Living beings are organized units that are capable of metabolism , reproduction , irritability , growth and evolution , among other things . Living beings have a decisive influence on the image of the earth and the composition of the earth's atmosphere ( biosphere ). Recent estimates suggest that 30 percent of the total biomass on earth is accounted for by microorganisms living underground . Recent living beings always descend from other living beings (theory of descent ). Intensive research is being carried out into the development of living beings from abiogenic preforms . Stromatolites in particular are among the oldest traces of terrestrial organisms .
Properties of living beings (overview)
|Mark||Example living beings||Example of non-living beings|
|export||Living beings as selectively open thermodynamic systems with subsystems (organs) that ensure entropy export. In this way the current entropy of the system can be kept below the maximum possible entropy that characterizes death.||Technical systems with mechanisms for self-repair. Data communication with error correction. As with living beings, redundancy ensures the required distance between the currently achieved and the maximum possible entropy.|
|Energy exchange with the environment|
|admission||Plants absorb light energy and generate biomass through photosynthesis ( primary production ).||Rocks heat up during the day by absorbing energy through light ...|
|Submission||All living beings, but especially mammals , give off energy directly as heat and indirectly in material excretions||... and give them back in the night|
|Exchange of substances with the environment|
|admission||Ingestion||Refueling a car with gasoline|
|Submission||Animals give carbon dioxide and water from||Car exhaust fumes consist (mainly) of carbon dioxide and water|
|Metabolism (chemical conversion of substances )||all creatures||burning candle|
|Exchange of information|
|Receiving information||Plants recognize the position of the sun||Light meter of the still camera measures light intensity|
|Sending information||Warning veil of wasps , language of bees and humans||Traffic lights|
|Reaction to stimuli from the environment|
|Adjustment / alignment||Plants align their leaves with the position of the sun||Solar cells tracking the sun|
|Volume increase||A yeast cell increases after the division of volume to||Growth of a common salt crystal|
Stem cells of the bone marrow .
Growth is the result of cell division (multiplication): Through growth, the surface area relative to the mass of the cell decreases. This reduces the cell's ability to export entropy. The division increases the surface again. More entropy can be exported again.
|“Cell division” is an originally organic term, so it cannot have an inorganic equivalent.|
|Self- reproduction ( procreation )|
|Multiplication||The cells that result from cell division are similar to their mother cells. Copy of DNA , i.e. inheritance .||Technical systems are not yet fully developed, but theoretically possible; Self-reproducing (see also recursion ) computer programs are common practice ( computer viruses ).|
|Basic building blocks||Biomolecules||Water molecule|
|Information carrier||DNA , RNA||Metal crystal ( metal grid )|
Characteristics characteristic of living beings can also be found sporadically in technical, physical and chemical systems . In particular, fire shows a large part of these properties, depending on the interpretation.
- All living organisms ("living beings") must apply all characteristics , at least at the cell level .
- Dead organisms had all the marks in their past.
- Organisms have latent life that do not have all the characteristics, i.e. are similar to dead organisms or inanimate objects, but can at any time become living organisms. (Examples: spores of bacteria or fungi ).
- Inanimate objects donot show all characteristicsat the time of their existence .
However, three essential properties have emerged that should apply as definition criteria for all living beings:
- Metabolism (metabolism) during at least one phase of life, which requires compartmentalization by a wall or membrane,
- Ability to self- reproduce and
- the genetic variability associated with self- reproduction as a condition of evolutionary development.
However, this restriction would categorically exclude many hypothetical early stages in the development of life as well as recent borderline forms of life such as viruses. This aspect is discussed in detail in the Living Things: Conceptual Problems section.
Structure of living beings
All living beings ( plants , animals , fungi , protists , bacteria and archaea ) are made up of cells or syncytia (multinucleated cell fusions, e.g. ciliates and many fungi). Both the individual cell and the entirety of the cells (of a multicellular organism ) are structured and compartmentalized , which means that they form a complex system of separate reaction spaces. They are separated from each other and from the outside world by biomembranes .
In the course of individual growth, the cells differentiate into various organs, each of which takes on certain functions for the overall system, the individual.
Chemistry of living things
In addition to carbon (C), hydrogen (H), oxygen (O) and nitrogen (N) as the main elements of the basic structure of the biomolecules, there are the elements phosphorus (P), sulfur (S), iron (Fe), magnesium (Mg) and potassium (K), sodium (Na) and calcium (Ca) exist in living things. Furthermore, chlorine (Cl), iodine (I), copper (Cu), selenium (Se), cobalt (Co), molybdenum (Mo) and some other elements occur only in traces, but are nevertheless essential.
