Water passes through the cell membrane constantly. For a human red blood cell, it crosses at roughly 100 to 300 molecules per microsecond through dedicated channel proteins. Diffusion through the lipid bilayer also occurs, but far more slowly. The 2010s-era resolution of the protein structure question settled a long debate: the membrane is not a passive sieve for water but an actively engineered filter.
That single answer explains why cells swell in fresh water, why kidneys can concentrate urine, and why freezing damages tissue. Water movement is passive, but the route it takes is anything but simple.
The bilayer gate: how water actually crosses
Cell membranes are phospholipid bilayers about 4 to 5 nanometers thick. Water is a small, uncharged, polar molecule, roughly 0.28 nanometers across. The lipid tails inside the bilayer are oily and hydrophobic, so water does not dissolve in them easily. Diffusion through this barrier is slow and temperature-sensitive, requiring a molecule to shed some hydrogen bonding to enter and re-form it on exit.
The dedicated route is aquaporins. Aquaporin-1, found in red blood cells and kidney proximal tubules, is a channel of about 0.28 nanometers at its narrowest point, lined with a conserved asparagine-proline-alanine motif. That motif strips the hydration shell from a water molecule, allowing one at a time to pass. Aquaporins can hit rates of roughly 10^9 water molecules per subunit per second. That is about a thousandfold faster than bilayer diffusion.
The counterintuitive detail is selectivity. Aquaporins do not reject solutes by size alone. The channel's electrostatics block charged ions while admitting the polar water molecule, and a proline residue disables hydrogen-bonding where the channel narrows. If water really passed through bare lipid, cells would be far less responsive to osmotic changes. An aquaporin-enabled membrane behaves more like a programmable filter than a leaky bag.
Quantified context: water flux is fast, tuned, and measurable
Water moves down its chemical potential gradient, which biologists describe by osmosis. A difference in solute concentration across a membrane creates a driving force. A 300-milliosmolar solution on one side is about 7.7 atmospheres of osmotic pressure, roughly 770 kilopascals at room temperature. That is enough to burst many cells if the gradient is unopposed.
Osmotic flow depends on the membrane's water permeability coefficient. In cells with abundant aquaporins, the permeability is roughly 10 to 100 micrometers per second. With aquaporin-1 in the red blood cell membrane, transport is fast enough that a typical red cell can register an osmotic challenge within milliseconds. Without aquaporins, the same cell would take much longer and could deform under stress.
Medical relevance is direct. In neuroscience, water flow in the brain is governed by aquaporin-4, densely expressed at astrocyte endfeet. When AQP4 is genetically deleted in animals, brain edema after stroke forms more slowly. AQP4 organizes in microdomain arrays; its clustering state appears to change how water and possibly other molecules reflect across the membrane. Water flow also sets the basis for cell volume control.
One biological scaling fact matters. Kidney tubules reabsorb roughly 150 to 180 liters of filtrate daily. Most of that water returns to the body through aquaporin-dependent transport. A 70-kilogram adult holds about 40 liters of total body water. The kidney's ability to move that volume repeatedly, and not let it swamp the cells, comes directly from controlled water channels.
Comparison table: routes of water across membranes
| Route | Speed | Regulated? | Where it dominates | Notes |
|---|---|---|---|---|
| Lipid bilayer diffusion | Slow, about 1 to 10 micrometers per second | Weakly | Cells with few aquaporins | Temperature-sensitive, sensitive to bilayer composition |
| Aquaporin channel | Fast, up to 10^9 molecules per subunit per second | Yes, by expression and trafficking | Kidneys, red cells, brain astrocytes | High water selectivity, some allow glycerol |
| Cotransport with ions or glucose | Variable, coupled stoichiometries | Yes | Intestinal and renal epithelia | Water moves with solutes, not alone |
| Pinocytosis/endocytosis | Bulk, low | Yes | Some epithelial cells | Captures extracellular fluid, not a water channel |
| Cell membrane damage/pores | Uncontrolled | No | Injured tissue | Nonspecific leak, a pathology not physiology |
The table reconciles several thresholds. The 1-to-10 micrometer-per-second figure refers to pure bilayer permeability, not aquaporin-enhanced flux. Rates in the 10^9 molecules per subunit per second range are single-channel estimates, not whole-cell transport. They differ by design, because one is a per-membrane area measure and the other is a per-channel measure.
Edge cases: when water movement becomes uncontrolled
Aquaporin-1 deficiency is rare but revealing. In humans, mutations in the AQP1 gene have been reported in a small number of individuals with reduced urinary concentrating ability and reduced red cell water permeability. AQP1 has also been studied in sickle cell disease; changes in red cell water permeability could affect dehydration and sickling. These are findings from specific studies, not general clinical rules.
