Chapter 36 Flashcards
How does water move up to the top of a 10-story high tree?
Water first enters the roots.
Then moves to the xylem.
Innermost vascular tissue.
Water rises through the xylem because of a combination of factors.
Most of that water exits through the stomata in the leaves.
Long-Distance Movement
Local changes result in long-distance movement of materials
Water and dissolved minerals travel great distances in xylem
Most of the force is “pulling” caused by transpiration.
Evaporation from thin films of water in the stomata.
Occurs due to cohesion (water molecules stick to each other) and adhesion (stick to walls of tracheids or vessels).
Movement of Water at Cellular Level
Water can diffuse down its concentration across a plasma membrane by osmosis
If the cell is placed into a hypotonic solution (concentration of solutes inside cell greater than that of the external solution)
The rate of water movement into or out of cells is enhanced by membrane water channels called aquaporins
Aquaporins speed up water movement across a membrane, but do not change its direction
Aquaporins
Water-selective pores in plasma membrane increase the rate of osmosis by facilitating the diffusion of water
Water Potential
Measured in units of pressure called megapascals (MPa)
is used to predict which way water will move
Water moves from higher to lower
Potentials are a way to represent free energy
Osmosis
If a single plant cell is placed into water.
Water moves into cell by osmosis.
Cell expands and becomes turgid.
If cell placed in high concentration of sucrose.
Water leaves cell.
Cell shrinks – plasmolysis.
Water potential has two components
Physical forces, such as gravity or pressure on a plant cell wall. The contribution of gravity is small, but the turgor pressure is significant. This is the pressure potential
The concentration of solute in each solution determines the solute potential
Total water potential is the sum of its pressure potential and solute potential
Water will always move, via osmosis, in the direction of lower water potential
Pressure Potential
As turgor pressure increases, Wp increases
Solutions that are not contained within a membrane cannot have turgor pressure and always have a Wp of 0 MPa
Turgor pressure generated from fluid within a cell pushing against the cell wall gives a turgid cell a Wp > 0 MPa
Solute Potential
Pure water has a Ws of 0 MPa
As a solution increases in solute concentration, it decreases in Ws making it < 0 MPa
When solutes are added, water molecules interact with the solute molecules.
Fewer free water molecules are available to move, which decreases the water potential.
Determining Water Potential
Ww = Wp + Ws
The total water potential of a plant cell is the sum of its pressure potential and solute potential
Represents the total potential energy of the water in the cell.
When Ww inside the cell equals that of the solution, there is no net movement of water
Water Absorption
Most of the water absorbed by the plant comes in through the region of the root with root hairs
Surface area further increased by mycorrhizal fungi.
Once absorbed through root hairs, water and minerals must move across cell layers until they reach the vascular tissues
Water and dissolved ions then enter the xylem and move throughout the plant
Three transport routes exist through cells
Apoplast route – movement through the cell walls and the space between cells
Avoids membrane transport.
Symplast route – cytoplasm continuum between cells connected by plasmodesmata
Transmembrane route – membrane transport between cells and across the membranes of vacuoles within cells
Permits the greatest control.
Inward Movement of Water
Eventually on their journey inward, molecules reach the endodermis
Any further passage through the cell walls is blocked by the Casparian strips
Apoplast route is blocked by waterproof material called suberin.
Molecules must pass through the cell membranes and protoplasts of the endodermal cells to reach the xylem
Movement of Ions
Mineral ion concentration in the soil water is usually much lower than it is in the plant
Active transport across endodermis is required for increased solute concentration in the stele.
Plasma membranes of endodermal cells contain a variety of protein transport channels
Proton pumps transport specific ions against even larger concentration gradients.
Xylem Transport
The aqueous solution that passes through the endodermal cells moves into the tracheids and vessel elements of the xylem
As ions are actively pumped into root or move via facilitated diffusion, their presence decreases the water potential
Water then moves into the plant via osmosis, causing an increase in turgor pressure
Root Pressure
Caused by the continuous accumulation of ions in the roots at times when transpiration from leaves is low or absent
Often at night.
Causes water to move into plant and up the xylem despite the absence of transpiration
Guttation is the loss of water from leaves when root pressure is high
Root pressure alone, however, is insufficient to explain xylem transport
Transpiration provides the main force.
Regulation of Water Movement
Water potential regulates the movement of water through a whole plant
Water moves from the soil into the plant only if water potential of the soil is greater than in the root
Water in a plant moves along a Ψw gradient from the soil to successively more negative water potentials in the roots, stems, leaves, and atmosphere
Cohesive Water Forces
Water has an inherent tensile strength that arises from the cohesion of its molecules
The tensile strength of a water column varies inversely with its diameter
Because tracheids and vessels are tiny in diameter, they have strong cohesive water forces
The long column of water is further stabilized by adhesive forces
Effect of Cavitation
Tensile strength depends on the continuity of the water column
A gas-filled bubble can expand and block the tracheid or vessel (process called cavitation)
breaks the tensile strength of a water column.
Damage can be minimized by anatomical adaptations
Presence of alternative pathways.
Pores smaller than air bubbles.
Tracheids and vessels are essential for the
bulk transport of minerals
Ultimately the minerals are relocated through the xylem from the roots to other metabolically active parts of the plant.
Phosphorus, potassium, nitrogen, and sometimes iron may be abundant in xylem.
Calcium, an essential nutrient, cannot be transported elsewhere once it has been deposited in a particular plant part.
Rate of Transpiration
Over 90% of the water taken in by the plant’s roots is ultimately lost to the atmosphere
At the same time, photosynthesis requires a CO2 supply from the atmosphere
Closing the stomata can control water loss on a short-term basis
However, the stomata must be open at least part of the time to allow CO2 entry
Guard Cells
Only epidermal cells containing chloroplasts
Have thicker cell walls on the inside and thinner cell walls elsewhere
Bulge and bow outward when they become turgid.
This causes the stoma between two guard cells to open.
Turgor in guard cells results from the active uptake of potassium chloride and malate
Addition of solute causes water potential to drop.
Water enters osmotically and cells become turgid.
Changing Transpiration Rates
Transpiration rates increase with temperature and wind velocity because water molecules evaporate more quickly
Several pathways regulate stomatal opening and closing
Abscisic acid (A B A) initiates a signaling pathway to close stomata in drought.
Opens K+. Cl-, and Malate channels
Water loss makes guard cells flaccid
Also Affecting Stomatal Opening
Close when CO2 concentrations are high
Open when blue wavelengths of light promote uptake of K+ by the guard cells
Close when temperature exceeds 34°C and water relations unfavorable
Crassulacean acid metabolism (C A M) plants conserve water in dry environments by opening stomata and taking in CO2 at night
