Osmosis

This lesson covers osmosis — a special case of diffusion — as required by the Edexcel GCSE Biology specification (1BI0), Topic 1: Key Concepts in Biology. You need to understand the definition of osmosis, its effects on plant and animal cells, and be able to describe Core Practical 2 — investigating the effect of different concentrations of sucrose solution on potato tissue.

---

What Is Osmosis?

Osmosis is the movement of water molecules from a dilute solution (high water concentration) to a more concentrated solution (lower water concentration) through a partially permeable membrane.

Key Points

• Osmosis is a special case of diffusion — it involves only water molecules.
• Water moves from where there is a higher water potential (dilute solution, more water molecules) to where there is a lower water potential (concentrated solution, fewer free water molecules).
• The partially permeable membrane allows water molecules to pass through but prevents larger solute molecules from passing.
• Osmosis is a passive process — it does not require energy (ATP).
• Osmosis continues until the water concentration (water potential) is equal on both sides of the membrane.