Aquaporins are channel proteins embedded in the plasma membrane that let water molecules cross quickly by facilitated diffusion, a form of passive transport that follows osmosis (high water potential to low water potential) without using ATP.
Water is a small but polar molecule, so it doesn't slip across the hydrophobic core of the membrane very easily on its own. Aquaporins fix that. They're channel proteins that form a water-specific pore through the membrane, so water can flood through fast when there's an osmotic gradient. Think of them as dedicated express lanes for water.
Because aquaporins move water down its gradient (from high water potential to low water potential) and don't burn ATP, they're a textbook case of facilitated diffusion, which is passive transport (CED 2.7.A, 2.5.A.2). The protein just provides a path; the gradient does the work. This matters most in cells that handle a lot of water movement, like kidney cells in osmoregulation or plant cells managing turgor through the central vacuole.
Aquaporins sit at the crossroads of several Unit 2 topics: facilitated diffusion (2.7), membrane transport (2.5), and tonicity/osmoregulation (2.7). They directly support [AP Bio 2.7.A] (how concentration gradients move molecules, including water moving from hypotonic to hypertonic regions) and [AP Bio 2.7.B] (how osmoregulation keeps organisms alive). They're also a concrete example of the membrane proteins described in [AP Bio 2.8.A]. The big-picture theme is structure and function: the membrane is selectively permeable, and aquaporins are why water gets through despite being polar.
Keep studying AP Biology Unit 2
Osmosis and Water Potential (Unit 2)
Aquaporins are the doorway; osmosis is the rule that decides which way water goes through it. Water always moves from high water potential to low (ψ = ψₚ + ψₛ), and aquaporins just speed that movement up.
Facilitated Diffusion and Channel Proteins (Unit 2)
Aquaporins are channel proteins, the same family as ion channels, but tuned for water. Both are passive, both need no ATP, and both just give polar or charged particles a path through the membrane's greasy interior.
Osmoregulation and the Contractile Vacuole (Unit 2)
Cells in a hypotonic environment, like protists with contractile vacuoles, must manage incoming water. Aquaporins control how fast that water enters, linking membrane transport to whole-organism homeostasis (2.7.B).
CFTR Protein and Selective Permeability (Unit 2)
CFTR moves chloride ions while aquaporins move water, but both show the same idea: each membrane protein is selective for what it lets pass. Compare them to see why the membrane isn't just one big sieve.
Aquaporins usually show up inside questions about osmosis, tonicity, and facilitated diffusion rather than as their own standalone topic. A classic MCQ stem describes water rushing into a red blood cell placed in a hypotonic solution until it bursts, then asks which membrane feature explains it. The answer hinges on selective permeability and water-conducting channels. You should be able to classify water transport through aquaporins as passive (no ATP) and explain that water still follows its gradient. Watch for trap questions that compare how fast different molecules cross: small nonpolar molecules diffuse straight through the bilayer, while polar water needs aquaporins. No released FRQ has used the term verbatim, but it fits any free-response asking you to explain water movement, predict cell swelling or shrinking, or describe how membrane proteins maintain water balance.
Aquaporins move WATER, and they're always passive. The Na⁺/K⁺ pump moves IONS and uses ATP (active transport), and voltage-gated channels move ions passively but open in response to voltage. Don't lump them together: aquaporins are water-only, ATP-free, and always open to water flow.
Aquaporins are channel proteins that let water cross the membrane quickly by facilitated diffusion.
They are passive transport, so they never use ATP; water still moves down its gradient from high to low water potential.
They exist because water is polar and crosses the hydrophobic membrane core slowly on its own.
On the exam they appear inside osmosis and tonicity questions, often explaining why a cell swells in a hypotonic solution.
Aquaporins are central to osmoregulation, helping cells like kidney cells and protists control internal water balance.
Aquaporins are channel proteins in the plasma membrane that create a water-specific pore, letting water cross by facilitated diffusion. They follow osmosis (high to low water potential) and don't use ATP.
No. Aquaporins are passive transport, so they require no energy. Water simply moves down its gradient through the open channel, which is what makes this facilitated diffusion rather than active transport.
Aquaporins move water passively with no ATP, while the Na⁺/K⁺ pump moves ions against their gradient using ATP (active transport). One is a free water lane; the other is an energy-powered ion pump.
Water is small but polar, so it doesn't pass easily through the membrane's hydrophobic core. Aquaporins give water a fast path through, which is why cells handling lots of water (like kidney cells) have many of them.
Yes, usually inside Unit 2 questions about osmosis, tonicity, and facilitated diffusion rather than as a standalone topic. You should be able to identify water movement through aquaporins as passive and connect it to water potential and cell swelling.