The Effect of Salinity on the Rate of Osmosis

Just as the surface area of an object affects the rate of osmosis, so too does the salinity of the solution that the object is in. The salinity of the solution will also determine whether water goes in or out of the object, or if there is no net osmosis.

As an example, consider a jellyfish. In the ocean, the salinity of the cells inside the jellyfish is exactly the same as the salinity of the ocean, so there is no net movement of water into or out of the jellyfish. But what would happen if the jellyfish was placed in fresh water? The jellyfish cells would have a higher salinity than the fresh water, and thus, assuming the fresh water had a very low salinity, water would continuously move into the jellyfish until it finally burst! Conversely, if it was placed in some water that was saltier than the ocean (about 3.5% salt), then water would move out of the jellyfish, causing it to shrivel up. There’s something to do with all the jellyfish that wash up on the beach…

Another example is when children pour salt on garden slugs. The salt dissolves in moisture on the outside of the slug, creating extremely salty water. Water moves out of the slug until it is completely dehydrated and dies.

But what exactly is the effect of salinity on the osmosis of living tissue? I used stems from a pigweed plant (Portulaca sp.) and a pigface plant (possibly Carpobrotus glaucescens) to answer this question. Both these plants are relatively fleshy, so I decided that they would show a greater rate of osmosis than a harder plant. I used 3 stems (around 4 or 5 cm long) from each plant, weighing around 2 grams.

To create solutions of varying salinity, I weighed out 0.50, 1.00, 2.00, 5.00 and 10.00 grams of table salt (NaCl), then added sufficient tap water to bring the total mass of each jar to 100.00 grams (±0.03 g). I also had pure tap water as 0% salinity. I weighed the stalks for each treatment group, placed them in the solution for 1 hour, then dried them with paper towel and reweighed them. The greatest change in mass observed was 0.11 g, so a scale with 0.01 g resolution is definitely necessary.

The results are shown in the graph below.

effect-of-salinity-on-the-rate-of-osmosis

Both plants reacted somewhat similarly to different salinities. The pigweed showed a nice curve, starting at around 4% in the tap water solution, and finishing at about -5% in the 10% salt solution. The pigface had a similar pattern, but as is often the case, the most interesting part is the anomalies. In the tap water solution, the increase in mass was far less than in the 0.5% or the 1% salt solutions (2% instead of 4 or 5%). Why would this happen? I didn’t do any replicates, so there is no way to tell whether a random error caused this result, or if it is something unique to pigweed. As most scientists know, where there is an anomaly, there is something interesting waiting to be discovered. Or else something went wrong with the experiment…

This definitely needs further experimentation to sort out what is going on, so if you’re interested, grab some pigweed and give it a go. All you need is a scale, pigweed, salt, a few jars, and some paper towel. This could easily turn into a complex experiment, maybe even an EEI. Other species could also be tested – do they show the same pattern, or do they have other mysterious anomalies? Explaining the anomalies would be the hardest bit.

Another interesting point to note is: where the graph crosses the x-axis, the change in mass is 0, so the salinity at that point must be the average salinity of the plant tissue. Makes sense when you think about it.

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