Cell as an Osmotic System!
The funnel described in the experiment (Fig. 1.2) may be compared roughly with the vacuolated plant cell. In such a cell, however, two layers—the cell wall and the layer of cytoplasm just within it—are placed between the cell sap and the environment.
The cell wall is permeable to water and solutes, but the cytoplasmic lining acts as a differentially permeable membrane.
Osmosis posses general problems of water balance in cells. A cell in a medium more dilute than its own solute concentration tends to swell up through osmotic absorption of water whereas in a medium more concentrated than its own, the cell tends to shrink owing to outward movement of water. Plant cells handle this problem of water balance in a simple way.
The cells are restrained from swelling by inwardly exerted cell wall pressure. The water pressure exerted by the cell contents against the cell wall is called turgor pressure. Turgor pressure is the actual pressure exerted by the protoplast of the turgid cell against the cell wall, while osmotic pressure, often used erroneously as a synonym of turgor pressure, is actually the maximum pressure which can be developed in the cell sap solution separated from pure water by a rigid membrane which is permeable to only water (Fig. 1.4).
A cell, the protoplasm of which exhibits turgor pressure, is said to be turgid or to possess turgidity. Turgidity is responsible for the rigidity of most soft tissues of plants, such as leaves and young stems, and is essential to their mechanical support.
When turgor pressure is lost these organs become limp and flabby, and are said to wilt. The ability of the plant cells to maintain a turgor pressure through osmotic absorption of water eliminates the need for a rigid skeleton to support the soft parts of the plant. Ordinarily, then, osmosis is not the cause of water balance difficulties in plants.
The amount of cell expansion is evidently determined by two factors, the osmotic concentration of its cell sap (which tends to draw water into the cell with a certain osmotic pressure) and the resistance of the wall to stretching (due to its wall pressure). The ability of a cell to absorb water (sometimes called as suction pressure or diffusion pressure deficit) is the osmotic pressure of its sap minus its wall pressure. Thus:
Diffusion pressure deficit (DPD) = Osmotic pressure (OP) – wall pressure (WP).
Where the wall pressure equals the osmotic pressure, no more water will enter the cell even though its osmotic concentration is higher than that of adjacent cells. A cell with a relatively low osmotic pressure but also a low wall pressure may absorb water from one which has higher osmotic pressure but also a higher wall pressure. Thus:
Osmotic Pressure (OP) = Turgor pressure (TP) + Suction pressure (SP)
Turgor Pressure (TP) = Osmotic pressure (OP) – Suction pressure (SP)
Suction Pressure (SP) = Osmotic pressure (OP) – Turgor pressure (TP)
The graph shows interrelationship of different types of pressures when a cell is placed in pure water. When osmotic pressure and turgor pressure are equal in magnitude, the diffusion pressure deficit (DPD) is zero. Hence —
DPD = OP – TP
The above mentioned relationships indicate that:
1. DPD of a cell before placing it in water would be equal to its OP or when a cell is flaccid its DPD = OP
2. When a cell is placed in water, its turgor pressure would be lesser than its osmotic pressure. Hence, DPD would exist and water would enter the cell, and turgor pressure would increase.
3. The increasing turgor pressure would lower the osmotic pressure of the cell.
4. When the increasing turgor pressure becomes equal to that of decreasing osmotic pressure, the entry of water into the cell would stop. This is called turgid condition of the cell. This can be expressed as —
OP – TP = 0 = DPD = 0
5. The entry of water into the cell is dependent on DPD and not OP alone. This can be exemplified as under:
A cell (A) with OP = 12 and TP = 5 is surrounded by the cells (B) with OP = 14 and TP = 12 then for cell A DPD = OP – TP
= 12 – 5 = 7
for cell B DPD = OP – TP
= 14 – 12 = 2
Since the DPD of cell A is more, it has less water and, therefore, water would diffuse from cell B into the cell A. The entry of water into the cell A would stop when DPD of both the cells becomes equal. In this way, water moves from a cell with less DPD into the cell with more DPD. Thus, the diffusion pressure deficit is the osmotic parameter determining the flow of water from one cell to another.
In view of above discussion, Diffusion Pressure Deficit (DPD) may be defined as under:
Each liquid has a specific diffusion pressure. Pure water has the maximum diffusion pressure. The solution prepared by dissolving solute (such as sugar or salt) in pure water has lesser diffusion pressure as compared to pure solvent or water (though the solution has higher osmotic pressure).
In this way, there is always a difference between the diffusion pressure of solvent and its solution. Therefore, diffusion pressure deficit (DPD) may be defined as “the difference between the diffusion pressures of a solution and a pure solvent, when both are subjected to the same atmospheric pressure.” To remove this deficit, the solution would absorb more solvent molecules. In this way, diffusion pressure deficit is the water absorption capacity of a solution. Therefore, DPD can also be called suction pressure.
Numerical Problems:
1. X and Y are the two adjacent cells in which osmotic diffusion of water can occur. Cell X has OP = 15 atm. pr. and TP = 8 atm. pr. and cell Y has OP = 11 atm. pr. and TP = 3 atm. pr. What is the direction of flow of solvent?
DPD of cell X = 15 – 8 = 7 atm. pr.
DPD of cell Y = 11 – 3 = 8 atm. pr.
Movement of water takes place from cell X to Y i.e., from lower DPD to higher DPD.
2. Osmotic pressure and turgor pressure in cell A are 25 atm. and 15 atm. respectively. But the osmotic pressure and turgor pressure of its adjacent cell B are 30 atm. and 10 atm. respectively. Determine, in which direction water would move?
DPD of cell A = 25 – 15 = 10 atm,
DPD of cell B = 30 – 10 = 20 atm.
We know that water moves from a cell with lower DPD to the cell with higher DPD. In this case DPD of cell A is less than the DPD of cell B. Thus water will move from cell A to cell B.