Transcript Chapter 38 - Workforce3One

Chapter 38
Transport in plants
How does water and mineral movement take
place in plants?
• Water enter through the roots and then moves through
the xylem (combination of different factors) to other
parts
• Major force is ‘pulling’ produced by transpiration
(loss of water/evaporation of water from stomata)
• This pulling involves cohesion (water molecules stick
to each other), adhesion (water molecules stick to
walls of tracheid or xylem vessel) and osmosis
How does water potential regulate
movement of water through plant?
• Potentials represent free energy
• Water potential (Ψw) is used to predict which
way water will move
• Measured in megapascals (MPa)
• Water will move from a cell or solution with
higher potential to a cell or solution with lower
potential- higher osmotic concentration
Calculation of water potential
• Two components of water potential are physical
forces such as plant cell wall or gravity and the
concentration of solute in each solution
• Pressure potential (Ψp) – Turgor pressure resulting
from pressure against cell walls
• Solute potential (Ψs) – is the smallest amount of
pressure needed to stop osmosis. Pure water has 0
value.
• Ψs decreases as solute concentration increases in
solution
• Ψs becomes negative as solute concentration
increases in solution
Calculation of water potential
• The water potential of a plant cell is sum of pressure potential
and solute potential
Ψw= Ψp+ Ψs
• Aquaporins are water-selective pores in plasma membrane that
increase rate of osmosis but do not alter the direction of water
movement
• Water moves into plants if soil water potential is greater than
that of the root and evaporation of water from leaf creates
negative water potential pulling water up the xylem
• Water potential gradients + transpiration = drive
movement of water up the xylem
Water and mineral absorption
• Root hairs and mycorrhizal fungi increase root absorption
surface area for water and minerals
• Minerals are actively transported from soil into roots causing a
greater internal solute potential and attraction for water
• Water and minerals follow three paths to enter vascular tissue
of root (figure 38.8).Apoplast pathway includes movement
through cell walls and spaces between cells
• Symplast pathway is a continuum of cytoplasm between cells
connected by plasmodesmata
• Transmembrane route permits control over what substances
enter and leave (membrane transport between cells and across
membranes of vacuoles within cells)
• In the transport tissues, casparian strips of endodermis control
water and nutrient flow to xylem
How does xylem transport take place?
• Continuous accumulation of ions in roots causes root
pressure (transpiration is low or absent)
• This results in high ion concentration within cells
which in turn causes more water to enter into root
hair cells by osmosis. Thus inspite of no
transpiration water enter into xylem
• When root pressure is high, it can force water up the
leaves – guttation (in form of dew)
• Guttation takes place through special cells present
near ends of veins
What is cohesion-tension theory?
• Bulk flow of water takes place because of cohesion
(hydrogen bonds forming water molecules) –
cohesion gives rise to tensile strength/tension
• The tensile strength of a column of water varies
inversely with diameter of column
• Smaller the diameter of column, the greater the
tensile strength and more flow of water (vessels have
more diameter than tracheids)
• The volume of liquid that can pass a particular point
in a column per second is proportional to r4 where r
is radius of column (greater the radius of column,
more water can be transported over a distance)
What is cavitation?
• Cavitation or embolism- Air bubble formation
disrupts the continuity of water column and tensile
strength (figure 38.10)
• Air bubbles can expand and block transport of
water to next tracheid or vessel.
• Leads to dehydration and death of part or all of the
plant
• Multiple connections among tracheids or vessels
provide alternative pathways minimizing damage
from cavitation
Factors affecting transpiration rates
• More than 90% of water taken in by roots is lost to atmosphere
• Transpiration rates increase with increase in temperature and
wind velocity. Rates decreases with increase in humidity.
• Occurs through stomata with intake of Co2 and release of O2.
• High concentration of Co2, the guard cells are triggered to
close stoma
• Blue light regulates stomatal opening and promote uptake of
K+ by guard cells
• Crassulacean acid metabolism (CAM) is alternative pathway
to reduce transpiration in succulent plants – collect Co2 at
night, decarboxylated during day, allows for photosynthesis in
day time with closure of stomata
How does turgor pressure in guard
cells cause stomata to open and close?
• Turgor in guard cells results in active uptake of K+,
Cl- and malate ions (because of ATP driven H+
pump). (figure 38.12)
• Solute concentration increases with decrease in water
potential and water moves via osmosis into turgid
guard cells. Stomata opens.
• Sucrose accumulates in photosynthetic guard cells (in
day time)
• Lose turgidity in night time due to pumping of
sucrose out of guard cells and guard cells close.
Different adaptations of plants to cope
water stress?
• Closing of stomata, reducing number of stomata,
having stomata located in pits on leaf surface, loss of
leaves, dormancy and covering leaf with cuticles and
woody trichomes.
• Flooding conditions deprived of oxygen: plants form
lenticels and adventitious roots
• Aquatic plants form arenchyma to ensure oxygen
transport to parts of plant below water (figure 38.15)
• Saline water – plants have pneumatophores for gas
exchange and salt secretion
How does phloem transport take place?
• Most carbohydrates formed and plant hormones are
transported from source (where formed) to sink (sites
of utilization) by phloem – translocation
• Transported both up and down- bidirectional
• Active transport of sugars into phloem causes
reduction in water potential (at source)
• Water flows into phloem and turgor pressure drives
contents into sink where sugar is transported into
cells – phloem unloading
• Water diffuses back into xylem to be reused
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