What is Dr. Zubair Khalid's research focus?

Dr. Zubair Khalid specializes in molecular virology, mRNA vaccine development, and computational biology, with a focus on avian pathogens like IBDV and Avian Reovirus.

Where is Dr. Zubair Khalid currently working?

Dr. Zubair Khalid is a Postdoctoral Research Associate at the University of Maryland (UMD), specifically within the Department of Animal and Avian Sciences.

Hypotonic Solution: Definition, Effects, and Examples

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-09-20

A hypotonic solution is a fluid whose concentration of dissolved solutes is lower than the concentration inside a cell, so water moves into the cell by osmosis. Because the cell gains water faster than it can lose it, the cell swells, and an animal cell without a rigid wall can eventually burst in a process called lysis.

That single definition drives a surprising amount of biology. Hypotonic solutions are how laboratories break open red blood cells to harvest hemoglobin and membranes, how plant cells generate the turgor pressure that holds leaves upright, and how a slug's cells would rupture if you dropped the animal into distilled water. The same principle explains why a red blood cell placed in 0.45% sodium chloride swells and why the same cell in distilled water disappears entirely. Tonicity is one of the clearest demonstrations that a cell is a physical object with a membrane, a volume, and a limit.

What Makes a Solution Hypotonic

Tonicity describes how a solution affects cell volume. A solution is hypotonic when it has a lower effective solute concentration than the cytoplasm on the other side of the membrane. Water always moves from the side with more water (fewer solutes) to the side with less water (more solutes), so a hypotonic bath pulls water into the cell.

Three words get used together and they are not synonyms:

Osmolarity is a physical measurement. It counts the total number of dissolved particles per liter of solution, regardless of what those particles are or whether the membrane lets them pass.
Osmolality is the same idea expressed per kilogram of solvent instead of per liter of solution.
Tonicity is a biological property. It counts only the solutes that cannot cross the membrane, called nonpenetrating or impermeant solutes, and it predicts what happens to cell volume.

This distinction is the single most common source of confusion in the topic. A solution can be isosmotic (same total particle count as the cell) and still be hypotonic, because the solute crosses the membrane freely. Urea is the classic example. Urea raises the osmolarity of a solution, but because it permeates the red blood cell membrane, it does not hold water outside the cell. The cell behaves as if the urea is not there, and the result is complete hemolysis, exactly as if the cells were in distilled water [1]. Sodium chloride behaves the opposite way. It is isosmotic and isotonic at physiological strength because the membrane is nearly impermeable to sodium and chloride, so the salt stays outside and balances the osmotic pull of the cytoplasm [1].

The practical rule: tonicity is about the solutes that stay put.

Why Water Moves at All

Osmosis is the net diffusion of water across a selectively permeable membrane. The membrane allows water through easily, largely via aquaporin channels and simple diffusion through the lipid bilayer, but restricts most dissolved ions and large molecules. Water crosses a red blood cell membrane at a rate of roughly 100 times the cell's own volume every second, with flow in both directions. Under balanced conditions the two directions cancel and there is no net change. When the balance breaks, the net flow becomes visible as swelling or shrinking [2].

The driving force is water potential, which in a cell is dominated by the concentration of impermeant solutes. Water moves toward the compartment with the higher concentration of impermeant particles. In a hypotonic solution, that compartment is the inside of the cell.

The Cell as an Osmometer

A cell behaves like a tiny osmometer. Its volume is set by the balance between the osmotic pull of its internal solutes and the mechanical resistance of its membrane and any wall around it. When the outside solution is diluted, the balance tips and water enters until either the osmotic gradient is erased or the membrane's tension limit is reached. For a red blood cell, that limit is close. The cell has a fixed surface area and a fixed internal hemoglobin concentration, so it can only stretch so far before the membrane fails [3].

The Comparison Table: Hypotonic, Isotonic, and Hypertonic

The table below summarizes the three tonicity states. It is the reference point for everything else in this article.

