Living being

A living being or organism is a material set of complex organization, in which molecular communication systems intervene that relate it internally and with the environment in an exchange of matter and energy in an orderly manner, having the capacity to carry out the basic functions of life, which are nutrition, relationships and reproduction, in such a way that living beings function by themselves without losing their structural level until their death.

95% of the matter that makes up living beings is made up of four elements (bioelements) which are carbon, hydrogen, oxygen and nitrogen, from which biomolecules are formed:

• Organic biomolecules or immediate principles: glucids, lipids, proteins and nucleic acids.
• Inorganic biomolecules: water, mineral salts and gases.

These molecules are constantly repeated in all living beings, so the origin of life comes from a common ancestor, since it would be very unlikely that two living beings with the same organic molecules have appeared independently. They have been found microfossils with an age of 3.770-4.280 million years, so life could have arisen on Earth during the Hadic. Molecular clocks also estimate it in the Hadic.

All living things are made up of cells (see cell theory). Within these, the sequences of chemical reactions, catalyzed by enzymes, necessary for life, take place.

The science that studies living beings is biology.

The coral reef is inhabited by a variety of living beings.

Definitions

Reproduction is a basic characteristic of living beings. At the top of the figure is seen a bacteria reproduced by binary fision.

It's usually easy to decide if something is alive or not. This is because living beings share many attributes. Likewise, life can be defined according to these basic properties of living beings, which allow us to differentiate them from inert matter:

• Organization. The basic units of an organism are the cells. A living being may be composed of a single (unicellular) cell or many (pluricellular).
• Homeostasis. Agencies maintain an internal balance, for example, actively control their osmotic pressure and the concentration of electrolytes.
• Relationship or Irritability. It is a reaction to external stimuli and allows living beings to detect or obtain information from the environment in which they live, make the right decisions and develop an adequate response to their survival. A response can be in many ways, for example, the contraction of a unicelular organism when touched or complex reactions involving the senses in the higher animals.
• Metabolism. Living organisms or beings consume energy to convert nutrients into cellular components (anabolism) and release energy by breaking down organic matter (catabolism).
• Development. Agencies increase in size by acquiring and processing the nutrients. Many times this process is not limited to the accumulation of matter but involves major changes.
• Reproduction. It is the ability to produce similar copies of themselves, both asexually from a single parent, and sexually from at least two parents.
• Adaptation. Species evolve and adapt to the environment.

Autopoiesis

An alternative way of defining living beings is through the concept of autopoiesis, introduced by doctors Humberto Maturana and Francisco Varela. The idea is to define living systems by their organization rather than by a conglomeration of functions. A system is defined as autopoietic when the molecules produced generate the same network that produced them and specify their extension. Living beings are systems that live as long as they maintain their organization. All their structural changes are to adapt to the environment in which they exist. To an observer outside the system, this organization appears as self-referring. Cells are the only primary living systems, that is, those capable of maintaining their autopoiesis autonomously. Multicellular organisms made up of cells have characteristics similar to those of cells, particularly the stable state, but their life is granted to them by the autopoietic organization of the cells that constitute them.

Span of Life

One of the basic parameters of living beings is their longevity. Some animals live as little as a day, while some plants can live for thousands of years. Aging can be used to determine the age of most organisms, including bacteria.

Chemical composition of living things

The protist Amoeba proteus (ameba) is an eukaryant organism that lives free in fresh water. It measures about 500 μm.

Organisms are physical systems supported by complex chemical reactions, organized in a way that promotes reproduction and to some extent sustainability and survival. Living things are made up of inanimate molecules; when these molecules are examined individually, it is observed that they conform to all the physical and chemical laws that govern the behavior of inert matter and chemical reactions are essential when it comes to understanding organisms, but it is a philosophical error (reductionism) to consider biology as solely physics or chemistry. The interaction with other organisms and with the environment also plays an important role. In fact, some branches of biology, for example ecology, are very far from this way of understanding living beings.

Organisms are open physical systems as they exchange matter and energy with their environment. Although they are individual units of life, they are not isolated from the environment around them; To function, they constantly absorb and release matter and energy. Autotrophs produce useful energy (in the form of organic compounds) from sunlight or inorganic compounds, while heterotrophs use organic compounds from their environment.

