Plant Physiology - Musiyenko, M. M. 2001

Chemical and molecular composition, structure, and functions of the plant cell
Turgor. Plasmolysis

A plant Cell Functions as an osmotic system. The Cell wall is generally fully permeable to solutes, meaning it cannot act as an osmotic barrier. However, plant Cells contain a large central vacuole, The Cell sap of which plays a key role in maintaining cellular osmotic pressure. There are also two vital membranes: the Plasmalemma and the tonoplast. Within the cell, the plasmalemma, Cytoplasm, and tonoplast act collectively as a single semipermeable membrane during osmosis. The magnitude of the osmotic potential is crucial for determining the force that drives Water uptake into the cell. Furthermore, determining osmotic potential is of great importance in ecological research, as its value reflects the maximum capacity of plant cells to absorb water from the soil and retain it under drought conditions.

Note that while the cell wall readily permits the passage of water and nutrients, it has a limited capacity for stretching. As water enters, the protoplast increases in volume, The Plasma Membrane stretches, and pressure on the cell wall rises. Nevertheless, the cell does not rupture because its cell wall is sufficiently rigid. Plant cell vacuoles are filled with cell sap—a solution of salts, sugars, organic acids, Amino Acids, and Other Compounds. Consequently, plant cells continuously absorb water via osmosis, generating internal hydrostatic pressure. This pressure is directed against the cell wall, conferring rigidity and turgidity to the cell. Therefore, the hydrostatic pressure in a plant cell is referred to as turgor pressure. It can be defined as the pressure developed within a cell As a result of osmosis. Turgor pressure is counterbalanced by an equal mechanical pressure exerted by the cell wall inward (pressure potential). Due to the limited extensibility of the cell wall, a point is eventually reached where the wall pressure entirely offsets the osmotic driving force of water influx. Recall that the direction of water movement is governed by The water potential. When a cell wall exerts additional pressure on an aqueous solution, water molecules are brought into closer proximity, increasing the system's energy and, consequently, The activity of the water molecules. Thus, the WATER POTENTIAL OF a cell depends primarily on the concentration of osmotically active solutes (osmotic potential), which is always negative, and the pressure potential, which is positive in most cases:

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If a cell is placed in contact with a hypertonic solution—that is, a solution with a lower water potential than the cell's own interior—water begins to flow out of the cell via osmosis across the plasma membrane. Cytoplasmic water is lost first, followed by the efflux of water from the vacuole through the tonoplast. The volume of the intracellular content decreases, the protoplast enclosed by the plasma membrane shrinks, and eventually, the plasmalemma pulls away from the cell wall (see Fig. 16).

The phenomenon where the protoplast detaches from the cell wall is known as plasmolysis. This process is reversible; if the cell is transferred back to pure water or a hypotonic solution, water will flow back into the cell.

In wilted plants, leaf cell protoplasts do not detach from the cell wall as they do during plasmolysis; instead, they compress and pull the wall inward, causing it to buckle. This phenomenon is termed cytorrhysis. A negative pressure is generated, causing the pressure potential to become negative. In this case, the water potential is determined not by the difference, but by the sum of the osmotic potential and the pressure potential:

Thus, the magnitude of water potential is determined by the degree of cellular Hydration: the lower the water saturation of the cell, the more negative its water potential.

Animal cells lack a cell wall, and their plasma membrane is too delicate to protect the cell against Swelling and rupture in a hypotonic solution. Therefore, animal cells rely on a defense system based on osmoregulation. In the state of plasmolysis, water exerts no pressure on the cell wall, and its counterforce is zero, making the water potential equal to the osmotic potential:

The more water enters the cell, the higher the turgor pressure and the resistance of the cell wall rise. Eventually, a point is reached where the cell wall is stretched to its maximum limit, the osmotic potential is completely counterbalanced by the cell wall resistance, and the water potential drops to zero:

Thus, under conditions of full turgor, the water potential of the cell is zero, whereas in the state of plasmolysis, the pressure potential (ψ_тиску=0) is zero, and the water potential equals the magnitude of the osmotic potential (-ψ = -ψ_осм).

Under normal conditions, the osmotic potential of a cell is not entirely balanced by the counter-pressure of the cell wall, allowing water to continue entering the cell. The difference between the osmotic potential of the cell sap and the resistance of the cell wall dictates the influx of water into the cell at any given moment. Water always moves in the direction of a more negative water potential: from a system where its Free energy is higher to one where its energy is lower.

The cell operates as a self-regulating system. Observing plasmolysis and turgor phenomena allows researchers to study various cellular properties. Plasmolysis demonstrates that a cell is alive and that its protoplast retains semipermeability. The rate and pattern of plasmolysis provide insight into cytoplasmic viscosity. Furthermore, plasmolysis enables the measurement of osmotic potential (the plasmolytic method). Indeed, the external solution that induces plasmolysis in a cell has an osmotic potential equal to that of the cell itself. By knowing the molar concentration of the external solution, one can calculate its osmotic potential, and consequently, the osmotic potential of the cell. The osmotic potential indicates the extent to which dissolved solutes lower water activity.

The osmotic potential can also be determined by measuring the concentration of the cell sap. The osmotic concentration of vacuolar sap in ROOT cells ranges from 0.3 to 1.2 MPa, whereas in the cells of above-ground Organs, it ranges from 1.0 to 2.6 MPa. This establishes a vertical gradient of osmotic concentration and suction pressure extending from the roots to the leaves.