How Does Water Cross the Cell Membrane?

Water crosses cell membranes through two main routes: it can slowly seep directly through the fatty lipid bilayer that forms the membrane itself, or it can flow rapidly through specialized protein channels called aquaporins embedded in that bilayer.¹ A third, less well-known route involves cotransporter proteins that drag water molecules along as they shuttle sugars or ions into the cell. Which pathway dominates in any given tissue depends on how many aquaporins are present, what signals the cell is receiving, and the physical forces pushing water one way or the other.

Seeping Through the Fat Layer

Every cell is wrapped in a lipid bilayer, a double sheet of fatty molecules with their water-repelling tails facing inward. Water is a polar molecule, so pushing it through that oily interior takes energy. Still, some water does make it through on its own, without any channel to help. The rate depends largely on how tightly the lipid molecules are packed together. Membranes whose lipids leave more space between neighbors let water slip through faster, while tightly packed membranes are more resistant.² Membrane thickness plays a secondary role, but the spacing between lipid molecules is the strongest predictor of how easily water crosses on its own.³

This baseline permeability is enough to keep some cells going, but it is far too slow for tissues that need to move large volumes of water quickly. Your kidneys filter roughly 180 liters of fluid per day and reabsorb most of it. Red blood cells squeeze through capillaries and must adjust their volume almost instantly. Relying on diffusion alone would be like trying to drain a swimming pool through a pinhole. That limitation is exactly why aquaporins evolved.

Aquaporins as Dedicated Water Highways

For decades, physiologists suspected that protein-based water channels must exist in cell membranes, because certain tissues moved water far faster than lipid diffusion could explain. The mystery was solved in the early 1990s when Peter Agre and colleagues isolated a 28-kilodalton protein from human red blood cells. When they inserted it into frog egg cells that are normally sluggish water movers, those cells suddenly swelled rapidly in dilute solutions, proving the protein was a dedicated water pore.⁴ The protein was eventually named aquaporin-1, and the discovery earned Agre the 2003 Nobel Prize in Chemistry.⁵

We now know the human body expresses at least 13 different aquaporin types, numbered AQP0 through AQP12, each distributed in specific tissues. AQP1 is widespread in red blood cells, kidney tubules, and the lining of blood vessels. AQP2 is the hormone-regulated water valve in the kidney’s collecting duct. AQP4 dominates in the brain. AQP5 is found in salivary and sweat glands. The diversity reflects how different tissues tailor water flow to their particular needs.

How an Aquaporin Lets Water Through but Blocks Almost Everything Else

The engineering of an aquaporin channel is remarkably precise. Each aquaporin protein functions as part of a four-unit cluster embedded in the membrane, and each unit contains its own narrow pore. The channel acts as a two-stage filter. First, a conserved structural motif deep inside the pore creates a selectivity region that orients water molecules into a single-file line. Second, a narrow constriction zone lined with specific amino acids physically excludes anything larger than a water molecule.⁶

Excluding dissolved salts and sugars is relatively straightforward when the pore is narrow enough, but blocking protons is a trickier problem. Protons normally hop along chains of water molecules through a quantum-mechanical relay, so you might expect them to ride right through a single-file column of water inside an aquaporin. They do not. The channel generates an electrostatic field near its center that creates a steep energy barrier for charged particles. This barrier is high enough that the time it would take a proton to cross the channel is, for all practical purposes, infinite on biological timescales.⁷ If aquaporins leaked protons, cells would lose the electrical and chemical gradients they depend on for energy production. The barrier that prevents this leakage matches the resistance of the lipid bilayer itself, so having aquaporins does not open up a weak point.⁸

What Pushes Water in One Direction

Aquaporins and lipid bilayers provide the pathways, but something has to push or pull water through them. Two forces do most of the work. Osmotic pressure arises whenever dissolved particles are more concentrated on one side of a membrane than the other; water naturally flows toward the more concentrated side to try to equalize the difference. Hydraulic pressure is the straightforward physical push of fluid, like blood pressure forcing water out of capillaries. When these two forces are not in balance, water moves across the membrane, changing cell volume in the process.⁹

This interplay matters in ways most people do not think about. When you eat a salty meal, the elevated sodium concentration in your blood draws water out of cells and into the bloodstream, which is why you feel thirsty. When you drink plain water faster than your kidneys can excrete it, your blood becomes dilute and water floods into cells, potentially causing dangerous swelling. Every moment, your cells are making fine adjustments based on the balance of these forces, and aquaporins determine how quickly those adjustments happen.

