xylem and transpiration - lqc 2b Flashcards
xylem function
transports water and mineral ions, structural/mechanical support
lignin and cellulose microfibrils in walls function
for strength to prevent vessel collapsing
lignin in walls function
to make the vessel waterproof
large cross-sectional area function
allows a larger volume of water to be transported
pits in walls function
allow lateral/sideways movement of water between xylem vessels
no end walls function
forms a continuous, dead hollow tube with no cell cytoplasm- reduces resistance/obstruction to water flow
lignin in rings or spirals function
to allow stretching/flexibility as the plant grows
water transport in a plant from the roots to the leaves
· water evaporates from the surface of the leaf cells into the air spaces and exits the leaf by the process of transpiration (water vapour diffuses out of the leaves though stomata , down a diffusion gradient)
· this creates a low hydrostatic pressure in leaves which draws water out of the xylem vessels by osmosis
· a continuous column of water molecules (transpiration stream) is pulled, under tension, up the xylem by capillary action
· due to cohesion - hydrogen bonding between polar water molecules in xylem
· and adhesion - hydrogen bonding between water and cellulose in the xylem walls
· water moves into roots by osmosis – creating a high hydrostatic pressure
· water moves along a hydrostatic pressure gradient, from high pressure in roots to low pressure in leaves, by mass flow