characteristics of living beings essay

What makes something living?

All living organisms share several key characteristics or functions: order, sensitivity or response to the environment, reproduction, adaptation, growth and development, homeostasis, energy processing, and evolution. When viewed together, these characteristics serve to define life. Different sources may use slightly different terms to describe these characteristics, but the basic ideas are always present.

Organisms are highly organized, coordinated structures that consist of one or more cells. Even very simple, single-celled organisms are remarkably complex: inside each cell, atoms make up molecules; these in turn make up cell organelles and other cellular inclusions. In multicellular organisms, such as the toad seen in Figure 1, similar cells form tissues. Tissues, in turn, collaborate to create organs (body structures with a distinct function). Organs work together to form organ systems.

In this class, we will be focusing on how cells function, so we will be concentrating on biological molecules, how they make up cells, and how those cells function.

Sensitivity or Response to Stimuli

Organisms respond to diverse stimuli. For example, plants can bend toward a source of light, climb on fences and walls, or respond to touch (Figure 2). Even tiny bacteria can move toward or away from chemicals (a process called chemotaxis ) or light ( phototaxis ).

Reproduction

Single-celled organisms reproduce by first duplicating their DNA, and then dividing it equally as the cell prepares to divide to form two new cells. Multicellular organisms often produce specialized reproductive germline (reproductive) cells that will form new individuals. When reproduction occurs, DNA is passed from the organism to that organism’s offspring. DNA contains the instructions to produce all the physical traits for the organism. This means that because parents and offspring share DNA ensures that the offspring will belong to the same species and will have similar characteristics, such as size and shape.

Growth and Development

All living things increase in size and/or change over their lifespan. For example, a human grows from a baby into an adult and goes through developmental processes such as puberty. Organisms grow and develop following specific instructions coded for by their genes (DNA). These genes provide instructions that will direct cellular growth and development, ensuring that a species’ young will grow up to exhibit many of the same characteristics as its parents, like the puppies seen in Figure 3.

Homeostasis and Regulation

In order to function properly, cells need to have appropriate conditions such as proper temperature, pH, and appropriate concentration of diverse chemicals. These conditions may, however, change from one moment to the next. Organisms are able to maintain internal conditions within a narrow range almost constantly, despite environmental changes, through homeostasis (literally, “steady state”)—the ability of an organism to maintain constant internal conditions. For example, an organism needs to regulate body temperature through a process known as thermoregulation. Organisms that live in cold climates, such as the polar bear (Figure 4), have body structures that help them withstand low temperatures and conserve body heat. Structures that aid in this type of insulation include fur, feathers, blubber, and fat. In hot climates, organisms have methods (such as perspiration in humans or panting in dogs) that help them to shed excess body heat.

Even the smallest organisms are complex and require multiple regulatory mechanisms to coordinate internal functions, respond to stimuli, and cope with environmental stresses. Two examples of internal functions regulated in an organism are nutrient transport and blood flow. Organs (groups of tissues working together) perform specific functions, such as carrying oxygen throughout the body, removing wastes, delivering nutrients to every cell, and cooling the body.

Energy Processing

All organisms use a source of energy for their metabolic activities. Some organisms capture energy from the sun and convert it into chemical energy in food (such as grass and bacteria that can perform photosynthesis); others use chemical energy in molecules they take in as food (such as the condor seen in Figure 5).

Unless otherwise noted, images on this page are licensed under CC-BY 4.0  by  OpenStax .

Text adapted from: OpenStax , Concepts of Biology. OpenStax CNX. May 25, 2017 https://cnx.org/contents/[email protected]:gNLp76vu@13/Themes-and-Concepts-of-Biology

Principles of Biology Copyright © 2017 by Lisa Bartee, Walter Shriner, and Catherine Creech is licensed under a Creative Commons Attribution 4.0 International License , except where otherwise noted.

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Characteristics of living things.

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When you look at the world around you, how do you categorise or group what you see? One of the broadest groupings is 'living' and 'non-living'. This may sound simple, but it is sometimes difficult to decide whether something is truly alive or not. Here we look at the characteristics of living things – using earthworms as an example.

All living things share life processes such as growth and reproduction. Most scientists use seven life processes or characteristics to determine whether something is living or non-living.

