Background
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Woody Hagge |
All living things must continually have water to survive. Some species are adapted
to spend their lives in fresh water. Others are adapted only to ocean (salty) water.
And some--like loons and manatees--can live in both! If YOU drank just half a glass
of saltwater, you'd very likely vomit--a good thing because it would protect your
body tissues from the life-threatening dangers of too much salt.
Why is salty water so dangerous to living things that aren't adapted for it? This
investigation using carrots to find out about osmosis can help shed light on that
question. Carrots, (like humans) are made mostly of water. Carrots are made of cells,
just as other organisms are. Every cell contains water, and a membrane or cell wall--rather
like a tiny water balloon-- surrounds every cell. Find out what saltwater does to
carrot cells.
Students will become familiar with the term and meaning of osmosis: the movement
of fluid through a semipermeable membrane until there is an equal concentration of
fluid on both sides of the membrane.
Materials
- A whole, large, raw carrot
- Salt
- Two glasses (made of see-through glass)
- String or dental floss
- Warm water
Activity
- Before you begin, establish that carrots contain water. Ask students to describe
raw carrots they've snacked on. Have they ever chosen a wet-looking carrot stick
over a dried-up looking carrot stick? Why? (Most people think carrots are better
when the water in them is not dried out.)
- Break (snap) the carrot in half. Wind 2 or 3 turns of string just above the broken
ends and tie a knot to hold it snug (but not cutting into the carrot).
- Fill the glasses half-full of warm water. In ONE of the glasses, dissolve 3 or
4 tablespoons of salt. (Salt dissolves faster in warm water.)
- Place the carrots in the glasses with the cut/snapped end down in the water.
Let them remain there overnight.
- Ask students to predict what will happen to each carrot.
- The next day, observe each carrot for changes. Lift out each carrot and tug on
the strings to see what happened to the carrot's size. What do students observe?
(One will have shrunk and the other will have expanded.)
Discussion
- Why did one carrot shrink and the other expand?
- What caused this? ( There was water and a lot of salt on one side of the carrot's
cell walls. On the other side was the cell's water. The only way for saltiness to
be balanced was for the water inside the cells to flow through the cell wall to the
saltwater to make it less salty. Osmosis happens until the amount of saltwater on
one side of the membrane equals the amount of saltwater on the other side of the
membrane. But there's too much salt in the saltwater for the water on both sides
of the membrane to ever be balanced. As a result, the cell loses its precious water,
collapses, and dies.)
- What could you do to prevent this from happening? (Someone might suggest coating
the carrot with Vaseline, etc.) Try your ideas.
- Why is drinking saltwater a bad idea for humans?
Extensions
1. Create water as salty as the ocean to understand saltwater's density and its
comparison to fresh water. Pass out a cup or glass to each student. To create a solution
roughly equal in salinity to ocean water, use these proportions: 4 ounces (118.8
cubic centimeters or milliliters) of warm water mixed with 5/6 teaspoon (4.1 cubic
centimeters or milliliters) of salt. Have students stir until the salt dissolves.
They can taste (but not drink) the solution. For a large-scale comparison, explain
that ocean water contains about 3.5 pounds (1.6 kilograms) of salt in every 100 pounds
or 45.4 kilograms (around 12 gallons or 45.4 liters) of water.
2. As a class, discuss what makes salt a health concern to people. Why do we hear
so much about low sodium diets and eating too much salt?
3. Interested students may want to find out what adaptations enable animals to live
in saltwater.
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