During osmosis, water molecules move through a semi-permeable membrane from an area with fewer dissolved particles to an area with more dissolved particles. This net movement continues until the concentration of dissolved substances is balanced on both sides, or until enough pressure builds up to stop further flow. Osmosis is the primary way water moves in and out of living cells, and it plays a critical role in everything from how your kidneys filter blood to how plants stay upright.
How Water Moves Through a Membrane
A semi-permeable membrane is a barrier with openings small enough to let water pass through but too small for larger dissolved molecules or ions. Think of it like a screen door that lets air through but keeps insects out. During osmosis, water molecules pass freely in both directions through this membrane at any given moment. But because there are more free water molecules on the dilute side (where fewer solutes are taking up space), more water crosses toward the concentrated side than crosses back. The result is a net flow of water toward the higher concentration of dissolved particles.
This process is a dynamic equilibrium. Water never stops moving in both directions, but the overall trend is always toward the side with more solutes. The force driving this movement is called osmotic pressure, and the side with the highest solute concentration generates the greatest osmotic pressure, pulling water toward it.
What Happens to Cells in Different Solutions
The concentration of dissolved particles surrounding a cell determines whether that cell gains water, loses water, or stays the same size. This property is called tonicity, and it has three basic categories.
In an isotonic solution, the concentration of dissolved particles outside the cell matches the concentration inside. Water moves in and out at equal rates, so the cell volume stays unchanged.
In a hypotonic solution, the surrounding fluid has fewer dissolved particles than the cell’s interior. Water flows into the cell by osmosis, causing it to swell. In animal cells, which lack a rigid outer wall, this swelling can eventually rupture the cell. In plant cells, the rigid cell wall prevents bursting, and the cell simply becomes firm and pressurized.
In a hypertonic solution, the surrounding fluid has more dissolved particles than the cell’s interior. Water flows out of the cell, causing it to shrink. Animal cells become wrinkled and compacted. Plant cells undergo a more dramatic change: the cell membrane pulls away from the rigid cell wall in a process called plasmolysis, and the plant tissue wilts.
Cells aren’t entirely passive in this process. When animal cells find themselves in a hypertonic environment, they activate transport proteins in their membranes that shuttle ions inward, raising the internal solute concentration to draw water back in. This response, called regulatory volume increase, helps cells recover from shrinkage over minutes to hours. With prolonged exposure, cells switch to a longer-term strategy, accumulating specialized organic molecules that raise internal concentration without disrupting the cell’s proteins.
How Aquaporins Speed Up the Process
Water can slowly seep through cell membranes on its own, but most cells dramatically accelerate osmosis using dedicated water channels called aquaporins. These are tiny protein tunnels embedded in the membrane that are shaped to let water molecules through in single file while blocking ions and other solutes. The selectivity comes from the physical size and electrical charge of the channel’s interior, which repels anything larger or differently charged than a water molecule.
Because each individual aquaporin moves a relatively small amount of water, cells that need high water flow pack their membranes densely, sometimes reaching 10,000 aquaporin channels per square micron of membrane surface. Your body expresses about a dozen types of aquaporins, concentrated in tissues where rapid water transport is essential: kidney tubules, salivary glands, the lining of the lungs, the eye’s cornea and lens, and the digestive tract. They also appear in less obvious places like red blood cells, fat cells, and the supportive cells of the brain and spinal cord.
Osmosis in Your Kidneys
Your kidneys are one of the most osmosis-dependent organs in the body. Every day, they filter a massive volume of fluid from your blood, then reclaim most of the water before it reaches the bladder. About 70% of filtered water is reabsorbed in the first stretch of the kidney’s filtering tubes (the proximal tubule), and another 20% is recovered in a deeper loop-shaped section called the thin descending limb of Henle.
This descending limb is impermeable to dissolved salts but highly permeable to water. As filtered fluid passes through it, the surrounding kidney tissue has a progressively higher concentration of solutes, creating an osmotic gradient that pulls water out of the tube and back into the body. This gradient is generated and maintained by a counter-current system that concentrates salts in the tissue around the loop. The result is that urine becomes more concentrated as it moves deeper into the kidney, and the body conserves water without needing to actively pump it.
How Osmosis Keeps Plants Upright
Plants rely on osmosis for their structural support in a way that animals don’t. When a plant cell absorbs water by osmosis, the expanding water pushes the cell membrane outward against the rigid cell wall. This internal pressure, called turgor pressure, is what makes plant tissue feel firm and keeps stems, leaves, and petals from drooping.
Turgor pressure serves two roles. At the level of individual cells, it stretches the cell wall and is thought to be the primary force driving cell growth. At the tissue level, the combined pressure from millions of turgid cells gives the plant its shape and structural strength. The outermost layer of cells bears the most mechanical stress, acting like a pressurized skin that holds the internal tissue together. When water is scarce and cells lose turgor, the tissue softens and the plant wilts. This is why a dehydrated houseplant droops but perks back up within hours of watering: the cells rapidly regain water through osmosis and re-pressurize.
Reverse Osmosis: Working Against the Flow
Osmosis naturally moves water toward higher solute concentrations, but applying enough external pressure can force it in the opposite direction. This is the basis of reverse osmosis, used widely in water purification and desalination. By pushing saltwater against a semi-permeable membrane at high pressure, pure water is forced through while salts and contaminants are left behind.
The pressure required depends on how salty the source water is. Brackish (mildly salty) water requires 15 to 30 bar of pressure, while seawater desalination demands 55 to 70 bar, roughly 55 to 70 times normal atmospheric pressure. That high pressure is needed to overcome the natural osmotic pressure of seawater, which would otherwise pull water back toward the salt. The same principle operates in household reverse osmosis filters for drinking water, though at much lower pressures since tap water has far fewer dissolved solids than seawater.