The table below describes seven characteristics of most living things and contains references to earthworms to explain why we can definitely say that they are 'living'.

Further classification

Based on the information above, we can confidently categorise earthworms as living things as they carry out all seven life processes.

It is now possible to classify them further into a series of hierarchical categories: kingdom, phylum, class, order, family, genus and species. Classifying living things into these categories is an important way for scientists to show how living things are related to each other. Most scientists classify living things into one of the following six kingdoms.

• Bacteria are single-celled microorganisms that don’t have a nuclear membrane.
• Protozoans are single-celled organisms that are generally much larger than bacteria. They may be autotrophic or heterotrophic.
• Chromists are a diverse group of plant-like organisms and range from very small to very large. They are found in almost all environments.
• Fungi are multicellular and rely on breaking down organic material as they are not able to make their own food.
• Plants are multicellular and autotrophic – they use photosynthesis to produce food using sunlight.
• Animals are multicellular. They are heterotrophic and rely on other organisms for food.

Which kingdom do you think earthworms belong to?

Animal characteristics

What did you decide? Sometimes people are surprised to find out that earthworms are actually animals – the same kingdom as humans, cats, dogs, dolphins and spiders! Just as living things share a set of common characteristics, animals have key characteristics that can help you to decide whether a living thing is an animal or not.

Key characteristics of animals include the following:

• They obtain energy by consuming other organisms (we say they are 'heterotrophic').
• They are able to physically move their bodies about at one or more stages of their life cycle.
• Their bodies are made up of multiple cells.
• Sexual reproduction occurs - a sperm and an egg cell combine to produce an embryo that grows into a juvenile animal.

Nature of science

Scientists need scientific vocabulary to communicate effectively. At an even more fundamental level, scientific language actually helps shape ideas and provides the means for constructing scientific understandings and explanations.

Related content

Read more on the classification system and taxonomy and learn about classifying marine organisms and how scientists classify ferns .

Activity idea

Students can explore their ideas about the characteristics of living things with this graphic organizer, Living or non-living?

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Living things

Living things n., singular: living thing [ˈlɪvɪŋ θɪŋ] Definition: any organism or a life form that possesses or shows the characteristics of life or being alive

Table of Contents

Living Things Definition

A living thing pertains to any organism or a life form that possesses or shows the characteristics of life or being alive. The fundamental characteristics are as follows: having an organized structure, requiring energy, responding to stimuli and adapting to environmental changes, and being capable of reproduction, growth, movement, metabolism, and death. Currently, living things are classified into three Domains: (Eu)Bacteria (true bacteria), Archaea (archaebacteria), and Eucarya ( eukaryotes ).

Etymology:  The term living came from the Old English lifende , meaning “living” or “having life”. The term thing came from the Old English þing , meaning “entity”, “being”, “body”, or “matter”. Synonyms: organism; life form; creature.

While Earth is presumed to be about 4.54 billion years old , life on Earth began later, probably around 3.5 billion years ago , although others believe that life may have started earlier than that.

Abiogenesis

The origin of life, also referred to as abiogenesis , refers to the natural process in which life came about from non-living matter or lifeless objects. How this occurred remains a matter of debate among scientists. Till now, there is no consensus as to how life on Earth began.

Primordial soup

The “primordial soup” refers to the hypothetical model of the primitive Earth wherein it accumulated organic material and water resembling a soup . This soup served as a site where organic compounds were synthesized. A widely-accepted research finding is that of the Miller–Urey experiment. Apparently, the simulated-primitive Earth favored the chemical syntheses of the basic structure of the cell membrane (e.g. phospholipids forming lipid bilayers) and organic compounds from inorganic sources. The primordial soup is also the edict of the heterotrophic theory of the origin of life proposed by Alexander Oparin and John Burdon Sanderson Haldane.

RNA world hypothesis

The transition from non-living to living entities may have occurred gradually. One of the popular theories held today is the RNA world hypothesis , which suggests that primordial life was based on RNA because it can act as both genetic material and catalyst. This RNA-based life could have served as the descendants of the current life on Earth.