Property	Hypotonic	Isotonic	Hypertonic
Solute concentration outside the cell	Lower than inside	Equal to inside	Higher than inside
Effective (nonpenetrating) solute	Low	Matched to cytoplasm	High
Net water movement	Into the cell	No net movement	Out of the cell
Animal cell outcome	Swells, then lyses	Stable volume	Shrinks (crenates)
Plant cell outcome	Swells, turgor pressure rises, wall resists lysis	Flaccid, no net change	Shrinks, plasma membrane pulls from wall (plasmolysis)
Example fluid	Distilled water, 0.45% NaCl, 0.3% NaCl	0.9% NaCl (normal saline), Ringer's solution	Concentrated seawater, high-sucrose syrup

Two entries deserve a note. First, the "effective solute" row is what makes the table biologically correct. A solution of urea can sit in the isotonic column for total osmolarity and in the hypotonic column for tonicity. Second, the plant cell row is not a minor exception. The cell wall changes the outcome completely, and that difference is why plants can live in fresh water while animal cells cannot.

What Happens to an Animal Cell in a Hypotonic Solution

Animal cells have no cell wall. Their only boundary is the plasma membrane, a fluid lipid bilayer with embedded proteins and a thin underlying cytoskeleton. That membrane can bend and fold, but it cannot stretch indefinitely.

Step by Step: From Swelling to Lysis

The gradient is set. The cell is placed in a solution with fewer impermeant particles than the cytoplasm.
Water enters. Net water flow crosses the membrane toward the higher internal solute concentration.
The cell swells. Volume rises, and the membrane unfolds from its reservoir of folds. A red blood cell changes from a biconcave disc to a sphere, a shape change that is visible under a microscope and measurable by light scattering [3].
The membrane reaches its limit. Once the cell is spherical and the membrane is fully stretched, further water entry raises membrane tension.
A pore forms and the cell lyses. A small defect opens, the cell cannot reseal fast enough, and hemoglobin escapes. This is hemolysis. In distilled water the process is complete, and every cell is destroyed [1].
Ghosts remain. After lysis, the empty membrane shells, called erythrocyte ghosts, can be collected by centrifugation. This is the standard first step for preparing red cell membranes and for loading drugs into red cells [4][5].

The swelling phase is fast. A red blood cell can go from normal disc to lysed in seconds to minutes depending on the size of the gradient. In a mildly hypotonic solution, only the most fragile cells burst and the rest survive, which is the basis of the osmotic fragility test [3].

Why Some Cells Resist

Not every cell pops at the same tonicity. Red cells from different donors differ, and stored red cells become more fragile over time. A proof-of-concept study found that after six weeks of hypothermic storage, about 9.5% of red cells had transformed into rigid spherocytes and sphero-echinocytes, and washing them in a mildly hypotonic saline (0.585 g/dL, osmolality about 222 mmol/kg) selectively burst those spherical cells while sparing the healthy discocytes. The treatment cut the spherical fraction roughly threefold and halved the exposure of phosphatidylserine on the surviving cells [6]. The lesson is that hypotonic lysis can be tuned. A gentle gradient removes the weakest cells, and a strong gradient removes all of them.

Some cells also fight back. Duck red blood cells placed in a nonhemolytic hypotonic medium first swell, then shrink back toward their original volume over a slower second phase. The recovery comes from a nearly isosmotic loss of potassium chloride and water, driven by a transient increase in potassium efflux. This volume-regulatory response works even when the sodium-potassium pump is blocked by ouabain, which shows it is a separate membrane mechanism that senses cell volume and adjusts ion loss accordingly [7]. Mammalian cells have their own regulatory volume decrease pathways, though they are usually slower than the initial osmotic swelling.

What Happens to a Plant Cell in a Hypotonic Solution

Plant cells behave differently because of the cell wall. The wall is a rigid mesh of cellulose and other polysaccharides outside the plasma membrane. It resists expansion, so as water enters, pressure builds inside the cell instead of the membrane stretching to failure.

That internal pressure is turgor pressure. Turgor is what keeps nonwoody plant tissue firm, drives cell expansion during growth, and provides the mechanical force for many plant movements. A plant cell in a hypotonic solution swells until the wall's resistance balances the osmotic pull of the cytoplasm. The cell reaches a new steady state and stops taking up water. It does not lyse under ordinary conditions because the wall carries the tension that the animal cell membrane cannot.