Another edge case is freezing. Ice formation extracts water from cells and concentrates solutes. Meats and vegetables change texture during freezing because water exits cells into extracellular ice and damages membranes. Cryobiologists add permeating compounds such as glycerol or dimethyl sulfoxide; these enter cells and alter freezing behavior, but the water channel is not the only route. Potent aquaporin-1 blockers remain experimental; clinical use would require evidence the block does not worsen dehydration or tissue stress.
Efflux is a specific and clinically striking case. In sickle cell disease, red cells undergo cation-driven dehydration; some experimental agents target the calcium-activated Gardos channel or other transport pathways. The FDA has approved the drug voxelotor (Oxbryta) for sickle cell disease, but its mechanism is hemoglobin-related, not aquaporin-related; in 2024 the manufacturer withdrew it from the market and the FDA announced a safety review. Water channels remain a research target, not a validated treatment pathway.
Misconceptions that persist
Misconception one: the cell membrane is freely permeable to water. The bilayer itself is only modestly permeable. Aquaporins are what make the membrane fast. A cell with many aquaporins behaves differently from one with few, and that difference is roughly 100-fold in some systems.
Misconception two: water channels are the key to detox or "hydration." Oral rehydration solutions work because sodium and glucose cotransport drives water absorption in the intestine, not because aquaporins are switched on by beverage choice. Water itself is absorbed across intestinal epithelia by multiple routes.
Misconception three: water and solutes cross equally. Aquaporins are narrow and electrostatic; charged ions such as sodium and chloride are rejected. That selectivity is what separates water balance from solute balance.
Misconception four: water movement requires an energy source. Osmotic flow is passive. The energy is spent building the osmotic gradient, for instance by active sodium transport by the sodium-potassium ATPase. A cell does not "pump" water; it pumps the solute that water follows. That single reframing resolves most confusion about which processes use ATP. It is the gradient, not the water, that costs energy.
Practical actions: thresholds you can act on
The knowledge gap is knowing when water movement matters and what to do with that knowledge. Here are concrete limits and actions.
If you need hydration during diarrhea, choose an oral rehydration solution with sodium roughly 75 and glucose roughly 75 millimoles per liter to exploit cotransport. A sports drink with sodium around 20 millimoles per liter is a poor replacement for severe diarrhea losses. Avoid drinking large volumes of plain water quickly after intense exercise. Hyponatremia has been reported at intake rates of roughly 1 to 1.5 liters per hour over several hours, mostly in endurance settings. Slow it down, and replace some water with sodium-containing fluids. If you use a "hydration multiplier" or salt packet, check the sodium content on the label. Acute dietary sodium recommendations for adults are about 2,300 milligrams per day (1,500 for high-risk groups), not a fluid protocol. For tissue preservation, freeze slowly or use cryoprotectants only under established protocols. That is not a home rationale for better ice; membrane damage during freezing depends on cooling rate, not just water channels. For research or laboratory work, do not infer channel abundance from water flow rate without a permeability test; whole-cell water transport depends on both bilayer composition and aquaporin density.
Limitations: what this answer does not cover
Water transport is more nuanced than a single channel. Aquaporin expression varies among cell types and diseases. Coexpression of different aquaporin isoforms, and regulation by phosphorylation or trafficking, changes the coefficient without obvious structural change. Water also crosses via the membrane's lateral gaps and through membrane defects, so the effective permeability is not simply the sum of the two pathways.
The 2024 withdrawal of the approved voxelotor (Oxbryta) illustrates why publication status and drug availability must be checked; a 2019 approval was not a permanent policy. That withdrawal is not a statement about aquaporins. It also should not be read as the end of red cell hydration research, but it is a reminder that trial evidence and labeling change.
The better answer for a clinician or a curious reader is that water passes through the cell membrane at a rate roughly 100 times higher with channels than without. The number is about permeability, not a philosophical point. Whether a kidney concentrates urine, a brain swells after injury, or a frozen steak loses juice comes down to the same physics: water follows its gradient, and whether the membrane has a fast lane is what decides the pace.
The deeper point: why "pass through" is the wrong question
The useful question is not whether water can pass, but how fast and under what control. A lipid bilayer is an insulating wall with a small water leak. An aquaporin is a selective, high-throughput pore. A cell's response to swelling, and a tissue's response to injury, depends on the numbers, not the word "pass."
That distinction explains why the kidney reabsorbs so much water, why red cells shrink or swell within seconds, and why the topic sits atop cell biology and clinical medicine at once. Water crosses the membrane. The mechanism is the membrane engineering, and the engineering is the scalable part.