The building blocks of RNA and DNA may have originated and formed in the asteroids from outer space and then reached the Earth through meteorites. According to NASA, they found RNA and DNA nucleobases such as adenine and guanine in meteorites. (1) These nucleobases could have led to the spontaneous creation of RNA and DNA.

These organic molecules might have been used by the first life forms to live and propagate. The earliest life forms might be the single-celled organisms that came into existence near the end of Hadean Eon or early in the Archean Eon. This is based on the discovery of graphite of biogenic origin in Western Greenland which was estimated to be 3.7 billion years old . (2) Organisms lacking membrane-bound organelles were likely the first living entities to have dominated the Earth. They are referred to as prokaryotes , a group comprised of bacteria and archaea.

Forum Question: Is DNA living or non-living thing ?     Featured Answer !

Endosymbiotic theory

The endosymbiotic theory posits that endosymbiosis between a larger cell and a prokaryote led to the first photosynthetic eukaryote. Based on this theory, the larger eukaryote might have engulfed prokaryotes that over time transformed into semi-autonomous organelles, such as chloroplasts and mitochondria inside the cell.

Multicellularity

Multicellular life probably began 600 million years ago and occurred several times in biological history. The most popular theory on the origin of multicellularity is Haeckel’s Gastraea Theory . Accordingly, multicellularity first occurred when cells of the same species group together in a blastula-like colony. Gradually, certain cells in the colony underwent cell differentiation. However, this theory is still inadequate in explaining the origin of multicellularity.

Ediacaran biota comprised of single-celled and multicellular organisms existed in the Ediacaran period, around 600 million years ago. In this biota , the earliest animals first appeared. They resemble sponges with sizes ranging from 1 cm to less than 1m. (3)

Cambrian explosion

Around 541 million years ago , a sudden burst of life occurred in the Cambrian period. This is referred to as the Cambrian explosion . Diverse plants and animals came into being. In the late Cambrian or Early Ordovician period, animals began to venture into the land. With the evolution of land plants, the animals, too, evolved and diversified. Eventually, they colonized terrestrial habitats, including farther inland.

Current population

In May 2016, the estimated total number of species on Earth is about 1 trillion. (4) In 2011, the Census of Marine Life estimated about 8.7 million eukaryote species on Earth. (5) Unfortunately, many of the living things (probably over five billion species) that ever lived on Earth became extinct.

Watch this vid about the origin of life on Earth:

Classification

Living things were initially classified as either a plant or an animal . While both animals and plants are eukaryotic, they are distinguished based on their defining characteristics, e.g. in terms of motility, mode of nutrition, and cellular features. Animals, basically, are living things that are motile and heterotrophic whereas plants are those that are non-motile, photosynthetic, autotrophic (i.e., capable of producing their own food), and have a cell wall. However, bacteria and archaea are neither plants nor animals mainly because they are prokaryotes (i.e. lacking in membrane-bound cytoplasmic organelles, including the nucleus ).

As for the distinction between bacteria and archaea, one of their differences lies in RNA polymerase. In archaea , it has ten subunits. In bacteria, it has four. Another example is the composition of the cell wall. The archaeal cell wall lacks peptidoglycan whereas the bacterial cell wall has.

At present, the modern biological taxonomy entails the classification of living things into three domains: (1) domain Eukarya , (2) domain Bacteria , and (3) domain Archaea. A biological domain is the highest taxonomic rank of organisms according to Carl Woese’s 3-domain system of taxonomy. Below the domain are seven major taxonomic ranks. In descending order, they are as follows:

Domain » Kingdom » Phylum » Class » Order » Family » Genus » Species

Domain Eukarya includes all living things that are eukaryotic. These include animals, plants, fungi, algae, and protists. They possess membrane-bound organelles within their cells.

Characteristics of Living Things

Living things are organisms that show the characteristics of being alive. What separates living things from non-living things is the following characteristics:

An organized structure

Living things are organized structures. It may be a single-celled such as a bacterial cell, or multicellular such as animals and plants that are made up of several cells. A cell is the fundamental biological unit of an organism . Various cellular processes are carried out by the cell in an orchestrated, systematized manner. A cell consists of protoplasm surrounded by a plasma membrane . Cytoplasmic structures (e.g. organelles ), each with specific roles and functions, are suspended in the cell’s cytosol .