The Kidney’s Water Valve

Nowhere in the body is water transport more tightly controlled than in the kidney’s collecting duct, the final stretch of tubing where your body decides how concentrated or dilute your urine will be. The key player is aquaporin-2, which sits on the inner surface of collecting duct cells. When you are dehydrated, your brain releases the hormone vasopressin (sometimes called antidiuretic hormone). Vasopressin binds to receptors on these kidney cells and triggers a cascade that shuttles aquaporin-2 channels from storage compartments inside the cell up to the cell surface facing the urine.¹⁰ With more channels in place, water pours out of the urine and back into the bloodstream, producing small volumes of concentrated urine.

When you are well hydrated, vasopressin levels drop, the aquaporin-2 channels are pulled back inside the cell, and more water stays in the urine, diluting it. This system is so sensitive that defects in aquaporin-2 or its regulation cause nephrogenic diabetes insipidus, a condition where the kidneys cannot concentrate urine and patients can lose liters of water per day. AQP3, present on the opposite side of the same collecting duct cells, provides a permanent exit route for water leaving the cell toward the blood.¹¹

A Third Route Through Cotransporters

There is a pathway for water movement across membranes that does not involve either passive diffusion or aquaporins. Certain transporter proteins that carry sugars or ions also drag water along for the ride. The best-studied example is the sodium-glucose cotransporter (SGLT1) in the lining of your small intestine. When SGLT1 moves one glucose molecule and two sodium ions into an intestinal cell, it brings along roughly 260 water molecules in the same stroke.¹² This happens even without an osmotic gradient pushing water in that direction, which distinguishes it from aquaporin-mediated flow.

The practical significance is substantial. Estimates suggest this cotransport mechanism accounts for several liters of water absorption in the human intestine daily. It also explains why oral rehydration solutions containing glucose and salt work so well for treating dehydration from diarrheal illness: the glucose activates SGLT1, which pulls sodium and water into the body simultaneously. SGLTs can also function as water and urea channels in their own right, adding further complexity to how water navigates epithelial tissues.¹³

How Cells Turn the Flow Up or Down

Cells do not just passively allow water through their membranes. They actively regulate how many channels are available and whether those channels are open. The kidney’s vasopressin-triggered shuttling of aquaporin-2 is one example of regulation through trafficking, physically moving channels to or from the membrane surface. But cells also use chemical modifications to control aquaporin behavior. Adding a phosphate group to specific spots on an aquaporin protein can change whether the channel is open, whether it stays at the cell surface, or whether it gets tagged for destruction.¹⁴

The picture is not always clean, though. For AQP4, the dominant water channel in the brain, one much-discussed phosphorylation site turned out to have no measurable effect on the channel’s water flow in careful experiments. Mutation of that site, chemical activation of the enzymes that should modify it, and molecular simulations all pointed to the same conclusion: at least for that particular site, phosphorylation does not gate the channel.¹⁵ This is a good reminder that regulation mechanisms vary across aquaporin types. What applies to AQP2 in the kidney does not automatically apply to AQP4 in the brain.

Aquaporins That Transport More Than Water

Not all aquaporins are strict water-only channels. A subclass called aquaglyceroporins have wider pores that also allow glycerol and certain other small uncharged molecules to pass through. AQP3 and AQP7 are the best-known members. Inside these channels, glycerol and water molecules compete for the same hydrogen bonds that guide them through the pore, and both can move through in either direction driven by thermal fluctuations.¹⁶

The biological consequences of glycerol transport are surprisingly visible. Mice engineered to lack AQP3 develop dry, inelastic skin because their skin cells cannot take up enough glycerol to maintain proper hydration and lipid production. Mice lacking AQP7 accumulate excess fat in their fat cells, because those cells cannot release glycerol efficiently.¹⁷ In yeast, an aquaglyceroporin called Fps1 is a critical part of the cell’s strategy for surviving sudden changes in environmental salt concentration; it regulates how much glycerol the cell retains as an internal osmotic buffer.¹⁸

Water Channels in the Brain

AQP4, the most abundant aquaporin in the central nervous system, is concentrated on the endfeet of star-shaped brain cells called astrocytes, which wrap around blood vessels. This positioning places AQP4 at the interface between the bloodstream and the brain’s interstitial fluid, making it a key gatekeeper for water entering and leaving brain tissue. AQP4 is also an integral part of the glymphatic system, the brain’s recently discovered waste-clearance network that flushes metabolic debris out of brain tissue during sleep.¹⁹

