Take an animal cell out of a body and put it in a glass of fresh water. Nothing will hold it back. Water will move in through the membrane, the cell will swell, and eventually it will burst. Put the same cell in concentrated salt water and the reverse happens: it shrinks away from its own contents.
A plant cell has a cell wall that resists both fates. A bacterial cell has one too. An animal cell has neither, and no skeleton, and no organ it can hand the problem to. Everything it does about this, it does with its own membrane and its own metabolism.
This is the part that sits underneath osmoregulation. When an animal controls the salt concentration of its blood, it is a colony of cells each doing the small private version of the same arithmetic.
The Cell Has No Pump for Water
The first thing to get straight is what a cell is actually able to move. Water moves by itself, down its own gradient, through any membrane it can cross. Nobody pumps it. There is no water pump to build, which means a cell cannot solve a water problem by working harder at the water.
What it can move is solute, and it can move some of those energetically. So a cell that wants to change how much water it holds has exactly one lever available: it changes what is dissolved inside itself, and lets the water follow.
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| Water moves toward the more concentrated side, and it stops when the two match — Image: KDS4444, CC0, via Wikimedia Commons |
This is why osmotic pressure is described as a colligative property. It depends on how many particles of solute are dissolved, not on what those particles are. Sugar and salt at the same molarity exert the same pull. The name means "bound together", because the property belongs to the collection rather than to the members.
The membrane itself is selective rather than open. Biological membranes generally refuse large and polar molecules. Ions, proteins and polysaccharides all stay out. Non-polar and hydrophobic ones like lipids pass freely, as do small molecules such as oxygen, carbon dioxide and nitric oxide. Water sits awkwardly in the middle. It is small enough to squeeze past, but it is polar, and that turns out to matter a great deal.
Two Problems That Are Not the Same
Most accounts collapse everything into one scale, more water or less water. A cell faces two genuinely different emergencies, and the fix for one is not the fix for the other.
| The outside is... | What the cell does | The end result |
| Hypotonic, with solutes the membrane cannot pass | swells, because water is drawn in and cannot leave by the same route | cytolysis: the membrane bursts and the contents escape |
| Hypertonic, with solutes the membrane cannot pass | shrinks, and compensates by pulling in salt and releasing water | dehydration, and if it continues, apoptosis |
| Isotonic, with matching total concentration | holds steady, and spends nothing on volume at all | nothing, which is why it is the cheap case |
The three cases, named by what the membrane will not let through rather than by what is dissolved in total
Tonicity is not the same as osmotic pressure
This is the distinction the original post never drew, and it is the one that makes the table above work.
Osmotic pressure counts every dissolved particle. Tonicity counts only the particles the membrane cannot pass. Those are different questions, so the two answers can differ completely.
A solution can so be strongly hypotonic and still leave the cell perfectly comfortable. Put urea in the water and the urea walks straight through the membrane until it is equally concentrated on both sides. Once it has equilibrated, it exerts no net pull at all, and no water moves. A solute that crosses freely cannot make a cell swell.
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| The same cells photographed under three different conditions, and the difference is entirely how much water they were holding — Image: Zephyris, CC BY-SA 3.0, via Wikimedia Commons |
This is why distilled water is described as the extreme hypotonic case. It has nothing in it, so nothing in it can cross over to even things out, and the cell has no defense at all.
What sets the solute in the first place
Before a cell can respond to a change outside, it has to be holding something in the first place, and that something is set by pumps.
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| Three sodium out for two potassium in, paid for with one ATP — Image: OpenStax, CC BY 4.0, via Wikimedia Commons |
The sodium-potassium pump sits in the membrane of every animal cell. For each ATP it spends, it exports three sodium ions and imports two potassium ones. That leaves sodium concentrated about five times higher outside the cell than inside, and potassium about thirty times higher inside than outside.
Every one of those ions is an osmole. The pump is so not just keeping nerves firing and cell volume stable. It is quietly setting the solute concentration that the whole water-balance arithmetic runs on. Jens Christian Skou found it in 1957, and got a Nobel Prize for it in 1997.
The cell that bursts
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| A red cell is a convenient subject, because it has no nucleus and no organelles to lose — Image: Blausen Medical 2014, CC BY 3.0, via Wikimedia Commons |
When a cell swells far enough, the membrane runs out of room. The process has a name, cytolysis, and water gets in by two routes at once: plain diffusion through the membrane, and the aquaporins, which make the flow much faster.