In multicellular organisms, their body is made up of cells, which are organized into tissues. The tissues , in turn, are organized into biological organs and systems . These structures, although with distinct functions, work together in a coordinated fashion. Thus, an organism, whether it is made up of one or more cells, is an assembly from a systematic, coherent design.

Energy-requiring

Living things require energy for survival. Energy is essential as it fuels numerous metabolic activities of a cell. One way that living organisms synthesize energy is by photosynthesis where light energy is converted into chemical energy. Another is by cellular respiration wherein biochemical energy is harvested from an organic substance (e.g. glucose ) and, then, stored in an energy-carrying biomolecule such as ATP for later use.

Reproductive capacity

A living thing is capable of reproducing. There are two ways by which living things can reproduce copies of themselves: sexual reproduction and asexual reproduction. In sexual reproduction , male and female sex cells of the two parents unite and form a zygote that will develop eventually into a being of their own kind. Asexual reproduction , in contrast, is a mode of reproduction that does not involve the fertilization of gametes or sex cells. The offspring comes from only one parent. Examples include binary fission , budding , vegetative propagation , sporogenesis , fragmentation , parthenogenesis , apomixis , and nucellar embryony .

Living things grow. At the cellular level , growth may refer to an increase in number or to an increase in size . The increase in the number of cells is through cell division . The stem cells of animals and the meristematic cells of plants divide to give rise to new living cells. As for the increase in cell size, it is attributed often to the increase in cytoplasmic mass.

The cell undergoes a series of phases in the cell cycle . Most of the time, the new cell produced by mitosis undergoes interphase . It is the phase in the cell cycle wherein the cell grows in size. Unless fully differentiated, the cell could replicate its DNA to prepare for the next cell division. In plants, new cells increase in volume by taking in and storing water inside a vacuole .

Some of the plant cells grow a secondary cell wall between the primary cell wall and the plasma membrane. At the tissue level , growth in vascular plants is of two types: primary and secondary. Primary growth entails vertical growth as primary xylem forms from the procambium whereas secondary growth is associated with lateral growth caused by the formation of secondary xylem from the vascular cambium.

In higher forms of animals, the growth of tissues follows a pattern and is genetically predetermined. Regeneration capacities are not as indefinite as those in plants. The extent of regeneration varies among species. For instance, salamanders can regenerate new eyes or new limbs whereas humans cannot. Nevertheless, humans are also capable of regenerating certain parts of their body, such as skin and parts of the liver .

Forum Question: What are the advantages and disadvantages of being a large organism ?    Featured Answer !

A living thing metabolizes. Metabolism refers to the various life processes that are responsible for the keeping up of the living state of a cell or an organism. Examples of those involved in cell growth, respiration, reproduction, response to stimuli, sustenance, biomolecular syntheses, waste elimination, and other homeostatic processes.

There are two forms of metabolism: catabolism and anabolism . In catabolism , living things carry out degradative chemical reactions that lead to the breaking down of complex molecules into smaller units and obtain energy that is released from the process. In anabolism , energy-driven chemical reactions build molecules from smaller units.

• Homeostasis

Living things tend to adjust to achieve a stable internal environment. They possess dynamic mechanisms (positive and negative feedback loops) that control and orchestrate biological systems in order to stay within the range of favorable or ideal internal conditions. Thermoregulation, osmoregulation, calcium homeostasis, and potassium homeostasis are just a few of the multifarious homeostatic processes vital to life. Achieving balance is crucial to the proper functioning and survival of an individual.

Responsive to stimuli

Living things respond to stimuli and adapt to environmental changes. A living organism can detect changes in the environment, especially by cells that function as receptors. For instance, humans have five fundamental senses: sight, hearing, smell, touch, and taste. Other senses are the vestibular sense (detects body movement, direction, and acceleration), sense for thermoception, kinesthetic sense (detects body part positions), internal sense (interoception), and so on. Apart from detecting changes in its surroundings, it can also adapt to these changes.

Adaptation and Evolution

Living things adapt and have the capacity to evolve. The ability of organisms to respond accordingly to changes in their environment enables them to adapt over time. This means the species has eventually acquired the trait that is functional, beneficial, and heritable . This, in turn, could lead to the gradual change and diversification of organisms across generations where species possess distinctive features driven by natural selection — an indication of  evolution .

