Use your cookie cutter to cut out 2 identical shapes
Animals that live in salty water have to control the amount of salt in their bodies, or they will not survive. But how do they do it? This activity will help show you.
Safety: This activity uses a sharp knife. Ask an adult to cut the potato slices for you.
The potato slice or shape in the fresh water should feel hard and firm, and should not fit back in the cookie cutter (or should measure slightly more than 5 x 5 centimetres). The shape in the salty water should feel a bit soft and floppy, and should easily fit back in the cookie cutter with space to spare (or should measure slightly less than 5 x 5 centimetres).
The potato pieces have enlarged in the fresh water and shrunk in the salty water because of a process called osmosis (oz-MOH-sis). The potato pieces are made of many tiny cells, which contain some water. Osmosis is the process of water moving across the membranes that surround the potato’s cells. It will move either in to or out of the cells, depending on how salty the water is. Water will always move from where it is less salty to where it is saltier. It does this to try and make the concentration, or amount, of salt inside and outside the cells the same.
So, for the potato in the fresh water, water will move from where it is less salty (in the bowl) to where it is saltier (inside the potato), making the potato swell up and feel firm. For the potato in the salty water, water will move from where it is less salty (inside the potato) to where it is saltier (in the bowl), making the potato shrink.
Animals that live in salty water use osmosis to balance the amount of salt and water in their bodies. They have to stop their bodies losing water, as the water will want to go from where it is less salty (in their bodies) to where it is saltier (the ocean).
Different animals have different ways of regulating the amount of salt inside them. Marine invertebrates (animals without a backbone), such as sea cucumbers, lobsters and crabs, have lots of salt inside their bodies. This stops water moving out – and also makes them taste salty.
Most fish constantly lose water by osmosis, so they have to take in lots of water to make up for it. Their gills have special cells that help get salt out of their bodies. Sharks and sea turtles also have a special gland that helps them get rid of salt.
Marine mammals, such as dolphins and whales, can get rid of salt by making their wee very salty. This wee can look much darker than the wee of us land-dwelling humans!
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25 November, 2022 at 11:58 am
What an elegantly simple and understandable way to demonstrate osmosis. Fantastic.