Chemical Elements

Living matter is made up of about 60 elements, almost all of the stable elements on Earth, except for the noble gases. These elements are called bioelements or biogenic elements. They can be classified into two types: primary and secondary.

• Them primary elements are indispensable to form organic biomolecules (glucids, lipids, proteins and nucleic acids). They constitute 96.2 % of living matter. They are carbon, hydrogen, oxygen, nitrogen, phosphorus and sulphur.

• Them side elements are all the remaining bioelements. There are two types: the indispensable and the variables. Among the first are calcium, sodium, potassium, magnesium, chlorine, iron, silicon, copper, manganese, boron, fluoride and iodine.

The bacteria Escherichia coli is a procarious organism present in the intestine of humans. It measures 1-4 μm.

The fundamental chemical element of all organic compounds is carbon. The physical characteristics of this element, such as its high bonding affinity with other small atoms, including other carbon atoms, and its small size allow it to form multiple bonds and make it an ideal basis for organic life. It is capable of forming small compounds containing few atoms (for example carbon dioxide) as well as large chains of many thousands of atoms called macromolecules; the bonds between carbon atoms are strong enough for macromolecules to be stable and weak enough to be broken during catabolism; silicon-based macromolecules (silicones) are virtually indestructible under normal conditions, which rules them out as components of a living being with metabolism.

Macromolecules

The organic compounds present in living matter show an enormous variety and most of them are extraordinarily complex. Despite this, biological macromolecules are made up of a small number of small fundamental molecules (monomers), which are identical in all species of living beings. All proteins are made up of only 20 different amino acids and all nucleic acids are made up of four nucleotides. It has been estimated that about 90% of all living matter, which contains many millions of different compounds, is actually made up of about 40 small organic molecules.

For example, even in the smallest and simplest cells, such as the bacterium Escherichia coli, there are about 5,000 different organic compounds, including about 3,000 different kinds of proteins, and it is estimated that in the human body can have up to 5 million different proteins; also none of the protein molecules of E. coli is identical to some of the human proteins, although several act in the same way.

Most of the biological macromolecules that make up organisms can be classified into one of the following four groups: nucleic acids, proteins, lipids, and carbohydrates.

Double DNA propeller.

A protein (hemoglobin).

Fosfolípidos organized in liposoma, micela and bicapa lipídica.

A glucose.

Nucleic Acids

Nucleic acids (DNA and RNA) are macromolecules made up of nucleotide sequences that living beings use to store information. Within nucleic acid, a codon is a particular sequence of three nucleotides that codes for a particular amino acid, while an amino acid sequence forms a protein.

Protein

Proteins are macromolecules made up of sequences of amino acids that, due to their chemical characteristics, fold in a specific way and thus perform a particular function. The following functions of proteins are distinguished:

• Enzymes, which catalyze metabolic reactions.
• Structural proteins, for example, tubulin and collagen.
• Regulatory proteins, for example, insulin, growth hormone and transcription factors that regulate the cell cycle.
• Signalizing proteins and their receptors, such as some hormones.
• Defensive proteins, for example, immune system antibodies and toxins. Sometimes toxins contain unusual amino acids such as canavanine.

Lipids

Lipids form the plasmatic membrane that constitutes the barrier that limits the interior of the cell and prevents substances from freely entering and leaving it. In some multicellular organisms they are also used to store energy and to mediate communication between cells.

Carbohydrates

Carbs (or carbohydrates) are the basic fuel of all cells; glucose is at the beginning of one of the oldest metabolic pathways, glycolysis. They also store energy in some organisms (starch, glycogen), being easier to break down than lipids, and form durable skeletal structures, such as cellulose (vegetable cell wall) or chitin (fungal cell wall, cuticle of cuticles). arthropods).

Structure

All living things are made up of units called cells; some are made up of a single cell (unicellular) while others contain many (multicellular). Multicellular organisms can specialize their cells to perform specific functions. Thus, a group of such cells forms a tissue. The four basic types of tissues in animals are: epithelium, nervous tissue, muscle, and connective tissue. In plants, three basic types of tissues can be distinguished: fundamental, epidermal and vascular. Various types of tissue work together in the form of an organ to perform a particular function (such as the heart's pumping of blood or as a barrier to the environment such as the skin). This pattern continues at a higher level with various organs functioning as an organic system that allow for reproduction, digestion, etc. Many multicellular organisms consist of several organ systems that coordinate to support life.

