Given: Plant cell in hypotonic solution (lower solute concentration outside)

(a) What happens to the cell:

Water moves into the cell by osmosis. The cell swells and becomes turgid (firm).

(b) Why water moves into the cell:

Osmosis: Water moves from high water concentration (low solute) to low water concentration (high solute) across a selectively permeable membrane.

• Outside: hypotonic (more water molecules, fewer solutes)
• Inside: hypertonic relative to outside (less water, more solutes)
• Water potential: $\Psi_{outside} > \Psi_{inside}$

Water moves down its concentration gradient into the cell.

Mechanism:

• Water is polar (partial charges due to bent shape and O-H bonds)
• Moves through aquaporins (water channel proteins)
• Follows concentration gradient until equilibrium

(c) What prevents bursting:

Cell wall provides structural support!

• Made of cellulose (rigid polysaccharide)
• Prevents excessive expansion
• Creates turgor pressure (pressure of cell contents against wall)
• Turgor pressure eventually equals osmotic pressure → equilibrium

Comparison to animal cells:

• Animal cells lack cell walls
• Would lyse (burst) in hypotonic solution
• Plant cells become turgid but don't burst

$\boxed{\text{Cell wall prevents bursting; turgor pressure maintains plant rigidity}}$

Given:

• Cell solute potential: Ψ_s = -0.5 MPa
• Cell pressure potential: Ψ_p = 0.3 MPa
• Solution water potential: Ψ_solution = -0.3 MPa

Step 1: Calculate cell water potential

$\Psi_{cell} = \Psi_s + \Psi_p$

$\Psi_{cell} = (-0.5) + (0.3)$

$\boxed{\Psi_{cell} = -0.2 \text{ MPa}}$

Step 2: Determine direction of water movement

Water moves from higher (less negative) to lower (more negative) water potential.

Compare:

• $\Psi_{cell} = -0.2$ MPa
• $\Psi_{solution} = -0.3$ MPa

Since $-0.2 > -0.3$:

$\boxed{\text{Water moves OUT of the cell into the solution}}$

Step 3: Explanation

The cell has a higher (less negative) water potential than the surrounding solution, so water flows out by osmosis.

What happens to the cell:

• Water exits the cell
• Cell becomes flaccid (loses turgor)
• May undergo plasmolysis (membrane pulls away from cell wall)
• Ψ_p decreases as cell loses water
• Eventually reaches equilibrium when $\Psi_{cell} = \Psi_{solution}$

Key Concept: Water always flows from high Ψ to low Ψ. More negative = lower water potential = less "free" water available.

$\text{Direction: Cell} \xrightarrow{\text{water}} \text{Solution}$