Lecture 24: Plant Transport II Flashcards
transpiration
- evaporation of water from a plant’s surface
- 95% of H2O is lost thru transpiration
how is water replaced in the xylem?
it’s replaced by bulk flow of water & minerals called** xylem sap**, from the steles of roots to the stems & leaves
at night transpiration is ____, which leads to
very low; root cells continue pumping mineral ions into the xylem of the vascular cylinder, lowering water (osmotic) potential
water flows in from the _____, and generates
root cortex; root pressure
positive root pressure is _____ ? and a _____ ? of xylem bulk flow
relatively weak; minor mechanism
what is gutttation
drops on edge of leaf as the result of root pressure pushing water up when transpiration is off/low; a weak force
explain the transpiration-cohesion-tension mechanism
- water is pulled upward by negative pressure in the xylem
- water vapor in the airspaces moves down its potential gradient and exits the leaf via stomata; lowers psi in the mesophyll
transpiration produces what type of pressure in the leaf, and it exerts what type of force
negative pressure; pulling force on water in the xylem, pulling water into the leaf
How is the transpiration pull on xylem sap transmitted
from the leaves to the root tips; even into the soil solution
transpiration pull is facilitated by ____ of water molecules to each other & ____ of water molecules to cell walls
cohesion; adhesion
water molecules are attracted to
cellulose in xylem cell walls through adhesion which helps offset the force of gravity
the thick walls of the tracheid & vessels do what
they withstand pressure & prevent collapse
cavitation
water expands when frozen & breaks tracheids/vessels, making ain air bubble leading to cavitation
How much of the water a plant loses escapes through stomata
95%
a stoma is flanked by a pair of what, which control the diameter of what by changing shape
guard cells; stoma
What force open and closes stomata, and what uptake occurs
changes in turgor pressure; uptake and loss of K+ ions by the guard cells
When do stomata open and close during the day
open during the day, close at night
What triggers stomatal opening
light, CO2 depletion, internal clock in guard cells
all eukaryotic organisms have internal clocks called
circadian rhythms
What can cause stomata to close during the day time
drought, high temperature, and wind
What hormone is produced in response to water deficiency & causes the closure of stomata
abscisic acid
If the lost water isn’t replaced by sufficient transport of water then what happens
the plant will lose water & wilt; turgor is lost throughout the plant
Another thing that transpiration also results in is
evaporative cooling, which can lower the temperature of a leaf and prevent denaturation of enzymes
Xerophytes
- plants adapted to arid climates
- some complete life cycle during rainy season
- others have leaf modifications reducing transpiration rate
