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A Handbook of Biology
Bulk flow is achieved either through a positive hydrostatic pressure
gradient (e.g. a garden hose) or a negative hydrostatic pressure
gradient (e.g. suction through a straw).
Bulk movement of substances in long distance through the conducting
tissues (xylem & phloem) is called translocation.
ÅBSØRPTÏØÑ ØF WÅTËR BÝ PLÅÑTS
Absorption of water and mineral occurs by diffusion through millions of
root hairs present at the root tips in the zone of maturation.
Root hairs increase the surface area for absorption.
The absorbed water is moved deeper into root layers by 2 pathways:
Apoplast pathway and Symplast pathway.
ÅPØPLÅST PÅTHWÅÝ:
It is the system of adjacent cell walls that is continuous throughout the
plant, except at the casparian strips of the endodermis in the roots.
Movement through the apoplast is dependent on the gradient. It does
not involve crossing the cell membrane.
It occurs exclusively through the intercellular spaces and the walls of
the cells.
Plasma membrane
symplast
apoplast
Xylem
Casparian strip
Plasmodesmata
The apoplast does not provide any barrier to water movement and
water movement is through mass flow.
As water evaporates into the intercellular spaces or the atmosphere,
tension develops in the continuous stream of water in the apoplast.
Hence, mass flow of water occurs due to the adhesive and cohesive
properties of water.