A Cool Story About an Antarctic Fish
by Jack Myers
Senior Science Editor
1 The key word is ice.
2 That describes the ocean around the edges of Antarctica. Far from land, a giant shelf of ice meets the ocean. At the underside of the shelf, a jumble of crushed ice and slush provides a home to a world of algae and tiny animals. In that icy soup there also lives a small fish, the Antarctic cod.
3 For forty years scientists have been curious about that fish. How does it live where most fish would freeze to death? It must have some secret. The Antarctic is not a comfortable place to work, and research has been slow in solving the problem. Now it seems we have an answer.
4 Research was begun by cutting holes in the ice and catching the fish with hook and line. Scientists studied the fish's blood and measured its freezing point, the temperature at which ice crystals just begin to form.
5 The fish were taken from seawater that had a temperature of 28.6 degrees Fahrenheit (F) and many ice crystals floating in it. The blood did not begin to freeze until its temperature was lowered to 28.3 degrees F. That small difference is enough that the fish can live at the freezing temperature of the ice-salt mixture.
6 The scientists' next research job was clear: Find out what kind of stuff in the fish's blood kept it from freezing. Their search led to some really weird stuff made up of a protein never before seen in the blood of a fish. When this stuff was removed, the blood froze at seawater temperature. When it was put back, the blood again had its antifreeze character and a lowered freezing point.
7 Making Ice Before deciding what to do next, the scientists thought about what happens to water when it freezes. That process takes place molecule by molecule. Water molecules easily moving around as a liquid suddenly become locked into position in an ice crystal.
8 In pure water, freezing begins to happen when it is cooled to 32.0 degrees F, which is its freezing point. Anything dissolved in the water is made up of atoms or molecules, which get in the way of water molecules. By crowding in, they make it harder for water molecules to lock together into an ice crystal. That lowers the freezing point.
9 Lots of substances can be used as antifreezes. Ethylene glycol works well in the radiators of automobile engines. Another antifreeze so cheap that we use it in winter on roads and sidewalks is plain old table salt. Seawater has enough salt to lower its freezing point to 28.6 degrees F.
10 Melting is just the opposite of freezing. It happens when water molecules get warm enough and zippy enough to bounce out of ice crystals and move around as liquid water. The lowest temperature at which that begins to happen is the melting point.
11 For water and for solutions of most substances, scientists take the melting point and freezing point to be the same temperature. Only a tiny temperature change determines whether ice crystals are forming or melting.
12 Super Antifreeze It was easy to find out that the new fish protein must be very different from any known antifreeze. Its molecules are about a hundred times more effective than salt molecules in lowering the temperature needed to form ice crystals. And the crystals that form take on oddball shapes.
13 The strangest thing about this new protein antifreeze is that it lowers the freezing point but not the melting point. Blood that has the antifreeze in it will not freeze above 28.3 degrees. Once that same blood is frozen, it will not begin to melt at 28.3 degrees. The blood will not melt until it goes all the way up to 32 degrees again. This is a big surprise, and means the stuff works in some way that scientists don't yet understand.
14 Study of the molecular structure of the new fish antifreeze showed that it is an unusual kind of protein. It has many small sugar molecules held in special positions within each big protein molecule. Because of its sugar content, it is called a glycoprotein. So it has come to be called the antifreeze fish glycoprotein, or AFGP.
15 There is one more part to the story of AFGP. How does it work to be such a powerful antifreeze?
16 We don't yet have a complete explanation, but we do have a pretty good idea. Chemists have learned to tell a lot about the behaviour of a molecule just from its structure. Their idea is that the sugar groups are all on one side of the molecule.
17 Sugars are so sticky to water molecules that they are called "water-loving" groups. They stick to the water molecules at the surface of an ice crystal.
18 The other side of the AFGP molecule has only "water-hating" groups. They tend to stay away from water, to stay dry. That gives the ice crystal a dry surface and makes it hard for water molecules to add onto an ice crystal.
19 You can see that it's easy to think of AFGP preventing the growth of ice crystals and the freezing of water. Of course this is just an idea yet to be proven. So there is still something more to be learned from fish that swim among ice crystals.
Copyright © 1998 by Highlights for Children, Inc., Columbus, Ohio.
1. According to the article, why have scientists waited so long to study the Antarctic cod?
Not enough scientists have been curious about the fish.
Algae and tiny animals clog the slush.
It is not easy to work in the Antarctic. ✔
The reason for the delay remains a secret.
2. What is the MAIN purpose of this article?
To persuade readers to support scientific study of Antarctic fish.
To entertain readers with a funny story about an unusual fish.
To explain how the Antarctic cod can live in freezing waters. ✔
To describe what it is like to live in a freezing cold ocean.
3. What is the MAIN purpose of paragraph 2?
To define important terms that will be used throughout the article.
To describe the setting and the main topic of the article. ✔
To capture the interest of the reader with simple, direct language.
To introduce the main characters and describe a problem that will be explored.
4. According to the article, what keeps the Antarctic cod's blood from freezing?
A layer of fat in its body
A salt mixture in its blood
Ice crystals around its body
A protein in its blood ✔
5. According to paragraphs 8 and 9, why is the freezing point of seawater lower than the freezing point of pure water?
Seawater contains several kinds of antifreeze.
Seawater particles stick together into ice crystals.
Seawater contains ethylene glycol.
Seawater contains a great deal of salt. ✔
6. According to the article, how do antifreezes work?
by sticking to water molecules
by raising the temperature of water to 32 degrees F
by making it more difficult for water molecules to attach to each other ✔
by dissolving ice crystals as they form
7. According to paragraph 14, what is the relationship between sugar and protein molecules in the fish antifreeze?
The sugar and protein molecules repel one another.
Large protein molecules are contained in a small sugar molecule.
Small protein molecules are contained in a large sugar molecule.
Small sugar molecules are contained in a large protein molecule. ✔
Hey Shannon,
ReplyDeleteIt look like you understand the story really well.
Keep up the good work
what is the main idea?
ReplyDeletewhat is the main idea?
ReplyDelete