Cambridge Syllabus Notes

3.2 Osmosis

Learning objective

State that water moves **into and out of cells** by osmosis through a partially permeable membrane.

What is osmosis?

Osmosis is the **passive** movement of water molecules across a membrane that is permeable to water but impermeable to most solutes. Water moves from an area of **higher water potential** (lower solute concentration) to an area of **lower water potential** (higher solute concentration) until the water potentials on both sides are equal.

Key features

Only water crosses the membrane; most solutes are retained.
The membrane must be partially permeable (e.g., the plasma membrane, dialysis tubing).
Movement continues until water potential (Ψ) is equal on both sides.
Water potential is the sum of solute potential (Ψ_s) and pressure potential (Ψ_p):
Ψ = Ψ_s + Ψ_p
No cellular energy (ATP) is required – osmosis is a form of passive transport.
Water can move **into** a cell (e.g., in a hypotonic solution) **or out of** a cell (e.g., in a hypertonic solution).

Water‑potential gradient

The direction of water flow is determined by the gradient in water potential (ΔΨ):

ΔΨ > 0 → water moves toward the side with the lower Ψ (higher solute concentration).
ΔΨ = 0 → no net movement; equilibrium is reached.

Factors influencing the rate of osmosis

Concentration (water‑potential) gradient: A larger difference in solute concentration speeds up osmosis.
Temperature: Higher temperatures increase kinetic energy, increasing the rate.
Surface area of the membrane: More area allows more water molecules to pass per unit time.
Permeability of the membrane: Membranes that are more permeable to water (e.g., contain aquaporins) facilitate faster movement.

Comparison: Diffusion vs. Osmosis

Aspect	Diffusion	Osmosis
Substance moved	Any small molecules or ions	Water only
Direction of movement	From high to low concentration	From high water potential to low water potential (i.e., from low to high solute concentration)
Membrane requirement	Can occur with or without a membrane	Requires a partially permeable membrane
Energy requirement	None (passive)	None (passive)

Types of solutions and cellular response

Solution type	Relative solute concentration	Effect on a typical animal cell	Effect on a typical plant cell
Hypotonic	Lower than inside the cell	Water enters → cell swells; may burst (lysis) if extreme.	Water enters → turgor pressure builds; cell becomes turgid.
Isotonic	Equal to inside the cell	No net water movement; cell size unchanged.	No net water movement; cell remains flaccid but functional.
Hypertonic	Higher than inside the cell	Water leaves → cell shrinks (crenation).	Water leaves → plasmolysis (plasma membrane pulls away from cell wall).

Biological significance

Plant roots: Osmosis draws water from moist soil into root cells.
Animal digestion: Water is absorbed by the walls of the small intestine.
Cell‑volume regulation: Cells maintain shape and function by balancing water influx and efflux.
Turgor pressure: In plants, the pressure of water inside the vacuole pushes against the cell wall, keeping stems upright.
Kidney function: Osmosis is essential for re‑absorption of water from filtrate back into the blood.

Practical investigations (Cambridge Core)

1. Osmosis with dialysis tubing

Cut a piece of dialysis tubing, seal one end and fill it with a small piece of potato or a few drops of egg‑white.
Place the tube in a beaker containing a sucrose solution of known concentration (e.g., 0.2 M).
After 5–10 min, remove the tube, blot dry and weigh it.
Result:
- Tube becomes heavier → water entered (outside solution was hypotonic to the contents).
- Tube becomes lighter → water left (outside solution was hypertonic).
Repeat with several sucrose concentrations to illustrate the effect of the concentration gradient.

2. Effect of solution concentration on plant tissue

Prepare three beakers:
- Distilled water (hypotonic)
- 0.3 M sucrose (isotonic for Elodea)
- 0.6 M sucrose (hypertonic)
Place a thin strip of onion epidermis or a small Elodea leaf in each beaker.
Observe after 5 min:
- Hypotonic: Cells become turgid; may burst in extreme cases.
- Isotonic: No visible change; cells remain flaccid.
- Hypertonic: Plasmolysis – the plasma membrane pulls away from the cell wall, visible as a clear gap.

Summary

Osmosis is the diffusion of water across a partially permeable membrane, moving from an area of higher water potential (low solute concentration) to an area of lower water potential (high solute concentration). The process is passive, requires no ATP, and can cause water to move **into** or **out of** cells depending on the relative solute concentrations of the internal and external environments. Osmosis underpins vital biological functions such as water uptake in plants, nutrient absorption in animals, and the maintenance of cell turgor and volume.

Suggested diagram: Water moving across a semi‑permeable membrane from a region of low solute concentration (high water potential) to a region of high solute concentration (low water potential), illustrating both inward and outward water flow.

State that water diffuses through partially permeable membranes by osmosis.