The elements silicon and aluminum , which occur far more frequently than carbon in the earth's crust, are not used as building blocks of life. Noble gases and elements heavier than iodine (atomic number 53) do not appear as functional building blocks of living beings.
Living beings are primarily characterized by the reproductive molecules they contain . The polynucleotides DNA and RNA are known today , but other molecules may also have this property. They also contain proteins ( proteins ), macromolecular carbohydrates ( polysaccharides ), and complex molecules such as lipids and steroids . All of these macromolecules and complex molecules do not occur in inanimate nature, they cannot be produced by inanimate systems. Smaller building blocks such as amino acids and nucleotides, on the other hand, can also be found in inanimate nature, for example in interstellar gases or in meteorites , and can also arise abiotic .
In addition, the cells of living beings largely contain water and inorganic substances dissolved in it.
All known life processes take place in the presence of water.
Systematics of living beings
The biological system tries to create a meaningful grouping of all living beings. The top level is formed by the domains . A distinction is made between three domains according to molecular biological criteria: the actual bacteria (Bacteria), the Archaea (Archaea), formerly also called Archaebacteria, and the Eukaryotes (Eukaryota). The first two domains mentioned contain all living organisms without a nucleus , which are called prokaryotes . The domain of the eukaryotes includes all living things with a cell nucleus, including animals (including humans ), plants and fungi as well as protists . The eukaryotes and archaea are more closely related to one another.
Living beings as systems
Properties of living things
The following properties of living beings also occur in inanimate systems of nature and technology:
- open : You are in a lifelong exchange of energy , material and information with the environment.
- complex : Life requires a certain complexity in the organization of the system.
- dynamic : At least on the biochemical level, they are constantlyexposed to stimuli and constraints from the environment, but can temporarily assume a steady state, i.e. they have a constancy of structure and performance. These changes are due on the one hand to the conditions inherent in the system (example: generation of genetic variation through recombination during reproduction ), and on the other hand to environmental influences and environmental stimuli. Living beings in turn have a changing effect on their environment. (Example: changes in the composition of the atmosphere through photosynthesis .)
- deterministic : Even if all properties of living beings are determined by the laws of nature, due to their complexity, especially for emergent properties, it is hardly possible to make mathematically exact statements about the predictability of their properties and development and their behavior: The reduction necessary for scientific investigations can be used Determine regularities for individual elements. However, regularities for the overall system cannot always be derived from this.
- stable and adaptive : living beings can maintain their structure and their inner milieu for a long time despite disturbing influences from the environment. On the other hand, they can also change in structure and behavior and adapt to environmental changes.
- autopoietic : living beings are self-replicating systems, whereby, on the one hand, the continuity of structure and performance is guaranteed over long periods of time, and, on the other hand, there are opportunities for evolutionary adaptation to environmental changes due to the imprecision of replication.
- self-sufficient : living things are to a certain extent independent of the environment. (See the discussion of the problem of self-sufficiency .)
The following forms of organization of living beings also occur in inanimate systems of nature and technology:
- As complex , heterogeneous systems , living beings consist of many elements of different structure and function, which are linked to one another through numerous, different interactions.
- Living beings are hierarchically structured: They consist of numerous different elements (subsystems) that are linked to one another through numerous relationships and themselves consist of numerous subunits, which themselves represent systems and consist of subsystems (for example, organs consist of cells, these contain organelles which are composed of biomolecules).
- Living beings are themselves elements of complex systems of a higher order (e.g. family group, population, biocenosis ) and are therefore also linked to numerous other systems (other living beings, inanimate and technical systems).
- All living beings are systems with special information channels and information stores.
The genetic program
Like the complex physical systems of inanimate nature (such as the solar system ), structures also arise in living beings through self-organization . In addition, in contrast to systems of inanimate nature, living beings have the genetic program , which, however, can also occur in a similar way in systems of technology (see genetic programming ). This program triggers, controls and regulates life processes. This also includes the reproduction of this program. This program is teleonomic , without being able to be teleological : it specifies the direction of the ontogenetic development and the behavior of the organisms and, to a certain extent, delimits them from other development possibilities and behaviors. If parts of the program are missing or malfunction, no viable organisms can develop in the long term outside of a tolerance range .
Evolution of life
The evolutionary history of life on earth ( evolutionary history ) has a unique course. Even if the initial conditions could be restored, a similar sequence would possibly result as it has already taken place, but most likely not exactly the same. The reason for this is the multitude of coincidental coincidences of influencing factors that have determined further development since the beginning of life. These random influences are partially compensated for by selection and adaptation processes, but an exactly identical development under real conditions is not likely.
