State that water moves into and out of cells by osmosis through the cell membrane.

Topic 3.2 – Osmosis

Learning Objective

• State that water moves into and out of cells by osmosis through the cell membrane.

What is Osmosis?

Osmosis is the passive diffusion of water molecules across the cell membrane (which is semi‑permeable) from a region of higher water potential (lower solute concentration) to a region of lower water potential (higher solute concentration). No cellular energy (ATP) is required.

Why Osmosis Matters in Living Organisms

Water is the universal solvent in cells; it carries nutrients, gases and waste products. In the IGCSE syllabus the following processes are specifically linked to osmosis:

• Transport of nutrients and gases in the bloodstream.
• Removal of metabolic waste in the urine (kidney function).
• Movement of digested food through the gut.
• Maintenance of turgor pressure in plant cells.

Cell‑Membrane Specifics

• The plasma membrane is a phospholipid bilayer that is **semi‑permeable** – it permits water to pass but restricts most solutes.
• In animal cells, specialised protein channels called aquaporins greatly increase the rate of water movement.
• In plant cells, the rigid cell wall works with the membrane; water entering the cell generates turgor pressure, which keeps the plant upright.

Key Features of Osmosis

• Occurs only through a semi‑permeable cell membrane.
• Water moves down its water‑potential gradient; no ATP is required.
• The direction of water movement is determined by the relative solute concentrations on either side of the membrane.

Factors Influencing the Rate of Osmosis

1. Water‑potential (concentration) gradient: A larger difference speeds up water flow.
2. Temperature: Higher temperature raises the kinetic energy of water molecules, increasing the rate.
3. Surface area of the membrane: More area allows more water to pass simultaneously.
4. Membrane permeability: Presence of aquaporins or other channels enhances permeability.

Osmotic Conditions and Their Effect on Cells

Illustrative Example – Red Blood Cells

• Isotonic solution: Water enters and leaves at equal rates – cells appear normal.
• Hypertonic solution: Water exits the cells – they shrink (crenation).
• Hypotonic solution: Water enters the cells – they swell and may burst (hemolysis).

Mathematical Representation (optional)

The net volume flux of water (J_w) can be expressed as:

$$J_w = L_p \, A \, (\Delta \pi - \Delta P)$$

• J_w = volume flux of water (m³ s⁻¹)
• L_p = hydraulic conductivity of the membrane (m s⁻¹ Pa⁻¹)
• A = membrane area (m²)
• Δπ = osmotic pressure difference (Pa)
• ΔP = hydrostatic pressure difference (Pa)

Suggested Practical – Investigating Osmosis with Dialysis Tubing

1. Cut a piece of dialysis tubing (molecular‑weight cut‑off ≈ 12 kDa) about 8 cm long and seal one end.
2. Fill the tubing with 10 mL of 0.5 % (w/v) glucose solution and seal the other end.
3. Weigh the sealed tube (initial mass = M₀).
4. Place the tube in three separate beakers containing:
   • Distilled water (hypotonic)
   • 0.9 % NaCl solution (isotonic)
   • 5 % glucose solution (hypertonic)
5. Leave for 30 minutes at room temperature, then remove, blot dry and weigh again (final mass = M₁).
6. Calculate the percentage change in mass:
   $$\%\,\text{change} = \frac{M_1 - M_0}{M_0}\times 100$$
7. Interpret the results: an increase in mass indicates water entry (hypotonic), no change indicates isotonic conditions, and a decrease indicates water loss (hypertonic).

Suggested Diagram

Summary

Osmosis is the diffusion of water across the cell membrane driven by differences in water potential. Water always moves toward the region of lower water potential (higher solute concentration). The resulting changes in cell volume depend on whether the external environment is isotonic, hypertonic or hypotonic. Because water is the main solvent in cells, osmosis underpins nutrient transport in blood, waste removal in urine, food movement through the gut, and turgor maintenance in plants. Mastery of these ideas satisfies the Cambridge IGCSE Biology learning objective for Topic 3.2.