Can Water Pass Through the Cell Membrane?

Water passes through cell membranes constantly, and it does so by two distinct routes: it can slowly diffuse directly through the fatty lipid bilayer that forms the membrane, and it can flow much more rapidly through specialized protein channels called aquaporins.¹ The balance between these two pathways varies dramatically from one cell type to the next, and the story of how scientists figured this out turns out to have broad implications for kidney function, brain injuries, plant survival, and even water purification technology.

Two Routes Across the Same Barrier

Early physiologists noticed something puzzling when they watched red blood cells swell and shrink in solutions of different concentrations. The water movement had two components: a slow, temperature-sensitive process and a much faster process that was relatively insensitive to temperature changes. The slow component behaved like diffusion, where individual water molecules wiggle their way through tiny transient gaps in the lipid bilayer. But the fast component looked more like water flowing through some kind of pore, because it happened too quickly and too easily to be explained by diffusion alone. Researchers found that mercury compounds could shut down the fast component, a strong hint that a protein was involved, since mercury binds to and disables certain proteins.

Direct diffusion through the lipid bilayer is the baseline path available to every cell. Water molecules are small enough and polar enough that they can, with some effort, squeeze through the interior of the membrane. But this route is slow, because the hydrophobic core of the membrane acts as an energy barrier. The rate depends heavily on membrane composition. Experiments with artificial membranes have shown that increasing cholesterol content significantly reduces water permeability. As cholesterol packs in among the other lipid molecules, it decreases the area each lipid occupies, stiffening the membrane and reducing the number of transient cavities that water molecules can slip through.² Molecular simulations confirm that cholesterol raises the energy barrier water must overcome to cross the hydrophobic core, mainly by making the lipid tails pack more tightly and leaving fewer gaps.³

This matters because different cells have very different cholesterol content in their membranes. A cell with a cholesterol-rich membrane lets far less water through by diffusion alone than one with a looser, more fluid membrane. The practical effect is that some cells are inherently more resistant to swelling or shrinking when the fluid around them changes concentration, purely because of their membrane recipe.

The Discovery of Aquaporins

The existence of dedicated water channels was suspected for decades, but nobody could identify the actual protein. The breakthrough came largely by accident in 1992, when Peter Agre’s lab at Johns Hopkins was studying proteins on red blood cell membranes and stumbled onto a small protein they had isolated for unrelated reasons.⁴ When they inserted this protein into frog egg cells and then placed those cells in dilute solution, the eggs swelled so dramatically that the first ones literally exploded. By adjusting the concentration of the surrounding fluid, they could measure swelling over a few minutes, and the numbers were striking: each subunit of the protein could shuttle roughly two billion water molecules per second.⁵ Despite this enormous water throughput, the channel did not allow passage of other small molecules like urea or glycerol, and it did not conduct any detectable electrical current. The protein was eventually named aquaporin-1, and Agre’s work on it earned him the Nobel Prize in Chemistry in 2003.

Since that initial discovery, researchers have identified a whole family of aquaporins. Humans express at least thirteen different types, each found in specific tissues where rapid water movement is critical.

How Aquaporins Let Water Through but Block Almost Everything Else

The selectivity of aquaporins is remarkable. Water molecules are only slightly smaller than many ions and other small solutes, yet aquaporins let water race through while almost completely blocking charged particles, including protons (hydrogen ions). The channel accomplishes this trick through its physical shape and its internal chemistry.

The pore narrows to about 3 ångströms at its tightest point, just wide enough for a single water molecule to pass.⁶ Water molecules file through in single file. In aquaporin-4, the predominant water channel in the brain, about eight water molecules can be found lined up inside each channel at any given moment.⁷ Blocking protons is especially important, because proton leakage would wreck the electrochemical gradients that cells depend on for energy production. Simulations have shown that the main barrier to proton passage is an electrostatic field generated by a highly conserved region of the protein, rather than a simple physical obstruction. The calculated energy barrier is high enough to match the natural resistance of the lipid bilayer itself, so aquaporins do not create a proton “leak” even as they massively increase water flow.⁸

Not All Aquaporins Are Strictly Water Channels

Some members of the aquaporin family break the “water only” rule. A subgroup called aquaglyceroporins can transport glycerol and certain other small uncharged molecules in addition to water.⁹ In humans, aquaporins 3, 7, 9, and 10 fall into this category. Their broader selectivity is physiologically important: aquaporin-7, for instance, is found in fat cells and helps release glycerol during the breakdown of stored fat, while aquaporin-9 in the liver imports that glycerol for use in making glucose. These channels link water transport to fat and sugar metabolism in ways that researchers are still untangling.

