What is the difference between osmosis and diffusion? Give factors affecting the rate of diffusion 5 marks Question
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Osmosis vs. Diffusion — Differences
| Feature | Diffusion | Osmosis |
|---|---|---|
| Definition | Net movement of solute (or any substance) from an area of high concentration to low concentration | Net movement of water across a semipermeable membrane from an area of low solute concentration to high solute concentration |
| Driving force | Concentration gradient of the diffusing substance | Osmotic pressure difference (caused by differences in solute concentration) |
| Membrane required? | Not necessarily — occurs in free solution or across membranes | Requires a semipermeable (selectively permeable) membrane |
| Substance moved | Any dissolved solute, gas, or ion | Water only |
| Direction | From high → low concentration | From low solute concentration → high solute concentration (high water concentration → low water concentration) |
Key distinction (Costanzo): "Osmosis of water is not diffusion of water: Osmosis occurs because of a pressure difference, whereas diffusion occurs because of a concentration (or activity) difference." — Costanzo Physiology 7th Edition
Guyton & Hall define osmosis as: "the process of net movement of water caused by a concentration difference of water" across a selectively permeable membrane. — Guyton and Hall Textbook of Medical Physiology
Factors Affecting the Rate of Diffusion
The rate of net diffusion (J) is described by Fick's Law:
J = P × A × (C_A − C_B)
where J = net rate of diffusion, P = permeability, A = surface area, and (C_A − C_B) = concentration difference.
1. Concentration Gradient (C_A − C_B)
The greater the difference in concentration across the membrane, the higher the rate of net diffusion. The net diffusion rate is directly proportional to (C_outside − C_inside). This is the primary driving force for diffusion.
— Guyton and Hall Textbook of Medical Physiology
2. Surface Area (A)
The greater the surface area of the membrane available for diffusion, the higher the rate. For example, lipid-soluble gases like oxygen and carbon dioxide have particularly high rates of diffusion across cell membranes partly due to the large surface area provided by the lipid bilayer.
— Costanzo Physiology 7th Edition
3. Thickness of the Membrane (ΔX)
The thicker the cell membrane, the greater the distance the solute must travel, and therefore the lower the rate of diffusion. Rate is inversely proportional to membrane thickness.
— Costanzo Physiology 7th Edition
4. Diffusion Coefficient (D) — Molecular Size & Viscosity
Governed by the Stokes-Einstein equation:
D = KT / (6πηr)
- K = Boltzmann constant, T = absolute temperature, η = viscosity, r = molecular radius
- Small molecules in non-viscous media have the largest diffusion coefficients and diffuse most readily.
- Large molecules in viscous solutions have the smallest diffusion coefficients. — Costanzo Physiology 7th Edition
5. Temperature (T)
Temperature is directly proportional to the diffusion coefficient (see Stokes-Einstein equation above). Higher temperature increases the kinetic energy of molecules, increasing their speed and therefore the rate of diffusion.
6. Lipid Solubility / Partition Coefficient (K)
The partition coefficient describes the solubility of a solute in lipid (oil) relative to water. Non-polar, lipid-soluble substances have a high partition coefficient and diffuse more readily through the lipid bilayer. Polar solutes have low partition coefficients and diffuse slowly or require carriers.
— Costanzo Physiology 7th Edition
7. Electrical Potential Difference (for ions)
For charged particles (ions), an electrical potential gradient across the membrane acts as an additional driving force — attracting ions of opposite charge and repelling same-charge ions. This can drive net diffusion of ions even without a concentration difference.
— Guyton and Hall Textbook of Medical Physiology
8. Pressure Difference
A higher hydrostatic pressure on one side of a membrane increases the kinetic energy of molecules, driving net diffusion toward the lower-pressure side. This is particularly relevant in capillary exchange.
— Guyton and Hall Textbook of Medical Physiology
Summary for 5-mark exam: Diffusion and osmosis both involve passive movement down a gradient, but differ in the substance moved (solute vs. water), driving force (concentration difference vs. osmotic pressure), and membrane requirement. The rate of diffusion is governed by Fick's Law and depends on concentration gradient, surface area, membrane thickness, molecular size/diffusion coefficient, temperature, lipid solubility, and electrical potential (for ions).
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