What is Osmosis and Diffusion?
Diffusion and osmosis are the two main ways molecules move passively across a cell membrane, without using energy. Diffusion moves any particle from high to low concentration; osmosis is the special case of water moving across a selectively permeable membrane.
Diffusion is the net movement of particles from an area of high concentration to low concentration; osmosis is the diffusion of water specifically, across a selectively permeable membrane, from a region of high water (low solute) concentration to low water (high solute) concentration.
- •Moves any type of particle
- •Occurs in liquids, gases, or across membranes
- •Driven by any concentration gradient
- •No membrane required
- •Moves water molecules specifically
- •Requires a selectively permeable membrane
- •Driven by a solute (water) concentration gradient
- •A special case of diffusion
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Step-by-step worked examples
Find the osmotic pressure of a 0.30 mol/L sucrose solution at 300 K.
π = M×R×T π = 0.30 × 0.0821 × 300 π ≈ 7.39 atm
A red blood cell is placed in distilled water (pure water, no solute). Predict what happens and why.
The cell's cytoplasm has a higher solute concentration than pure water (a hypotonic environment). Water moves by osmosis from the low-solute water into the high-solute cell, following the concentration gradient. The cell swells and may burst (lyse) because water keeps entering.
Find the molarity needed to produce an osmotic pressure of 12.3 atm at 273 K.
π = M×R×T → M = π / (R×T) M = 12.3 / (0.0821 × 273) M = 12.3 / 22.41 ≈ 0.55 mol/L
Flashcards
Quick quiz
Q1.Which best describes osmosis?
Q2.A cell placed in a hypertonic solution will most likely…
Q3.In π = MRT, if molarity M doubles while T stays constant, osmotic pressure π…
Q4.Does diffusion require energy (ATP) input?
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Common mistakes
Osmosis and diffusion are the same thing. — Correct: Osmosis is a specific type of diffusion — only for water, and only across a selectively permeable membrane.
Water always moves toward the side with more water. — Correct: Water moves toward the side with more solute (less water concentration), i.e., down its own concentration gradient.
Diffusion requires a membrane. — Correct: Diffusion can happen anywhere there's a concentration gradient — in air, liquid, or across a membrane.
A hypertonic solution makes a cell swell. — Correct: A hypertonic solution draws water OUT of the cell, causing it to shrink, not swell.
FAQ
What is osmosis and diffusion?
Diffusion is particles spreading from high to low concentration; osmosis is the diffusion of water specifically across a selectively permeable membrane.
What is the osmosis formula?
Osmotic pressure is calculated with π = MRT, where M is molarity, R is the gas constant, and T is temperature in Kelvin.
What are examples of osmosis and diffusion?
A tea bag coloring water is diffusion; a wilted plant reviving after watering, or a red blood cell swelling in fresh water, are examples of osmosis.
How do you calculate osmotic pressure?
Multiply the solute's molarity by the gas constant (0.0821) and the absolute temperature in Kelvin: π = M × R × T.