Anatomy and Physiology 12 · Biology · Lesson 2.2
Passive transport
- Level
- Grade 12
- Reading time
- 14 min reading
- Review
- Last reviewed 2026-09-03
- Standard
- Accuracy standard
- Errata
Abstract
Diffusion and osmosis.
What is osmosis?
Short answer
Osmosis is the passive movement of water across a selectively permeable membrane from a region of lower solute concentration to one of higher solute concentration, until they are balanced. It requires no energy. Cells use osmosis to gain or lose water: a cell in a hypertonic solution loses water and shrinks, while one in a hypotonic solution swells.
Concept preview
What you will be able to do
- 01Define diffusion and explain net movement down a concentration gradient.
- 02List and explain the factors that affect the rate of diffusion.
- 03Distinguish simple diffusion from facilitated diffusion through channel and carrier proteins.
- 04Explain osmosis in terms of water potential.
- 05Predict what happens to animal and plant cells in hypertonic, hypotonic and isotonic solutions.
A living cell is forever exchanging materials with its surroundings — taking in oxygen and nutrients, releasing carbon dioxide and wastes. Remarkably, a great deal of this traffic happens for free, driven not by the cell's own energy but by the restless, random motion of the molecules themselves. Passive transport requires no direct metabolic-energy input. Neutral solutes diffuse down concentration gradients; ions respond to electrochemical gradients, including both concentration and voltage. Water moves down a water-potential gradient, which also depends on pressure. In this topic we meet its three faces: simple diffusion straight through the bilayer, facilitated diffusion through proteins, and osmosis, the special case that concerns water. Each moves down its relevant potential gradient. For osmosis across a membrane impermeable to the solute, and with equal pressures, net water movement is toward the side with more dissolved solute.
School Plan
Passive transport
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Related topics
Frequently asked questions
- How is osmosis different from diffusion?
- Osmosis is specifically the diffusion of water across a selectively permeable membrane; diffusion is the general spreading of any particles.
- What happens to a cell in a hypotonic solution?
- Water moves in and the cell swells — and may burst if the membrane cannot withstand the pressure.