mass transport in plants

lignin

insoluble polymer found in xylem cells impermeable to water

suberin

waxy waterproof substance controlling movement of water into xylem from root tissue by preventing water from travelling through cell wall space

transpiration/ cohesion tension theory

water evaporates from spongy mesophyll to enter air spaces and water vapour diffuses out of stomata (transpiration), water potential of leaf cells is lowered, water moves down water potential gradient generating transpirational pull, water pulled up xylem in contnuous column due to cohesion of water molecules by h bonds and adhesion of h2o molecules to side of xylem

transfer of sucrose into sieve elements from photosynthesising tissue - first stage of mass flow theory

sucrose formed by parts of plants with chloroplasts, sucrose facilitated diffusion down conc gradient into companion cells, H+ ions actively transported into cell wall spaces, protons diffuse down conc gradient through carrier proteins into sieve tube element, sucrose molecules transported alongside protons in co-transport with co-transport proteins

mass flow of sucrose through seive tube elemtns

sieve tubes have low water potential due to sucrose entering via active transport, water moves into phloem from xylem via osmosis increasing hydrostatic pressure, sugar solution moves down phloem down pressure gradient, at sink sucrose used in respiration or stored as starch causing lower water potential so water moves from sieve tubes into sink cells, maintains hydrostatic pressure gradient between source and sink

ringing - investigating translocation

removal of ring of surface tissues including phloem from stem of plant while leaving core intact, sugars of phloem accumulate above ring causing swelling, region below ring withers/dies

tracer - investigating translocation

radioactive tracer (14co2 readily absorbed by leaves and used in photosynthesis to produce sucrose) movement can be tracked