Water Transport in Plants: Xylem Flashcards
(28 cards)
Phloem
tissues primarily are responsible for the movement of nutrients + photosynthesis products
Xylem
tissues primarily are responsible for the movement of water
- plants move water from their roots to shoots through the combination of water potential, evapotranspiration, and stomatal regulation
- not cellular energy used
Water Potential
measure of the potential energy in water based on potential water movement between two systems (difference in potential energy between a water sample and pure water)
- potential of pure water is ZERO
- expressed in pressure-megapascals (MPa)
- can be + or - , and is calculated from the combined effects of solute concentration and pressure
Water Potential Equation
Ysystem = Ys + Yp
WP: Factors
- adding more dissolved solutes will decrease the WP; removing will increase it
- adding more pressure will increase the WP; removing will decrease it
Water Movement
WATER ALWAYS MOVES FROM REGIONS OF HIGH TO LOW WP UNTIL IT EQUILIBRATES THE WP OF THE SYSTEM
WP Equilibrium
at equilibrium there is NO difference in WP on either side of the system meaning:
Ysoil > Yroot > Ystem > Yleaf > Yatmosphere
Solute Potential (Ys)
AKA osmotic potential
- pure water is zero
- as long as the WP of plant root cells is lower than that of the soil, water will move into the root cells via OSMOSIS
- plant cells metabolically manipulate Ys by adding or removing solute molecules to increase water uptake in dry conditions
Pressure Potential (Yp)
AKA turgor potential - may be positive or negative
- positive pressure (compression) increases Yp, while negative pressure (vacuum) decreases it
- plant cells can indirectly manipulate it by manipulating Ys and osmosis
- if a plant increases cytoplasmic solute then Ys will decline and water will move into the cell, causing Yp to increase
- stomatal openings allows water to evaporate, causing Yp to decrease + Ytotal while increasing the WP difference between the water in the leaf and petiole, allowing water to flow into the leaf
IMPORTANT RELATIONSHIP
Ysoil > Yroot > Ystem > Yleaf > Yatmosphere
- must occur for TRANSPIRATION to occur (continuous movement of water through the plant from the soil to the air without equilibrating)
- relies on a WP gradients, where WP decreases at each point as it passes through plant tissue
- is disrupted if soil is too dry, which decreases Ys
- if WP becomes sufficiently lower in the soil than the roots, water will move out of the plant into the soil
Pathways in Roots
water moves through the ground tissues along its WP gradient through one of three possible routes BEFORE entering the xylem:
1. Symplast
2. Transmembrane Pathway
3. Apoplast
Symplast
“shared cytoplasm”; water + minerals move from the cytoplasm of one cell into the next via PLASMODESMATA that physically join different plant cells, eventually reaching the xylem
Transmembrane Pathway
water moves through WATER CHANNELS present in the cell’s PM, from one cell to the next until it reaches the xylem
Apoplast
“outside the cell”; water + dissolved minerals never move through a cell’s PM but travel through their POROUS CELLS surrounding plant cells
Pathways in Roots: FILTRATION
- water + minerals that move into a cell THROUGH the PM has been “filtered” as it passes through water/other channels within the PM
- water + minerals that move via the APOPLAST are not filtered until they reach the ENDODERMIS
Endodermis
layer of cells which separate the vascular tissue (stele in the roots) from the ground tissue in the outer portion of the root
- ONLY PRESENT IN ROOTS
- checkpoint for materials entering the root’s vascular system
Casparian Strip
waxy region made up of SUBERIN present in the endodermal cell walls; forces water + solutes to cross the endodermal cell PM instead of slipping between cells
- ensures only materials required by roots enter the endodermis
- blocks toxic substances and pathogens
Water Movement in the Xylem
there are three - not mutually exclusive - hypotheses that explain movement of water up a plant against gravity. each contribute to the movement of water in a plant
1. Root Pressure
2. Capillary Action
3. Cohesion-Tension
Root Pressure
relies on positive pressure that forms in roots as water moves from the soil into roots
- water moves via osmosis due to the lower solute potential in roots
- water intake increases Yp in the root xylem, “pushing” it up
- in extremes, root pressure can result in guttation
- root pressure can only move water a few meters - not sufficient for movement in tall trees
Guttation
secretion of water droplets from stomata in leaves
- can be cause by closed stomata to prevent water evaporation by root pressure
Capillary Action
tendency of a liquid to move up against gravity when confined in a narrow tube (capillary), forming a meniscus
- occurs due to surface tension, adhesion, and cohesion
- not strong enough to move water up a tall tree
CA: Surface Tension
occurs because hydrogen bonding between water molecules is stronger at the air-water interface than among the molecules within the water
CA: Adhesion
molecular attraction between “unlike” molecules
- (EX: water molecules + xylem cell wall)
CA: Cohesion
molecular attraction between “like” molecules
- (hydrogen bonding between water molecules)
