explain diffusion pressure deficit with an example cleanly
Diffusion pressure deficit (DPD), is simply the net force or pressure that causes water to enter a plant cell. It was previously called suction pressure. On the other hand, it can be defined as reduction in diffusion pressure (diffusing power) of pure water due to addition of solute in it. So, its value increases with solute concentration.
The water of the system/ solution /cell which has more value DPD (due to presence of solute in it) cannot diffuse out to other system/solution/cell. Instead, this system diffuses water in itself from other system due its high DPD (water absorbing capacity). This is the reason that the direction of movement of water is from low to high DPD. That is why DPD is defined as the net force or pressure responsible for entry of water in cell.
In case of cell, its value is influence by osmotic pressure and turgour pressure (wall pressure). If a plant cell is placed in pure water, then water enters into it is due to its osmotic pressure (OP), but this entry is further restricted by developing turgour pressure (TP). Therefore, actual pressure due to which water enters into a cell is OP minus TP.
Diffusion pressure deficit (DPD)= Osmotic pressure (OP)- Turgor pressure (TP)
The condition at which OP equalises TP, DPD becomes zero, that means the cell becomes fully turgid and stops taking in water.
Let Us understand this with the help of example.
Cell A has osmotic pressure 10 atm and turgor pressure 7atm while Cell B has osmotic pressure 8 atm and turgor pressure 3 atm. Both of these cells are contiguous. The value of diffusion pressure deficit in cell A and B will be 3 atm and 5 atm respectively (because, Diffusion pressure deficit = Osmotic pressure -Turgor pressure ). The direction of movement of water will be from low DPD to high DPD site, i.e from Cell A to cell B.
To know more about osmotic pressure and turgour pressure
Osmotic pressure is the pressure that is required to be applied to stop water from diffusing in through semi permeable membrane. It depends on the solute concentration. More the solute concentration more is the osmotic pressure. Numerically it is equal to osmotic potential but of opposite sign(positive). Osmotic pressure decreases with the movement of water in a cell. Thus, higher the solute concentration of cell, higher will be osmotic pressure and so higher will be value of DPD.
Turgour pressure is the outward hydrostatic pressure that is generated within the cell against cell wall as a result of entry of water into it due to osmosis. It increases with the movement of water in a cell and opposes endosmotic entry of water. Thus, developing turguor pressure decreases DPD of plant cell.