Water Transport in Plants Flashcards
How do Roots absorb water from soil?
Soil - Plant - Atmosphere
How does water flow thru?
- continuous path
- passive process - no energy expenditure overall
What is involved in water flow?
- different driving forces
- mechanisms of transport
What does soil particles help?
- adsorb and hold water
- compete with plant roots for water
What is matric potential?
in soils, adhesion of water to soil particles
What happens to dry soils?
- remain water cannot be easily absorbed by roots because it is tightly held by the soil particle
- Ψm is negative
- the drier the soil becomes - the more negative the soil becomes
Soil changes
- when soil is at field capacity in wet soil, water pervades all of the channels between soil particles and easily available to roots
- roots absorb water from their immediate environment
- creates air pockets that are ‘refilled’ by water present in nearby larger channels
what happens in extremely dry soils?
- water is tightly bound in smallest channels
- cannot replace water removed by the roots
- large air pockets form
- water is not available to roots
What are continuous in plant tissues?
cell walls
cytosol
What is apoplast?
- continuum of non-living cell walls and extracellular spaces
What is symplast?
- continuum of living cytosol connected by plasmodesmata
Movement of water and soil nurtients start where?
roots
What stimulates root growth?
search for water
what does root growth involve?
- absorption of water & nutrients
- radial transport (water & nutrients)
- vertical transport thru plants
Root hair characteristic
- each root hair is a single cell
Root pathway
- Water enters root hairs by osmosis
- water pass the root, from cell to cell by osmosis; seeps between cells
- water drawn up xylem vessels
what happens when an increase in solute concentration in xylem cells?
cause decrease solute potential compared to cortex
What has a specific carrier proteins which can actively accumulate and concentrate solute inside the root’s vascular cylinder?
Plasmalemma of endodermis
what is endodermis?
- cell layer with Casparian Strip
- cell walls contain suberin
- forms a hydrophobic belt
- separates cortex apoplast from apoplast of root stele
What happens in apoplast?
- water/minerals from soil move into cortex
- movement is passive transport
What happens in Symplast?
- water and minerals may pass from cortex into stele without crossing and membranes only thru symplast because Casparian Strips in endodermis prevent movement in apoplast
what are cells via connected called?
Plasmodesmata
what membranes control access to symplast?
selectively permeable membranes
water/ions enter root stele ONLY
thru the symplast after entering endodermal cells
which part of the root is the most absorptive?
apical growing part
what happens to older roots?
have hydrophobic surface
What makes hydrophobic surface?
containing suberin
what is the movement of fluid in vascular tissues?
driven by water potential differences
what are the opposite ends of xylem?
tracheids (in gymnosperms)
vessels (in angiosperms)
steps of bulk flow in stems/tree trunk
- past endodermal barriers, water and solute leave symplast
- active transport moves ion across membranes increasing ion concentration in apoplast, lower water potential
- water follows water potential gradient and enters apoplast thru osmosis (passive transport) and empty cells
- water carries minerals thru xylem cells of root and stems to leaves
Wood
xylem
bark
xylem cells dead at maturity
problem - water under tension transmits large inward force on xylem walls
lead –> collapse xylem cell walls
solution:
- tracheids and vessel elements have thick secondary walls with spiral thickening
- tracheids and vessel elements are lignified with lignin
- taller plants (trees) have denser wood
problem - water under tension
lead - expansion of air bubbles if present, cause “cavitation” and development of an ‘air-lock- that could impede water flow
solution
- symplastic path in root “filters out” bubbles
- mechanism needed to minimize problem or repair cavitation –> produce a new layer of xylem each year
What is transpiration?
evaporation from leaves
who controls transpiration?
conditions inside the leaf
what does transpiration produce?
negative pressure (tension) in leaf
- pulling force on water in xylem - pulling water into leaf
- reverse of loading water into xylem in roots
Who facilitate transpiration and how?
leaves
- larges volume of air
- stomata
- short cell distances to xylem
Transpiration does what to leaves?
cools leaves by water loss form leaves
how is transpirational water loss minimized?
waxy cuticle of leaves/stems - reduces CO2 flow
- 5% of water loss from leaves thru cuticle
how can transpirational be controlled?
stomata in leaves
What does stomata do?
in epidermis allow gas exchange (CO2 and O2)
water flow (loss)
What does guard cells do?
- control opening and closing of stomata
When would the stomata open?
- light is intense enough to maintain photosynthesis
When would the stomata close?
- too much water is being lost
What happens when stomatal opening?
- proton pump activated by blue light
- pump H+ out of guard cells to epidermal cells (active transport)
- take up K+ (membrane potential) and Cl- ions (cotransport) from nearby cells causing water to flow thru osmosis, causing cells to swell (turgid) and opening stomata
What happens when stomatal closing?
- proton pumps stop pumping H+ in dark
- guard cells lose K+ and Cl- ions (facilitated diffusion) causing water to follow thru osmosis
- guard cells go flaccid causing stomata to close
Elements in water transport in xylem
- transpiration from leaves
- tension - negative pressure potential in xylem
-cohesion and adhesion of water molecules
What does not require energy?
Transpiration, cohesion, tension mechanism
How does water move?
passively toward more negative water potentials
how does mineral ions move?
in xylem sap rise passively in solution
How does transpiration contribute to plant?
- temperature regulation
= cooling plants in hot
How does water and solute mineral transport?
driven by water potential differences
How does water move?
from regions of higher (roots) to regions of lower (in leaves)