Aquaglyceroporins are not unique to mammals. The malaria parasite Plasmodium falciparum has its own version, which helps it take up glycerol for building the lipid membranes it needs as it multiplies inside red blood cells.¹⁰ Other protozoan parasites also carry aquaporin genes, making these channels potential drug targets.¹¹

How Cells Regulate Their Water Channels

Having permanently wide-open water channels would be dangerous. If a cell suddenly found itself in a more dilute environment, uncontrolled water entry could swell and burst it. Cells manage this risk by regulating aquaporins at multiple levels: they can change how many channels sit in the membrane, they can open or close existing channels (a process called gating), and they can adjust these responses in seconds to minutes.

Gating has been studied in detail for several aquaporin types. Changes in pH, for example, can cause conserved amino acids within the channel to rearrange and physically block the pore.¹² In the lens of the eye, aquaporin-0 is regulated by calcium levels. When calcium rises inside the cell, a signaling protein called calmodulin binds to the tail end of aquaporin-0 and, through a chain of structural shifts spanning the length of the protein, squeezes the pore shut. This allosteric mechanism lets the cell fine-tune water flow in the lens, which needs precise internal fluid balance to stay transparent.¹³

The kidney provides one of the most dramatic examples of aquaporin regulation. In the collecting duct, the hormone vasopressin (also called antidiuretic hormone) controls how much water is reabsorbed from urine back into the bloodstream. Vasopressin triggers the movement of aquaporin-2-containing vesicles from storage compartments inside the cell to the cell surface, rapidly increasing the number of water channels facing the urine side. It also ramps up production of new aquaporin-2 protein. When vasopressin levels drop, those channels are pulled back inside the cell, and the collecting duct becomes much less permeable to water, so more water stays in the urine.¹⁴ Defects in this system cause conditions like nephrogenic diabetes insipidus, where the kidneys cannot concentrate urine properly and patients produce enormous volumes of dilute urine.¹⁵

What Happens When Water Movement Goes Wrong

Because water moves freely across cell membranes in response to concentration differences, anything that changes the solute concentration outside cells will shift water in or out. When the fluid surrounding cells becomes too concentrated (hypertonic), water leaves cells and they shrink. This shrinkage is not just a minor inconvenience. Severe hypertonicity can cause organ damage and death.¹⁶ In the opposite direction, when surrounding fluid is too dilute, water rushes in and cells swell. Cells respond to these osmotic shocks within seconds, with membrane tension tracking volume changes in real time as the cell scrambles to re-equilibrate.¹⁷

Brain edema illustrates how aquaporins can be both part of the problem and part of the solution. Aquaporin-4 is heavily concentrated in the brain, particularly in the end-feet of astrocytes, the support cells that form barriers between brain tissue and fluid compartments like blood and cerebrospinal fluid.¹⁸ In cytotoxic edema, the kind caused by stroke or water intoxication, water rushes into brain cells and swells them. Mice engineered to lack aquaporin-4 are partially protected from this type of swelling, with a roughly 35% reduction in brain enlargement after an experimental stroke.¹⁹ But those same mice fare worse when edema is caused by fluid leaking out of blood vessels (vasogenic edema), because aquaporin-4 is also needed to clear excess water out of brain tissue. The channel that lets water in during one type of injury is the same channel the brain needs to pump water out during another type. This dual role makes aquaporin-4 a complicated therapeutic target.

Lab studies show just how dominant aquaporin-4 is in brain water transport. When researchers measured water permeability in cultured brain astrocytes from mice lacking the channel, the rate was about seven times lower than in normal astrocytes.²⁰ In other words, in these cells, diffusion through the lipid bilayer accounts for only a fraction of total water movement; the vast majority flows through aquaporin-4.

