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Wize University Biology Textbook > Plant Physiology

Water Transport

Plant Tissue Systems and OrgansPrevious Section
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Water Transport



Short Distance Water Transport

  • Plants use active and passive transport of solutes
  • Plants use primarily Hydrogen pumps
  • Not Sodium pumps as in animals
  • Plants use ATP to pump Hydrogen against its gradient
  • Use this Hydrogen gradient to take in neutral solutes (e.g. sucrose) through cotransport
  • Cotransport: coupled transport of substance A using the energy from the favorable movement of substance B down its concentration gradient
  • Plants gain or lose water through osmosis
  • Osmosis: diffusion of free water across a membrane
  • Water Potential: physical property that predicts which way water will move across a membrance
  • Depends on solute concentration and pressure
  • Water moves from high water potential to low water potential Example: If a plant is put into a solution with a lower water potential than the plant, water will move out of the plant
  • The greek letter Psi (Ψ\Psi) is used to symbolize water potential
  • Ψ\Psi uses the units MPa (megapascal), a unit of force
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Water Potential Equation

  • Water potential is determined using the water potential equation:
Ψ=ΨS+ΨP\boxed{\Psi=\Psi_S+\Psi_P}
  • Ψ\Psi = Water potential
  • Describes which way the water will move
  • Negative values = water leaving the cell
  • Positive values = water entering the cell
  • ΨS\Psi_S = Solute potential
  • Proportional to the molarity
  • Also called osmotic potential
  • ΨP\Psi_P = Pressure potential
  • Physical pressure of the water
  • Can be positive or negative
Photo by Katiestu | CC BY
Wize Tip
Water potential refers to the waters potential energy, so if a solution has HIGH water potential, that means the solution can do work (like push water into or out of a cell). This is the same as a ball on a hill, the ball has high potential energy and can use that energy to roll down the hill.


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Effects of water on plant cells

  • Flaccid: plant is limp, wilted
  • Water has left the cell
  • Occurs when you put a plant in a solution with greater solute concentration Example: when you forget to water your plants and they become wilted
  • Plasmolysis: the act of the plant cell shrinking and pulling away from the rigid cell wall
  • Turgid: plant is firm
  • Water has entered the cell
  • Occurs when you put a plant in a solution with less solute concentration Example: when you put wilted flowers in a vase with pure water and they perk up


Photo by LadyofHats | CC BY

Long Distance Water Transport

  • Bulk Flow: water travels long distance due to a pressure gradient
  • Does not rely on solute concentration
  • Moves through the Xylem and Phloem
checklist
Mark Yourself Question
  1. Grab a piece of paper and try this problem yourself.
  2. When you're done, check the "I have answered this question" box below.
  3. View the solution and report whether you got it right or wrong.
Calculate the water potential of a plant cell where the solute potential is -0.5 MPa and the pressure potential is -0.3MPa.
What direction will water move?
When placed into an unkown solution, a plant went from being flaccid to turgid.
What was the solution?
Extra Practice
Practice: Membrane Transport 2
Sea urchin eggs are isotonic to seawater. Consider these conditions:
a) Eggs are in seawater.
b) Eggs are in 65% seawater (35% distilled water).
Practice: Membrane Transport 2
Sea urchin eggs are isotonic to seawater. Consider these conditions:
a) Eggs are in seawater.
b) Eggs are in 65% seawater (35% distilled water).
Practice: Membrane Transport 2
Sea urchin eggs are isotonic to seawater. Consider these conditions:
a) Eggs are in seawater.
b) Eggs are in 65% seawater (35% distilled water).
Practice: Membrane Transport 2
Sea urchin eggs are isotonic to seawater. Consider these conditions:
a) Eggs are in seawater.
b) Eggs are in 65% seawater (35% distilled water).
Water lettuce is a type of plant that grows on top of freshwater ponds. Their cells are used to water with little to no solutes in them. What would happen if you place them in seawater instead? Select all that apply
Practice: Membrane Transport 2
Sea urchin eggs are isotonic to seawater. Consider these conditions:
a) Eggs are in seawater.
b) Eggs are in 65% seawater (35% distilled water).
Why does water travel up a tall tree?
Which is the hardest route for water to travel in plant tissues?
What would happen to both an animal and a plant cell if they are placed in a hypotonic solution?
What would likely happen to an animal cell in a hypotonic solution?
What would happen to both an animal and a plant cell if they are placed in a hypotonic solution?
Which of the following would cause a plant cell to become turgid?