This is why distilled water is lethal to animal cells but harmless to most plant cells. It is also why a wilted plant recovers when watered. The soil solution becomes hypotonic relative to the cytoplasm, water enters the cells, turgor returns, and the tissue stiffens.

Plant scientists exploit the same physics in the laboratory. Isolated plant protoplasts, which are cells stripped of their walls by enzymatic digestion, are spherical and behave like animal cells. A standard assay perfuses protoplasts with an isotonic solution and then a hypotonic solution, video-records the swelling, and fits the volume change over time to calculate the osmotic water permeability coefficient of the membrane [8]. Removing the wall removes the safety margin, which is exactly what makes protoplasts useful for measuring membrane water transport.

Tonicity Versus Osmolarity: The Distinction That Matters

Students often treat osmolarity and tonicity as interchangeable. They are not, and the difference has real consequences.

Osmolarity is calculated from the total solute concentration. A 300 mOsm/kg solution of urea and a 300 mOsm/kg solution of sodium chloride have identical osmolarity. Tonicity asks a different question: which of those solutes can cross the membrane? Sodium chloride cannot cross a red cell membrane quickly, so it stays outside and holds water out. Urea crosses freely, so it equilibrates across the membrane and its osmotic effect disappears. The urea solution is isosmotic but hypotonic, and red cells in it hemolyze completely [1].

The general rule follows from this:

A solution is hypotonic if it has fewer nonpenetrating solutes than the cytoplasm.
A solution is isotonic if its nonpenetrating solute concentration matches the cytoplasm.
A solution is hypertonic if it has more nonpenetrating solutes than the cytoplasm.

Penetrating solutes do not count toward tonicity, no matter how much they raise the osmolarity. This is why the tonicity of a solution cannot be read off a label that lists only total osmolarity.

A Worked Example

Suppose a red blood cell has an internal nonpenetrating solute concentration equivalent to about 0.9% sodium chloride. Place it in 0.45% sodium chloride. The outside has half the impermeant solute of the inside, so the solution is hypotonic. Water enters, the cell swells, and because 0.45% NaCl is still a fairly mild gradient, hemolysis is incomplete. Some cells burst and others survive [1]. Now place the same cell in distilled water. There is essentially no impermeant solute outside, the gradient is maximal, and hemolysis is complete [1]. The two outcomes differ only in the size of the gradient.

Examples of Hypotonic Solutions in Biology and the Laboratory

Hypotonic solutions are not exotic. They show up across cell biology, medicine, and biotechnology.

0.45% Sodium Chloride

A 0.45% sodium chloride solution has roughly half the salt concentration of normal plasma and is hypotonic to it. Red cells suspended in 0.45% NaCl swell and undergo incomplete hemolysis, meaning a fraction of the cells burst while others remain intact [1]. The same solution is used in the laboratory as a controlled osmotic challenge. In one drug-loading study, red cells were exposed to buffers of 0.5%, 0.6%, and 0.7% NaCl to open pores in the membrane, allow pravastatin to enter, and then reseal the cells. The highest loading, about 34%, was achieved at 0.6% NaCl with 10 mg/mL drug over a 60-minute incubation, and the loaded cells showed no increase in fragility or oxidative stress markers compared with controls [9]. The hypotonic step is what makes the cell temporarily permeable.

Distilled Water and Complete Hemolysis

Distilled water is the extreme case. With no solutes outside, water floods into red cells until every membrane fails. Distilled water produces complete hemolysis, as does isosmotic urea, for the same underlying reason: both are hypotonic to the cell even though their total osmolarities differ enormously [1].

Hypotonic Lysis as a Purification Tool

The fragility of red blood cells is useful. Red cells lyse at milder hypotonicity than most nucleated cells, so a brief hypotonic wash removes red cells from a mixed sample while leaving the cells of interest intact.

