Transport in Plants - Notes | Class 11 | Part 2: Plant

Cell Cycle and Cell Division - Mitosis

Plant-Water Relations

Water is a universal solvent.
Protoplasm is mainly water in which different molecules are dissolved and suspended.
Soft plant parts mostly contain water. E.g., watermelon has 92% water.
Herbaceous plants have only 10-15% dry matter.
Dry seeds and woody parts also contain little water.
A mature corn plant absorbs 3 liters of water daily.
A mustard plant absorbs water equal to its own weight in about 5 hours.

Water Potential (Ψw)

It is the potential energy of water, measuring the ability of water molecules to move freely in solution.
It is expressed in pressure units such as Pascals (Pa).
Water molecules have kinetic energy. In liquid and gaseous forms, they show random, rapid, and constant motion.
As the concentration of water in a system increases, its kinetic energy (‘water potential’) also increases. Hence, pure water has the greatest water potential.
Water molecules move from a higher energy system (higher water potential) to a lower energy system (lower water potential). This movement down a gradient of free energy is called diffusion.
Water potential (Ψw) of pure water at standard temperatures, not under any pressure, is zero.
If a solute is dissolved in pure water, water potential decreases due to a decrease in the concentration (free energy) of water. Hence, Ψw of solutions is lower than pure water.
The magnitude of lowering of water potential due to dissolution of a solute is called solute potential (Ψs) or osmotic potential.
Ψs is always negative. The more solute molecules, the lower (more negative) is the Ψs.
For a solution at atmospheric pressure, Ψw = Ψs.
If a pressure greater than atmospheric pressure is applied to pure water or a solution, its water potential increases, equivalent to pumping water from one place to another.
When water enters a plant cell due to diffusion, it causes pressure against the cell wall, making the cell turgid. This increases the pressure potential (Ψp).
Pressure potential is usually positive, though negative potential or tension in the water column in the xylem plays a major role in water transport up a stem.
Water potential of a cell is affected by solute potential and pressure potential. The relationship is:

Ψw = Ψs + Ψp

Osmosis

It is the spontaneous diffusion of water across a differentially- or semi-permeable membrane.
Cell membrane and tonoplast (membrane of vacuole) are important determinants of molecule movement in or out of a plant cell. The cell wall is permeable to water and substances in solution, so it is not a barrier.
Vacuolar sap in the large central vacuole contributes to the solute potential of the cell.
The net direction and rate of osmosis depend on pressure gradient and concentration gradient.
Water moves from a region of higher chemical potential (concentration) to a region of lower chemical potential until equilibrium is reached. At equilibrium, both chambers have the same water potential.

Solution A: High water potential, high solute potential.

Solution B: Low water potential, low solute potential.

Potato Osmometer

Create a cavity in a potato tuber and pour a concentrated sugar solution into it. This setup is called a potato osmometer.
Place it in water. Water enters the cavity due to osmosis.

A Demonstration of Osmosis

A thistle funnel filled with sucrose solution is inverted in a beaker containing pure water.
The sucrose solution is separated from water by a semi-permeable membrane (e.g., egg shell membrane).
Water moves into the funnel, causing the solution level to rise until equilibrium is reached (figure a).
If external pressure is applied from the upper part of the funnel, no water diffuses into the funnel through the membrane (figure b).
This pressure, required to prevent water diffusion, is the osmotic pressure, a function of solute concentration. Higher solute concentration requires greater pressure to prevent diffusion.
Numerically, osmotic pressure is equivalent to the osmotic potential, but with opposite signs: osmotic pressure is positive, and osmotic potential is negative.

Plasmolysis

If an external solution balances the osmotic pressure of the cytoplasm, it is called isotonic.
In an isotonic solution, there is no net flow of water into or out of the cell (water flow is in equilibrium). Such cells are flaccid.
If the external solution is more dilute (higher water potential) than the cytoplasm, it is hypotonic. Cells swell and become turgid in hypotonic solutions.
If the external solution is more concentrated (more solutes) than the cytoplasm, it is hypertonic.

When a cell is placed in a hypertonic solution, water moves from the cell (area of high water potential) to the outside (area of lower water potential), causing the cell to shrink. This is called plasmolysis. Water is first lost from the cytoplasm and then from the vacuole.
During plasmolysis, the cell membrane and protoplast shrink away from the cell wall. Such cells are plasmolysed.
Plasmolysis is usually reversible. When placed in a hypotonic solution, water diffuses into the cell, and the cytoplasm builds up pressure against the wall, known as turgor pressure.
The pressure exerted by the protoplasts due to water entry against the rigid walls is called pressure potential (Ψp). The cell does not rupture due to the rigidity of the cell wall. Turgor pressure causes enlargement and extension growth of cells.

Imbibition

It is a type of diffusion in which water is absorbed by solids (colloids), increasing their volume. E.g., absorption of water by seeds and dry wood.
The pressure due to the swelling of wood can split rocks.
Seedlings emerge from the soil due to imbibition pressure.
Imbibition requires: