees
- salt glands that excrete salt crystals (grey mangrove)
- root cell membranes that mostly exclude salt ions (red)
- store salt in vacuoles, keeping cell turgid (red)
internal factors affecting rate of transpiration 4
- root to shoot ratio
- surface area of leaves
- number of stomata per unit leaf area
- leaf struture eg presence of hair
external factors affecting rate of transpiration
- light
- wind
- temperature
- humidity
- water availiablity
how does temp affect rate of transpiration
temp incr = transpiration incr
- incr temp = incr energy for water evaporation + decr humidity
(but if temp too high for enzymes, stomata close, transp. rate falls)
how does humidity affect transpiration rate
incr humidity = decr transpiration
- concentration gradient for diffusion of water vapour between air spaces and atmosphere: more/less steep
how does light affect transpiration rate
light intensity incr = transpiration incr
- stomata closed in dark; incr LI = open stomata, water escapes
- photons provide energy for evaporation
how does wind affect transpiration rate
wind velocity incr = transpiration incr
- low wind = incr humidity = decr water vapour conc gradient = decr transpiration
using a porous pot, how is cohesion-tension due to evaporation demonstated via a model
- water saturates the porous pot, evaporates into surrounding air
- cohesion tension extends down tubing into beaker, water drawn up
using filter paper, how is cohesion-tension due to capillary action demonstated via a model
cohesion of water + adhesion to cellulose fibres in paper draws water up
what are potometers used for
indirect measurement of water loss = rate of transpiration measurement of water uptake via water level/ movement of air bubbles
measuring transpiration rates using mass
measuring decr in mass from plant in sealed container
- BUT does not account for mass incr due to photosynthesis
struture of stomata
- structure: 2 elongated guard cells, attached to epidermal cells
- pore appears when they seperate
how do stomata open and close
due to turgor pressure of guard cells
water absorbed by guard cells = push epidermal cells beside them = pore develops
- closes when water is lost and guard cells become flaccid
what are root hairs
extensions of individual epidermal cells, relatively short lived
3 possible routes for water movement thru plant cells and tissues
- mass flow (apoplast pathway)
- occurs thru interconnecting free spaces betw cellulose fibres of cell walls (incl xylem) - diffusion (symplast pathway)
- occurs thru cytoplasm and via cytoplasmic connections betw cells (organelles = resistance) - osmosis
- from vacuole to vacuole, driven by osmotic pressure gradient
- AT of mineral ions in roots = absorption of water via osmosis
- not as significant pathway for across plant transport
how active uptake is achieved – movement of water and ions from roots to xylem by structure
- vascular tissue in roots contained by endodermis
- Casparian strip at endodermis: waxy strip in radial walls – blocks water passage momentarily
- water passes thru endodermis via osmosis
- HENCE cytoplasm of endodermal cells AT ions from cortex to endodermis
outline ion uptake
- by active transport
- highly selective process – reflects needs of the plant
- involves protein pumps – needs ATP + specific
how do ions reach the cell membranes during active uptake of ions in roots
- mass flow of water thru free spaces = delivers fresh soil solution to root hair cell plasma membranes
- active uptake of selected ions
- ions diffuse from higher conc outside apoplast –> lower conc adjacent to protein pumps
- mutualistic r/s with soil-inhabiting fungi: fungal hyphae receive supply of sugar from plant root cells, in exchange they release ions + death & decay also releases ions
active uptake of ions in soil but mnore chemistry
- neg charged clay particles – pos charge ions attach
- minerals needed: Mg2+, nitrates, Na+, K+, PO43-
- AT uploading into roots
- root cells have protein pumps – expel H+ ions into soil = displaces pos mineral ions
