Water potential and osmosis — plasmolysis and turgidity in plant cells

Question

Define water potential and its components. Explain osmosis, plasmolysis, and turgidity in plant cells. What happens when a plant cell is placed in hypotonic, hypertonic, and isotonic solutions?

(NCERT Class 11 — frequently asked in NEET)

Solution — Step by Step

Water potential ( $\Psi$ ) is the potential energy of water in a system compared to pure water at standard conditions. Pure water has a water potential of zero.

\Psi = \Psi_s + \Psi_p

$\Psi_s$ = Solute potential (osmotic potential) — always negative (adding solutes lowers water potential)
$\Psi_p$ = Pressure potential (turgor pressure) — usually positive in turgid plant cells

Water always moves from higher water potential to lower water potential (down the gradient).

Osmosis is the movement of water molecules through a selectively permeable membrane from a region of higher water potential to a region of lower water potential.

The cell membrane and tonoplast act as selectively permeable membranes. The cell wall is freely permeable (it doesn’t restrict water movement).

Hypotonic solution (external water potential > cell water potential):

Water enters the cell by osmosis
The cell swells and becomes turgid
The cell wall prevents bursting (unlike animal cells, which lyse)
Turgor pressure ( $\Psi_p$ ) increases until equilibrium: $\Psi_{cell} = \Psi_{solution}$

Hypertonic solution (external water potential < cell water potential):

Water leaves the cell by osmosis
The cell shrinks
The cell membrane pulls away from the cell wall → plasmolysis
The space between membrane and wall fills with the external solution

Isotonic solution (equal water potential):

No net water movement
Cell is flaccid (neither turgid nor plasmolysed)

Plasmolysis is the shrinking of the protoplast (cell contents) away from the cell wall in a hypertonic solution. The point at which the membrane just begins to pull away is called incipient plasmolysis.

Deplasmolysis is the recovery — when a plasmolysed cell is placed back in a hypotonic solution, water enters and the cell regains turgidity.

Plasmolysis is reversible (initially) but becomes irreversible if the cell is left in the hypertonic solution too long (cell death).

Why This Works

Water potential is a quantitative way to predict water movement. The equation $\Psi = \Psi_s + \Psi_p$ captures the two main forces: solutes attract water (lowering $\Psi$ ), and pressure pushes water (raising $\Psi$ ). In a turgid cell, the positive turgor pressure balances the negative solute potential, and the cell reaches equilibrium.

Plant cells don’t burst in hypotonic solutions (unlike animal RBCs) because the rigid cell wall exerts an inward pressure (wall pressure) equal to turgor pressure. This wall pressure-turgor pressure balance is essential for maintaining plant structure — wilting happens when cells lose turgor.

Alternative Method — Direction of Water Flow

For NEET, the simplest rule: water moves from high $\Psi$ to low $\Psi$ . If a cell has $\Psi = -5$ bar and the solution has $\Psi = -2$ bar, water moves from solution (-2, higher) into cell (-5, lower). Remember: on the number line, -2 is greater than -5. This trips up students who think “5 is bigger than 2.”

Common Mistake

The most common error: students say “water moves from low concentration to high concentration.” This is vague and confusing. The correct statement: water moves from higher water potential to lower water potential (or equivalently, from lower solute concentration to higher solute concentration). Also, don’t confuse plasmolysis (happens in plant cells placed in hypertonic solution) with crenation (shrinking of animal cells in hypertonic solution). Plasmolysis involves the membrane pulling away from the wall — crenation has no wall involved.

Question

Solution — Step by Step

Why This Works

Alternative Method — Direction of Water Flow

Common Mistake

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