Vegetable cells. Within these and in green are chloroplasts appreciated.

The Cell

The cell theory, proposed in 1839 by Schleiden and Schwann, establishes that all organisms are composed of one or more cells; all cells come from other pre-existing cells; all the vital functions of a living being occur within cells, and cells contain hereditary information necessary for the cell's regulatory functions and for transmitting information to the next generation of cells.

All cells have a plasma membrane that surrounds the cell, separates the interior from the environment, regulates the entry and exit of compounds thereby maintaining membrane potential, a saline cytoplasm that makes up most of the volume of the cell and hereditary material (DNA and RNA).

Based on the location and organization of the DNA, two types of cells are distinguished:

• Procariot cells (of the procariot organisms), which lack nuclear wrap, so DNA is not separated from the rest of the cytoplasm.
• Eukaryotic cells (of the eukaryotic organisms), which have a well-defined nucleus with a wrapping that encloses DNA, which is organized in chromosomes.

All cells share several abilities:

• Cell division reproduction (binary fission, mitosis or meiosis).
• Use of enzymes and other proteins encoded by DNA genes and built via a messenger RNA on ribosomes.
• Metabolism, including obtaining the constructive components of the cell and energy and excretion of waste. The functioning of a cell depends on its ability to extract and use the chemical energy stored in organic molecules. This energy is obtained through the metabolic chains.
• Response to external and internal stimuli, for example, temperature changes, pH or nutrient levels.

Body symmetry

It is the arrangement of body structures with respect to some axis of the body. They are classified in:

• Asymmetric: when they do not present a defined form, such as amoebas.
• Radial: it is presented by organs in the form of a wheel or cylinder and its body parts depart from a shaft or central point. Example: hedgehogs and sea stars.
• Bilateral: it is presented by most living beings, it is that in which when passing an axis through the center of the body two equivalent parts are obtained. Example: vertebrates.

Ecology

Living things can be studied at many different levels: chemical, cellular, tissue, individual, population, community, ecosystem, and biosphere. Ecology proposes an integrating vision of living beings with the environment, considering the interaction of different organisms with each other and with the physical environment, as well as the factors that affect their distribution and abundance. The environment includes both local physical factors (abiotic factors), such as climate and geology, and other organisms that share the same habitat (biotic factors).

Prokaryotes and eukaryotes have evolved according to different ecological strategies. Prokaryotes are small and simple: this gave them the possibility of a high rate of growth and reproduction, which is why they reach high population sizes in a short time, which allows them to occupy ephemeral ecological niches, with dramatic nutrient fluctuations. By contrast, larger and more complex eukaryotes have slower growth and reproduction, but have developed the advantage of being competitive in stable environments with limited resources. One must not fall into the error of considering prokaryotes as evolutionarily more primitive than eukaryotes, since both types of organisms are well adapted to their environment, and both have been selected until today due to their successful ecological strategies.

Proposed forms

Apart from the living beings properly mentioned, it has also been proposed to include other biological forms such as viruses and subviral agents (satellite viruses, viroids and virusoids), nanobios, nanobacteria that are generally not considered living beings because they do not comply with all the characteristics that define living things. The article (Proposed ways of life) can be consulted for information on the arguments for and against its inclusion. In 2012, the discovery of a cellular organism named Parakaryon myojinensis that does not fit into any of the three existing domains was carried out. It differs from prokaryotes in that it has a nucleus and from eukaryotes in that it lacks organelles. In addition, the genetic material is stored in filaments and not in linear chromosomes, and the cell wall is made up of peptidoglycans, characteristics similar to those of bacteria. A characteristic that distinguishes it from both prokaryotes and eukaryotes is the absence of flagella and cytoskeleton. Due to this, some authors consider that it should form its own domain Parakaryota.

Classification of living things

Archaea.	Bacteria.	Protist.
Fungi.	Plant.	Animalia.
Parakaryon.

