BIOL 005B Lecture Notes - Lecture 25: Storage Organ, Tuber, Apoplast
BIOL 005B Lecture 25: Transport in Plants
●Bulk Flow in the Xylem
○Plants lose a large volume of water from transpiration, the evaporation of water
from a plant’s surface
○Water is replaced by the bulk flow of water and minerals, called xylem sap, from
the steles of roots to the stems and leaves
○How does sap rise? Is it pushed or pulled?
●Pushing Xylem Sap: Root Pressure
○At night, when transpiration is very low
, root cells continue pumping mineral ions
into the xylem of the vascular cylinder, lowering the water potential
○Water flows in from the root cortex
, generating root pressure
○Positive root pressure is relatively weak
and is a minor mechanism of xylem bulk
flow (but guttation can occur)
●Pulling xylem sap: the transpiration-choesion-tension mechanism
○Water is pulled upward by negative pressure in the xylem
○Water vapor in the airspaces of a leaf moves down its water potential gradient and
exits the leaf via stomata. This lowers the Ψ in the mesophyll
○Transpiration produces negative pressure (tension) in the leaf, which exerts a
pulling force on water in the xylem, pulling water into the leaf
●Cohesion and adhesion in the ascent of xylem sap
○The transpirational pull on xylem sap is transmitted all the way from the
leaves to the root tips and even into the soil solution
○Transpirational pull is facilitated by cohesion of water molecules to each other
and adhesion of water molecules to cell walls
○Water molecules are attracted to cellulose in xylem cell walls through adhesion
which helps offset the force of gravity
○Thick walls of tracheid and vessels prevent collapse
○What happens with damage or freezing? Cavitation
●Stomata: major pathways for water loss
○About 95% of the water a plant loses escapes through stomata
○Each stoma is flanked by a pair of guard cells, which control the diameter of the
stoma
by changing shape
○Changes in turgor pressure open and close stomata
○These result primarily from the reversible uptake and loss of K
+
ions
by the guard
cells
●Stimuli for stomatal opening and closing
○Generally, stomata open during the day and close at night
to minimize water loss
○Stomatal opening at dawn is triggered by light, CO
2
depletion, and an internal
“clock”
in guard cells
○All eukaryotic organisms have internal clocks; circadian rhythms are 24-hour
cycles
○Drought, high temperature, and wind can cause stomata to close during the
daytime
Document Summary
Pushing xylem sap: root pressure from a plant"s surface. Plants lose a large volume of water from transpiration , the evaporation of water. Water is replaced by the bulk flow of water and minerals, called xylem sap , from the steles of roots to the stems and leaves. At night, when transpiration is very low , root cells continue pumping mineral ions. Water flows in from the root cortex , generating root pressure. Positive root pressure is relatively weak and is a minor mechanism of xylem bulk into the xylem of the vascular cylinder, lowering the water potential. Water is pulled upward by negative pressure in the xylem. Water vapor in the airspaces of a leaf moves down its water potential gradient and exits the leaf via stomata. Transpiration produces negative pressure (tension) in the leaf, which exerts a pulling force on water in the xylem , pulling water into the leaf.
