Osmosis (AQA GCSE Biology Combined Science): Revision Notes
Osmosis
Osmosis is a special type of diffusion that happens with water. It's an important process that affects how cells work and survive.
What is osmosis?
Osmosis is when water moves from one place to another through a special barrier. The water always moves from where there's lots of water (dilute solution) to where there's less water (concentrated solution).
Think of it like this: water wants to balance things out. If one side has more water than the other, water will move to even things up. This natural tendency to reach equilibrium is what drives the osmosis process.
How osmosis works
Water moves through something called a partially permeable membrane. This is like a very selective door:
- It lets small things through - like water molecules
- It blocks big things - like sugar molecules or ions
Cell membranes work exactly like this. Water can pass through them easily, but larger molecules like sugars get blocked. This selective permeability is crucial for maintaining proper cell function and preventing harmful substances from entering cells.
The direction of water movement
Water always moves in the same direction during osmosis:
- From dilute solutions (lots of water, few dissolved substances)
- To concentrated solutions (less water, lots of dissolved substances)
This happens because water molecules spread out to balance the concentration on both sides of the membrane. Water will continue moving until equilibrium is reached, or until other forces prevent further movement.
Investigating osmosis
Scientists use Visking tubing to study osmosis. This artificial membrane works like a cell membrane and provides a controlled way to observe osmosis in action.
Laboratory Demonstration: Visking Tubing Experiment
Step 1: The tubing is filled with sugar solution
Step 2: It's placed in a beaker of pure water
Step 3: Water moves into the tubing through the membrane
Step 4: The liquid level inside the tubing rises over time
This experiment proves that water moves from the dilute solution (pure water) into the concentrated solution (sugar water).
Osmosis in living cells
Osmosis affects real cells too. The surrounding solution concentration determines what happens to cells.
Real-World Example: Red Blood Cells
- In concentrated solutions: Water leaves the cells, making them shrink (crenation)
- In dilute solutions: Water enters the cells, making them swell up (potentially bursting)
This shows how important it is for cells to be in the right environment. Too much water in or out can damage or destroy cells.
This is why medical IV fluids must be carefully balanced to match the concentration of body fluids. Incorrect concentrations can cause serious damage to blood cells and other tissues.
Summary
Key Points to Remember:
- Osmosis is the movement of water from dilute to concentrated solutions
- It happens through partially permeable membranes that let water through but block larger molecules
- Cell membranes are partially permeable - this is why osmosis affects living cells
- Water always moves to balance concentrations on both sides of the membrane
- Osmosis can make cells shrink or swell depending on their surroundings