Osmosis (Including Required Practical)

This lesson covers osmosis — a special type of diffusion involving water — as required by the AQA GCSE Combined Science Trilogy specification (8464). You need to understand the definition, the effects of osmosis on plant and animal cells and be able to describe and analyse the Required Practical investigation.

---

What Is Osmosis?

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

More precisely:

$\text{Osmosis is the net movement of water molecules across a partially permeable membrane from a region of higher water potential to a region of lower water potential.}$Osmosis is the net movement of water molecules across a partially permeable membrane from a region of higher water potential to a region of lower water potential.

Key points:

• Osmosis is a passive process — it does not require energy from respiration.
• It is a special case of diffusion, but specifically involves water molecules.
• A partially permeable membrane (also called a selectively permeable membrane) has tiny pores that allow small molecules (like water) through, but block larger molecules (like sugar or protein).
• Water moves down its own concentration gradient — from where there is more water (dilute solution) to where there is less water (concentrated solution).