Three cells A, B and C are joined in linear manner. Demonstrate the direction of movement of water in them.

Key Points

Osmotic pressure (OP)

• It is the pressure required to prevent solvent from passing through a semipermeable membrane from a region of higher to lower concentration of solute.
• The minimum pressure required to prevent the inward flow of a solution’s pure solvent through a semipermeable membrane.
• If there are two solutions one pure and another impure, separated by a semipermeable membrane then solute from the impure solution will move into the pure solution.
• This is because in impure solution the concentration of solute is more, so solute will flow from high concentration to low concentration.
• If we apply a certain amount of external pressure on only  impure solution, the solute will stop flowing towards the pure solution.
• This amount of external pressure is called osmotic pressure for that solution.
• The more the concentration of solute, the more the osmotic pressure required.

Turgor pressure (TP)

• Turgor pressure is the pressure of the cell contents against the cell wall in plant cells.
• In hypotonic solution, water diffuses into the cell and thereby, cell enlargement and growth occurs.
• As water diffuses in, cytoplasm builds up a pressure against the cell wall. This pressure is called turgor pressure.
• As a result of this, the cell membrane gets stretched and the osmotic pressure of the cell becomes reduced.
• Osmotic pressure become reduced because since some solute moved to pure water, less pressure is now required to stop flow of solutes.
• So, when a cell is kept in pure water, solute from cytoplasm will start moving into the water.
• But the cell and water solution will not become isotonic because the cell wall is applying turgor pressure that prevents the flow of all solutes.

Diffusion Pressure Deficit (DPD)

• DPD is the proportion by which the diffusion pressure varies between two solutions.
• Diffusion pressure deficit (DPD) is the pressure which opposes the diffusion of water from the region of higher chemical potential to lower chemical potential.
• The two factors which determine the DPD are osmotic pressure(OP) and turgor pressure (TP).
• Water is shifting from lower DPD to greater DPD.
• DPD = OP - TP
• In flaccid cell, TP = 0; DPD = OP
• When cell is flaccid, the cell wall does not exert any pressure on cell so turgor pressure is zero.
• DPD is equal to osmotic pressure.
• In turgid cell, TP = OP; DPD = 0
• When a cell is fully inflated, its osmotic pressure becomes equals to turgor pressure, and the diffusion pressure deficit becomes zero.
• At a time When DPD becomes zero, the water cannot enter the cell.

Explanation:

• DPD for A = OP - TP = 50 - 45 = 5
• DPD for B = OP - TP = 40 - 30 = 10
• DPD for C = OP - TP = 30 - 23 = 7
• In the given figure, the DPD of three cells A, B and C can be calculated as 5, 10 and 7 atmospheres respectively.
• Water moves from lower DPD to greater DPD.
• Thus, water from cells A and C will move to cell B.

Therefore, A → B ← C demonstrates the direction of movement of water.