Membranes, osmosis and active transport
Choose the correct transport mechanism from its gradient and energy requirements.
Before you begin
- Cells and diffusion
- Solutions and concentration
Describe osmosis precisely
Osmosis is the net movement of water through a partially permeable membrane from higher water potential to lower water potential. In a simple dilute-solution comparison at the same pressure, adding solute lowers water potential. The membrane's permeability matters: a net movement of water is not the same as the solute itself moving freely.
Distinguish active transport
Active transport moves substances against their concentration or electrochemical gradient using an energy source and membrane proteins. In school examples this is often linked to energy released through cellular respiration. Facilitated diffusion also uses proteins, but it moves down a gradient without a direct metabolic energy supply. Protein involvement alone therefore does not identify active transport.
Worked example
A plant cell is placed in pure water. Explain why it becomes turgid rather than expanding indefinitely.
Show the worked solution
- Water initially enters by osmosis down the water-potential gradient.
- The vacuole expands and the contents press against the cell wall.
- The wall provides resistance; pressure builds until net water uptake stops under stable conditions.
The cell becomes turgid because the wall resists further expansion.
Try it yourself
Which feature specifically indicates active transport in this comparison?
Common mistakes
- A concentrated solution does not 'pull' every substance through any membrane.
What can you explain now?
An animal cell is placed in a sufficiently dilute solution. Predict the direction of initial net water movement.
Compare with the explanation
Into the cell, potentially causing swelling and lysis.
The outside has a higher water potential in this simple comparison. Unlike a plant cell, an animal cell lacks a rigid cell wall that limits expansion.
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