I. Measuring Water Potential in Plants: PRE-LAB QUESTIONS What is the definition
ID: 164705 • Letter: I
Question
I. Measuring Water Potential in Plants:
PRE-LAB QUESTIONS
What is the definition of water potential?
Water potential is expressed in units of __________. (Fill in the blank.)
Write the equation for plant water potential and explain the equation in one short sentence.
What is the water potential of pure (i.e. distilled) water in an open vessel at standard atmospheric pressure?
Is the water potential of a solution (i.e., water with something dissolved in it) greater or less than that of pure water at standard atmospheric pressure?
Define and briefly explain the term “osmosis”. Include the term “semipermeable membrane” in your explanation.
A semipermeable membrane is made into a small bag and is filled with a sugar solution. The membrane is permeable to water but not to the solute (sugar). What will happen to the bag and its contents after it is placed in an open container of pure water? Explain fully.
How can plants regulate their loss of water under hot and dry (i.e. water limited) environmental conditions? List at least 2 mechanisms.
Explanation / Answer
1. Water potential is the potential energy of water per unit volume relative to pure water in reference condition. It is the measure of relative tendency of water to move from one area to another and is represented by the Greek letter Psi.. It is the measure of free energy available to do work. Water moves from the region of higher water potential to region of lower water potential.As it moves down its potential gradient it releases free energy which allows it to do work. It is caused by Osmosis, Gravity, Mechanical Pressure
Its expressed in the units of MPa (Megapascal)
Water potential = Pressure potential + solute potential+ gravitational potential+ metric or capillary potential, - As solute is added water potential of the solution drops and water tends to enter the solution.
Water Potential for pure water is Zero.
It will be less as pure water has highest water potential, because it has highest concentration of water molecules.
Osmosis is defined as the diffusion of water across a semi-permeable membrane from an area of lower concentration of impermeable solute to an area of higher concentration of impermeable solute to equalize the concentrations.
The bag will inflate with water until water no longer fills the bag due to physical constraints. This would occur as the solution in the bag has higher solute concentration. In order to try to equlize the concentrations and equalize the water potential, pure water flows into the bag.
Plants regulate the water loss by the opening and closure of the stomata.Opening of the stomata allows the diffusion of the gases as well as transpiration,while closure prevents diffusion of gases as well as transpiration.