Why Ice Floats:

Water has maximum density at 4°C. When water freezes at 0°C, it forms a crystalline structure with hydrogen bonds holding molecules in fixed positions, creating more space between molecules.

Key Points:

• Ice has lower density (0.92 g/cm³) than liquid water (1.0 g/cm³)
• Hydrogen bonds in ice create hexagonal lattice structure
• This structure is less dense than liquid water's arrangement

Biological Significance:

1. Insulation: Ice layer on top insulates water below, keeping it liquid
2. Habitat preservation: Aquatic organisms can survive in liquid water under ice
3. Temperature moderation: Water below ice stays near 4°C (maximum density)
4. Prevents complete freezing: Lakes freeze from top down, not bottom up

Ecological Impact:

Without this property, lakes and ponds would freeze solid from the bottom up, killing most aquatic life. The ice layer acts as an insulating blanket, allowing fish, plants, and other organisms to survive winter in the liquid water below.

$\boxed{\text{Ice floats because hydrogen bonding creates a less dense crystalline structure}}$

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.

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