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Plant water relations overview We use the term water potential (Ψw) to describe the chemical potential...

Plant water relations overview

We use the term water potential (Ψw) to describe the chemical potential of water, and our understanding of diffusion of water depends on our understanding of how various factors (such as temperature, pressure, presence of dissolved solutes) affect the water potential in various parts of a system. Since diffusion of a substance tends to occur downward along a chemical potential or free energy gradient, it also is important to understand how such gradients are established.

By definition, at atmospheric pressure the water potential (Ψw)of pure water = 0, and for any aqueous solution at atmospheric pressure Ψw<0.

Water potential can be comprised of several components, but in most cases that we will consider it consists of osmotic or solute potential Ψπand the hydrostatic pressure potential Ψppcan be positive, negative (in which case it is frequently referred to as tension), or zero. Ψπis always negative. The key relationship among these potentials, fundamental to our understanding of cellular water relations is represented by this equation:

Ψw= Ψπ+ Ψp

Ψwis usually described in terms of energy or pressure. Most commonly used is megapascals (MPa).

In solving problems dealing with osmotic quantities and the water potential of plant cells, it is useful to remember the facts presented above and to be aware of certain assumptions that can be made under certain conditions. First, remember the following, which always apply:

  1. Ψw= 0 for pure water at atmospheric pressure.
  2. Because we define aqueous solutions at atmospheric pressure to have a Ψp=0, we know that for a solution in an open vessel Ψw= Ψπ, and is less than zero.

3.  At equilibrium,  Ψwsolution= Ψwcell

4.  We define a cell that is at incipient plasmolysis to be one in which Ψp= 0

And two assumptions frequently used:

  1. The Ψπof a solution in which a cell is placed does not change in the course of an experiment (valid if the volume of the solution is much greater than that of the cell).
  2. The Ψπof a cell doesn’t change in the course of an experiment (valid if cell volume does not change and solutes neither enter nor leave the cell).

Please answer the questions below by reading above text.. please answer in detail. thanks

  1. A plant cell with an osmotic potential of -2 MPa and a hydrostatic pressure potential of 0.8 MPa is placed in a glycerol solution with an osmotic potential of -0.3 MPa. The system is at atmospheric pressure.
  1. What are the initial solute, hydrostatic pressure, and water potentials of the solution and the cell before you place the cell into the solution? (3 pts)
  2. After equilibrium due to water movement has been reached, what will be thehydrostatic pressure potential (Ψp)of the cell?

  1. Consider a leaf cell with the following initial characteristics:
    1. at incipient plasmolysis (flaccid)

Ψπ= -2.6 MPa

ε (elasticity) = 17 MPa

  1. What is the Ψwof the cell under these conditions?
  2. The leaf is then put into a solution of pure water and allowed to come to equilibrium reaching full turgor. What is the finalΨw,Ψpand percent change in volume that the cell underwent?
  3. Would this percent change in volume be MOREor LESSif the elasticity (ε) was 12 instead of 17 MPa?
  4. Consider a cell an elasticity (ε) of 9 MPa that is placed in pure water at atmospheric pressure and undergoes an increase in pressure potential of 1.5 MPa. What is the percent change in volumethat the cell experienced?

please answer the above questions in detail. thank you. please answer all the questions and show some calculations thanks

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