Water moves across cell membranes by osmosis to try to equalize the concentration of solutes on both sides of the membrane. In this lesson, learn how osmosis works and examine some examples of its importance in biology.
We also recommend watching Osmosis, Diffusion and Saturation and Passive Transport in Cells: Simple and Facilitated Diffusion & Osmosis
Definition
Did anyone ever tell you that you should sleep with your chemistry book under your pillow to 'learn by osmosis'? Sounds like an easy way to study for an exam, but as anyone who's ever tried it knows, it doesn't actually work.
But osmosis is real! Osmosis is the flow of water down its concentration gradient, across a semi-permeable membrane. Osmosis is an example of diffusion, which is when molecules tend to distribute themselves evenly in a space.
What's a semi-permeable membrane? It's a membrane or barrier that allows some molecules or substances to cross, but not others. An everyday example is the plastic wrap in your kitchen: it allows air and water vapor to travel across it, but not water or food. The membranes of cells are semi-permeable, too. They allow water and certain solutes (small molecules that are dissolved in asolvent such as water) to cross, but other solutes cannot cross.
Osmosis Changes Cell Volume
Depending on the direction that water flows across a plasma membrane, osmosis can cause a cell to shrink or swell. Take a look at this diagram to understand why, then read further to learn about the different types of solutions.
Hypotonic Solutions
When the overall concentration of solutes is lower on the outside of a cell than in the cytosol, we say that the cell is in a hypotonic solution (hypo means low). In hypotonic solutions, water flows into the cell by osmosis to try to equalize the concentration of solutes on both sides of the membrane. This means that in hypotonic solutions, our cells swell up. They can even burst!
But wait, didn't we say earlier that water flows across membranes down its concentration gradient? However, the diagram shows the water in the hypotonic solution moving from where there is a lowconcentration of solutes to where there is a high concentration. Wouldn't that be up the concentration gradient? Well, yes, it would be up the solute's concentration gradient, but still down the water's concentration gradient. Just remember that, where there's a higher concentration of solutes, there is a lower concentration of water, and vice versa. So everything checks out: the water is still flowing down its concentration gradient.
Hypertonic Solutions
Hyper means high, so a hypertonic solution is one in which the overall concentration of solutes is higher than it is in the cytosol. In a hypertonic solution, water flows out of the cell to try to even out the concentration of solutes on both sides of the membrane. This makes cells shrink or shrivel up.
Isotonic Solutions
So if our cells want to stay the same volume, without swelling or shrinking, they need to be in isotonicsolutions, where the concentration of solutes is the same as the concentration in the cytosol. Here, the osmotic flow of water into and out of the cell is the same, so the cell doesn't grow or shrink.
Blood Is Isotonic
The image below shows red blood cells in hypertonic, isotonic and hypotonic solutions. As you can see, the cells shrivel up, look normal, and swell up, respectively. The liquid part of our blood is an isotonic solution, which keeps our blood cells 'happy'. If we need intravenous injections into our blood stream, nurses and doctors make sure to use only isotonic solutions.
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Cell Walls Help Cells Control Their Volumes
Plant, algal, fungal and bacterial cells have tough cell walls surrounding their plasma membranes. This means that in a hypotonic solution, the cells swell up but don't burst. Instead, the pressure inside the cell increases. That's one reason why plant stems can stand upright. Animal cells don't have cell walls, so they have to regulate their volumes in other ways, such as controlling ion transport.