Extreme Temperatures and Life

Extreme Temperatures and Life

Extremely hot or extremely cold temperatures are not normally compatible with life. There are several reasons for this, includ- ing the chemical reactions discussed earlier. But one of the most important reasons of all is that life depends on water.

The human body is about 65 percent water by weight. All forms of life, whether plant or animal, are composed of little compart- ments called cells, which are surrounded by a membrane made mostly of lipids (fats). The fluid inside and outside of cells is water that contains molecules critical to all of the chemical reactions necessary for life. At boiling or freezing temperatures, these solu- tions can no longer exist—and neither can life.

Even smaller changes can have a lot of effects; it does not take temperatures as extreme as boiling and freezing points to cause problems. As mentioned earlier, the rate at which chemical reac- tions proceed depends on temperature—in general, the higher the temperature, the faster the reaction. But plants and animals have many different reactions going on at any given time, with the out- put of one reaction providing the input of another. These chains of linked reactions must work together, and most life-forms require the reaction rates to be carefully controlled. Temperatures must be relatively constant to do so.

Another important concern involving chemical reactions is the molecules that help them to occur. These molecules are known as enzymes, which typically help reactions to occur by bringing together the reactants. The reactants are the molecules participat- ing in the reaction, and most of them are free to move around. If they had to meet each other by chance in order to react, the reaction would seldom occur. Enzymes often work by temporarily binding the reactants so that they will encounter each other. Most enzymes are large molecules called proteins. As described in the section “Body Temperature,” most proteins have a specific shape or geometric configuration that is critical to their function, and this is certainly true of enzymes. Biologists call this shape the protein’s conformation. An enzyme, for example, may form pockets where two or more reactants are bound and held until they react. But

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temperatures even just a few tens of degrees warmer than body temperature cause a protein to lose its conformation because the bonds that form the shape are broken.

Human beings cannot afford to let the body temperature get too high, because vital proteins will stop functioning and chemical reactions will proceed too quickly. On the other hand, chemical reactions will proceed too slowly if the body temperature gets too low. Maintaining the proper temperature within strict limits is essential. All animals, especially small ones such as microor- ganisms, can be killed by heat or cold. Surgeons sterilize their instruments in order to destroy microorganisms such as bacteria that could invade the body as the surgery proceeds. One way to sterilize an instrument is to put it in boiling water for several minutes.

But scientists made a surprising discovery in the middle of the 20th century. They found tiny microorganisms inhabiting hot springs and other places having temperatures that biologists had formerly assumed could not possibly harbor life.

It turns out that many species of one branch of life, called Archaea, actually exist in extreme environments. They are known as thermophiles (the name means heat lovers). These microorgan- isms eat substances that other animals are not able to metabolize (digest), such as sulfur, hydrogen, and compounds containing these elements. They survive, even thrive, in volcanic vents, acidic hot springs, hot-water vents deep in the ocean, and other inhos- pitable environments. (They can also thrive in artificial environ- ments such as power plants and hot-water heaters.)

Thermophiles survive in extreme temperatures because their proteins are exceptionally sturdy. Somehow these proteins manage to maintain their shape even when the atoms and molecules are jiggling around violently. The bonds holding together the shape must be extraordinary.

There is a steep price to pay for this amazing ability—thermo- philes cannot survive elsewhere. They not only thrive on heat, but they also require it. Surgeons need not worry that their sterilization procedure does not kill these microbes, because they are not likely to invade the patient’s body. It is too cold for them to grow!

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Hydrothermal (hot water) vents like this one on the ocean floor spew out hot, mineral-rich water from beneath Earth’s crust. The cold water of the ocean causes some of the minerals to collect and solidify, forming particles that make the emission appear dark in color. (NOAA/OAR/NURP/P. Rona)

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But these microbes have proven to be important in other ways.

A remarkable enzyme from one of these thermophiles is used in laboratories across the globe, in procedures as wide-ranging as medical diagnosis and forensics (analysis of crime scene evidence). Since this enzyme can withstand high temperatures, it can be used in techniques that require molecules to be repeatedly subjected to heating. One technique, called polymerase chain reaction (PCR), replicates DNA molecules so that they can be sequenced and iden- tified. PCR helps in diagnosing genetic diseases as well as identify- ing blood and hair samples found at crimes scenes.

In the opposite extreme, at frigid temperatures, chemical reac- tions do not go fast enough to support life. Freezing is also a big concern. But freezing is beneficial as a way to maintain dead tissue from decomposing. It can do this because freezing is so incompat- ible with life; microorganisms that would otherwise cause decay and decomposition cannot survive. People store certain foods in the freezer—or in the refrigerator, if freezing is not required or desirable—to stop or slow down the activity of bacteria and other microbes.

Freezing has another potential function, to persevere people who have recently died. Called cryonics, the idea is to keep a deceased person’s body from decomposing until some unspecified time in the future, when whatever accident or disease that caused the death can be reversed or treated. Because it is so expensive to maintain something as large as body in cold storage, sometimes only the head or brain is preserved. Cryonics is not very popular, but thousands of people have done it.

The unfortunate truth is that cryonics will probably not be successful. The reason is simple and has been noted several times already: water expands when frozen. As a person’s tissues freeze, the body’s huge amount of water also freezes. Ice forms and grows too big for the delicate membranes that enclose the body’s cells, disrupting the membranes. There is little chance that such massive cellular damage can be repaired any time in the near future, and even freezing will not preserve tissue forever. Cryonics consumes a great deal of resources, usually drained from the deceased’s estate (which would otherwise go to surviving relatives), and a belief in

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cryonics requires perhaps too much optimism about future soci- ety’s charity, economy, and medical technology.

Life has adapted well to thermodynamics, but there are limits to any successful strategy. Although the structure of proteins and the chemistry underlying life may survive at extreme temperatures in special circumstances, most often there is a narrow comfort zone that must be maintained. Whether living beings generate most of their own body heat or get it from the environment, temperature is a concern for all.