Plasmolysis: Definition, Types, Forms, Examples and Significance

Introduction to Plasmolysis

Plasmolysis was first defined by de Vries, who developed method to measure turgor pressure of plants using hypertonic solutions.

In nature, molecules and ions are continually in motion. They have natural tendency to distribute uniformly within available space.

They move from regions of higher concentration to lower concentration through process of diffusion to achieve state of equilibrium.

One of types of diffusion is osmosis, which involves movement of water molecules from region of higher water potential to lower water potential.

Osmosis and Turgor Pressure in Plant Cells

When cell is placed in hypertonic solution, water flows out of cell, causing cytoplasm to contract and detach from cell wall.

This phenomenon results in visible shrinkage of protoplasm and is known as plasmolysis. During plasmolysis, plant cell shrinks as result of water deficit.

What is Plasmolysis

Plasmolysis is biological process in which cytoplasm of plant cell shrinks away from cell wall due to loss of water when water potential inside cell is greater than that of outside. This occurs due to movement of water as result of osmosis.

• Plasmolysis occurs when plant cells are exposed to hyperosmotic solutions, such as those containing substances like sucrose, mannitol, or sorbitol.
• In hyperosmotic conditions, water moves out of cell due to higher concentration of solutes outside compared to inside.
• Plant cells contain several specialized cellular organelles which differ from animal cells. Plasma membrane, cytoplasm, and various cell organelles work together to sustain overall vitality of plant.
• Within cytoplasm, fluid-filled vacuoles play vital role in storing water in plant cells.
• Vacuole helps in maintaining turgor pressure (internal pressure) within cell.
• In hyperosmotic conditions, water is released from vacuole, leading to loss of turgor pressure.
• When water is lost, protoplast contracts, separating plasma membrane from cell wall.
• Plasmolysis is reversible process. Addition of hypotonic solutions can expand protoplast again and restore initial turgor pressure.
• Plasmolysis specifically takes place in cells fully submerged in solution, lacking any air spaces around them.

Types of Plasmolysis

Two main types of plasmolysis based on final structure of cytoplasm are concave and convex plasmolysis.

Concave Plasmolysis

• In concave plasmolysis, plasma membrane separates from cell wall and forms small concave pockets.
• Protoplasm shrinks inwardly in concave plasmolysis.
• Process is reversible by placing cell in hypotonic solution by deplasmolysis process.
• Concave plasmolysis suggests higher protoplasmic viscosity or stronger binding between plasma membrane and wall.

Convex Plasmolysis

• In convex plasmolysis, protoplast separates from smaller walls of each cell, forming rounded and symmetrical convex ends.
• Unlike concave plasmolysis, this process is irreversible.
• Convex form indicates relatively lower viscosity or weaker binding between plasma membrane and cell wall.

Plasmolysis Forms

Process of plasmolysis is not uniform across all cells. Different forms occur and these variations depend on type of cell being studied and nature of plasmolytic solution used.

• Incipient plasmolysis - refers to initial stage where plasma membrane just begins to separate from cell wall. It can vary among different cells in tissue. Widely used to measure osmotic pressure of cells.
• Cap plasmolysis - first reported by Kuster (1929) as event induced by prolonged exposure to alkaline salts, particularly potassium (K+). It causes swelling in cell and forms characteristic cap at ends of cylindrical protoplasts.
• Systrophe - phenomenon during plasmolysis where cytoplasm accumulates as ball, often around nucleus or other parts of cell. Systrophe is entirely reversible process and does not seem to cause cellular damage.
• False plasmolysis - also known as stimulative or induced plasmolysis, occurs when cells are suspended in hypotonic solutions. Characterized by plasmolysis-like changes, even though cells are in hypotonic environment. May result from increased permeability of membranes to water.

Examples of Plasmolysis

Plasmolysis rarely occurs in nature and is most often used in laboratory experiments, for example, by submerging plant cells in high-salt or high-sugar solutions.

• Shrinkage of vegetables in hypertonic conditions
• Contraction of blood cells in hypertonic conditions
• Salt deposition onto land during coastal flooding
• Preservation of food with high salt concentrations hindering microorganism growth

Significance of Plasmolysis

Laboratory and Educational Significance

• Used in lab experiments for understanding process of osmosis helping to learn about movement of water in and out of cells.
• Can be used as method for distinguishing between living and dead cells as plasmolysis doesn’t occur in dead cells.

Food Preservation

• Plays crucial role in preserving food items. Salting of food raises osmotic pressure, inducing plasmolysis in bacteria and fungi and prevents food spoilage and extends shelf life.

Agriculture

• Can also be used as tool for weed control. Chemical weedicides induce plasmolysis in weed cells, causing dehydration and eliminating unwanted plants.

Research and Biotechnology

• Extensively used to study and measure various physiological processes like investigating plant cell water relations, determining solute concentrations in individual cells and exploring freezing tolerance.
• Plays fundamental role in tissue culture studies as initial step in isolating protoplasts. Protoplast isolation is crucial for biotechnology, including genetic engineering and plant regeneration.

Summary

Plasmolysis first defined by de Vries to measure turgor pressure using hypertonic solutions involves diffusion and osmosis movement of water from higher to lower water potential. When cell placed in hypertonic solution water flows out causing cytoplasm to contract detaching from wall visible shrinkage of protoplasm. Defined as cytoplasm shrinking away from wall due to water loss when internal water potential greater than outside via osmosis, occurring in hyperosmotic sucrose, mannitol, sorbitol solutions, with plasma membrane, cytoplasm, vacuoles storing water maintaining turgor, releasing water losing turgor contracting protoplast, reversible by hypotonic deplasmolysis, in fully submerged cells. Two main types are concave plasmolysis forming small concave pockets inward shrinkage reversible indicating higher viscosity stronger membrane-wall binding, and convex plasmolysis forming rounded symmetrical convex ends irreversible indicating lower viscosity weaker binding. Other forms include incipient initial separation used to measure osmotic pressure, cap plasmolysis reported by Kuster 1929 from alkaline potassium salts forming cap at cylindrical ends, systrophe cytoplasm accumulating as ball around nucleus reversible without damage, and false stimulative induced plasmolysis in hypotonic solutions from increased water permeability. Rare in nature used in lab with high salt sugar causing vegetable shrinkage, blood cell contraction, salt deposition during coastal flooding and food preservation hindering microbes. Significance includes teaching osmosis, distinguishing living dead cells, food preservation by salting inducing plasmolysis in bacteria fungi, weed control via weedicides, studying water relations solute concentration freezing tolerance and protoplast isolation for genetic engineering.

References

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