A living thing moves. Since a living thing can detect stimuli from its surroundings, it can respond accordingly. For example, animals move to forage, escape predators, and seek a potential mate. While animals can move at will, plants have a rather limited form of movement, referred to as nastic movement (e.g. thigmonasty , nyctinasty ).

Living things die. A living thing has life and this life ends eventually. At a certain point in the typical life cycle of organisms, there will come a time wherein an individual starts to experience a gradual decline in physical robustness, function, and capacity. This marks the period of senescence.

Senescence refers to biological aging. It is when living things gradually deteriorate over the course of their life. The organism gradually loses its ability to function and deal with stressors. As such, it becomes more vulnerable to diseases and dysfunction. At the cellular level, the cell no longer divides although it may still be metabolically active. One of the natural causes of cellular senescence is the shortening of telomeres leading to DNA damage. Conversely, some living beings are regarded as immortal because they seem to circumvent death. Examples include the age-reversing jellyfish Turritopsis doohmii , the regenerating flatworms , and the seemingly indestructible tardigrades. Thus, different species could have variations in their life stages. While some organisms have shorter life stages, others have longer lifespans owing to the complexity of their growth and development.

Non-cellular life

Are viruses living things? This question has sparked major debate among biologists for so long. Some would consider viruses as living things since they appear to be alive when they are inside their host. They possess genetic material, replicate themselves, and evolve by natural selection. However, others do not take them as living things because they are essentially dead when outside their host. Viruses cannot reproduce independently.

They rely on the host cell’s machinery to do so. Thus, viruses are not absolutely living or non-living. When outside their host, viruses are inactive and seemingly inanimate. When inside their host, they became active, capable of utilizing the host cell ‘s structures and replicating. Viroids are another group that seems to be a non-cellular life. They are infectious and pathogenic short strands of circular, single-stranded RNA.

The study of living things is called biology (also called biological science). An expert in this field is called a biologist . Several areas of biological studies include morphology , anatomy , cytology , histology , physiology , ecology , evolution , taxonomy , and pathology.

“Living things that mimic non-living things”

One of the distinctive features of a living thing is its ability to adapt . Some organisms, though, have leveled up a notch by possessing the ability to “mimic” their surroundings. Because of such an astounding deceptive strategy to “hide”, these organisms can trick others into believing they are non-living things when, in fact, they are alive.  Here are some of them!

• Lithops sp. also known as “living stones” may look like rocks but they’re actually a group of succulent plants.

• Watch out! This venomous stonefish can look like a stone or rock.

• Flounders matching the sandy or rocky substrate on the ocean floor

Choose the best answer. 

Send Your Results (Optional)

• Non-living thing
• NASA – NASA Researchers: DNA Building Blocks Can Be Made in Space . (2011, January 1). Retrieved from Link
• Ohtomo, Y., Kakegawa, T., Ishida, A., Nagase, T., & Rosing, M. T. (2013). Evidence for biogenic graphite in early Archaean Isua metasedimentary rocks. Nature Geoscience, 7 (1), 25–28. Link
• The History of Animal Evolution . (2000, January 1). Retrieved from Link
• Researchers find that Earth may be home to 1 trillion species NSF – National Science Foundation. (2016, January 1). Retrieved from https://www.nsf.gov/news/news_summ.jsp?cntn_id=138446
• Census of Marine Life. (2011, August 24). How many species on Earth? About 8.7 million, new estimate says. ScienceDaily . Retrieved from Link

Further reading

• Characteristics of living things – Science Learning Hub . (Additional resource material that depicts the characteristics of living things).

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Last updated on August 17th, 2023

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What is life?

• Open access
• Published: 27 July 2021
• Volume 48 , pages 6223–6230, ( 2021 )

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• Jaime Gómez-Márquez   ORCID: orcid.org/0000-0001-6962-1348 1  

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Many traditional biological concepts continue to be debated by biologists, scientists and philosophers of science. The specific objective of this brief reflection is to offer an alternative vision to the definition of life taking as a starting point the traits common to all living beings.