A cell wall prevents this, which is why plants and bacteria do not burst. Cytolysis is specific to animal cells and protozoa, the two groups with no wall. The reverse process, in which a walled cell pulls away from its contents as water leaves, is called plasmolysis.
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| Failure looks like this: intact cells give up their contents to the surrounding fluid — Image: Vioximiko, CC BY-SA 4.0, via Wikimedia Commons |
Rupturing red blood cells and releasing their contents into the surrounding fluid is hemolysis, and it is the same event happening to one very specific cell type. In a body it can be caused by bacterial and fungal toxins called hemolysins, and it can also be caused by nothing more sinister than intense physical exercise.
The Protist Solved It Without a Kidney
The last point is historical, and it is the reason this is a cell problem rather than a vertebrate problem.
A contractile vacuole is a sub-cellular structure involved in osmoregulation. It is found predominantly in protists, including unicellular algae, and it used to be called a pulsatile or pulsating vacuole instead.
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| One cell, no organs, solving the same arithmetic — Image: Deuterostome, CC BY-SA 3.0, via Wikimedia Commons |
A Paramecium is a eukaryotic unicellular ciliate, and it lives in freshwater, brackish and marine water alike. It is common in stagnant basins and ponds, and it is easy to culture. Because it divides and conjugates on demand, it has been used in classrooms for a very long time. It is called the lab rat of its phylum for that reason.
Freshwater is the hardest possible case, because it is the most hypotonic thing a cell can sit in. Water arrives continuously and never has anywhere to go. The contractile vacuole is how that cell answers. It answers with a membrane-bound organelle, inside a single cell, with no kidney and no circulation anywhere in the picture.
Which puts the whole subject in order. Volume regulation is not a clever adaptation that vertebrates invented. It is one of the oldest problems a cell has, and every solution since, aquaporins and osmosensors and kidneys alike, is a variation on it.
The Doorway Is a Protein, Not a Hole
If water were moving like a small non-polar molecule, the phospholipid bilayer would be enough. It is not moving like one. Water is polar, and the bilayer slows it down badly.
So cells built dedicated machinery for it. Aquaporins are channel proteins, belonging to a larger family of major intrinsic proteins, which form pores in the membrane and move water between cells. Bacteria, fungi, plants and animals all carry them.
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| The channel, resolved as a structure rather than drawn as a speculative cartoon — Image: David Goodsell, CC BY 3.0, via Wikimedia Commons |
An aquaporin is built from six membrane-spanning alpha-helical domains, with both the carboxylic and the amino ends sitting on the cytoplasmic side. Two hydrophobic loops carry a conserved asparagine-proline-alanine sequence, the NPA motif, and these loops form a barrel around the central pore.
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| Water passes; most of what is dissolved does not, which is what makes the barrel a filter rather than a hole — Image: Opossum58 and Peter Wolber, CC BY-SA 3.0, via Wikimedia Commons |
The effect is that water crosses far faster than it would on its own, which effectively raises the water permeability of the membrane. In one direction that is a fair trade. In the other it is a problem, because a cell that cannot stop water coming in has to be able to stop it fast.
The discovery was important enough to be recognised. The 2003 Nobel Prize in Chemistry went to Peter Agre for the discovery of aquaporins, and to Roderick MacKinnon for his work on the structure and mechanism of potassium channels.
Where This Becomes an Organ
At the level of one cell, all of this is manageable. Across billions of cells it becomes physiology, and that is where the existing posts on this blog take over. Only the outline is needed here.
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| The organ-level machinery that a single cell runs locally — Image: Holly Fischer, CC BY 3.0, via Wikimedia Commons |
Aquaporin 1 is the version that has been characterised most thoroughly. It sits in the basal and apical membranes of the proximal tubules, in the descending limb of the loop of Henle, and in the descending part of the vasa recta. It is also in red blood cells, in vascular endothelium, in the gastrointestinal tract, sweat glands, lungs and the central nervous system.
So the cell-level rule set out above is not confined to kidney cells. It is running in a red blood cell with no nucleus, and in a neuron, using the same channels and the same arithmetic.
When the machinery is inherited in a broken state, the consequences are clinical. Genetic defects involving aquaporin genes are associated with nephrogenic diabetes insipidus, where the kidney cannot concentrate urine, and with neuromyelitis optica, a disease of the optic nerve and spinal cord.
What the Cell Actually Moves
Here is the part that surprises people. To change its volume, a cell does not take in or push out patches of its own membrane.
