As the sun's direct point gradually moves to the southern hemisphere, the winter's footsteps begin to come to us, and the polar bear living in one of the coldest regions of the earth is about to start its long hibernation. When the animal is hibernating, the metabolism of the body's material is slowed down, and it is almost unnecessary to supplement the food outside so that it can survive the long and cold winter. These observations naturally inspired the imagination of science fiction writers. When the subject of the universe adventure is sought after by readers, the hibernation machine is almost the standard configuration on all spaceships. In 1968, Kubrick's famous film "2001 Space Odyssey" with almost no lines appeared in them. Regardless of the theme of science fiction, the world at low temperatures and even ultra-low temperatures has always attracted great interest from physicists and biologists. Many substances have some very interesting and wonderful changes at low temperatures and ultra-low temperatures. For example, transparent air will turn into a light blue liquid at 190 ° C, liquid helium becomes superfluid, and the resistance of many metals and compounds will Completely disappeared into a superconductor. 2001 has already passed, and the current manned spacecraft has only been to the moon. The cryopreservation technology of the human body is still a science fiction theme as it was decades ago. What is hindering this dream from becoming a reality?
Low-temperature danger
The damage of high temperature to life is easy to understand. As the temperature rises, the structure of proteins and nucleic acids disintegrates. The temperature rises continuously, which directly leads to the separation of phospholipids on the cell membrane. The cells that are the basis of life completely disintegrate. Of course, if the temperature continues to rise, in the case of aerobic conditions, most of the organic molecules will be completely oxidized, and the dust will return to the earth. The phoenix nirvana is only a mythical legend. Even if the dust that floats once again enters the life cycle, It has nothing to do with the previous life. However, it is not easy to understand the damage of low temperature in depth.
The world of life basically belongs to a chemical world, and the speed of chemical reactions always slows down with the temperature drop and eventually stops. Common food spoilage caused by microorganisms is a typical chemical reaction that can be delayed by low temperatures. Long ago, people knew that ice cubes were stored in winter for long-term preservation of food, and the invention of the refrigerator popularized the life that the nobles could enjoy. Therefore, in theory, the course of life should also gradually slow down and eventually freeze as the temperature drops, and then recover again as the temperature rises.
Indeed, people have observed in the natural world that some creatures have amazing anti-freezing ability, they can remain in the "suspended" state and wait for the temperature to rise. After the pear trees are at a temperature of minus 20~33 °C and the apple tree is dormant at a low temperature of minus 46 °C for winter, it is still able to bloom in the spring. In some temperate waters, the temperature in winter nights can drop to minus 30~30°C. The mollusks on the beach, such as mussels and oysters, are straightened into ice sculptures, but when the tide rises, they wake up again like a sleeping beauty. But more animals and plants are very sensitive to low temperatures, their lives are drifting in the cold wind, and there is no longer a chance to wake up.
For decades, many biologists have been working to explore the underlying causes of low-temperature threats to life. By studying the antifreeze properties of antifreeze organisms and freezing experiments on cells, there are currently two complementary theories. A basic explanation is given to this: chemical damage caused by low temperature and ice crystal damage.
There are many reasons for chemical damage, but the most basic ones come from the powerful destructive power of oxygen and its derivatives. Most of the substances that build life are lost due to oxidation. When the first life form that produced glucose from sunlight, water, and carbon dioxide was born on Earth, oxygen was released into the atmosphere as a by-product of this process, accompanied by a gradual increase in oxygen concentration, most of the life at that time. It was eliminated by ruthless oxidation, which is the first biological extinction that paleontologists have identified. Today, oxygen is not called poison gas but life, because, hundreds of millions of years ago, a microbe "learned" to control oxygen, using its powerful oxidizing power, decomposing organic molecules to gain a lot of energy. For reasons that we don't quite understand, the microbes gave up an independent lifestyle and evolved into mitochondria in primitive eukaryotic cells. It is precisely because of the rapid and large supply of energy that makes multi-cell life forms possible, and we are descendants of the original eukaryotic cells.
However, no matter how important oxygen is, it cannot hide its oxidative decomposition and destroy the essence of life molecules. Especially in the case where cells actively use oxygen, a large amount of free radicals are generated which are more active than molecular oxygen. Under normal conditions, cells use a series of enzymes, directly or indirectly, to protect against damage caused by oxygen and its free radicals. However, as the temperature decreases, although the rate of destruction also decreases, the ability to accelerate due to the catalysis of the enzyme is now dramatically reduced as the activity of the enzyme is drastically reduced, and the originally balanced chemical reaction eventually collapses. One side. Fundamentally speaking, life as an orderly structure is originally a kind of dance on the wire. If the balance is disintegrated, life will naturally end. Of course, the speed of the collapse of the balance is very different for different life forms. Many plants' seeds only need a low temperature of about zero degrees, which can significantly extend the storage period without significant chemical damage. But to store animal cells for long periods of time requires much lower temperatures.
As for ice crystal damage, it is related to the nature of water. Water is a rare substance that does not decrease in volume after crystallization solidification. This is the reason why ice can float on the water. The freezing of the water pipe allows you to visually imagine the scene of freezing and bursting of cells. In addition, the rigid structure of the ice crystals will exclude other substances, resulting in a large increase in the concentration of solute in the liquid that has not been frozen. Excessive salt concentration and harmful substances can directly damage the cells.