The development of different types of living beings is dealt with in the theory of evolution . This branch of biology founded by Charles Darwin explains the diversity of life forms through mutation , variation , inheritance and selection . The theories of evolution aim to explain the changes in life forms over time and to make the emergence of the earliest forms of life comprehensible. There are a number of concepts and hypotheses for the latter (for example RNA world , see also chemical evolution ).
The oldest fossil traces of living beings found so far are microscopic threads that are considered to be the remains of cyanobacteria . However, these deposits, found in rocks that are 3.5 billion years old, are not generally viewed as traces of life, as there are purely geological explanations for these formations.
The currently most popular theory on the origin of autotrophic life postulates the development of a primitive metabolism on iron-sulfur surfaces under reducing conditions , such as those found in the vicinity of volcanic evaporation. During the early phase of the evolution of earthly creatures that in geological period prior to 4.6 to 3.5 billion years ( Precambrian ) took place, was the Earth's atmosphere probably rich in gases such as hydrogen , carbon monoxide and carbon dioxide , while the hot oceans relatively high concentrations of Ions of transition metals such as dissolved iron (Fe 2+ ) or nickel (Ni 2+ ) contained. Similar conditions can be found today in the vicinity of hydrothermal vents that were formed during plate tectonic processes on the sea floor. Near such as black smoke ( English black smokers vents thrive) designated thermophilic methanogenic archaea based on the oxidation of hydrogen and the reduction of carbon dioxide (CO 2 ) to methane (CH 4 ). These extreme biotopes show that life can thrive independently of the sun as a supplier of energy, a fundamental requirement for the creation and maintenance of life before photosynthesis occurred .
Superlative living beings
- The pando , an aspen colony in Utah (USA) with a common root system, is considered the oldest (80,000 years) and with a total weight of about 6,000 tons the heaviest known creature on earth. Pando spreads over 43.6 hectares.
But there are probably even more extensive stocks of clonally growing plants such as reeds , blueberries or bamboo .
- In the Malheur National Forest in Oregon (USA) was a copy of Dark Honey Fungus -Pilzes discovered the mycelium covers about 9 square kilometers (depending on the source to the 880 hectares or 965 hectares). According to the current state of knowledge, this mushroom is the largest living being on earth and the largest mushroom on earth in relation to its area. Its weight is an estimated 600 tons.
- The General Sherman Tree in California (USA), a specimen of the giant sequoia , weighs 1,950 tons (as of 1938) and is probably the heaviest non- clonal creature on earth.
Living things: conceptual problems
Definition of the physical limit
Here the outermost limit is ultimately the cell membrane , the pellicle , the cell wall or some other enveloping and delimiting structure. At higher organizational levels, closing and covering tissue such as epidermis , epithelium , skin or cortex take on this function.
Many organisms release substances into the environment and thus create their own close-up environment, a microenvironment. Example: Pneumococcus mucous capsule . Here the physical delimitation of the individual depends on the question.
Definition of the individual
The delimitation of a single living being from other, independent living beings is not trivial. According to its Latin origin, the word individual means something indivisible. In this sense, it is not practical for all living beings. It is true that one cannot divide most of the higher animals without killing them or the separated part. So they are not divisible. Addressing a dog as an individual is therefore not a problem. On the other hand, you can cut off an offshoot from an "individual" tree and let it grow into a new specimen. This means that the supposed tree individual is basically a “dividual”, because it is not two parts of a tree individual that live on, but one specimen has grown into two, the original specimen was multiplied. Many plants use this method of spreading systematically, e.g. B. by offshoots. Often whole lawns or forests grow in this way, which actually belong to a single contiguous specimen, but which could be divided into several specimens at any point at any time.
The possibility of cloning creates the ability to separate a new viable specimen, also for mammals (see clone sheep Dolly ). This means that the term individual is more or less obsolete in many areas of biology and would have to be replaced there by another, more appropriate one, such as the term specimen .
In the case of slime molds and single-cell colonies ( e.g. Eudorina ), individual, self-sufficient cells can be distinguished. However, at least temporarily they enter into connections with each other in which they give up their individuality and independence, i.e. resemble a multicellular organism.
- So living beings are never self-sufficient in terms of energy, they are always dependent on an external energy source, which is usually given by the sun. Organisms that only need light or the chemical energy of inorganic substances as an energy source, i.e. do not depend on other living beings as energy suppliers, can be regarded as energetically self-sufficient.