Volume changes happen without any major change in membrane area, meaning no large-scale exocytic insertion of membrane and no endocytic retrieval of it. Instead, volume is regulated almost entirely by transporting potassium, sodium, chloride and organic osmolytes across the membrane. The membrane stays the same size. What changes is how much is dissolved inside it, and the water follows that change around on its own.
The specific machinery for the swollen case is the volume-regulated anion channel, or VRAC. These channels carry chloride ions and organic osmolytes such as taurine and glutamate across the plasma membrane, and some research has suggested they are water-permeable as well.
The process is called regulatory volume decrease. A cell that has swollen opens these channels, loses chloride and its organic cargo, and the water that was following them leaves too. The specific role of VRACs in volume regulation is this decrease step.
It is worth being clear that these channels are not a specialized valve opened only in emergencies. Volume has to be managed throughout every stage of a cell's life, and VRACs appear to be widely expressed in mammalian cells, possibly everywhere. The same channels also turn up in cell proliferation, migration and apoptosis.
Why Salt Is Not Enough
Given all this, the obvious design would be to move a lot of salt and leave it there. Cells do exactly that, and then they do something else as well, for reasons that are worth understanding.
The extra something is osmolytes, low-molecular-weight organic compounds that influence the properties of biological fluids. They regulate osmotic pressure and hold cellular homeostasis together, particularly when conditions change.
| Class | Examples named in the sources | What they do |
| Amino acids | taurine, and the amino acids proper | contribute to volume control without behaving like salt |
| Sugars and polyols | the sugar alcohols | raise solute concentration and steady protein folding |
| Methylamines | including betaine and its relatives | carry charge while staying chemically uncharged overall |
| Methylsulfonium compounds | sulfur-based analogues of the methylamines | add solute without disturbing the protein environment |
| Urea | the protein-breakdown product | balances osmotic stress, at the cost of being a denaturant at high levels |
The five classes of osmolyte, and why a cell keeps small organic molecules in reserve as well as salt
Their main protective role is subtler than pushing back on water. They affect the viscosity, the melting point and the ionic strength of the fluid inside the cell, which is how they hold its integrity together. They also interact directly with the cell's other constituents, and they influence how proteins fold.
That is the key. Salt is a blunt instrument: it works osmotically, and it also competes with everything else that needs those ions. A cell can hold its osmotic balance with a compound that gets in nobody's way. It can then stockpile a lot of that compound without disturbing the proteins that make the cell work.
Taurine shows why this needs a different sort of molecule. Its IUPAC name is 2-aminoethanesulfonic acid. It carries a sulfonic acid group where an amino acid would carry a carboxylic acid. That single swap is why it is not used to build proteins, and why it is not part of the genetic code.
It is abundant across mammalian tissues, a major constituent of bile, and the human liver makes it from cysteine and methionine. It is named after the Latin for a bull or ox, because it was first isolated from ox bile in 1827. Despite all that, it is not an essential human nutrient, and it has no recommended intake. It is also not quite an amino acid, and it sits outside the standard definition of one.
Betaine is built the same way and for the same reason. It has a permanently positive quaternary ammonium group with no hydrogen attached to the nitrogen, and a separate carboxylate group. It is a zwitterion that cannot flip between forms, because there is no labile hydrogen to move. Historically the word referred specifically to trimethylglycine, which works in methylation reactions and in clearing homocysteine.
Nobody Is Running the Cell
All of this assumes the cell notices. It does, and the arrangement is the one physiologists have recognised since the nineteenth century.
The idea of regulating the internal environment was developed by the French physiologist Claude Bernard. The word for it, homeostasis, was coined by Walter Bradford Cannon in 1926. The mechanism they describe is negative feedback: sensors detect the change, control mechanisms coordinate a response, and effectors act to counteract the deviation.
Water balance in an animal works the same way at the organ level, and osmoregulation is formally described as being detected by osmoreceptors. But the same three-part arrangement works inside a single cell, with membrane proteins as the sensors and effectors.
There is one more move available to a cell that wants to change its water permeability quickly, and it is worth naming because it is counter-intuitive. Rather than altering the channels already sitting in the membrane, the cell can move them.
Aquaporin 2 is the clearest example. It is found in the apical membranes of the collecting duct cells of the kidney, and also in intracellular vesicles distributed through the cell. Moving vesicles to the surface changes how many water channels the membrane has in it, and so how fast water can move.
