Lesson 5 of 6 / Movement of substances
Diffusion, osmosis or active transport?
Which clues tell you which process is happening?
In this lesson: Choose the process using the substance, membrane and direction of movement.
About 3 min
The key ideaName the substance first. Then check the membrane, the gradient and whether energy from respiration is needed.
Explore the idea
What changes between diffusion, osmosis and active transport?
Diffusion
Net movement down the concentration gradient of the substance.
Osmosis
Net movement of water from higher water potential to lower water potential.
Active transport
Moves a substance against its concentration gradient, from lower to higher concentration.
Arrows show the net direction of transport. The narrow green band represents a partly permeable membrane; the rounded green shape represents a carrier. Diffusion and osmosis do not mean that individual particles move in only one direction.
Explanation
For a water question, look for a partially permeable membrane and a difference in water potential. Together, these identify osmosis. State that water moves from higher to lower water potential.
For a substance such as oxygen or carbon dioxide, compare its concentrations. Net movement from higher to lower concentration is diffusion. A membrane is not required for diffusion, although diffusion can occur across one.
For movement across a cell membrane from lower to higher concentration, identify active transport. Include the energy supplied by respiration. It is not enough to say that a substance moves into a cell.
Several processes can happen in the same cell at once. A root hair cell can gain water by osmosis while taking up mineral ions by active transport. Explain each substance separately.
Step by step
- 1
What is moving?
Use the exact substance named: water, oxygen, glucose or mineral ions.
- 2
What gradient is given?
Use water potential for osmosis and the concentration of the named substance for diffusion or active transport.
- 3
Does the mechanism fit?
Check the membrane requirement and whether energy from respiration is needed.
Worked example
Two processes, one root hair cell
Soil water has a higher water potential than the cell contents, but a lower mineral ion concentration. Explain how both water and mineral ions can enter.
One way to explain it
Water enters by osmosis, from higher water potential in the soil water to lower water potential inside, through the partially permeable cell membrane. Mineral ions enter by active transport against their concentration gradient, using energy from respiration.
Why this answer works
- Explain water and mineral ions separately.
- Use the appropriate gradient for each.
- Mention energy only for active transport.
Is this true? "Osmosis is the movement of any particles across a membrane."
Osmosis specifically concerns water through a partially permeable membrane. A reference to a membrane alone cannot identify the process.