Living things comprise about 1.9 million described species and are classified into domains and kingdoms. The most widespread classification distinguishes the following taxa:

• Archaea (arches). Procariot organisms that present large differences with bacteria in their molecular composition. About 500 species are known.
• Bacteria (bacteria). Typical proximate agencies. Some 10,000 species are described.
• Protozoos. Generally unicellular eukaryotic organisms. With some 55,000 species described.
• Fungi. Eukaryotic, unicellular or pluricellular suchophytic and heterotrophic organisms that make an external digestion of their food. Understand some 100 000 species described.
• Plantae (plants). Generally multicellular, autotrophic and tissue-based eukaryotic organisms. It comprises some 310 000 species.
• Animalia (animals). Eukaryotic, multicellular, heterotrophic organisms, with a variety of tissues that are characterized, in general, by their ability to locomotive. It's the largest group with 1 425 000 species described.
• Parakaryota. Recently discovered unicellular organisms that do not share the characteristics to be considered part of some of the existing domains and kingdoms. Only one species has been described.

Origin

Earth formed at the same time as the Sun and the rest of the solar system about 4.57 billion years ago, but until 4.3 billion years ago it was too hot to support life. The oldest known fossils are microfossils from Canada dated to between 3.770–4.28 billion years old suggest that life could have arisen on Earth during the Hadic period more than 4.3 billion years ago. Under early Earth conditions (or in outer space and brought by meteorites) the simplest biomolecules could be formed. These include amino acids, nucleotides, and phospholipids, which can spontaneously assemble under certain conditions, forming precellular structures called protobionts. The appearance of biomolecules and the formation of protobionts may have started as early as 4,410 Ma.

Stromatolytes are known as those that form the current cyanobacteria with an antiquity of up to 3700 million years.

From these monomers the proteins, nucleic acids and membranes that make up the protocells are formed. However, here a problem arises: proteins are excellent catalysts for chemical reactions, but they cannot store genetic information, that is, the information necessary for the synthesis of another protein. For their part, nucleic acids store genetic information, but for their duplication they require enzymes, that is, proteins. This raises the dilemma of which came first, proteins (according to metabolism models first) or nucleic acids (gene models first). The theory of the world of iron-sulfide fits into the models of the first type, which assume that the emergence of a primitive metabolism could have prepared a propitious environment for the subsequent appearance of the replication of nucleic acids. The hypothesis of the world of iron RNA, which is widely considered, falls between models of the second type and is based on the observation that some RNA sequences can behave like enzymes. This type of compound is called a ribozyme, meaning an enzyme made of ribonucleic acid.. The RNA world hypothesis assumes that the origin of the molecular and cellular components of life involved the following steps:

• The random chaining of nucleotides to form RNA molecules could have originated ribozymes that would be capable of self-replication and that could possess mechanisms of self-insertion and self-elimination of nucleotides.
• Natural selection processes for greater diversity and efficiency would lead to ribozymes that catalyze peptides and then small proteins, as these compounds are better catalysts. Thus the first ribosome emerged and protein synthesis begins.
• Proteins become dominant biopolymers and nucleic acids (RNAs and DNA) are restricted to predominantly genome use.
• Phospholipids, for their part, can spontaneously form lipid bicapas, one of the two basic components of the cell membrane. The membranes would assist the replication and synthesis of nucleic acids and proteins according to two possible models: cytoplasm inside and cytoplasm outside. In the latter case, nucleic acids and proteins evolve into the outer part of the membrane and only later become internal to form the first cells.

It is not ruled out that the world of RNA could in turn be preceded by other simpler genetic systems, such as ANP, ANT or HAP.

Evolution

Tree of living beings on the basis of symbiogenetic and phylogenetic relationships. Procariots appear in the fossil record at least 3700 Ma, while the origin of the eukaryotic cell was given by symbiogenesis between an archaea and a bacterium makes at least 2100 Ma.

A hypothetical phylogenetic tree of all organisms, based on data from genetic sequences of the RNA 16S, showing the evolutionary history of the three domains of life, Bacteria, Archaea and Eukarya. Originally proposed by Carl Woese.

Extensive horizontal transfer of genes between domains and an ancestral colony as the root of the phylogenetic tree of living beings.

In biology, the universal common ancestor theory holds that all organisms on earth have a common origin. The theory is supported by evidence that all living organisms share numerous common traits. In Darwin-Wallace's time it was based on the visible observation of morphological similarities, such as the fact that all birds have wings, even those that are flightless. Currently genetics reinforces this statement. For example, every living cell makes use of nucleic acids as genetic material and uses the same twenty amino acids as building blocks for proteins. The universality of these traits strongly supports a common ancestry, since it would be highly unlikely that two living beings with the same organic molecules would have appeared independently.

