Wednesday, September 26, 2012

Why Does Salt Melt Snails (And Not Us)?

If you’ve ever seen a kid put salt on a snail (something you would never do, right?), you know they whither like the Wicked Witch of the West in a hot tub. But why is something as commonplace as table-salt so deadly to these little critters? And if salt is so lethal, why do some people pay spas beaucoup bucks to apply a salt scrub all over their bodies?

The Wicked Witch of the West melts after Dorothy dumps a bucket of water on her. Illustration by William Wallace Denslow from the original The Wonderful Wizard of Oz, by L. Frank Baum, 1900. Image available at Wikimedia Commons.
The cells that animals are made up of are essentially bags of salty water (with a little bit of specialized biological machinery). The outside of these bags, called cell membranes, are not waterproof but they are somewhat solute-proof (Solutes are all the dissolved stuff in the cell, including salts). That is, water can freely pass through the membranes, but salts (and other solutes) either can’t pass through or do so very slowly.

When solutes are in water, they like to be evenly spread out. This is pretty easy if you just have a glass of water with salt in it. But let’s say you put a membrane in this glass that allows water, but not salts, to pass through it. Now if you put saltier water on one side of the membrane and less salty water on the other side, what do you think would happen? If the salt can’t cross to the less salty side, then the water will cross to the more salty side.

If a membrane holds more salt (pink dots) on one side than the other, the water will
move to the side with more salt so that the salt and water can be evenly spaced out.
This principle isn’t just true for saltwater in a glass, it is also true for the contents of animal cells in an environment that is more or less salty than the cells are. For animals that live in freshwater, their cells generally have more solutes than the water around them. These animals and their cells are hyperosmotic to their environment, meaning water is constantly entering the cells and the water pressure is higher inside the cell pressing outwards. Salts slowly leak out of these cells, which helps to prevent them from exploding. But this process causes a problem for these animals: Left unchecked, these animals would gain too much water and lose too many salts, leaving them too diluted to function properly. They have to work to push out water and take in salts. Freshwater animals are okay with this, because this is what their species are used to.

If a cell is hyperosmotic to its environment, the water will move into it and salts will
leak out so that the salt and water can be evenly spaced out. Eventually, the cell
won’t function properly unless it can reabsorb some of these salts.
But what would happen if you take an animal that is used to a freshwater environment and suddenly expose it to high levels of salt? Water would rush out of the cells to mix with the salt on the outside! That is exactly what happens when you put salt on a freshwater snail: The salt mixes with the water in the mucous layer on the animal and almost all the water inside the animal’s body comes rushing out to mix with the resulting salty paste. The snail’s cells haven’t had to bring water in quickly before (remember, they are used to pushing water out), so the snail’s body just isn’t prepared to prevent the rapid loss of body water.

A freshwater ramshorn snail has nightmares about kids with salt-shakers.
Image by Alan R Walker at Wikimedia.
But then why don’t we shrivel up if you put salt on us? Unlike freshwater snails, which have bodies adapted to life in water, we are terrestrial animals and we have bodies adapted to life out in the air. In the heat, we run the risk of losing too much water by evaporation. To prevent this water loss, our skin is more waterproof than the skin of a snail. Your more waterproof skin can protect you from quickly losing all of your body water when the lady at the spa rubs salt paste all over your body for a “salt treatment”. So enjoy it – I promise you won’t melt.

4 comments:

  1. Very interesting.
    Why do we get wrinkles in our finger tips when we have been for a long time in water?
    Thank you!

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    1. Great question! Our fingers and toes wrinkle in the bathtub for the opposite reason to why snails melt in salt. Our cells contain more solutes than the freshwater of a bath, so our cells are prone to absorbing water. The cells that make up the outermost layers of our palms and bottoms of our feet have properties that cause them to absorb and retain water even more than other cells do. Because they are connected to the cells underneath, their swelling causes the skin to pucker unevenly, creating the pruny-fingers/toes effect.

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  2. When i provided a similar answer to why our fingers get pruney in my 300-level Animal Physiology class, one of my students mentioned the following study published in Nature in June 2011:

    http://www.nature.com/news/2011/110628/full/news.2011.388.html

    Interesting food for thought ...

    --Erick

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    1. Fascinating theory Erick. Thanks for the link!

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