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(Running Time: 10:45)

Two cold-blooded creatures have evolved very different physiologies to cope with the extreme cold of their environments. The winter flounder can live in sea water of -1.8 degrees C. The wood frog actually freezes - its heart stops beating and its liver converts glycogen to glucose, thereby lowering the temperature at which ice crystals form and protecting its cells from the ice when freezing does occur. Scientists hope further understanding of these mechanisms might be used to preserve human organs for transplants.

Curriculum Links
Activity: Chill Out: Organic Antifreeze
Activity: Cryogenics: The Big Chill





change of phase,

freezing and thawing,
water cycle


cold-blooded animals,
cryogenics, yeast

freezing point
depression (supercooling)



As a substance (solute) dissolves in another substance (solvent), it produces a mixture called a solution. The relative amounts of solute and solvent can affect the physical properties of the solution. For example, glucose acts as a solvent to lower the freezing point of water. When this happens, it is known as freezing point depression or supercooling.
An automobile's engine is cooled by jackets of water. In winter, water in these channels can freeze and expand. The expansion is so forceful it can crack an engine block. Antifreeze is a liquid solute (ethylene glycol) that dissolves easily in water. In solution and added to a car's engine, it lowers the freezing point of water, preventing it from freezing and ruining the engine.

Glucose, like the antifreeze ethylene glycol, dissolves in water. In this activity, think of the sugar solute as a form of glucose that depresses the freezing point of water. In cells, the lowering of the freezing point of the liquid cell contents protects the living system from freezing and ripping open with fatal results. The wood frog's liver converts glycogen to glucose, which is able to actually prevent ice crystals from forming in the frog's cells through freezing point depression.

  • basin or container
  • thermometer
  • 2 small plastic cups
  • ice
  • stirring rod
  • graduated cylinder
  • water
  • salt
  • sugar packets
  • marker


Investigate the role of glucose (sugar solution) when used as an antifreeze.


  1. Fill a small basin with about 500 mL of an ice-and-water mixture. Use a thermometer to measure the temperature of this mixture until it reaches 0 degrees C.

  2. Once the temperature has stabilized, add 1/4 cup of salt to the mixture. Stir well. After five minutes remeasure the temperature of the solution. Did it change? If so, how?

  3. Add 10 mL of ice-cold water (no ice) to two small cups labeled A and B.

  4. Mix 1/2 packet of sugar into cup B.

  5. Place both cups into the large basin that has been filled with salted ice-water.

  6. After ten minutes, compare the contents of the two cups.


  1. What happened to the water in cups A and B?
  2. What might account for any observed differences in the final appearance of the cup contents?


  1. The water in cup A froze; the solution in cup B remained liquid.

  2. The dissolved sugar lowered the freezing point of the water, thus preventing it from freezing.


As you observe on FRONTIERS, some organisms withstand frigid temperatures by shutting down their energy needs. In a "suspended state," their cells, tissues and organs require very little energy. The demands of such a "quasi-living" state can be satisfied by a very slow metabolic rate. Metabolism refers to the physical and chemical processes that make energy available to an organism. Metabolism is affected by temperature. The colder the temperature, the slower the reaction rate. When the rate of these life-sustaining reactions drops beneath a critical level, the organism will die.

In this activity, you'll observe the relationship between temperature and metabolism. The subjects for this experiment are Saccharomyces cerevisiae - one-celled organisms more commonly known as baker's yeast. These cells have been specially packed, treated and stabilized so they can remain in a "suspended" but viable state for several months. When placed in warm water, the cells activate. As the metabolism awakens, the cells generate carbon dioxide gas. By observing the presence of this gas, you'll be able to make inferences about metabolism. You'll see that both the yeast and the multicellular organisms seen on FRONTIERS can survive states of suspended animation or low metabolic activity.

  • 4 16-ounce clear beverage containers
  • 8-inch to 10-inch balloons
  • 2 packages of dry baker's yeast (or dried yeast in jars)
  • warm water
  • small basin filled with ice water
  • small basin filled with warm water (about 40 degrees C)
  • thermometer
  • sugar
  • spoon
  • magnifying glass


Observe the relationship between temperature and metabolism.


  1. Open a package of baker's yeast and carefully remove several small granules. Examine the grains of pressed dried yeast with a hand lens. Does the yeast appear alive? Explain.

  2. Divide the contents of this opened package into two equal portions (about 1 1/8 teaspoons each). Place one portion into a small, clean, dry beverage container labeled A and the other portion into a similar container labeled B.

  3. Open and divide the contents of a second yeast package into two equal portions. Place one portion into a beverage container labeled C and the other portion into a container labeled D.

  4. Do not activate the yeast in container A. The yeast in containers B, C and D should be activated according to the instructions printed on the yeast package, including adding about 1/2 teaspoon of sugar per container.

  5. Stretch and secure a balloon over the mouth of each of the four containers.

  6. Set containers A and B on a desktop. Place container C in a basin filled with warm water. Place container D in a basin filled with ice water.

  7. Examine the setups after 15 minutes. Record any change in the balloons' appearance.

Note: You can use dried yeast either in packets or in less expensive jars. The experiment will work even if amounts of yeast are not precise. You might want to try varying amounts of yeast to see what happens. Most instructions call for the use of sugar to activate the yeast; try variations of the instructions to see what happens.

CREDIT: These activities were contributed by science writer Michael Dispezio, author of the book The Science of HIV, part of a new video and activities curriculum package available from NSTA.


Scientific American Frontiers
Fall 1990 to Spring 2000
Sponsored by GTE Corporation,
now a part of Verizon Communications Inc.