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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.

You will need

• Large potato
• Chopping board
• Large sharp knife
• Metal cookie cutter, or sharp knife and ruler
• 2 small bowls
• Table salt
• Water
• Measuring cup
• Tablespoon
• Blank label
• Pen

What to do

1. Put the potato onto the chopping board and get an adult to cut it into slices, about one centimetre thick.
2. Add half a cup of water to each bowl.
3. Add a tablespoon of table salt to one of the bowls and stir until it is dissolved. Label this bowl so you remember which is which.
4. Use the cookie cutter to cut two shapes from the potato slices. If you haven’t got a cookie cutter, ask an adult to help you cut squares of potato exactly 5 x 5 centimetres in size.
5. Place one potato shape into each bowl and leave them for one hour.
6. Take the potato shapes out of the bowls. What do you notice? Does one slice feel softer and floppier than the other?
7. Try to fit the shapes back into the cookie cutter. Which one fits in? If you cut squares of 5 x 5 centimetres, measure the slices and see if they are bigger or smaller than 5 x 5 centimetres.

What’s happening?

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.

In real life: Osmosis in the ocean

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!