The Last Universal Common Ancestor (LUCA) is the name of the hypothetical single-celled organism from which all of us descend. However, this concept presents some difficulties, since it is possible that the different molecular and cellular components of living beings today come from a community of ancestral organisms, rather than from an individual organism. Molecular data show an atypical distribution of genes among different groups of living beings, and phylogenetic trees built from different genes are incompatible with each other. The history of genes is so convoluted that the only reasonable explanation is extensive horizontal gene transfer. Therefore, each molecule in a living being has its own molecular history and it is possible that each molecule has a different origin (in an organism or not). This is the reason why phylogenetic trees of living things have different branching structures, particularly near the root.

Geology and planetary science also provide information about the early development of life. Life has not only been a passive subject of geological processes but has also actively participated in them, such as in the formation of sediments, the composition of the atmosphere, and the climate.

Eukaryotic organisms are estimated to have arisen about 2.5 billion years ago (the earliest recognizable fossils date from 2.2 billion years ago), so the time required for living matter to emerge from inanimate matter was nearly four times less than that necessary for the eukaryotic cell to arise from the prokaryotes. This observation is not without surprise, since the level of complexity of a eukaryotic cell does not seem to justify the amount of time that elapsed until its appearance. One hypothesis that would explain this is that prokaryotes, upon establishing themselves, became effective competitors that decreased the number of appearances of evolutionary novelties in ecological niches where they did not provide an adaptive advantage. Evolutionary novelties may initially decrease the survival of the new lineage to some degree, and if there is competition they may be eliminated.

Phylogeny

Philogenetic tree of the prokaryota filos where Eukaryota is seen as a sub-clad inside Archaea.

The phylogenetic relationships of living beings are controversial and there is no general agreement among different authors. It now seems clear that eukaryotes derive from archaea with the incorporation by endosymbiosis of a bacterium, but there is disagreement as to where to put bacteria and archaea.

There are two main types of bacteria:

• Gramnegative bacteria present a cellular wrap composed of cytoplasmic membrane, cellular wall and outer membrane. This is, they have two distinct lipid membranes (they are didérmicas), while the rest of the organisms have a single lipid membrane (they are monodermic). Some groups of these bacteria are thermophiles and chemoautrophos such as Aquificae and others can perform annoxygenic photosynthesis, as Chlorobi does today. Later the cyanobacteria improved the mechanism of photosynthesis that became oxygenic, beginning the release of large amounts of molecular oxygen to the environment.

• The grampositive bacteria have a single membrane and the peptidoglycan wall (mureine) is much thicker. The hypertrophy of the cell wall increases its resistance but prevents the transfer of lipids to form the outer membrane. These organisms were probably the first to colonize the soil.

For their part, archaea are monodermic, like gram-positive bacteria, but their cell wall is different because it is not made of peptidoglycan, but of glycoproteins, pseudomurein, etc. Furthermore, the lipids of archaeal cell membranes differ from those of bacteria (they are composed of prenyl ether instead of acyl ester). The metabolism of archaea is very varied, comprising many types of chemoautotrophic organisms and others that carry out anoxygenic photosynthesis such as Haloarchaea. Many species are hyperthermophilic or are adapted to other extreme environments (hyperacidophiles, hyperhalophytes, etc).

Possible relationships between the two groups of bacteria and the archaea are as follows:

• Archaea is the oldest domain, or alternatively, Archaea and Bacteria are equally ancient.

• The grampositive bacteria are the oldest, deriving the gramnegatives and arches from them.

• Gramnegative bacteria are the oldest ones, deriving the grampositives and arches from them.

On the other hand, molecular studies support that both bacteria and archaea are monophyletic groups, which implies that they evolved together from the last universal common ancestor and not from each other.

Recent discoveries support that the Eukarya domain derives from Archaea, specifically from Proteoarchaeota, with the archaea of the Asgard clade being the closest to eukaryotes. Furthermore, mitochondria are considered to derive from the endosymbiosis of alpha proteobacteria, while the chloroplasts of plants do it from a cyanobacterium. Eukaryotes used their new cell surface glycoproteins as a flexible shell that gave rise to phagocytosis for the first time in the history of life, and through the acquisition of mitochondria ultimately led to change in the structure of cells. the cell (nucleus, endomembrane, cytoskeleton, etc). This change is reflected in the profound differences between the prokaryotic and eukaryotic cell.