Plasmolysis, De-Plasmolysis, and Water Absorption in Plants – Definition, Process & Importance

Study notes August 10, 2025

 

1. Introduction

Plants rely heavily on water for survival, growth, and physiological functions. Understanding processes like plasmolysis, de-plasmolysis, and the mechanisms of water absorption helps explain how plants maintain water balance and survive in different environmental conditions.

2. What is Plasmolysis?

Plasmolysis is the shrinkage of a cell’s protoplast away from the cell wall when placed in a hypertonic solution. This occurs due to exosmosis, where water exits the cytoplasm and central vacuole.

Stages of plasmolysis:

  • Limiting plasmolysis: The cell becomes flaccid as its pressure potential (Ψp) reaches zero, and internal osmotic concentration equals the external solution.

  • Incipient plasmolysis: The protoplast starts retracting from the cell wall corners.

  • Evident plasmolysis: Protoplast detaches except at one or few points, leaving spaces between it and the cell wall.

3. What is De-Plasmolysis?

De-plasmolysis is the reverse of plasmolysis, where a plasmolysed cell swells again when placed in a hypotonic solution. This happens via endosmosis as water re-enters the cell. It is only possible immediately after plasmolysis; prolonged plasmolysis can cause permanent cell damage.

4. Importance of Plasmolysis

  • Proves that the cell membrane is semi-permeable.

  • Shows that the cell wall is elastic and permeable.

  • Helps measure osmotic pressure of cells.

  • Used to determine whether a cell is living or dead.

  • Agricultural applications such as weed control by salting and preservation of pickles and jams by preventing microbial growth.

  • Highlights risks of overusing chemical fertilizers.

5. Absorption of Water in Plants

Plants mainly absorb water from the soil through their roots. The process depends on soil composition and root anatomy.

5.1 Soil Composition

  • Inorganic Mineral Particles: Sand, silt, and clay formed by rock weathering.

  • Organic Matter (Humus): Improves aeration and water retention.

  • Soil Water: Exists as gravitational, capillary, hygroscopic, and chemically bound water.

  • Soil Air: Essential for root respiration.

5.2 Root Anatomy

  • Epidermis: Outermost layer with root hairs for water absorption.

  • Cortex: Stores water and nutrients.

  • Endodermis: Controls movement of water via Casparian strips.

  • Stele: Contains xylem (water transport) and phloem (food transport).

5.3 Pathways of Water Movement

  1. Apoplast Pathway: Water moves through cell walls and intercellular spaces.

  2. Symplast Pathway: Water moves through cytoplasm connected by plasmodesmata.

  3. Vacuolar Pathway: Water passes from vacuole to vacuole between cells.

6. Mechanism of Water Absorption

Active Absorption: Requires metabolic energy; can be osmotic or non-osmotic.
Passive Absorption: Driven by transpiration; root cells are passive in the process.

7. Factors Affecting Water Absorption

External (Soil) Factors:

  • Soil Temperature: Low or high extremes reduce absorption.

  • Soil Aeration: Poor aeration slows root activity.

  • Soil Solution Concentration: High solute levels can cause water loss from roots.

  • Water Availability: Best absorption occurs between field capacity and permanent wilting point.

Internal (Plant) Factors:

  • Root System Size: Larger root systems absorb more water.

  • Transpiration Rate: Higher transpiration increases water uptake.

8. Conclusion

Understanding plasmolysis, de-plasmolysis, and water absorption helps in agriculture, plant physiology, and environmental science. Proper soil management and balanced irrigation are essential for optimal plant water uptake.

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