3.5-Mass transport in plants

what is the phloem involved in?

transport of organic substances and other soluble products

from leaves to other parts of the plant

what is the xylem involved in?

transport of water and dissolved minerals

from soil, through roots, the stem and to the leaves

what does the lignin in xylem cell walls help with?

strengthens xylem against tension within them

waterproof

faster flow

what is the function of the dead cells of the xylem?

leaves hollow lumen with no cytoplasm that offers little resistance to mass flow of water and minerals

waterproof

what is the function of the pits in the walls of the xylem vessels?

if a vessel becomes blocked or damaged, the water can be diverted laterally

upwards movement of water can continue in an adjacent vessel

what does it mean if the vessels lose their end walls?

form a continuous column for water movement from root to leaves

two advantages of the spiral thickening of a xylem vessel?

less material- less wasteful

more flexible/lower mass

what is the cohesion tension theory?

evaporation of water from leaves pulls up more water from the xylem in the stem

cohesion- water molecules form hydrogen bonds with each other

adhesion- water molecules form hydrogen bonds with xylem walls

what are some adaptations of xerophytic plants?

hairs on lower epidermis of leaf

stomata sunken in pits

thick waxy cuticle

rolled leaves

how do the adaptations help to limit water loss?

water vapour is trapped

reduces water potential gradient between air spaces inside leaf and atmosphere

lower rate of diffusion

less transpiration

what is the evidence FOR mass flow hypothesis? (6 points)

high hydrostatic pressure in phloem (release of sap when cut)

conc. of sucrose higher in leaves (source) than roots (sink)

downward flow in phloem occurs in day, but ceases when leaves are shaded, or at night

increase in sucrose levels, then increase in phloem little later

metabolic inhibitors/lack of oxygen inhibit translocation

companion cells- many mito

what is the evidence AGAINST mass flow hypothesis (4 pts)

function of sieve plats unclear

not all solutes move at same speed- should if moved by mass flow

sucrose delivered at same rate to all sinks rather than going fastest to lowest sucrose conc.

movement due to gravity

what do radioactive tracers do?

carbs synthesised by photosynthesis using radioactively labelled co2