Q.Water movement between the cells is due to .............

Water Potential: The "Thirst" of Water

Imagine you have two glasses of water. One is pure water, the other has a spoonful of sugar dissolved in it. If you connect them with a straw, which way does the water move? Your intuition probably says: from the pure water into the sugar water. But why? The water molecules in the pure glass are "freer" — they have more freedom to move around. The sugar molecules get in the way, trapping some water molecules and reducing their ability to escape.

That "freedom" is what water potential measures. It is the potential energy of water per unit volume — essentially, how eager water is to move from one place to another. Water always moves from where it has higher potential (more free energy) to where it has lower potential (less free energy), until equilibrium is reached.

Where:

• $\Psi_w$ = water potential (measured in pressure units, typically MPa or bars)
• $\Psi_s$ = solute potential (always negative or zero)
• $\Psi_p$ = pressure potential (can be positive, negative, or zero)

The Reference Point: Pure Water

By convention, pure water at standard conditions (1 atm pressure, room temperature) has a water potential of zero. This is the maximum possible value. Anything that reduces the free energy of water makes its potential negative.

Think of it like altitude. Sea level is zero. Everything else is either above (positive) or below (negative). Water flows downhill in the energy landscape, from higher potential to lower potential.

The Two Components

Solute Potential ($\Psi_s$)

When you dissolve anything in water — sugar, salt, ions — the solute particles surround themselves with water molecules (hydration shells). This "ties up" some water molecules, reducing their freedom to move. The more solute particles, the lower (more negative) the solute potential.

The formula for solute potential (also called osmotic potential) is:

Where $i$ is the ionization constant (number of particles a solute dissociates into), $C$ is molar concentration, $R$ is the gas constant, and $T$ is absolute temperature. For a non-electrolyte like sucrose, $i = 1$. For NaCl, $i \approx 2$.

Pressure Potential ($\Psi_p$)

This is the physical pressure exerted on the water. In a plant cell, the cell wall pushes back against the expanding protoplast — this is turgor pressure, and it is positive. In the xylem (water-conducting vessels), tension from transpiration creates negative pressure.

Putting It Together: The Direction of Flow

Water moves from higher $\Psi_w$ to lower $\Psi_w$. Here is the key insight: pure water has the highest possible water potential (zero). Everything else is lower.

Consider a plant cell:

• Pure water outside: $\Psi_w = 0$
• Cell interior: $\Psi_s = -0.9$ MPa, $\Psi_p = +0.4$ MPa, so $\Psi_w = -0.5$ MPa

Water moves from outside (0) into the cell (-0.5). As water enters, the cell swells, $\Psi_p$ increases, and eventually $\Psi_w$ inside equals $\Psi_w$ outside — equilibrium.

• Water potential is always negative in biological systems (except pure water at zero).
• Water moves down the potential gradient (high to low).
• The more negative the water potential, the "thirstier" the system.
• Solute potential always lowers water potential; pressure potential can raise or lower it.

A Concrete Example

Take a raisin (dried grape) and put it in a glass of water. The raisin's cells have concentrated sugar solutions — very negative $\Psi_s$, and since the raisin is flaccid, $\Psi_p \approx 0$. So $\Psi_w$ of the raisin is very negative. The surrounding water has $\Psi_w = 0$. Water moves into the raisin, it swells, and becomes a plump grape again.

Now reverse it: put a fresh grape in concentrated sugar syrup. The syrup has very negative $\Psi_s$ (and $\Psi_p \approx 0$), so its $\Psi_w$ is very negative. The grape's cells have $\Psi_w$ around -0.5 to -1.0 MPa. Water moves out of the grape into the syrup. The grape shrivels.

This is why you cannot drink seawater — the salt makes its water potential so negative that your cells would lose water rather than gain it.

As part of the Transport in Plants chapter in NCERT Class 11 Biology, Water Potential is a well-established fixture of the CBSE biology syllabus, and searches like "Water Potential class 11 important questions" or "Water Potential diagram" reflect how often this plant-physiology topic is tested in school and NEET exams.