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LIFE
Fundamental Trait
Old Version - 6, updated Wed, Apr 5, 2017 at 18:35:23, 20113 bytes - by relopez.
This article is for information and educational purposes only and is not
intended to give medical, legal or professional advice.
Life is a fundamental trait which distiguishes living organisms from minerals and is associated with the integration of information and energy by complex structures.
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INTRODUCTION
Experience teaches us that living things, which include us, as humans, are different in their behavior from other objects that surround us. Their behavior is more complex and in some way less amenable to mathematical description. We are able to identify behavior which is common to all living creature and this allows us to categorize living creatures into a strict category which distinguishes them from non living objects. However, even tough this unique behavior is the result of life, it does not define the essence of life itself. Therefore in order to define life we must first determine what life does, what behavior make it different from non-living objects and then we must look into the structure of living organisms at the most foundational level to try to determine the source of this complex behavior.
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BEHAVIORAL TRAITS OF LIVING ORGANISMS
These are traits which we can use to define living organisms and distinguish them from non-living objects. It is important to note that these are characteristics of living organisms, they tell us what life does. They do not constitute a definition of life.
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1. Growth
The size of living organisms changes through time. Almost all living organisms begin their independent existence small and they get bigger with time. As the organism reaches the end of its existence it may have undergo a small reduction in size before it dies and decomposes.
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2. Reproduction
Living organisms reproduce, they have offsprings, babies. Some organism produce intermediate forms like seeds and eggs, some produce immature forms like babies and some simple organisms split in half. Reproduction is a form of growth because it causes the growth of the population. Furthermore, growth is generally caused by the reproduction of the smaller parts of the organism. For example, the growth of large plants and animals is caused by the reproduction of small units called cells within the organism.
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3. Motion
Living organisms move. Some move faster than others, but even plants move slowly to follow the sun. Sometimes the motion might be microscopic. For example, when nerves are activated, tiny molecular valves open to allow charged molecules in and out of the cells.
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4. Responsiveness
Living organisms respond to stimuli. This response almost always involves some form of motion, so it is related to the trait of motion.
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5. Adaptation
Living organisms adapt to their environment. They adapt in two different way. (A) There is a fast short-term adaptation by individual organisms. This is seen primarily in some complex living organisms, mostly animals, which adapt as individuals by the process of learning. (B) All kinds of living organisms are able to adapt as a group to their environment by producing offsprings which are slightly different from each other. Therefore, some offsprings will be better suited to certain environments and tasks than others.
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6. Consumption
Living organisms eat. In order to accomplish the tasks above they have to absorb chemicals and energy from their environment. Plants absorb nutrients through the roots and energy through their leaves, although some carnivorous plant have specialized leaves capable of capturing and digesting insects. Animals extract both energy and nutrients from their food. Some textbooks call this category metabolism, but that focuses on the use of energy. However, living organisms must also consume chemicals in order to maintain their body structure.
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Subordinate Traits
Many lists of traits of living organisms include various internal traits which are found in all living organisms. These are qualities which characterize living things on the inside. These traits can be considered to be subordinate to the other traits. In other words, the things which living things do are so complicated that they need to have structures which are stable and complex. Therefore, in a way, the purpose of these traits is to permit the existence of the external traits. These traits are not as clearly defined as the external traits, and there are several ways to describe the internal characteristics of organisms. In the list below we find one common way to do it. This section describes general internal traits (not components). In the next section will examine more closely specific internal components of living organisms.
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(A) Homeostasis (stability and balance) – living things have to maintain a stable environment inside their bodies in order to function properly. For example, they must keep the concentration of salts and minerals within a certain range and the more complex animals must also keep their body temperature close to an ideal temperature. When these conditions vary the organism will experience thirst, fever, hunger or other sensations, and if the environment is not restored to its ideal range the organism will die. Homeostasis is based on the principle of stability and balance. Not too hot, not too cold, not too salty, not too watery, not too acidic, not too basic, etc. In order to maintain homeostasis the different parts (or organs) of the organism must communicate with each other and respond to the other parts. This communication between organs creates feedback loops and regulatory networks which are the basis for homeostasis.
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(B) Organization (complexity and hierarchy) – living things are often called organisms because they are composed of smaller parts which are in turn composed of even smaller parts. Since living organisms perform many tasks, these tasks are assigned to specialized units. These units can be arranged into larger units which perform even more complex tasks. For example, animals are composed of organs, which are made up of tissues, which are made of cells, which are divided into compartments (the nucleus, cytoplasmic space and mitochondria) which are similar to the simpler bacterial cells, which contain organelles, which are made up of proteins and other complex molecules. This is based on the principles of complexity and hierarchy.
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EXCEPTIONS TO THE TRAITS WHICH DEFINE LIVING ORGANISMS
There are some objects which we consider to be non-living which share some of the traits found in the list above. We could call these life-like non-living entities. Furthermore, some living organisms seem to be missing some of these traits. We could call these seemingly defective living organisms.
