Plant Water and Solute Movement Notes
Movement of Water and Solutes in Plants
Learning Objectives
Understand the three principles of water movement and their mechanisms.
Explain transpiration and its necessity for plants.
Identify the primary location of water loss in plants.
Describe the sequence of events leading to stomatal opening and closing.
Name the triggers influencing stomatal opening and closing (e.g., temperature).
Identify the key solutes involved in stomatal regulation.
Define cavitation and embolism, and determine where they are more likely to occur (tracheids or vessel elements) and why.
Explain root pressure and how it can be observed.
Define hydraulic redistribution.
Mechanisms of Water and Mineral Transport
Diffusion
Osmosis
Bulk Flow
Transpiration drives the transport of water and minerals from the roots to the shoots.
Mechanisms of Transport: Passive Transport
Diffusion
Movement of dissolved particles from an area of higher concentration to an area of lower concentration.
Diffusion is more rapid in:
Gases than liquids
Higher rather than lower temperatures
Smaller rather than larger molecules
Uncharged molecules
Example: In non-photosynthetic cells, O2 is used up quickly, maintaining a steep concentration gradient that draws additional O2 from outside to inside the cell.
Osmosis
Water moves across a selectively permeable membrane:
From a region of:
Higher water potential
Lower solute concentration
Higher osmotic potential
To a region of:
Lower water potential
Higher solute concentration
Lower osmotic potential
Water Potential \psi
Water Potential \psi: The potential energy in water relative to the potential energy of pure water.
Measured in Megapascals (MPa).
Solute concentration and pressure are major drivers of water movement in plants.
If flow isn’t restricted: Water moves from an area of higher water potential to an area with lower potential.
Solute Potential \psi_s
Pure water in an open container = 0 MPa.
Solutes lower \psi; any solute at atmospheric pressure has a negative \psi_s (-\psi chemical potential).
Any 0.1 molar solution = -0.23 MPa.
H_2O moves through a membrane from high (0 MPa) to lower (-0.23 MPa).
Pressure Potential \psi_p
Pressure can be positive or negative.
Positive Pressure
Applying physical pressure.
Negative Pressure
Tension or suction.
Water Potential Equation
\psi = \psip + \psis
Adding solute to one side lowers \psi_s, causing water to move to that side.
Applying positive pressure to one side increases \psi_p, causing water to move to the other side.
Applying negative pressure to one side lowers \psi_p, causing water to move to the first side.
Turgor Pressure
Plant cells normally concentrate high solute concentrations, including salts, sugars, organic acids, and amino acids, which leads to low water potential.
Results in water absorption via osmosis, increasing pressure inside the cell.
In animal cells (without cell walls), this causes cell rupture (lysis).
In plant cells, the rigid cell wall tolerates increased internal pressure.
Turgor pressure: Pressure against the rigid plant cell wall from movement of water into the cell.
Turgor Pressure Provides Stiffness to Plant Cells
Turgor Pressure and Support
Turgor pressure is especially important in supporting non-woody plant parts.
If a turgid plant cell is placed in a solution with lower water potential:
Water leaves the plant cell, resulting in a loss of turgor (plants become flaccid and wilt).
If excessive, the plasma membrane pulls away from the cell wall = Plasmolysis.
Solutions and their effects on cells
Hypotonic solution: Animal cell lyses, Plant cell becomes Turgid (normal).
Isotonic solution: Animal cell is Normal, Plant cell is Flaccid.
Hypertonic solution: Animal cell Shrivels, Plant cell becomes Plasmolyzed (Plasma membrane separated from cell wall).
Bulk Flow
The overall movement of water due to differences in water potential.
Water and solutes move together through the xylem (tracheids and vessel elements).
Water is drawn from the soil up through the plant into the atmosphere.
Water Movement in Roots
Water can move through roots via:
Apoplastic route
Symplastic route
Transcellular route
Apoplastic Route
Water and minerals are taken up by the hydrophilic walls of the root epidermis and diffuse along the permeable cell walls into the root cortex.
The water and minerals encounter the Casparian strip, a waxy barrier in the apoplast that forces anything in the apoplast to cross a cell membrane for filtration before entering the vascular cylinder.
The filtered solution is released back into the apoplast on the other side of the Casparian strip by endodermal cells and living stele cells.
Water and minerals in the stele apoplast enter the xylem (which is dead and part of the apoplast), where it flows by bulk flow up the roots.
Symplastic Route
Water and minerals are immediately filtered as they cross a root hair cell's cell membrane, entering the symplast.
The water and minerals move from cell to cell through plasmodesmata toward the vascular cylinder.
Because these minerals and water are already in the symplast (and so already filtered by a membrane), they bypass the Casparian strip.
Transpiration
Transpiration: Loss of water vapor through plant tissues.
Loss can occur via any aboveground part of the plant body.
Leaves are the major source of water loss:
90% of water loss occurs via stomata.
10% of water loss occurs via the waxy cuticle.
Nearly 90+% of water absorbed by roots is lost via transpiration.
Global transpiration returns 60% of total precipitation to the atmosphere.
Transpiration via Stomata
Pores found in the epidermis of the leaves, stems, and other organs that facilitate gas exchange.
Stomatal transpiration involves two steps:
Evaporation of water from cell walls bordering intercellular air spaces.
Diffusion of resulting water vapor from intercellular spaces into the atmosphere via stomata.
Stomata Density
Stomata occur on all aboveground parts (shoots), but their greatest density is on leaves.
Density varies:
Ex: Ratio of stomata on leaf bottom / stomata on leaf top
Avena sativa (Oat) – 45/50
Zea mays (Corn) – 108/98
Nicotiana tabacum (Tobacco) – 190/50
Quercus velutina (Oak) – 405/0
The Photosynthesis-Transpiration Compromise
Plants cannot gain carbon dioxide without simultaneously losing water vapor.
How to get as much CO2 in as possible while retaining maximum H2O?
H2O diffuses 1.6x faster than CO2.
H2O leaving influences CO2 entering.