Mice genetically engineered to lack AQP4 show markedly abnormal brain fluid transport, with stagnation of interstitial fluid and impaired glymphatic clearance.²⁰ This has sparked intense interest in whether AQP4 dysfunction contributes to neurodegenerative diseases where toxic proteins accumulate in the brain. Researchers have investigated AQP4 as a potential drug target for reducing brain swelling after stroke or traumatic injury, though no specific therapeutic agents that inhibit water flow through aquaporins have been developed for clinical use yet.²¹

How Plants Use Aquaporins Differently

Plants face water-management challenges that are in some ways more extreme than those of animals. A plant rooted in drying soil has no option to walk toward a water source. Instead, it fine-tunes the permeability of its root cell membranes by adjusting aquaporin expression. Plants have a remarkably large family of aquaporins, sometimes dozens of different types in a single species, reflecting how central water control is to their survival.²²

During drought, some plants actually reduce their aquaporin levels in root cells. This seems counterintuitive, but it prevents water from leaking back out of roots into even-drier soil. In tobacco plants, drought stress decreases the production of certain aquaporin types in roots, effectively tightening the membrane and reducing water loss. When rain returns, the plants restore aquaporin levels and reopen the pathway for water uptake.²³ Different grapevine varieties use different aquaporin strategies to cope with the same drought conditions, with some maintaining higher root water flow than others by keeping their aquaporin genes more active.²⁴

Aquaporins, Cell Migration, and Cancer

One of the more unexpected discoveries about aquaporins is their involvement in cell movement. When a cell crawls forward, it extends a thin sheet of membrane at its leading edge. That extension requires rapid local water influx to swell the advancing part of the cell. Cells with high aquaporin expression can manage this faster, and experiments show that cancer cells with elevated aquaporin levels migrate more aggressively and invade surrounding tissue more readily.²⁵ Mice lacking AQP1 show defective tumor blood vessel formation because the endothelial cells lining those vessels migrate too slowly.

At least seven different aquaporins have been linked to various aspects of cancer invasion and metastasis, from boosting the growth of new blood vessels feeding tumors to helping cancer cells detach from one another and squeeze through tissue barriers.²⁶ This has made aquaporins a target of interest for cancer researchers, though as with brain edema, the challenge of developing drugs that selectively block specific aquaporin types without disrupting normal water balance throughout the body remains unsolved.

Aquaporins Across All Domains of Life

Aquaporins are not unique to mammals or even to animals. Bacteria, archaea, fungi, and protozoa all carry aquaporin genes. The water channel AqpZ in E. coli shares a similar structure with human AQP1, pointing to an origin deep in evolutionary history. For bacteria, aquaporins matter most when osmotic conditions shift abruptly, such as when a sudden rainstorm dilutes the salt concentration of a pond. AqpZ helps E. coli shuttle water in or out fast enough to prevent lysis or shriveling, and its gene is regulated in response to both growth phase and external salt levels.²⁷

The evolutionary trajectory also reveals something interesting about how vertebrates adapted to life on land. Genomic analyses show that the cluster of aquaporin genes including AQP2, AQP5, and AQP6 is a feature unique to the lineage leading to land vertebrates, traceable to ancestral forms in coelacanths and lungfishes.²⁸ AQP2 is the very channel your kidneys use to conserve water. Its emergence in the genomes of early land-venturing vertebrates suggests that fine-tuned water conservation was part of the biological toolkit that made terrestrial life possible.

Aquaporin-Inspired Water Purification

The extreme selectivity of aquaporins has caught the attention of engineers working on water purification and desalination. Conventional reverse-osmosis membranes force water through synthetic barriers under high pressure, which is energy-intensive and creates a trade-off between throughput and filtering quality. Membranes incorporating aquaporin proteins, or synthetic channels that mimic their structure, can achieve high water flow with strong rejection of dissolved contaminants at lower pressures.²⁹ Several companies have commercialized forward-osmosis membranes embedding aquaporin proteins for applications ranging from drinking-water production to industrial wastewater treatment. The technology is still maturing, and challenges remain around long-term protein stability and manufacturing scale, but it represents one of the more tangible cases of biology inspiring industrial engineering.

Paracellular Water Movement

Not all water that crosses an epithelial barrier goes through cells. Some slips between cells, moving through the gaps sealed by structures called tight junctions. This paracellular route and the transcellular aquaporin route are not entirely independent. In salivary glands, for example, mice that lack AQP5 show not only reduced water flow through cells but also a halving of the paracellular barrier’s permeability, suggesting a molecular crosstalk between the two pathways.³⁰ How exactly aquaporin expression on cell surfaces signals changes in the junctions between cells is still being worked out, but the finding reinforces that water transport across tissues is a more integrated system than a simple “through the cell or around the cell” dichotomy would suggest.