WEBVTT 00:00:00.000 --> 00:00:06.040 (MUSIC) 00:00:06.460 --> 00:00:11.130 When we were kids, growing up in West Texas, our winters would be cold, but rarely experienced 00:00:11.130 --> 00:00:12.130 snow. 00:00:12.130 --> 00:00:14.880 But we did have ice, which resulted in the roads being salted. 00:00:14.880 --> 00:00:19.660 As the salt mixes in and dissolves into water on the road, this can lead to a lower freezing 00:00:19.660 --> 00:00:22.760 point, which can help prevent the roads from icing over. 00:00:22.760 --> 00:00:26.779 And while this is great for making the roads more safe, it wasn’t so great for the plants 00:00:26.779 --> 00:00:28.739 that lived right along the roadside. 00:00:28.739 --> 00:00:30.579 It often caused them to die. 00:00:30.579 --> 00:00:35.550 Now winter can be hard for many plant species, but I’m talking about this salt affecting 00:00:35.550 --> 00:00:37.820 even some hardy plant life. 00:00:37.820 --> 00:00:41.440 This issue with salt and plants isn’t limited to winter. 00:00:41.440 --> 00:00:46.539 During hurricanes near the coast, salty ocean water can be dumped in large quantities into the 00:00:46.539 --> 00:00:47.539 soil. 00:00:47.539 --> 00:00:49.920 This can eventually kill plants- including 00:00:49.920 --> 00:00:53.199 trees- that had originally survived the hurricane. 00:00:53.199 --> 00:00:54.309 Why? 00:00:54.309 --> 00:00:57.019 Do plants just dislike salt that much? 00:00:57.019 --> 00:01:00.909 Well, it's actually related to a term called osmosis. 00:01:00.909 --> 00:01:05.170 When you are talking about osmosis, you are talking about the movement of water through a 00:01:05.170 --> 00:01:08.370 semi-permeable membrane, like a cell membrane. 00:01:08.370 --> 00:01:13.020 Water molecules are so small that they can travel through the cell membrane unassisted, 00:01:13.020 --> 00:01:18.000 or they can travel in larger quantities through protein channels like aquaporins. 00:01:18.000 --> 00:01:22.170 The movement of water molecules traveling across a cell membrane is passive transport, 00:01:22.170 --> 00:01:25.430 which means, it does not require energy. 00:01:25.430 --> 00:01:30.740 In osmosis, water molecules travel from areas of a high concentration (of water molecules) 00:01:30.740 --> 00:01:33.020 to a low concentration (of water molecules). 00:01:33.020 --> 00:01:36.860 But there’s another way to think about water movement in osmosis. 00:01:36.860 --> 00:01:42.820 A low water concentration likely means there is a greater solute concentration. 00:01:42.820 --> 00:01:48.560 Solutes are substances like salt or sugar that can be dissolved within a solvent like 00:01:48.560 --> 00:01:49.560 water. 00:01:49.560 --> 00:01:55.570 Water has the tendency to move to areas where there is a higher solute concentration, which 00:01:55.570 --> 00:01:58.590 would mean less water concentration. 00:01:58.590 --> 00:02:03.820 So, if you want to easily figure out where the water will travel in osmosis, look to the side 00:02:03.820 --> 00:02:07.259 where there is a greater solute concentration. 00:02:07.259 --> 00:02:11.550 Unless we bring in another variable, like pressure, water will generally have a net 00:02:11.550 --> 00:02:14.900 movement to the area of higher solute concentration. 00:02:14.900 --> 00:02:16.290 So, let’s bring out a U-tube! 00:02:16.290 --> 00:02:17.300 Ha, U-tube. 00:02:17.300 --> 00:02:18.320 That’s funny. 00:02:18.320 --> 00:02:21.150 There’s a semi-permeable membrane in the middle of it. 00:02:21.150 --> 00:02:26.080 Let’s assume that it is similar to a cell membrane and that water molecules can squeeze 00:02:26.080 --> 00:02:29.680 through it—the molecules are quite small—but salt can’t. 00:02:29.680 --> 00:02:33.010 Right now, there is just water in this U-tube. 00:02:33.010 --> 00:02:36.360 The water levels on side A and side B are equal. 00:02:36.360 --> 00:02:39.910 That doesn’t mean that the water molecules aren’t moving---water molecules like to 00:02:39.910 --> 00:02:43.210 move---but the net movement across the two sides is zero. 00:02:43.210 --> 00:02:47.020 That means, the overall change in the direction of movement is zero. 00:02:47.020 --> 00:02:52.910 Now let’s imagine on side B, you dump a huge amount of salt there. 00:02:52.910 --> 00:02:59.400 So, which direction will the water initially move towards, A or B? 00:02:59.400 --> 00:03:01.870 Think about what we mentioned with osmosis. 00:03:01.870 --> 00:03:04.440 The answer is B! 00:03:04.440 --> 00:03:11.080 Side B has a higher solute concentration than side A. Water moves to areas of higher solute 00:03:11.080 --> 00:03:14.200 concentration, which is also the area of lower water concentration. 