Diffusion and Osmosis: Principles of Passive Transport
Understanding Diffusion
Diffusion is the net movement of particles from an area of high concentration to an area of low concentration.
Key Characteristics of Diffusion
- It happens in liquids and gases.
- Particles move randomly all the time.
- It is a passive process.
- It does not need energy from the cell.
Diffusion in Biological Cells
Cell membranes are partially permeable. This means:
- Small molecules can pass.
- Large molecules cannot pass easily.
Diffusion happens across the membrane when there is a concentration difference.
Factors Affecting the Rate of Diffusion
- More surface area = faster diffusion
- Higher temperature = faster diffusion
- Greater concentration difference = faster diffusion
- Greater distance = slower diffusion
The Dialysis Bag Experiment
A dialysis bag contains glucose and starch. The bag is placed in water.
After some time, glucose diffuses out, but starch stays inside. Why? Because glucose molecules are small enough to pass through the membrane, but starch molecules are too large.
Why did starch not diffuse? Because starch molecules are too large to pass through the partially permeable membrane.
Why does water move? Because water moves from high water potential to low water potential.
Repeated Observation: Dialysis Bag Experiment
A dialysis bag contains glucose and starch. The bag is placed in water. After some time, glucose diffuses out, while starch stays inside. Why? Because glucose molecules are small enough to pass through the membrane, but starch molecules are too large.
Why did starch not diffuse? Because starch molecules are too large to pass through the partially permeable membrane.
Why does water move? Because water moves from high water potential to low water potential.
Examples of Diffusion in Action
Sugar in Water
Because molecules are always moving, sugar molecules spread through the water. Eventually, the sugar molecules become evenly spread.
Why does sugar spread? Because diffusion causes sugar molecules to move down the concentration gradient.
Gas Exchange in the Alveoli
In the lungs, alveoli contain a high oxygen concentration. Blood arriving at the lungs has a low oxygen concentration. Because of this difference, oxygen diffuses from the alveoli into red blood cells.
Why? Because oxygen moves down its concentration gradient.
Why does oxygen enter the blood? Because oxygen concentration is higher in the alveoli than in the blood.
Rehydration and Solute Balance
Challenge Question: If you had to rehydrate quickly by drinking a lot of water, you could do so more safely by drinking water containing dissolved sugar.
Drinking water with sugar is safer because it helps maintain the solute balance. If you drink too much pure water, too much water can enter cells by osmosis and make them swell.
Understanding Osmosis
Osmosis is the net movement of water molecules from a region of high water concentration to a region of low water concentration through a partially permeable membrane.
Key Characteristics of Osmosis
- Osmosis only involves water.
- It is a passive process.
- No energy is required.
Osmosis in Plant Cells
If there is a higher concentration of solute (and so a lower concentration of water molecules) than the cytoplasm inside the cell, water will leave the cell by osmosis and the cytoplasm will shrink.
If there is a lower concentration of solute (and so a higher concentration of water molecules) than the cytoplasm inside the cell, osmosis will result in water entering the cell.