Results and Conclusions

Thus, I define life as a process that takes place in highly organized organic structures and is characterized by being preprogrammed, interactive, adaptative and evolutionary. If life is the process, living beings are the system in which this process takes place. I also wonder whether viruses can be considered living things or not. Taking as a starting point my definition of life and, of course, on what others have thought about it, I am in favor of considering viruses as living beings. I base this conclusion on the fact that viruses satisfy all the vital characteristics common to all living things and on the role they have played in the evolution of species. Finally, I argue that if there were life elsewhere in the universe, it would be very similar to what we know on this planet because the laws of physics and the composition of matter are universal and because of the principle of the inexorability of life.

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Introduction

Life is a wonderful natural process that occurs in highly organized dynamic structures that we call living beings. Today, thanks to the enormous advance of Biology, we know and understand much better the vital phenomenon, the molecular biology of the cells, the enormous biodiversity on our planet, the evolutionary process, and the complexity of ecosystems. However, despite these enormous advances, biology still lacks a solid theoretical framework necessary to understand the vital phenomenon and to answer questions such as what is life? or are viruses living entities? To answer these and other fundamental questions related to life, in addition to the universal laws of physics, biology needs its own principles to help us find answers to major theoretical challenges such as the origin of life, the construction and maintenance of genomes, or the concept of life itself. Regarding the principles governing life, there have been several contributions from different perspectives (e.g. [ 1 , 2 , 3 , 4 , 5 ]) and I myself have proposed a series of principles (named as the commandments of life) to explain and understand the vital phenomenon from an evolutionary perspective, far from any vitalist, pseudo-scientific or supernatural considerations [ 6 ].

In the words of B. Clark, a definition of life is needed more than ever before to provide defendable objective criteria for searches for life on other planets, to recognize critical distinctions between machine life and robots, to provide insight into laboratory approaches to creating test-tube life, to understand the profound changes that occurred during the origin of life, and to clarify the central process of the discipline of biology [ 7 ]. It is worth noting what E. Koonin wrote about the complexity of defining life: “In my view, although life definitions are metaphysical rather than strictly scientific propositions, they are far from being pointless and have potential to yield genuine biological insights” [ 8 ]. However, despite its importance there is no widely accepted definition of what life is and some of the most commonly employed definitions (see below) face problems, often in the form of robust counter-examples [ 5 , 9 ]. Even some scientists and philosophers of science suggest that it is not possible to define life [ 5 , 8 ].

We can define life in very different ways depending on the context and the focus we want to give to the definition. For example, we can define life as the period from birth to death or as the condition that occurs only in living organisms. We can also say that life is a wonderful and ever-changing process that occurs in highly organized receptacles that we identify as living entities. Likewise, the popular encyclopedia Wikipedia define life as “a characteristic that distinguishes physical entities that have biological processes ….. from those that do not …” [ 10 ]. However, with these expressions we are not defining precisely what life is and therefore we need to create a definition that concisely but informatively reflects our scientific knowledge of the vital phenomenon. We have to distinguish between life and living matter, which is the place where life lives, and between living beings and non-living matter. In reality, when we ask ourselves “what is life?” we are asking “what are the characteristics that distinguish a living organism from a non-living entity?

