Osmosis

This lesson covers osmosis as required by the Edexcel GCSE Combined Science specification (1SC0). You need to define osmosis, explain the concept of water potential, describe the effect of osmosis on plant and animal cells, and understand the practical investigation using potato cylinders.

---

What Is Osmosis?

Osmosis is the movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution) through a partially permeable membrane.

Key points:

• Osmosis is a special case of diffusion — it specifically involves water molecules.
• A partially permeable membrane has tiny pores that allow small molecules (water) to pass through but block larger molecules (e.g. sugar, starch).
• Osmosis is a passive process — it does not require energy from the cell.
• Water moves down the water potential gradient.

Exam Tip: The Edexcel specification uses the term "water potential" rather than "concentration". Water moves from high water potential to low water potential. A dilute solution has a higher water potential than a concentrated solution.

---

Understanding Water Potential