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Life-Like Non-Living Entities
Automobiles – they move and consume gasoline and oil.
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Crystals – they grow and consume minerals in solution.
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Fire – grows, consumes fuel and reproduces.
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Clouds – grow, absorbs energy and humidity from the environment (eats), and reproduce (clouds cause the formation of other clouds).
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Computer viruses – they reproduce, move and consume energy and disc space (they eat) and a few even adapt.
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However, these examples are not considered to be living organisms for the two following reasons. (A) First of all, these objects do not share all the traits of living organisms. Most living organisms posses all the defining traits listed above. (B) Secondly, living organisms do not just act like they are alive, they are alive. They contain life. The objects listed above might resemble living creatures in some ways but they do not posses life. A later section discusses a formal definition of life.
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Seemingly Defective Living Organisms
Some living creatures do not posses all of the traits which categorize most living things.
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Some plants move very little – For example, large woody trees are basically fixed into a rigid position. However, even trees often have parts which move slowly, like their leaves and seed pods. Also, sap and nutrients move through their internal vascular structure. Furthermore, these trees had more motility when they were young and growing.
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Some organisms cannot reproduce – Old and injured animals (like old people, for example) cannot reproduce, and some organisms are born without the ability to reproduce. This is true of unusual crosses like the mule. Mules cannot produce more mules. However, they themselves are the product of reproduction, they are produced by crossing a horse and a donkey. Furthermore, the cells inside these animals are constantly reproducing themselves and that is what keeps these animals alive.
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Some organisms can exist in a state of suspended animation where they no longer have to eat – This is true of seeds, for example. However, one could say that seeds are not actually alive in the full sense of the word, but are only potentially alive. They become alive when the seed absorbs water and the tiny plant inside the seed begins to absorb nutrients and energy stored in the seed.
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Many animals quit growing when they reach a pre-determined size – However, all these animals had a period of significant growth early in life. Furthermore, even many mature animals can still grow if they over eat, but this growth is unhealthy.
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The traits which characterize living organisms are most strongly expressed in young organism. As an organism ages and approaches death the expression of the traits which characterize living organisms begins to diminish.
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MICRO-STRUCTURE OF LIVING ORGANISMS
In order to arrive at a definition of life it is important to look at the components of the simplest examples of living organisms. These simple organisms belong to a category called prokaryotes, which are singled celled organisms without a nucleus or other internal membrane bound structures or organelles. Prokaryotes are divided into two groups, bacteria and a related group of organisms called the Archaea. Archaea are most well known as the microbes which give some hot springs their brilliant colors. These two groups are structurally very similar and differ mainly in the chemistry of their metabolism and the use of different enzymes in translation and transcription. These differences are not important for the definition of the essence of life.
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The Cell
All living organisms are composed of small structures called cells. Prokaryotes also take the form of simple cells. Cells are enclosed in a lipid membrane which is partially impermeable to water, ions, and other molecules. The lipid membrane, because it is basically composed of oil, is completely impermeable to water and to water soluble substances but the membrane contains pores of different types composed of various types of complex proteins which control the entry and exit of water, ions and other molecules. The cell membrane is essential for maintaining a high concentration of essential ions, enzymes and complex proteins within its enclosed space. Without this high concentration of essential components, life would grind to a halt and the components would disperse into nothingness. This concentration of molecules serves as a form of energy storage. Cells convert the concentration gradients produced by metabolism into chemical energy by the production of high energy molecules like ATP. The cell membrane allows the cell to maintain a fairly stable concentration of these components. This stability is called homeostatis.
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Dna
All living cells contain two types of nucleic acid polimers. Deoxyribonucleic acid (DNA) is the primary vehicle for storing genetic information in the cell and Ribonucleic acid (RNA) is the primary vehicle for transferring and processing information in the cell. Some of this information is used to make the hundreds or thousands of specialized proteins found in each cell and other information is used to control the operation of the cell. Each nucleic acid molecule is composed of long sequences of four different types of units called nucleotides. Each nucleotide is composed of a sugar, either ribose or deoxyribose and one of four different nucleobases. They are bound into long strands by means of phosphate bonds. The information is coded by the sequence of these different nucleobases. Every amino acid in every protein within a cell is coded by a particular sequence of three bases.
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DNA and RNA are very similar except that the sugar in DNA is missing an oxygen atom and one of the nucleotides is different in RNA compared to DNA. Nevertheless, these small differences causes DNA to take the form of a stiff double stranded double helix while RNA is a flexible single stranded chain. This flexibility allows RNA to combine with protein to form complex nano-robotic multimolecular structures which are involved in transcription, translation and replication of genetic information.