00:03:14.200 --> 00:03:20.340 You will also see the water level on side B rise as the water moves to that area. 00:03:20.340 --> 00:03:25.630 You can almost think of the water as trying to equalize the concentrations diluting 00:03:25.630 --> 00:03:30.540 side B. Once equilibrium is reached, the net movement of water across the two sides will 00:03:30.540 --> 00:03:35.080 be zero, but remember that water still likes to move and movement still occurs. 00:03:35.080 --> 00:03:40.840 Now here’s some vocabulary to add in here---we call side B hypertonic. 00:03:40.840 --> 00:03:44.790 That means higher solute concentration! 00:03:44.790 --> 00:03:49.240 But we can’t just say something is hypertonic without comparing it to something else. 00:03:49.240 --> 00:03:55.450 We say side B is hypertonic to side A because it has a higher solute concentration than 00:03:55.450 --> 00:04:00.560 side A. In osmosis, water moves to the hypertonic side. 00:04:00.560 --> 00:04:08.740 We say side A is hypotonic (hypo rhymes with low which helps me remember that it is the low solute concentration) 00:04:08.740 --> 00:04:12.550 when compared to side B. Let’s get a little more real life now instead 00:04:12.550 --> 00:04:13.550 of just the U-tube. 00:04:13.550 --> 00:04:18.470 As you know, water is important for your body and many processes that occur in the body. 00:04:18.470 --> 00:04:23.560 When someone gets an IV in a hospital, it may look like the fluid in the IV is just 00:04:23.560 --> 00:04:24.560 pure water. 00:04:24.560 --> 00:04:26.300 But it is certainly not pure water. 00:04:26.300 --> 00:04:29.039 That would be a disaster because of osmosis, let’s explain. 00:04:29.039 --> 00:04:32.680 Let’s say hypothetically pure water was in an IV. 00:04:32.680 --> 00:04:37.810 Now an IV tube typically runs through a vein, so that you have access to your blood stream, 00:04:37.810 --> 00:04:40.310 really useful for running medication through. 00:04:40.310 --> 00:04:44.120 Blood actually consists of many different types of components and red blood cells are 00:04:44.120 --> 00:04:45.730 a great example. 00:04:45.730 --> 00:04:49.210 So, what do you think has a higher solute concentration, 00:04:49.210 --> 00:04:52.849 the hypothetical pure water in this IV tube 00:04:52.849 --> 00:04:54.660 or the red blood cells? 00:04:54.660 --> 00:04:58.600 Well, cells are not empty vessels, they contain solutes. 00:04:58.600 --> 00:05:02.710 The pure water that hypothetically is running through this IV tube has no solutes. 00:05:02.710 --> 00:05:04.889 So, where does the water go? 00:05:04.889 --> 00:05:11.210 It goes to the area of higher solute concentration which in this case is inside the cells. 00:05:11.210 --> 00:05:16.259 The cells are hypertonic compared to the pure water in the IV tube because the cells have 00:05:16.259 --> 00:05:18.620 a greater solute concentration, 00:05:18.620 --> 00:05:21.379 the cells would swell and possibly burst! 00:05:21.379 --> 00:05:22.749 Exploding red blood cells are not good. 00:05:22.749 --> 00:05:27.139 If a person needs fluids, they typically will receive a solution that is isotonic to their 00:05:27.139 --> 00:05:29.060 blood plasma. 00:05:29.060 --> 00:05:33.849 Isotonic means equal concentration, so you won’t have any swelling or shrinking red 00:05:33.849 --> 00:05:34.849 blood cells. 00:05:34.849 --> 00:05:37.060 Another example, let’s talk about the aquarium. 00:05:37.060 --> 00:05:41.189 I have always wanted a saltwater fish tank, ever since I was a little kid. 00:05:41.189 --> 00:05:42.930 But I’ve only had freshwater tanks 00:05:42.930 --> 00:05:44.020 so far. 00:05:44.020 --> 00:05:48.300 I did often question when I was a kid, why is it that a saltwater fish can’t be in 00:05:48.300 --> 00:05:49.379 my freshwater tank? 00:05:49.379 --> 00:05:53.280 Well, let me explain one reason why this would be dangerous to a saltwater fish and how it 00:05:53.280 --> 00:05:55.490 relates to osmosis. 00:05:55.490 --> 00:05:59.080 First ask---where is there a higher solute concentration? 00:05:59.080 --> 00:06:00.930 In the saltwater fish cells 00:06:00.930 --> 00:06:05.460 or in the freshwater that the fish would be hypothetically placed in? 00:06:05.460 --> 00:06:07.409 Definitely in the saltwater fish cells. 00:06:07.409 --> 00:06:08.860 So, where would the water go? 00:06:08.860 --> 00:06:14.409 It goes to the area where there is a higher solute concentration----the hypertonic side----so 00:06:14.409 --> 00:06:17.219 it goes into the cells of that poor saltwater fish. 00:06:17.219 --> 00:06:19.090 If not rescued, it could die. 00:06:19.090 --> 00:06:24.710 Now one thing to clarify: saltwater fish and freshwater fish are not necessarily isotonic to 00:06:24.710 --> 00:06:25.710 their surroundings. 