There are numerous definitions of life formulated from different characteristics of living beings (replication, metabolism, evolution, energy, autopoiesis, etc.) and from different approaches (thermodynamic, chemical, philosophical, evolutionary, etc.). Often, definitions of life are biased by the research focus of the person making the definition; as a result, people studying different aspects of biology, physics, chemistry, or philosophy will draw the line between life and non-life at different positions [ 11 ]. These strategies create a panoply of alternative definitions that makes it very difficult to reach a consensus on the best definition of life because they all have pros and cons. [ 12 , 13 ]. Let me briefly discuss some of the most representative definitions of life. There is a short definition “Life is self-reproduction with variations” [ 14 ] that is interesting for its brevity and because it includes two fundamental characteristics of living organisms: reproduction and evolution. However, this minimalist definition is clearly insufficient [ 8 ] and it does not include some of the most important traits we see in living things. Along these lines, there is also the definition coined by NASA: “Life is a self-sustaining chemical system capable of Darwinian evolution” [ 15 , 16 ]. This is a more complete and, I believe, better definition than the previous one, as it incorporates, in addition to reproduction and evolution, metabolism. However, both definitions are unsatisfactory because neither cell nor multicellular organisms are self-sufficient as there is always a dependence on other organisms and external factors to live and reproduce. Furthermore, these definitions say nothing about the chemical nature of living matter, the interactions with the environment or the low entropy of living things. Apart from that, reproduction being essential for the perpetuation of species and evolution, not all living beings are able to reproduce (e.g. the mule, most bees, etc.) and do not thereby lose their living status. A more recent definition of life states: “Life is a self-sufficient chemical system far from equilibrium, capable of processing, transforming and accumulating information acquired from the environment” [ 17 ]. Although this definition is more comprehensive than the previous ones and includes a reference to thermodynamics, in my opinion it has four drawbacks: (i) the term “self-sufficient” is not adequate because the quality of life does not provide self-sufficiency; (ii) the thermodynamic component does not highlight how fundamental low entropy or high order is for any living being; (iii) information can be acquired from “within” and not only from the environment; (iv) life is not a system is a process and living beings are the system where that process occurs (I discuss this point below). From a very different perspective it was defined life as “matter with the configuration of an operator, and that possesses a complexity equal to, or even higher than the cellular operator” [ 18 ]. This proposal introduces a new term, the operator, which is somewhat confusing, excludes viruses and makes a strange classification of living systems. On the other hand, some scientists have also attempted to define life from a handful of key features. Thus, seven “pillars” (the essential principles by which a living system functions) have been proposed on which life as we know it can be defined [ 19 ], but no definition was provided. Life has also been considered as any system that from its own inherent set of biological instructions and the algorithmic processing of that "prescriptive information" can perform the nine biofunctions [ 20 ] which are basically the same as the "pillars" mentioned above. However, no definition of life was proposed, and again it was considered as a system rather than a process. Both definitions exclude viruses as living beings, mainly because the existence of a membrane, a metabolic network and self-replication are set as conditions for life. In short, there are many more definitions of life but as R. Popa says “We may never agree on a definition of life, which will remain forever subject to a personal perspective” [ 21 ].

My definition of life

Traits are measurable attributes or characteristics of organisms and trait-based approaches have been widely used in systematics and evolutionary studies [ 22 ]. Since any definition of life must connect with what we observe in nature, my strategy for finding a definition of life was to establish what are the key attributes or traits common to all living things. What do bacteria, yeasts, lichens, trees, beetles, birds, whales, etc. have in common that clearly differentiates them from non-living systems? In my opinion, living organisms share seven traits: organic nature, high degree of organization, pre-programming, interaction (or collaboration), adaptation, reproduction and evolution, the last two being facultative as they are not present in all living beings.

Organic nature and highly organized structures. Living matter is organic because it is based on carbon chemistry and molecular interactions take place following the laws of chemistry. As R. Hazen wrote “Carbon chemistry pervades our lives. Almost every object we see, every material good we buy, every bite of food we consume, is based on element six. Every activity is influenced by carbon—work and sports, sleeping and waking, birthing and dying.” [ 23 ]. Living organisms are highly organized structures that maintain low entropy (the vital order) by generating greater disorder in the environment, thus fulfilling the postulates of thermodynamics [ 24 , 25 ]; when this vital order is lost, life disappears and the only way to restore life is to generate a new vital organized structure through reproduction [ 6 ]. Living organisms resist entropy thanks to biochemical processes that transform the energy they obtain from nutrients, sun or redox reactions. It could be said that vital order and energy are two sides of the same coin.

Pre-programming. Every living entity has a software (a pre-programme) in its genetic material that contains the instruction manual necessary for both its construction (morphology) and its functioning (physiology). This programme has been modified in the course of evolution, as a consequence of contingency and causality, so it is not a static or immutable program but a dynamic one. Furthermore, there is also another preestablished program that conditions the vital phenomenon and that I have called the principle of inexorability [ 6 ]. Let me give few examples of the principle of inexorability at different levels of complexity. The shape of ribosome is determined (pre-programmed) by the chemical bonds that are established between ribosomal proteins and rRNA. A similar example is the λ phage morphogenesis that depends only on interactions protein–protein and protein-DNA. Evolutionary convergence or the need for wings to fly are other examples of this inexorability guided by the laws of nature.