Key Terms in Plain English
These are the words in this article that would send you to a dictionary. Each one is given here the way it is actually used above.
| Term | What it means here |
| Osmosis | water moving on its own across a membrane it can pass, toward the side holding more of what it cannot pass. Nobody pumps it |
| Osmotic pressure | the pressure that would have to be applied to stop that movement. It counts dissolved particles and ignores what they are, which is why it is called a colligative property |
| Tonicity | the same question asked only about solutes the membrane cannot pass. Different answer from osmotic pressure, because a freely crossing solute does not count |
| Aquaporin | a protein channel built specifically to let water through a membrane quickly. Found in bacteria, fungi, plants and animals |
| Water channel | another name for an aquaporin. It describes what it does rather than what it is made of |
| Osmole | one dissolved particle that counts towards osmotic pressure. The count is of particles, not of substances |
| Semipermeable | passing the solvent but holding back the solute. It is the arrangement that makes osmosis possible in the first place |
| Cytolysis | a cell bursting because too much water got in. Only happens in cells with no cell wall, which means animal cells and protozoa |
| Hemolysis | the same bursting, restricted to red blood cells, with the contents released into the surrounding fluid |
| Osmolyte | a small organic molecule kept in reserve to manage osmotic pressure and stabilise proteins, rather than a salt |
| Zwitterion | a molecule carrying both a positive and a negative charge, while overall being neutral |
| Contractile vacuole | the freshwater osmoregulatory organelle of protists and unicellular algae, once called a pulsatile vacuole |
| Homeostasis | holding internal conditions steady by negative feedback: sensors detect, control mechanisms respond, effectors correct |
These are the terms that carry the argument above. Every one of them is used here in the sense given here, and nowhere else.
Key Takeaways
- A cell cannot pump water. It can move solute, and water follows solute, so every water-balance mechanism is really a solute mechanism.
- Osmotic pressure counts every dissolved particle. Tonicity counts only the particles the membrane cannot pass, so the two can disagree entirely.
- A freely crossing solute cannot make a cell swell, because it equilibrates across the membrane and then exerts no net pull.
- Water is polar and crosses the bare lipid bilayer only slowly. Aquaporins are dedicated channels that make it fast, and won a Nobel Prize in 2003.
- Volume is regulated by moving potassium, sodium, chloride and organic osmolytes, not by inserting or retrieving patches of membrane.
- Swelling is corrected by regulatory volume decrease, in which volume-regulated anion channels let chloride and osmolytes out and the water leaves with them.
- Osmolytes exist because salt is a blunt tool: small organic molecules raise solute concentration without disturbing the proteins that make the cell work.
- Failure has two directions. Too much water means cytolysis; prolonged shrinkage leads to dehydration and then apoptosis.
- The contractile vacuole solves the same problem in a single protist cell with no organ at all, which shows how old the problem is.
Frequently Asked Questions
Why do animal cells swell but plant cells do not?
The cell wall. Water gets into a plant cell just as readily, but the rigid wall outside the membrane resists the expansion and stops the membrane tearing. Cytolysis is specific to cells with no wall, which in practice means animal cells and protozoa.
Is tonicity the same as osmotic pressure?
No. Osmotic pressure depends on the total concentration of dissolved particles. Tonicity depends only on those particles the membrane cannot pass. Because only impermeant solutes can hold water on one side, they are the only ones that affect a cell's volume.
Can a solution be hypotonic but not cause swelling?
Yes, and this is where the distinction earns its keep. If the extra solute can cross the membrane, it diffuses through until it is equally concentrated on both sides, and then it creates no net osmotic pull at all. The solution is hypotonic by total concentration while exerting no tonicity whatsoever.
Do cells only use aquaporins when they are in trouble?
No. Aquaporin 1 is described as widely expressed. It appears in red blood cells, in vascular endothelium, in the gastrointestinal tract, in sweat glands and in lungs. It is also in the central nervous system, and of course in the kidney. Constant water traffic across a membrane is normal, not an emergency.
Why does a cell use organic molecules when salt would work?
Because salt is a blunt instrument. Osmolytes raise the solute concentration and influence protein folding, and they do it without competing for the ions the cell needs for everything else. A cell can accumulate a great deal of them without disrupting the proteins it depends on.
What happens if a cell cannot regulate its volume?
The two directions end differently. Swelling leads to lysis. Shrinking leads from dehydration to apoptosis, which is a programmed and rather more deliberate death than bursting.
Sources: Wikipedia articles on osmosis, tonicity, aquaporins, aquaporin 1, aquaporin 2, the sodium-potassium pump, contractile vacuole, Paramecium, cytolysis, hemolysis, volume-regulated anion channels, osmolytes, taurine, betaine, plasma membrane, osmoregulation and homeostasis. Images: Wikimedia Commons, with authors and licenses noted in each caption.