A 0.3% NaCl solution effectively removed red blood cells from adipose tissue aspirates within a 10-minute incubation and did not affect the survival of adipose-derived stem cells. Its lysis efficiency matched that of 155 mM ammonium chloride, the traditional lysis reagent [10].
Exposure of cord blood to 0.2% saline for 5 minutes selectively lysed non-reticulocytes and increased the reticulocyte count about 3.6-fold. The remaining cells supported invasion and growth of the malaria parasite Plasmodium falciparum, which made the method useful for malaria research [11].
Hypotonic ammonium chloride lysis is a standard first step before flow sorting rare cell populations such as very small embryonic-like stem cells from umbilical cord blood [12].
Hypotonic lysis is also the standard way to prepare erythrocyte ghosts for membrane protein analysis, including in nonmammalian species such as the red-eared turtle [5], and to produce nano-erythrocyte membranes for drug delivery vehicles [4].

One caution comes from the same literature. Red cells that lyse release microvesicles, and those vesicles can transfer phosphatidylserine to the surface of nearby nucleated cells. The transferred lipid makes those cells stain positive with Annexin V, which is normally a marker of apoptosis, even though the cells are healthy and continue to proliferate [13]. Any protocol that uses hypotonic lysis before a viability assay has to account for this false signal.

Hypotonic Lysis in Biotechnology and Microbiology

The same principle scales up. Halophilic and thermophilic bacteria that produce polyhydroxyalkanoate bioplastics are naturally susceptible to hypotonic shock because their cytoplasm is adapted to high salt. Diluting them into a hypotonic sodium dodecyl sulfate solution at elevated temperature disrupts the cells and releases the polymer granules. The method recovered polyhydroxyalkanoate at purities above 99% and yields close to 1 [14]. Here the hypotonic gradient is not a side effect but the primary cell disruption mechanism.

Hypotonic Buffers in Cell Fractionation

Hypotonic swelling is also a routine step in subcellular fractionation. Cells are swelled in a hypotonic buffer, then gently homogenized and centrifuged to separate nuclei from cytoplasm. The swelling weakens the cell so that mechanical disruption releases intact organelles instead of shredding them [15]. The method works well for cultured cells and soft tissues but not for material with high intrinsic ribonuclease activity or tough tissue such as muscle [15].

Turgor in Plants

In plants, hypotonic conditions are the normal state. Soil water is generally more dilute than the cytoplasm, so water enters root cells, turgor rises, and the tissue becomes rigid. The cell wall converts what would be a lethal osmotic load in an animal into a structural advantage.

How Hypotonic Effects Are Measured

Several standard methods turn the swelling response into a number.

Osmotic fragility test. Red cells are suspended across a range of hypotonic salines, and the fraction that lyses at each concentration is measured. The resulting curve describes the population's resistance to osmotic stress. Light-scattering instruments can track individual cells as they sphere and burst, and the data yield estimates of cell volume, surface area, hemoglobin concentration, and membrane elasticity [3].
Hemolysis assays. Hemoglobin released into the supernatant is measured by absorbance or against hemoglobin standards, giving the proportion of cells that lysed. This is the basis of the classic teaching experiment in which red cells are placed in solutions of differing osmolarity and tonicity [1].
Blood film observation. A modified blood film technique applies different osmotic solutions directly to a film and lets students watch red cell shape change under a microscope, including the transition from discocyte to spherocyte to lysed ghost [2].
Volume tracking in single cells. For cells other than red blood cells, swelling is recorded by video microscopy and the volume change over time is fitted to a model to extract the membrane's water permeability coefficient. Isolated plant protoplasts are a common subject [8]. Mammalian cells such as COS-7 have also been measured this way, and the analysis revealed that water permeability is asymmetric, with endosmotic flow several times faster than exosmotic flow, a phenomenon called rectification [16].
Tonicity effects on adhesion. Tonicity also changes how cells stick. Normal red cells showed increased adhesion to the matrix protein thrombospondin-1 as they became dehydrated, while sickle cells showed the opposite trend with increasing hydration [17]. This shows that tonicity is not only about volume.

