Transport in Plants

apoplastic

non-living pathway of water uptake
water travels through cell walls (low resistance, as cellulose has a mesh like structure which allows water to flow freely)
water moves by diffusion
it is blocked by casparian strip (rejoins symplastic - higher levels of control)

symplastic

living pathway of water uptake
water travels through cytoplasm/plasmodesmata (high resistance)
water moves by osmosis
symplastic does not have to cross casparian strip

potometer common improvements

assembled underwater - no air bubbles enter, continuous column of water between potometer and leaves

dry leaves - water on leaves prevents transpiration (no conc gradient)

syringe + tap - allows repeats by resetting bubble position

xylem vs phloem

both have cell walls with cellulose
both have tubes

xylem vessels contain lignin, phloem does not
xylem has no end walls, phloem has sieve tubes
phloem contains cytoplasm, xylem does not
phloem has companion cells, xylem does not

mass-flow hypothesis

sugars are loaded into sieve tubes, lowering water potential so water from xylem enters phloem, hydrostatic pressure in phloem increases and water moves down pressure gradient, sugars converted to insoluble starch at root hair cells and absorbed by sink, this increases water potential in sieve tube so water moves out

cohesion-tension theory

water evaporated (transpired) from stomata of leaves, therefore water potential gradient created, leaves have low water potential water is polar, so is cohesive and is drawn up as a column of water, adhesion of water molecules to xylem walls

pros/cons of mass flow

function of sieve plates unclear - would seem to hinder mass flow
not all solutes move at same speed
sucrose delivered at more or less the same rate to all regions

lack of oxygen inhibiting translocation
pressure within sieve tubes
concentration of sucrose higher in source than sink
companion cells possess many mitochondria and readily produce ATP
downward flow in the phloem occurs in daylight, ceases at night
increase in sucrose levels in leaf are followed by sucrose levels in phloem a little later

transpiration definition

movement of water out of leaf stomata via evaporation

how does water move into root hair cells

osmosis

how do minerals move into root hair cells

active transport via carrier proteins

adaptations of phloem cells

have companion cells providing ATP for active transport as energy needed to transport organic molecules. sieve tubes have no organelles so free flow of compounds

what is bubble potometer measuring vs mass potometer

bubble potometer: water absorbed
mass potometer: water absorbed - water lost

why is more water lost than absorbed

some water is used in photosynthesis to maintain turgor

high wind speed causes what to transpiration

increase in rate as diffusion gradient is steeper

plant adaptations when very dry

curled, hairy, grooved leaves that trap still moist air around stomata

xylem growth and adaptations

protoxylem → grows as walls not fully lignified

metaxylem → tissues die and become lignified, end wall cells break down
xylem is a continuous tube with no living content
strong due to lignin
has pits in walls to allow water to move out

cambium purpose

unspecialised cells which divide to form xylem/phloem