Osmosis (AQA GCSE Biology): Revision Notes
Osmosis
What is osmosis?
Osmosis is a special type of diffusion that only involves water. It happens when water moves from one place to another through a barrier.
Key Definition: Osmosis is the movement of water from a weak solution to a strong solution through a partially permeable membrane.
Understanding the terminology is crucial for grasping osmosis concepts:
- Dilute solution = weak solution (lots of water, not much dissolved stuff)
- Concentrated solution = strong solution (less water, lots of dissolved stuff)
How osmosis works
Water always moves from where there's more of it to where there's less of it. This creates a net movement of water in one direction.
The driving force behind osmosis is the difference in water concentration between two solutions. Water molecules naturally move to balance out these concentration differences.
Partially permeable membranes
These special barriers are key to understanding how osmosis functions:
- They let small molecules like water pass through
- They block larger molecules like sugar (sucrose)
- Cell membranes work like this too
Tea Bag Analogy: Imagine a tea bag in hot water. The bag lets water in and tea out, but keeps the tea leaves inside. This is similar to how partially permeable membranes work in osmosis.
Investigating osmosis with Visking tubing
Scientists use Visking tubing to study osmosis because it acts like a cell membrane.
Practical Investigation: Osmosis with Visking Tubing
Setup:
- Soak Visking tubing in water first to soften it
- Fill the tubing with sugar solution
- Tie it at the bottom with cotton thread
- Place it in a beaker of pure water
- Attach it to a thin glass tube (capillary tube)
Observations:
- Water moves from the beaker into the sugar solution
- The liquid level in the glass tube rises
- This proves water has moved by osmosis
Osmosis in living cells
Red blood cell example
When red blood cells are placed in different solutions, the effects of osmosis become clearly visible:
Scenario 1: Red Blood Cells in Concentrated Sugar Solution
What happens:
- Water moves out of the cells
- Cells shrink and become smaller
- This occurs because the sugar solution is stronger than the cell contents
Scenario 2: Red Blood Cells in Pure Water
What happens:
- Water moves into the cells
- Cells swell up and may burst
- This occurs because pure water is weaker than the cell contents
These observations demonstrate that cell membranes are partially permeable - they allow water to pass through but not larger molecules like sugars.
The behaviour of red blood cells in different solutions provides clear evidence that osmosis occurs across cell membranes in living organisms.
Key Points to Remember:
- Osmosis only involves water movement - not other substances
- Water moves from dilute to concentrated solutions - always this direction
- Partially permeable membranes are essential - they control what passes through
- Cell membranes are partially permeable - this is why osmosis happens in living things
- Osmosis can make cells shrink or swell - depending on the solution they're in