Back To CourseCollege Biology: Help and Review
24 chapters | 433 lessons
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Katy teaches biology at the college level and did her Ph.D. work on infectious diseases and immunology.
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.
But wait...what is a semi-permeable membrane? It is 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 a solvent such as water, to cross, but other solutes cannot cross.
And what does it mean for water to flow down its concentration gradient? It means that it flows from where it is at a higher concentration to where it is at a lower concentration in order to try to equalize the concentration. The directions across concentration gradients are named like the currents of rivers: the direction that molecules or rivers tend to flow is called 'downstream,' and the direction that takes work is called 'upstream.'
So unlike chemistry information, which is at a higher concentration in your textbook than in your brain, it cannot simply cross your pillow to enter into your memory. Water does travel across our membranes by osmosis. Let's look at why osmosis matters and some examples of it.
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 we will discuss different types of 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 low concentration 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.
It's important to note that 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.
'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.
So, if our cells want to stay the same volume without swelling or shrinking, they need to be in isotonic solutions, 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 cells don't grow or shrink.
This image 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.
There are many everyday examples of osmosis. You can try this one yourself: if you put a potato into pure water, it swells up over time. This is because there's a much higher concentration of starch and other solutes inside the potato's cells than in the water, so water flows into the potato cells by osmosis. The same thing happens if you have a limp or wilted carrot stick. Put it in water for a while and it will plump up again!
Another example that some of us cruelly tried in our childhood is when you sprinkle salt onto a slug. The high concentration of salt on the outside of the slug causes water to come out of its cells by osmosis. The poor little slug shrivels up and dies.
Apart from these experiments, how is osmosis used in nature? Plant root cells take up water from the soil by osmosis. Additionally, our stomach and intestinal epithelial cells use osmosis to reabsorb water from our food as it digests so that we don't get dehydrated.
One final example is that some pathogenic bacteria (like Vibrio cholerae) can use toxins to interfere with our intestinal cells' ion transport channels. This creates a hypertonic environment in the intestinal lumen, which draws water out of the intestinal cells by osmosis. This is the cause of watery diarrhea that is a major symptom of cholera. It's handy for the cholera bacteria, too; they can hold on tight to the intestinal cells, while the normal bacteria that live in our gut get washed away. This gives the cholera bacteria plenty of territory to replicate and grow! Clever little bugs, right?
Let's review. We've learned that osmosis is the flow of water down its concentration gradient across semi-permeable membranes, like our cells' plasma membranes.
In hypotonic solutions, where the concentration of solutes is lower outside than inside the cell, water flows into cells and makes them swell. In hypertonic solutions, on the other hand, osmosis makes water flow out of cells, which shrivels them up. In isotonic solutions, cell volumes stay the same because the net flow of water in both directions is balanced.
|Osmosis||the flow of water down its concentration gradient, across a semi-permeable membrane|
|Diffusion||when molecules tend to distribute themselves evenly in a space|
|Semi-permeable membrane||a membrane or barrier that allows some molecules or substances to cross, but not others|
|Solutes||small molecules that are dissolved in a solvent such as water|
|Hypotonic solutions||the overall concentration of solutes is lower on the outside of a cell than in the cytosol|
|Hypertonic solutions||one in which the overall concentration of solutes is higher than it is in the cytosol|
|Isotonic solutions||the concentration of solutes is the same as the concentration in the cytosol|
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Back To CourseCollege Biology: Help and Review
24 chapters | 433 lessons