Water is fundamental to plant life. It serves as a solvent, reactant, transport medium, and a regulator of temperature and turgor. The entry, movement, and utilization of water in plants are governed by key physical processes—diffusion, imbibition, and osmosis. Understanding these, along with osmotic pressure and plasmolysis, is essential in plant physiology.
1) ROLE AND SIGNIFICANCE OF WATER
- Turgidity: Water maintains cell turgor, giving mechanical strength to non-woody tissues.
- Cell expansion: Turgor pressure drives cell enlargement and organ growth.
- Solvent and medium: Most metabolic reactions occur in aqueous phase.
- Reactant: Water splits in photosynthesis to provide electrons and protons.
- Enzyme function: Proper hydration maintains enzyme conformation and activity.
- Transport: Dissolved minerals move in the xylem with the transpiration stream.
- Temperature regulation: High specific heat and evaporative cooling stabilize plant temperature.
- Stomatal function: Guard-cell hydration status regulates stomatal aperture and gas exchange.
2) DIFFUSION
Definition: Passive movement of molecules or ions from a region of higher concentration to a region of lower concentration until equilibrium is reached.
- Energy independent (no metabolic energy required).
- Driven by the concentration gradient and random molecular motion.
- Rate increases with higher temperature, steeper gradient, and shorter diffusion distance.
- Exchange of O2 and CO2 through stomata and lenticels.
- Water vapor diffusion during transpiration.
3) IMBIBITION
Definition: Absorption of water by hydrophilic colloids (e.g., cellulose, pectins, proteins), resulting in swelling and increase in volume.
- Type of diffusion where water moves along a matrix potential gradient into dry colloids.
- Generates large imbibitional pressure, capable of producing significant mechanical effects.
- Seed imbibition: Initiates germination by hydrating enzymes and macromolecules.
- Initial water entry into root tissues and dry plant parts.
- Mechanical effects: Swelling pressures aid natural processes like seed coat rupture.
4) OSMOSIS
Definition: Net movement of water across a semipermeable membrane from a region of higher water potential (dilute solution) to lower water potential (concentrated solution).
- Semipermeable membrane (plasma membrane, tonoplast).
- Difference in water potential (solute concentration or pressure).
- Endosmosis: Water enters the cell; cell becomes turgid (e.g., raisins in pure water).
- Exosmosis: Water leaves the cell; cell shrinks (e.g., cells in strong salt/sugar solution).
- Hypotonic: External solution more dilute than cell sap → endosmosis → turgidity.
- Isotonic: External solution with equal solute potential → no net water movement.
- Hypertonic: External solution more concentrated → exosmosis → shrinkage/plasmolysis.
- Magnitude of water potential difference (solute and pressure components).
- Membrane permeability and surface area.
- Temperature (affects kinetic energy and diffusion rate).
- Cell metabolic status (affects maintenance of gradients via transporters).
5) SIGNIFICANCE OF OSMOSIS IN PLANTS
- Water absorption: Root hairs absorb soil water predominantly by osmotic gradients.
- Turgor maintenance: Essential for mechanical support in herbaceous plants.
- Cell enlargement and growth: Turgor-driven wall stretching enables elongation.
- Stomatal regulation: Osmotic changes in guard cells open/close stomata, controlling transpiration and gas exchange.
- Short-distance transport: Water and solutes move cell-to-cell through membranes and plasmodesmata via osmotic/pressure gradients.
- Seedling vigor: Rapid hydration sets up internal water relations for early growth.
- Stress response: Osmotic adjustment (accumulation of compatible solutes) helps maintain turgor during drought/salinity.
6) OSMOTIC PRESSURE
Definition: The hydrostatic pressure required to prevent the net influx of water across a semipermeable membrane separating a solution from pure water. It reflects the solution’s tendency to draw water in.
- Greater solute concentration → higher osmotic pressure (greater “pull” for water).
- In plant water relations, osmotic pressure relates inversely to solute potential (more negative solute potential corresponds to higher osmotic pressure demand to stop osmosis).
- Determines direction and magnitude of water movement between cells and their environment.
- Helps interpret cell sap concentration and drought/salinity tolerance (via osmotic adjustment).
- Important for calculating water potential gradients in tissues and across membranes.
7) PLASMOLYSIS
Definition: Contraction of the protoplast and its separation from the cell wall due to water loss by exosmosis when a living cell is placed in a hypertonic solution.
- Incipient plasmolysis: The plasma membrane just starts to detach from the wall; turgor pressure ≈ 0.
- Full (evident) plasmolysis: The protoplast retracts markedly; strands of cytoplasm may remain attached.
- Concave plasmolysis: Typical, with pockets of separation; generally reversible if returned to hypotonic medium.
- Convex plasmolysis: Extreme, the protoplast rounds off; usually irreversible and may lead to cell death.
- Re-entry of water and restoration of turgidity when plasmolyzed cells are returned to hypotonic or pure water.
- Extent of recovery depends on duration and severity of the hypertonic treatment.
8) ADVANTAGES / APPLICATIONS OF PLASMOLYSIS
- Demonstrates semipermeability: Classic proof that living cell membranes are selectively permeable to water and not to solutes.
- Determination of cell sap concentration: Point of incipient plasmolysis is used to estimate osmotic potential of cell sap.
- Food preservation: High salt/sugar in pickles, jams causes microbial cells to lose water (plasmolysis), inhibiting growth.
- Experimental tool: Used to study water relations, permeability changes, and effects of stress on cells.
- Agronomic insight: Explains wilting under salinity/drought and guides management of osmotic stress.
9) COMPARATIVE SUMMARY: DIFFUSION vs. IMBIBITION vs. OSMOSIS
| Feature | Diffusion | Imbibition | Osmosis |
| Medium/Membrane | No membrane required; any medium (air/liquid) | No semipermeable membrane; involves solid colloids (matrix) | Requires a semipermeable membrane |
| Driving Force | Concentration gradient (chemical potential) | Matrix potential (affinity of colloids for water) | Water potential gradient (solute + pressure) |
| What Moves | Solutes or gases (and water vapor) | Water absorbed into dry hydrophilic matrix | Water across membrane |
| Energy Requirement | Passive (no metabolic energy) | Passive (physical swelling) | Passive (no metabolic energy) |
| Typical Example | CO2 entry via stomata | Seed swelling during hydration | Root hair water uptake |
10) KEY TERMS AND CLASSIC CLASSROOM EXPERIMENTS
- Water potential (Ψ): Tendency of water to move; decreases with solute addition and increases with pressure.
- Solute potential (Ψs): Component of water potential due to dissolved solutes (more solute → more negative Ψs).
- Pressure potential (Ψp): Hydrostatic pressure component; positive in turgid cells.
- Turgor pressure: Pressure of the protoplast against the cell wall in a turgid cell.
- Incipient plasmolysis: Stage where turgor is just lost; start of membrane detachment.
- Raisin experiment: Raisins swell in water (endosmosis) and shrink in concentrated sugar solution (exosmosis).
- Elodea leaf plasmolysis: Cells in saline solution show retracted protoplasts; deplasmolyze in distilled water.
- Potato osmometer: Sugar solution in a semi-permeable tube inserted into a tuber shows rise due to osmotic influx.
Water relations underpin plant life. Diffusion governs gas and solute spread, imbibition enables hydration of dry matrices, and osmosis drives water flow across membranes to maintain turgor, growth, and stomatal function. Osmotic pressure quantifies the pull for water, while plasmolysis illustrates cellular responses to hypertonic environments and has valuable experimental and practical applications.
