What is life?

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Life is a very broad concept! For example: big trees and small grass, one tall and one short; elephants and whales that live in the jungle and aquatic habitats, both have large body shapes, but elephants have long noses, big ears and four sturdy legs, while whales not only have no legs, but also no ears and noses. There is one or two holes on the huge head, which is the nose of the whale. Also, lively monkeys and bacteria that are hard to see with the naked eye, they are all living things, although they vary in size and lifespan, but they are all living things.

How do you know that living things are living things? Just look at the similarities between them and you can draw a conclusion. All living things have cell structures, a bacterium has only one cell, while a human has 1012 to 1014 cells.

All living things transform the nutrients in food into their own components and store energy in the process of material transformation, which is called assimilation; living things also decompose their own components in the process of growth and development, while releasing energy, which is called dissimilation; assimilation and dissimilation are collectively called metabolism. All living things can produce offspring under normal conditions, which is called reproduction.

All living things have differences as well as similarities between the upper and lower generations. Black cows can give birth to white calves, flower dogs can give birth to white cubs or black cubs. This situation of similarity between the upper and lower generations is called inheritance, and the difference between the upper and lower generations is called variation.

All living things are coordinated with the environment. Different living things live in different environments. If living things cannot keep up with the pace of environmental changes, they only have one way to die. In other words, living things in the living environment are in a competition of “survival of the fittest, elimination of the unfit”.

From the common points of living things, we can understand life as having cell structure, metabolism, reproduction, genetic variation and adaptation to the environment.

Life is the most surprising and charming natural phenomenon in nature. People’s understanding of life starts from concrete living things (i.e., living things). Today, different biologists have studied animals, plants, and microorganisms in great detail, and there is also a lot of information about life activities.

But because living things are the most complex systems in nature, and life has uncertainty, limitation and finiteness, studying life is the most meaningful thing, but with the knowledge that people currently have, there is still a considerable gap from truly understanding the essence of life. It is still difficult to give a precise definition of life that is universally recognized.

In the vast nature, are there any objects that are both non-living and living? Yes, these are the weird viruses. Viruses are complexes of proteins wrapped around nucleic acids. These complexes are the same as nucleic acids and proteins existing separately outside cells. They have no metabolic ability or reproductive ability. Of course, they also have no genetic variation and adaptation to the environment. They are non-living substances. But once they enter cells, they become living things.

Viruses that can only enter animal cells are called animal viruses. Viruses that can only enter plant cells are called plant viruses. Viruses that can only enter fungi or bacteria are called fungal viruses and bacterial viruses respectively. Bacterial viruses also have a name called "phage", which is famous for its ability to "devour" bacteria.

There is a kind of phage that specializes in "eating" Escherichia coli. Under an electron microscope, this virus shows a polygonal head and a tubular tail. The head and tail are composed of proteins. In the middle of the head, there is a nucleic acid molecule hidden. This nucleic acid molecule is deoxyribonucleic acid, abbreviated as DNA.

When phages come into contact with bacteria, their tails immediately adsorb on the surface of bacterial cells and secrete an enzyme that can dissolve cell walls. They make a hole on the surface of bacteria; then phage DNA passes through this hole and enters bacterial cells directly. The protein of phage stays outside bacteria.

After phage DNA enters bacteria, it immediately uses various substances inside bacteria to synthesize new substances according to its own needs. The new substances include phage DNA, phage protein, substances that help assemble phage protein and phage DNA into phages, and substances that can destroy bacteria and release phages.

In this way, after a phage DNA enters a bacterial cell, the phage multiplies more phages by converting new and old substances, that is, metabolism. And genetic variation and environmental adaptation also occur in the process of reproduction. It can be seen that after entering cells, viruses really become living things.

American scientist Carleton Gajdusek conducted a long-term study on "kuru disease" in Papua New Guinea and finally showed that "kuru disease" is caused by a “prion” that is smaller than many viruses. If humans and chimpanzees are infected with prions, they will have symptoms of muscle twitching and limb spasms after 12 to 14 months, and they will lose their memory and become mentally confused as they continue to develop. They will make a painful laugh before death.

Carleton Gajdusek further found that kuru disease and Creutzfeldt-Jakob disease (CJD) in the population are the same kind of disease.

What exactly is a prion? In 1982, another American scientist, Stanley B. Prusiner, found out by experiment that prions are purely proteins. "Prions" are also called "prions", but "prions" are only infectious proteins, and proteins themselves are not living. Because of their outstanding achievements in the study of "kuru disease" and "prions", they both won the Nobel Prize in Physiology or Medicine in 1976 and 1997 respectively.

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