What is Diffusion and Osmosis?
Diffusion and osmosis are two ways molecules move passively across a concentration gradient without using energy. Diffusion moves any solute or gas from high to low concentration, while osmosis specifically moves water across a selectively permeable membrane.
Diffusion is the net movement of particles from high to low concentration; osmosis is the diffusion of water specifically, across a selectively permeable membrane, from an area of low solute concentration to high solute concentration.
- •Moves any type of particle (solute or gas)
- •High to low concentration
- •No membrane required
- •Example: perfume spreading in a room
- •Moves water specifically
- •Low to high solute concentration (across membrane)
- •Requires a selectively permeable membrane
- •Example: water entering a plant root cell
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Step-by-step worked examples
Calculate the osmotic pressure of a 0.3 mol/L glucose solution at 298 K (glucose does not dissociate, so i = 1).
π = iMRT π = 1 × 0.3 × 0.0821 × 298 π = 7.34 atm
A red blood cell is placed in a hypertonic solution (higher solute concentration outside than inside). What happens to the cell?
Water moves from low solute (inside cell) to high solute (outside) concentration by osmosis The cell loses water and shrinks — this is called crenation
A drop of food coloring is added to a glass of still water. Describe what happens over 10 minutes using the concept of diffusion.
Color molecules start concentrated at one point (high concentration) They move toward the surrounding low-concentration water (net movement high→low) After ~10 minutes, the color spreads evenly — the system reaches equilibrium
Flashcards
Quick quiz
Q1.What does diffusion always move from high to low?
Q2.Osmosis specifically refers to the movement of what?
Q3.A cell placed in a hypertonic solution will…
Q4.In π = iMRT, what does 'i' represent?
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Common mistakes
Diffusion and osmosis are the same process. — Correct: Osmosis is a special case of diffusion that applies only to water across a membrane; diffusion applies to any particle.
A hypertonic solution makes a cell swell. — Correct: A hypertonic solution has more solute outside, so water leaves the cell and it shrinks.
Diffusion requires energy (ATP). — Correct: Diffusion is passive transport — it needs no cellular energy, only a concentration gradient.
Osmosis can move solutes directly. — Correct: Osmosis moves water only; the membrane is selectively permeable and blocks most solutes.
FAQ
What is diffusion in biology?
Diffusion is the passive net movement of particles from an area of higher concentration to lower concentration until equilibrium is reached.
What is the formula for osmotic pressure?
Osmotic pressure is π = iMRT, where i is the van't Hoff factor, M is molarity, R is the gas constant, and T is temperature in kelvin.
What are examples of diffusion and osmosis?
Diffusion: perfume spreading in air, or ink spreading in water. Osmosis: water moving into a plant root, or a red blood cell shrinking in salty water.
How do you calculate osmosis effects on a cell?
Compare solute concentrations inside vs outside the cell — water always moves toward the higher solute (more concentrated) side by osmosis.