- Autotrophic organisms are materially self-sufficient in the sense that they produce the body's own organic substances from inorganic substances and metabolize them to inorganic substances. In this way, a photosynthetically active plant can be kept alive in a glass vessel that is closed off from the ambient air with sufficient lighting, since a balance between photosynthesis and respiration can be established. However, growth and reproduction are only possible in this system as long as the supply of water and nutrient salts is sufficient. In this sense, heterotrophic organisms are not self-sufficient because they depend on the nutrients prepared by other living beings.
- Superordinate systems such as a community ( biocenosis ) can in turn achieve energetic and material self-sufficiency if certain groups of organisms are available in sufficient numbers and with a balanced rate of reproduction. (See ecological balance .) In the deep sea, an autarkic community has developed between chemoautotrophic bacteria , tube worms , crabs and fish . The Ecology examined, among other things, that minimum requirements have completed community must meet in order to be self-sufficient, that is, to allow a closed material cycle. Ultimately, the totality of all living beings on earth can be understood as an autarkic community (compare the Gaia hypothesis , which understands the earth as an organism.)
- All living beings are self-sufficient with regard to a program inherent in the system, the genetic system. This enables them to initiate, control and regulate their own life processes. (See system behavior ). (In this sense, viruses and viroids would also be self-sufficient, but their program is not complete, they are also dependent on the programs of their hosts). This self-sufficiency is complete insofar as the programming , i.e. the creation of the genetic source code, does not have to be carried out externally by a “higher level programmer”. On the other hand, the programs are not sufficient to determine all life processes : For example, the brain cannot develop completely without the influence of the environment. The visual cortex would not be able to function fully in complete darkness .
- All living beings are self-sufficient in terms of growth , repair and reproduction . They produce the system elements characteristic of them ( biomolecules , cell organelles , cells ) themselves, compensate structural disturbances within certain limits with the help of repair mechanisms and are able to produce similar copies of themselves. The production of identical copies is in principle not possible at any system level due to physical and chemical laws. The resulting inevitable variation, in interaction with the environment, leads to evolution on all system levels. (See system theory of evolution )
In the development of systems theory by physicists, mathematicians and technicians, they repeatedly went into analogies in the structure and behavior of living things. This view of living beings as systems led to the fact that concepts of cybernetics , computer science and systems theory found their way into biology, most recently and comprehensively in the systems theory of evolution .
Living beings as open systems have always been far removed from thermodynamic equilibrium since their existence . They have a high degree of order and thus a low entropy . These can only be maintained if the increase in the degree of order is energetically coupled with processes that supply the energy required for this. (Example: Build-up of organic substances with low entropy such as glucose , DNA or ATP , with inorganic substances with high entropy such as carbon dioxide , water and mineral salts through photosynthesis and metabolism .) When death occurs, thermodynamic equilibrium is established, the high degree of order cannot more are maintained, the entropy increases. Life can be understood thermodynamically as the feedback of an open system with its environment, which maintains its own order at the expense of this. This definition is consistent with one of the possible formulations of the 2nd law of thermodynamics , according to which the change in the entropy of an entire system is zero or greater than zero. In order for the order of a system to be maintained or to increase, the disorder of the environment must increase at least to the same extent, so that the total change in the overall system is at least zero.
Classification of the viruses
Viruses occur on the one hand as naked nucleic acids in the host cells, on the other hand outside of cells as virions , which consist of the nucleic acid and a protein shell. Most scientists don't count viruses as living things. If, for example, a cell structure is viewed as a fundamental characteristic of living beings, viruses are not included in living beings because they are neither cells nor are they made up of cells. Two other criteria are even more important: Viruses do not have their own metabolism and they do not reproduce independently. Their reproduction takes place exclusively through the biosynthesis machinery of the host cells, which is controlled by the virus nucleic acid.
Classification as a "borderline case of life" is however obvious. The existence of viruses could indicate a transition from “not yet alive” to “alive” in evolution. However, the viruses could also have regressed from “real” organisms such as bacteria .
It has now been possible to artificially generate a nucleic acid with the sequence of the poliovirus through DNA synthesis; In the same way, many more DNA and RNA sections have already been generated for genetic engineering experiments. If you then smuggle DNA strands generated in this way into cells, complete, natural polioviruses result. The experiment shows that the boundary between living things and non-living things is difficult to determine.
Viruses are by mutations and selection of Evolution subject. In a broader sense, this also applies to many non-living beings, for example to individual genes (see the egoistic gene ), but also to behaviors and cultural achievements such as tools, techniques and ideas (see meme theory). The evolution of viruses is therefore not sufficient evidence that viruses are living things.
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