00:06:25.710 --> 00:06:30.550 But they have special adaptations that allow them to live in their environment and usually 00:06:30.550 --> 00:06:35.150 cannot make a major switch from a saltwater environment to a freshwater one. 00:06:35.150 --> 00:06:37.810 Now---not all fish have this problem. 00:06:37.810 --> 00:06:43.689 There are some fish that have this amazing adaptations to switch between fresh and salt water, and 00:06:43.689 --> 00:06:45.919 they have to deal with this osmosis problem. 00:06:45.919 --> 00:06:47.159 Salmon for example. 00:06:47.159 --> 00:06:50.880 I think if I could pick to be a fish, I’d be a salmon. 00:06:50.880 --> 00:06:51.880 Osmosis 00:06:51.880 --> 00:06:54.509 explains how many kinds of plants get their water. 00:06:54.509 --> 00:06:56.229 Sure, many plants have roots. 00:06:56.229 --> 00:06:58.729 But how does the water get into the roots? 00:06:58.729 --> 00:07:01.689 When it rains, the soil becomes saturated with water. 00:07:01.689 --> 00:07:06.370 The root hair cells generally have a higher concentration of solutes within them than 00:07:06.370 --> 00:07:09.340 the solute concentration in the saturated soil. 00:07:09.340 --> 00:07:14.310 The water travels into the root hair cells as the root hair cells are hypertonic compared to 00:07:14.310 --> 00:07:15.560 the hypotonic soil. 00:07:15.560 --> 00:07:21.789 By the way, you may wonder---well, why don’t those root hair cells burst with all the 00:07:21.789 --> 00:07:23.409 water that is going in them. 00:07:23.409 --> 00:07:28.860 That brings us to our next osmosis topic and why plant cell walls are amazing! 00:07:28.860 --> 00:07:34.980 So, let’s bring in another variable that can influence osmosis: pressure potential. 00:07:34.980 --> 00:07:40.370 This is when it’s very useful to understand how one can calculate water potential. 00:07:40.370 --> 00:07:45.800 Water potential considers both solute potential AND pressure potential. 00:07:45.800 --> 00:07:50.080 In osmosis, water travels to areas of lower water potential. 00:07:50.080 --> 00:07:55.919 So, the formula is water potential is equal to the pressure potential plus the solute potential. 00:07:55.919 --> 00:08:00.620 Adding solute actually causes the solute potential to have a negative value and the overall water 00:08:00.620 --> 00:08:02.960 potential to lower. 00:08:02.960 --> 00:08:06.479 Water will travel to areas of lower water potential. 00:08:06.479 --> 00:08:12.599 But exerting pressure can raise the pressure potential, a positive value, therefore raising 00:08:12.599 --> 00:08:13.860 the total water potential. 00:08:13.860 --> 00:08:16.430 So, let’s give a quick example. 00:08:16.430 --> 00:08:20.979 In the popular water potential in potato cores lab---all kinds of neat variations of this 00:08:20.979 --> 00:08:26.900 lab procedure exist online---you can calculate the water potential in potato cores using 00:08:26.900 --> 00:08:28.889 the water potential formula. 00:08:28.889 --> 00:08:34.451 When a potato core is first put into distilled water—that’s pure water---the potato core 00:08:34.451 --> 00:08:36.340 cells start to gain water. 00:08:36.340 --> 00:08:37.770 You’d expect that. 00:08:37.770 --> 00:08:40.270 The water is moving towards the higher solute concentration. 00:08:40.270 --> 00:08:46.030 Thanks to their higher solute concentration, they have a lower solute potential. 00:08:46.030 --> 00:08:50.820 That means a lower total water potential than the surroundings and water travels to areas 00:08:50.820 --> 00:08:53.020 of lower water potential. 00:08:53.020 --> 00:08:58.340 But over time as the potato core cells gain water, the water that has entered exerts pressure 00:08:58.340 --> 00:09:02.710 against the plant cell walls from inside the plant cells, 00:09:02.710 --> 00:09:07.280 therefore raising the overall water potential in the potato core cells. 00:09:07.280 --> 00:09:12.171 We want to point out that this turgor pressure that results in plant cells, thanks to osmosis 00:09:12.171 --> 00:09:17.210 and plant cell walls, is critical for overall plant structure and the ability of plants 00:09:17.210 --> 00:09:19.880 to grow upright and not wilt. 00:09:19.880 --> 00:09:22.490 Turgor pressure is definitely something to explore. 00:09:22.490 --> 00:09:25.740 In summary, where would living organisms be without osmosis? 00:09:25.740 --> 00:09:32.230 After all, it involves movement of one of our very valuable resources: water. 00:09:32.230 --> 00:09:35.080 Well, that’s it for the Amoeba Sisters and we remind you to stay curious!