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Complex Proteins
All living cells contain complex macromolecular protein structures which are involved in metabolism (energy production), in translation, transcription and replication (information processing), in the maintenance and functioning of the cell membrane (used to maintain homeostasis) and in various forms of motility. The most complex of these are the proteins involved in information processing. Many of these, particularly the ribosome, are composed of a mixture of various proteins and several RNA strands. The largest and most complex of these is the Ribosome which is involved in translation, the creation of protein using information stored in DNA. Most of these protein structures function as molecular machinery with complex moving parts. The speed at which these molecules move is impressive.
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ESSENCE OF LIFE
Many biology textbooks attempt to describe living organisms by the traits which most of them exhibit. This is a phenomenological definition of life. In other words, life is that which is contained by objects which manifest the traits or phenomena on the list. However, we saw that some non-living objects share some of these traits, and some living organisms lack one or two of these traits. This shows us that there is something missing in such descriptions of a living organism. That is because a living organism is best defined as an object which contains life. This life allows it to do many unique things which only living organisms do best. However, for that we need to define what life actually is.
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Historical Definitions Of Life
Throughout history many philosophers and scientists have tried to define life. Some of the earliest definitions which have survived to the present were made by a Greek philosopher named Empedocles around the year 430 BC. He believed that life is the result of a precise mixture of elements. Since then various theories about the nature of life have been put forward. These theories can be grouped into three categories.
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1. Materialism – this is one of the oldest view of life and holds that life is produced by the proper mixture of elements. This is the view held by Empedocles.
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2. Hylomorphism – this view was developed by Aristotle around 322 BC and holds that life is produced by the existence of a proper form or design. It is this form or structural design which produces life.
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3. Vitalism – This view was developed in its modern form by Stahl in the 17th century. It held that life is in essence immaterial. Some popular descriptions of vitalism see life as an immaterial fluid, a force or an energy field. The force in the Star Wars double movie multi trilogy appears to be based loosely on the concept of vitalism. In the Star Trek series they have technology which is able to detect life signs. This also assumes that life is some sort of substance that can be detected by instruments. Even though these are works of fiction, it shows that many people tend to define life using a vitalistic point of view.
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Most modern biologists would state that these ancient theories have been discredited, at least in their original form, although they each might hold an element of truth. Scientific progress in the last century has allowed us to understand more accurately the structure and function of the cell, the smallest unit of life. In spite of this, there is no general agreement about the definition of life. However, there is one definition which seems to explain many aspects of life.
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A Possible Definition Of Life
As scientist were able to explore the ever smaller structures within the cell they came across something which sets living cells apart from all other material objects. This is the existence of a complex molecule called DNA which stores enormous quantities of information, and the existence of complex structures within the cell which allows this information to control all the activity of the cell. No other naturally occurring object contains discretely encoded information. Therefore, information must be an essential component of life. Using this fact and some of the behavioral traits described above, we can suggest a tentative definition of life and examine it logically to see how well it stands up to scrutiny.
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A possible definition of life – Life has something to do with the integration of information and energy. In other words, energized information is an important, if not the essential, component of life.
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This definition has the advantage of being based on two things that are measurable. We can measure the amount of energy an organism consumes, and we can measure the amount of information contained in the genetic material of the cells of the organism. It also implies a definition for living organisms.
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A living organism is a physical object which integrates information and energy.
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LEVELS OF LIFE
Living organisms process information at various different levels. The way an organism processes information is useful as a way to categorize organisms.
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Bios (Life)
All living creatures process information at the molecular level using complex molecules to process the information stored in DNA.
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The Greek term for life is ‘bios.’ This is the level that uses DNA, RNA, Ribosomes and other macromolecules to process information.
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Anima (Soul)
Animals process information at the cellular level using specialized cells called nerves to process the information stored in the brain.
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Some organisms have a nervous system where specialized thread-like cells transmit information quickly using electrical signals. We call these organisms ‘animals’ and call non-animals ‘plants’, or ‘plant-like’ organisms. Animals are able to process and use information much more quickly and efficiently than plants. Ancient people called this ability to process and use information ‘anima’ and called the system which processes this information a ‘soul’.
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Sentience (Spirit)
Humans are able to process complex abstract symbolic information, to communicate it to other humans and to store it in ways that later generations can access it.
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There is one animal which is able to process and communicate abstract information using symbolic language. This animal is the human being, which, of course, includes you and me. We are not sure how or why we are able to process information this way. However, it distinguishes us from all other living physical organisms. We call the processing of abstract information ‘thought’ or ‘thinking’ and the ability to think is called ‘sentience.’ As far as we know, humans are the only physical living organisms that can think. Some people call whatever it is that allows us to think the ‘spirit’. This creates a hierarchy of life. All living creatures posses ‘bios’ or life, animals posses ‘anima’, or a soul, and humans, the highest living creatures, posses ‘sentience,’ or a spirit.
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KEYWORDS
The following keywords are alternate terms which can be used to access this page.
life
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SEE ALSO
Living Organisms
Dna
Rna
Mitochondria
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REFERENCES
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BIBLIOGRAPHY
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