Why Tonicity Matters Beyond the Laboratory

Tonicity is a core concept because cells are constantly exposed to solutions that differ from their interior. The kidney adjusts urine concentration to protect cell volume. The gut absorbs water along osmotic gradients. Every cell culture medium is formulated to be isotonic so that cells do not swell or shrink during experiments. When the balance fails, the consequences are immediate and physical.

The red blood cell is the standard model because it is simple, abundant, and easy to observe. It has no nucleus or internal membranes to complicate the picture, and its lysis is easy to detect because hemoglobin is brightly colored. Almost everything known about osmotic behavior at the cell level was first worked out with red cells, and the same principles apply to every other cell type.

Common Mistakes and Limitations

A few errors show up repeatedly when students work with tonicity.

Treating osmolarity and tonicity as the same thing. They are not. A solution can be isosmotic and hypotonic at the same time, and urea is the standard counterexample [1]. Always ask whether the solute can cross the membrane.
Assuming all hypotonic solutions cause complete lysis. They do not. Incomplete hemolysis occurs in mildly hypotonic saline, and only the most fragile cells burst [1]. The degree of lysis depends on the size of the gradient and the condition of the cells.
Forgetting that plant cells have a wall. A plant cell in a hypotonic solution does not lyse under normal conditions. It swells until turgor pressure balances the osmotic gradient. The wall is the reason.
Ignoring cell history. Stored red cells become more fragile over time, and a hypotonic wash can selectively remove the damaged subpopulation [6]. Fresh and stored cells do not respond identically.
Overlooking artifacts from lysis. Hypotonic lysis releases microvesicles that can transfer phosphatidylserine to other cells and create false apoptotic signals in Annexin V assays [13]. Lysis is not a clean separation step.
Assuming volume regulation is instant. Some cells swell and then actively shrink back toward their original volume by losing ions and water [7]. The initial swelling is not always the final state.
Applying the concept to individual patients. Tonicity is a general principle. What it means for a specific person or a specific fluid depends on clinical context and requires a veterinarian or physician.

Quick Review

A hypotonic solution has a lower concentration of nonpenetrating solutes than the cell interior, so water moves in by osmosis.
Tonicity counts only impermeant solutes. Osmolarity counts all dissolved particles. A solution can be isosmotic and hypotonic, as with urea [1].
Animal cells swell and can lyse because they have no wall. Red cells in distilled water hemolyze completely [1].
Plant cells swell but resist lysis because the cell wall generates turgor pressure.
0.45% NaCl is hypotonic to plasma and causes incomplete hemolysis. 0.3% NaCl is used to lyse red cells during stem cell isolation [1][10].
Hypotonic lysis is a standard laboratory tool for purifying cells, preparing membranes, and disrupting microbial cells [14][4][5].
Lysis releases microvesicles that can falsely mark healthy cells as apoptotic in Annexin V assays [13].

Frequently Asked Questions

What is a hypotonic solution in simple terms?

A hypotonic solution has less dissolved solute than the inside of a cell, so water flows into the cell and makes it swell. If the cell has no wall, it can burst.

What is the difference between hypotonic and isotonic?

A hypotonic solution has fewer impermeant solutes than the cytoplasm, so cells gain water and swell. An isotonic solution matches the cytoplasm, so there is no net water movement and cell volume stays stable.

Why do plant cells not burst in a hypotonic solution?

The cell wall resists expansion. As water enters, pressure builds inside the cell until turgor pressure balances the osmotic pull, and the cell stops swelling before the membrane fails.

Can a solution be isosmotic but hypotonic?

Yes. If the dissolved solute can cross the membrane freely, it does not contribute to tonicity. Urea is isosmotic with blood but causes complete red cell hemolysis because it permeates the membrane [1].

What happens to red blood cells in distilled water?

They swell and burst completely. Distilled water has essentially no impermeant solute, so the osmotic gradient is maximal and hemolysis is complete [1].

Why is hypotonic lysis used in the laboratory?

Red blood cells are more fragile than most nucleated cells, so a brief hypotonic wash removes them from a mixed sample while leaving the cells of interest intact. It is used to purify stem cells, prepare erythrocyte membranes, and disrupt microbial cells [10][14][5].