Understanding Osmosis Revision Notes | GCSE OCR Foundation Biology

What this lesson covers

Learn what osmosis is and how water moves across cell membranes from dilute to concentrated solutions through partially permeable membranes.

• Define osmosis as the diffusion of water through a partially permeable membrane
• Explain why water moves from dilute to concentrated solutions
• Compare osmosis with simple diffusion
• Apply knowledge of osmosis to predict water movement in cells

Key ideas to keep in view

Use this lesson to stay inside B2.1 Supplying the cell while connecting the detail back to the wider B2: Scaling up topic.

• B2: Scaling up
• B2.1 Supplying the cell
• GCSE OCR Foundation Biology

Lesson notes preview

Read the core explanations from Understanding Osmosis before testing yourself from memory.

• Picture two glasses of water separated by a special membrane. One glass has pure water, the other has sugar dissolved in it. Leave them overnight and something fascinating happens - water moves from the pure side to the sugary side, even though nothing is pushing it.
• This movement has a name: osmosis. It's happening in your body right now - in every cell, keeping you alive.
• The diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane
• Trap: Many students think osmosis needs energy or a pump. Actually, it's completely passive - water moves down its concentration gradient naturally, just like diffusion.
• Osmosis can't happen through any old barrier. It needs a partially permeable membrane - think of it as a molecular bouncer that's very picky about who gets through.
• A membrane that allows some molecules to pass through but not others - water can pass but larger solute molecules cannot
• Cell membranes are partially permeable. Water molecules are tiny and can pass through, but many solute particles cannot. Glucose and proteins are too large to pass through easily, and ions are also restricted by the membrane's selective properties.
• This creates the perfect setup for osmosis. Water can move to balance things out, but the dissolved substances can't follow.
• 3. Why Water Moves - The Science Behind It
• Here's the key insight: water doesn't actually 'want' to move anywhere. It's all about concentration gradients and random molecular motion.

Continue through the topic

Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.