How Osmosis Moves Water Through Cells

Osmosis is the net movement of water molecules through a partially permeable membrane from a region with a higher concentration of water to a region with a lower concentration of water. Another way to say this is that water moves from a more dilute solution towards a more concentrated one. The membrane lets water pass but blocks at least some dissolved particles.

Water molecules actually move in both directions because they are always in motion. The word net matters. If more water crosses one way than the other, the overall result is movement in that direction. Osmosis continues until the difference becomes smaller or another force, such as pressure, balances the movement.

The dissolved substance does not pull water with a tiny rope. A concentrated solution simply contains fewer free water molecules per unit volume than a dilute solution. Thinking about the relative amount of water on each side is often easier than trying to imagine the solute giving orders.

An animal cell placed in a very dilute solution may gain water by osmosis. Because it has only a flexible cell membrane around it, the cell can swell and may eventually burst. In a concentrated solution, water leaves the cell, causing it to shrink. Cells therefore need surroundings that keep water movement within a safe range.

A plant cell also gains water in a dilute solution, but its rigid cell wall resists further expansion. Pressure builds inside the cell and makes it firm, or turgid. This pressure supports leaves and stems. The wall prevents the cell from bursting under ordinary conditions.

In a concentrated solution, a plant cell loses water. Its membrane may pull away from the cell wall as the contents shrink, a condition called plasmolysis. A wilted plant often has cells with reduced turgor pressure. Watering the soil can restore that pressure if the roots and tissues are still healthy.

When you face an osmosis question, identify the membrane and compare the solutions on its two sides. Then follow the water rather than becoming distracted by every dissolved particle. The final size change depends on the cell type and whether a supporting wall is present.

Use this order:

• •Find the partially permeable membrane.
• •Decide which side is more dilute.
• •Move water towards the more concentrated side.
• •Predict whether the cell gains or loses water.
• •Check whether a cell wall limits expansion.

A classic school experiment makes osmosis measurable. You cut equal cylinders of potato, weigh them, and leave each one in a different sugar or salt solution, from pure water to a strong solution. After some time you blot and reweigh them. Pieces in dilute solutions gain mass because water enters the cells, while pieces in concentrated solutions lose mass. If you plot percentage change in mass against concentration, the point where the line crosses zero shows roughly the concentration at which the potato tissue neither gains nor loses water overall.

You can see the same process in food. Sprinkling sugar on sliced strawberries draws juice out of the fruit, forming a syrup without any added liquid. Salting sliced cucumber pulls water from its cells, which is why cooks sometimes salt and drain it before making a salad. The reverse works too. Limp lettuce or celery often crisps up after soaking in cold water, because the cells take in water and their turgor pressure returns. Each case follows the same rule: water moves towards the more concentrated side of the membrane.

Osmosis matters in medicine. Fluid given through a drip into a vein must have roughly the same concentration of dissolved particles as blood, described as isotonic. A common choice is saline containing about 0.9 per cent salt. If pure water were put directly into the blood, red blood cells would take in water, swell and could burst. In a much stronger salt solution, they would lose water and shrivel. Matching the concentration keeps the cells stable while fluid is replaced.

The same principle explains why drinking seawater makes dehydration worse. Seawater is saltier than your body fluids, so your kidneys must use extra water to get rid of the excess salt, and you end up losing more water than you drank. Fish face this problem every day. A saltwater fish constantly loses water to the sea, so it drinks seawater and actively pumps out salt through its gills. A freshwater fish has the opposite problem. Water keeps entering its body, so it rarely drinks and produces large amounts of very dilute urine.

The inner layer of a cell membrane is made of fatty material that water crosses only slowly. For a long time scientists suspected that many cells must have dedicated channels for water, because some, such as red blood cells, swell or shrink far faster than a bare membrane would allow. In the early 1990s the American scientist Peter Agre and his colleagues identified the first of these proteins, now called aquaporins. They let water molecules pass in single file at great speed while blocking ions. The discovery earned Agre a share of the Nobel Prize in Chemistry in 2003.

Your kidneys use aquaporins to adjust how much water you keep. When you are short of water, the brain releases antidiuretic hormone, also called vasopressin. It signals cells lining the collecting ducts of the kidney to insert extra aquaporins into their membranes. Water then moves by osmosis out of the forming urine and back into the body, leaving a small volume of dark, concentrated urine. When you have drunk plenty, less hormone is released, fewer channels are inserted and more water passes out. Alcohol reduces the release of this hormone, which is one reason it makes people urinate more.

Osmosis can push hard enough to lift a column of liquid. Tie a partially permeable membrane over the mouth of an upturned funnel, fill it with sugar solution and stand it in pure water. Water moves in, and the liquid climbs up the funnel's narrow tube until the weight of the raised column presses back hard enough to balance the inflow. The pressure needed to stop osmosis completely is called the osmotic pressure. It rises with the concentration of dissolved particles, which is why seawater, at around 3.5 per cent salt, has an osmotic pressure more than 25 times atmospheric pressure.

Desalination plants turn this around. In reverse osmosis, pumps push seawater against a membrane at a pressure well above its osmotic pressure, often more than twice as high. Water is forced through from the salty side to the fresh side, leaving most of the salt behind as a concentrated brine. Reverse osmosis has become the main way of desalinating seawater worldwide, and smaller units purify tap water in homes and laboratories. Maple syrup producers use it too, removing much of the water from thin sap before boiling, which saves a large amount of fuel.

Osmosis is the net movement of water across a partially permeable membrane from a more dilute region to a more concentrated one. Animal cells can swell or shrink, while plant cell walls create supporting pressure and reduce the risk of bursting. Compare the two sides, follow the water and the result becomes much easier to predict.