[7.7] transport of water in the xylem Flashcards
what does the xylem transport?
- water
- mineral ions
what does a mature xylem vessel look like?
- a stack of cell walls on top of each other
- there are no internal structure so there is an unimpeded stream of water flowing through it
how is the xylem vessel adapted for it function?
- lignin in walls
- spiralised thickening
why is lignin a useful structural adaptation?
- extremely difficult to break down
- waterproofs it so water doesn’t leak out
why is spiralised thickening a useful structural adaptation?
- gives plant extra strength
- means pressure inside doesn’t cause xylem vessel to collapse inwards due to negative pressure
what is the role of the xylem?
to transport water through the stem by transpiration, until it diffuses through the stomata
describe the movement of water through the stomata
- higher ψ in atmosphere than in air spaces next to stomata
- ψ gradient from stomata to atmosphere
- if stomata are open, water vapour molecules diffuse out of air spaces into surrounding air
- water lost by diffusion from air spaces is replaced by water evaporating from cell walls of surrounding mesophyll cells
- plants can control their rate of transpiration by changing size of stomata
how can water travel through the plant?
- apoplast pathway (cell wall pathway)
- symplast pathway (cytoplasm pathway)
how does water enter the root hair cell from the soil?
- osmosis (low to high water potential)
- sometimes, active transport of mineral ions can manipulate water potential, but mineral ions are also beneficial
what are mineral ions needed for?
- potassium: for sodium-potassium pump in co-transport mechanism
- magnesium: to make chlorophyll, which absorb sunlight for photosynthesis
- nitrate: for amino acids, which form proteins
- phosphate: for nucleic acids (structure) and the phospholipid bilayer
due to what force is water effectively drawn up the transpiration stream?
- cohesive forces between water molecules
- this is due to hydrogen bonds between water molecules
why is water needed?
- metabolite (eg. photosynthesis)
- keeps mesophyll turgid so it is upright so maximum sunlight can be absorbed for photosynthesis
- cooling - reduces heat generated from plant’s metabolic reactions
describe the movement of water across the cells of a leaf (symplast / cytoplasmic pathway)
- mesophyll cells lose water to air spaces by evaporation due to heat from sun
- these cells now have a lower ψ
- water enters by osmosis from neighbouring cells
- loss of water from neighbouring cells lowers their ψ
- they also take in water from their neighbours by osmosis
- ψ gradient is established
- water is pulled from xylem across leaf mesophyll into the atmosphere
describe the apoplast / cell wall pathway
- water moving in apoplast pathway is done almost exclusively through cell walls
- after the root cortex cells, there is an epidermic later
- this barrier prevents apoplast pathway through epidermis so water must go via the cells
- therefore, must cross the selectively permeable cell membrane
- this means that water, and anything dissolved in it, must cross at least one cell membrane before it gets into the xylem vessels, at which point water will be distributed everywhere else in the plant
how can water moving in apoplast pathway be done almost exclusively through cell walls?
because cytoplasm is freely permeable
what happens when unwanted things build up in the epidermic layer?
it has enzymes which can break them down
what does the cohesion-tension mechanism help with?
moving water up plants, from roots to leaves, against the force of gravity
describe the movement of water up the stem in the xylem based on the cohesion-tension mechanism
- water evaporates from mesophyll cells in leaves
- atmosphere is less humid than air spaces; water vapour diffuses out through stomata
- ψ of these cell is lowered, creating tension that pulls more water in from neighbouring cells by osmosis
- water molecules are cohesive; when a water molecule leaves the leaf, another one is pulled along with it, drawing more water up into leaf from stem and roots
- continuous column of water in xylem is pulled upwards to leaves from roots: the transpiration stream
how is the transpiration stream passive?
relies on energy from the sun
what is negative pressure?
in context of water movement through a plant
- water is pulled upwards from the roots due to a reduction in pressure in the leaves
- ∴ water moves towards an area of lower pressure
what evidence is there to support the cohesion-tension theory?
- diameter of tree trunks changes according to the rate of transpiration
- if a xylem vessel is broken and air enters it, the tree can no longer draw up water
- if a xylem vessel is broken, water does not leak out, as it would do if it were under pressure; air is drawn in, which means it is under tension
describe the changes in a tree’s diameter throughout a day, which supports the cohesion-tension theory
- fastest rate of transpiration during the day
- faster rate = more tension in xylem
- more tension means walls of xylem vessels pull inwards
- trunk shrinks in diameter
- opposite at night, where transpiration is slowest
what is a potometer?
- apparatus used to estimate effect of different factors on rate of transpiration
- measures plant’s water uptake
- assumes that water uptake will be directly proportional to, but not exactly the same as, water leaving the leaf by transpiration
describe a method to measure the uptake of water using a potometer
- cut shoot underwater to prevent air entering the xylem
- fill potometer with water. ensure there are no air bubbles
- place shoot into capillary tube in potometer underwater
- remove potometer from underwater
- seal joints (eg. with vaseline) to prevent water loss
- introduce air bubble into graduated capillary tube
- record start position of air bubble in capillary tube
- using a stopwatch, record bubble movement in a set time
- rate of bubble movement is an estimate of rate of transpiration (only change one variable at a time)
how do you calculate water uptake?
record:
- distance moved by bubble
- time taken to move
- volume of capillary tube
- units = incoporate volume and time eg. cm³ s⁻¹
calculation:
- vol of capillary tube = πr² × distance bubble moved (h)
- rate = (volume of capillary tube / scale) / time taken
what factors affect the rate of transpiration?
- temperature
- humidity in the atomsphere
- wind
- light intensity
- concentration of carbon dioxide
how does increased temperature affect the rate of transpiration?
- increased rate of transpiration
- increased rate of evaporation & diffusion of water out of stomata
- higher temp = higher KE
how does increased humidity in the atompshere affect the rate of transpiration?
- decreased rate of transpiration
- lower water potential gradient between leaf and atmosphere
how does increased wind / air movement affect the rate of transpiration?
- increased rate of transpiration
- moist air cannot cumulate outside stomata
- this creates a steep water potential gradient
how does increased light intensity affect the rate of transpiration?
- increased rate of photosythesis so increased rate of transpiration
- increased heat energy from sun hits leaf so increased rate of evaporation
how does increased concentration of carbon dioxide affect the rate of transpiration?
- increased rate of photosynthesis so increased rate of transpiration
- stomata have to be open to allow carbon dioxide in so water can also leave
why is uptake of water not necessarily equal to transpiration?
- water is used in photosynthesis
- water is used in hydrolysis
- water is used in vacuoles to maintain cell turgidity
- in a potometer system, there could be leaks