Plants Depend on Aquaporins Even More Than Animals

Plants face a constant challenge: they need to move large amounts of water from roots to leaves, and they need to do it through cells that are enclosed by rigid cell walls. Aquaporins are central to this process. Plants have far more aquaporin genes than animals, often dozens, and they use them to rapidly adjust water permeability at key bottleneck points in the root and leaf. By changing how many aquaporins are present in a membrane and whether those channels are open or closed, plants can modify how easily water flows through specific tissue layers, acting almost like adjustable valves.²¹

This regulation works in concert with stomata, the pores on leaf surfaces that control gas exchange and water loss. When soil water is plentiful, aquaporins can be upregulated to keep pace with transpiration. Under drought stress, plants dial down aquaporin activity to conserve water. The speed of these adjustments, happening within minutes through protein modification and trafficking, gives plants a faster response than relying on gene expression alone.

Bacteria and the Oldest Water Channels

Aquaporins are ancient. Even bacteria have them, which makes sense given that single-celled organisms living in soil or water face sudden, dramatic swings in osmotic conditions. When E. coli experiences a sudden drop in the salt concentration of its surroundings, water floods in and can increase the pressure on the cell membrane four- to fivefold. The bacterium’s aquaporin is normally repressed when cells are growing in high-salt conditions. By suppressing the channel under those circumstances, the cell slows the rate of water entry if it suddenly encounters a more dilute environment, buying time to activate other defenses like mechanosensitive channels that can release pressure.²² This is a surprisingly sophisticated bit of regulation for a single-celled organism, and it hints at how fundamental water management has been throughout the history of life.

How Pathogens Exploit Host Water Channels

Some pathogens have figured out how to hijack a host cell’s aquaporins for their own purposes. The parasite Cryptosporidium parvum, a common cause of waterborne diarrheal disease, needs to push its way into the cells lining bile ducts. Research has shown that the parasite recruits host aquaporin-1 and a glucose transporter to the spot where it attaches to the cell. Glucose uptake at that site drives localized water influx through aquaporin-1, and the resulting water entry helps inflate a membrane protrusion that the parasite uses to pull itself inside the cell. Blocking either the glucose transporter or aquaporin-1 hampers the parasite’s ability to invade.²³

Plant pathogens have their own strategies. Certain bacterial plant pathogens deliver proteins into plant cells that form water- and solute-permeable channels, effectively poking extra holes in the host membrane. Experiments in the model plant Arabidopsis showed that cells expressing one of these bacterial proteins swelled more than control cells that only had their normal aquaporins, demonstrating that the bacterial channel increases water permeability beyond what the plant’s own channels provide.²⁴ This flooding of the host cell likely helps the pathogen access nutrients and disrupts normal cell function.

Aquaporin-Inspired Water Purification

The extraordinary selectivity and speed of aquaporins have caught the attention of engineers working on water filtration. Traditional desalination membranes face an inherent trade-off: making a membrane more permeable to water tends to let more contaminants through as well. Aquaporin-based biomimetic membranes aim to sidestep this trade-off by embedding aquaporin proteins (or synthetic mimics of them) into artificial membrane structures, achieving high water flow while maintaining tight salt rejection.²⁵

One recent design used a ceramic substrate to support a layer containing aquaporin vesicles, achieving water flux rates that outperform many conventional polymer membranes while maintaining high selectivity.²⁶ Fully synthetic artificial water channels, designed to mimic aquaporin structure without using biological protein, are also under development for desalination applications.²⁷ These are still largely in the research stage, but the concept of borrowing a design that evolution spent billions of years refining is appealing.

The Search for Aquaporin-Targeting Drugs

Given how many diseases involve disordered water transport, the idea of drugs that can block or enhance specific aquaporins has obvious appeal. Conditions like brain edema, certain cancers, glaucoma, and obesity all involve aquaporin subtypes that might, in principle, be druggable.²⁸ In practice, though, finding effective inhibitors has been frustratingly difficult. The channels are small, compact proteins with pores that are hard to access with conventional drug molecules. Many early reports of aquaporin inhibitors failed to hold up when other labs tried to reproduce the results, possibly because of technical issues with the assays used to measure water transport in the original studies.

There has been recent progress. Researchers solved the structure of human aquaporin-7 bound to a specific inhibitor using cryo-electron microscopy, showing the drug wedged into the intracellular side of the channel. Cancer cells treated with this compound showed reduced proliferation, suggesting a link between glycerol transport and tumor growth.²⁹ Other research groups are exploring whether targeting the regulatory machinery around aquaporins, rather than plugging the pore directly, might be a more practical approach.³⁰ No aquaporin-targeting drug is currently approved for clinical use, so this remains an area where the biology is far ahead of the pharmacology. But the structural blueprints are getting sharper, and the medical motivation is strong enough that the field keeps pushing.