Osmosis moves water passively across selectively permeable membranes, often through aquaporins, toward regions with higher solute concentration; Aquaporins make this water movement faster and more physiologically responsive.
Coverage 2024–2025 · Updated 15 Jul 2026
Osmosis is passive net movement of water across a membrane that is permeable to water but not freely permeable to the solutes involved.
Water molecules move randomly both ways. A difference in solute concentration creates a difference in water potential, so net water movement is from the lower-solute, higher-water-potential side toward the higher-solute, lower-water-potential side.
Aquaporins are selective protein pores that greatly increase water permeability. They change the rate of osmosis, not the direction set by the water-potential difference.
A cell in a solution with higher solute concentration than its cytoplasm loses water and shrinks; aquaporins allow that volume change to occur faster.
Do not state only that 'water follows solute'. Identify the selectively permeable membrane, compare the two solutions and distinguish net movement from random movement in both directions.
This objective is assessed through structured response, commonly using Describe / Identify / Define.
Describe / Identify / Define / State / Explain / Distinguish
Build the answer around this relationship: Osmosis is passive net movement of water across a selectively permeable membrane.
Describing movement of solute instead of net movement of water.
Representative question
Describe transport across cell membranes by osmosis.
The SL membrane model is a decision system. The bilayer forms because phospholipids are amphipathic, and the hydrophobic core creates selective permeability. Small non-polar molecules diffuse directly; water moves by osmosis and often through aquaporins; ions and polar molecules use channels or transporters; pumps use ATP for movement against gradients. Proteins and glycocalyx components add transport, recognition, and model evidence.
