Topic 2.7 Notes – Tonicity and Osmoregulation

1. How Concentration Gradients Drive Water Movement

Cell membranes are selectively permeable. Small nonpolar molecules pass easily, but many solutes (salts, sugars) cannot cross freely. Water can move, and it does so by osmosis.

Osmosis = diffusion of water across a membrane from high water concentration to low water concentration.

A quick reminder about gradients:

• A concentration gradient is a difference in concentration between two areas.
• Molecules move down their gradient (high → low).
• No ATP is required. This is passive transport.

For water, we often think in terms of solute:

• Water moves toward higher solute concentration.
• Or from low solute concentration → high solute concentration.
• Or from high water potential → low water potential.

All three statements describe the same idea.

Students often mix this up: water is not “chasing salt.” It’s moving because there is more free water on one side. Adding solute reduces the amount of free water.

This movement of water is constant, and growth and homeostasis depend on it.

2. Tonicity and Its Effects on Cells

Tonicity describes how an external solution affects a cell’s water balance. It compares solute concentration outside the cell to inside the cell.

There are three conditions you need to instantly recognize:

Isotonic

• Solute outside = solute inside
• No net water movement
• Cell size stays the same

Hypotonic solution (outside the cell)

• Lower solute outside than inside
• Water moves into the cell
• Cell swells
  • Animal cell → may lyse (burst)
  • Plant cell → becomes turgid (firm and healthy)

Hypertonic solution (outside the cell)

• Higher solute outside than inside
• Water moves out of the cell
• Cell shrinks
  • Animal cell → crenates
  • Plant cell → plasmolysis (membrane pulls away from cell wall)

Here’s the comparison clearly:

Water always moves toward the hypertonic side. That sentence alone can save you on multiple choice questions.

On tests, they often describe a scenario instead of using the words hypotonic or hypertonic. Translate it into solute comparison first, then decide water direction.

3. Water Potential and the Water Potential Equation

When the question gets quantitative, you use water potential (Ψ) to predict movement.

Water moves from:

• Higher (less negative) Ψ → Lower (more negative) Ψ

Pure water has $Ψ = 0$.

The Water Potential Equation

$$Ψ = Ψ_p + Ψ_s$$

• $Ψ_p$ = pressure potential
• $Ψ_s$ = solute potential

Solute Potential

$$Ψ_s = -iCRT$$

• $i$ = ionization constant (NaCl = 2, glucose = 1)
• $C$ = molar concentration
• $R = 0.0831 \text{ L} \cdot \text{bar} \cdot \text{mol}^{-1} \cdot \text{K}^{-1}$
• $T$ = temperature in Kelvin

Important patterns:

• Increasing solute concentration → more negative $Ψ_s$
• More negative total $Ψ$ → water moves toward it

You’re rarely asked to do heavy math. More often, they give two solutions and you decide which has the lower water potential.

Pressure Potential $Ψ_p$

• Physical pressure on a solution
• In plants, this is turgor pressure
• Usually 0 in an open beaker
• Can be positive inside plant cells

When water enters a plant cell:

• $Ψ_p$ increases
• The cell wall prevents bursting
• The plant stays upright

This is why plants wilt when they lose water.

4. Osmoregulation and Why It Matters

Osmoregulation is maintaining internal water balance and solute concentration.

Without it, cells would:

• Swell and burst
• Shrink and stop functioning
• Lose proper enzyme activity

Examples you should recognize:

Freshwater Protists

Environment is hypotonic. Water constantly enters. They use a contractile vacuole to pump excess water out.

Paramecium with contractile vacuole

Plant Cells

• Large central vacuole stores water
• Maintains turgor pressure
• Essential for structure and growth

Animals

• Kidneys regulate water and ion balance
• Maintain stable internal conditions despite changing environments

The big idea is dynamic homeostasis. Water is always moving. Organisms must constantly regulate it to survive.