Interaction and adaptation. If we look at nature in its purest state or at the complex human society, we can see countless interactions between living beings and with their environment necessary for survival and reproduction. We can see interactions at the molecular level (e.g., allosteric interactions, metabolic pathways, cellular signaling, quorum sensing), in the relationships between organisms of the same or different species (e.g., sexual reproduction, symbiosis, infection, parasitism, predator–prey, or sound language), or between living forms and the environment (e.g., photosynthesis or physiological/anatomical interactions for swimming or flying). Interaction is collaboration, it is cooperation at all levels [ 6 ], the ecosystem being the best example of multiple collaborative interactions between very different organisms. In terms of adaptation, living organisms show a great capacity to adapt both to their surroundings and to environmental circumstances; furthermore, adaptations involving new biological characteristics can be seen as an opportunity to find a different way to evolve. In this sense, the evolutionary process reflects this continuous adaptation and anatomy, physiology and genome bear witness to this. Life is adaptative because species adapt to environmental changes modifying their physiology or metabolism, for instance reducing heartbeat during hibernation (e.g. the grizzly bear Ursus arctos horribilis ) or synthesizing fat from excess sugar to increase the energy reserves of the body (e.g. Homo sapiens ). In addition to these temporal adaptations in response to environmental changes [ 26 ], there are also changes in genotype or phenotype since the adaptation process is the result of natural selection acting upon heritable variations [ 27 ]; a well-known example of this is the peppered moth Biston betularia whose allele frequencies of the locus that controls the distribution of melanin in the wings changed with the industrial revolution in England [ 28 ]. Epigenetic variations also contribute to rapid adaptative responses [ 29 , 30 ].

Reproduction and Evolution. Another property of living beings is their ability to perpetuate themselves and thus make it possible for the species not to disappear and to evolve. Reproduction can be observed at the molecular (DNA replication), cellular (mitosis, meiosis, binary division), and organismal (sexual and asexual) levels. From a different perspective, reproduction is also the way to overcome the second law of thermodynamics and the tyranny of time because when we reproduce, we are creating a new order and resetting the vital clock to zero [ 6 ]. What about individuals such as the mule or the male and female of a species, or the hermaphrodite that cannot self-fertilize, who cannot reproduce because they are sterile or because they need another member of their species to reproduce? Are not these organisms living beings? Of course, they are! In this context, reproduction must be considered as a facultative trait because not all living organisms are fertile or can produce offspring on their own but maintain all other traits necessary for the life process. If an individual is sterile, the species will continue to exist because the evolutionary process must be analyzed at the population level, not at the level of individual organisms; obviously, if the entire population were sterile, then the species would disappear and there would be no life. All species have the capacity to evolve, and this property is unique to life. Evolution allows living beings to adapt to new circumstances and the best genomes are selected and transmitted to the next generations. The concept of evolution (reproduction with variations and permanence in time) allows us to interpret the reality of the life we observe now and to guess what it has been like in the past. We cannot predict the future because evolution is not a finalistic process, it is, to use the words of J. Monod, the fruit of chance and necessity.

There is nothing on this planet, apart from a living being, that complies with all these characteristic features of living beings. It should therefore be possible to define life by logically combining them. Consequently, I define life as a process that takes place in highly organized organic structures and is characterized by being preprogrammed, interactive, adaptative and evolutionary. If life is the process, a living organism is the system in which that process takes place and which is characterized as organic, highly organized, pre-programmed, interactive, adaptative, and evolutionary. Why do I say that life is a process and not a system? According to the Merriam-Webster dictionary, a process is a natural phenomenon characterized by gradual changes that lead towards a certain result. A second meaning defines it as a continuous natural or biological activity or function; and a third one as a series of actions or operations conducing to an end. These three meanings of what a process is fit very well with what we observe happening in living beings, which is none other than the vital process or life. The dictionary itself defines a system as a regularly interacting or interdependent group of items forming a unified whole, and as an assemblage of substances that is in or tends to equilibrium or a group of body organs that together perform one or more vital functions. Once again, these definitions fit very well with what a living being represents.

What is the difference between life, living being and a robot? [ 31 ] Life is the vital process and the living being is the system, the “container” in a metaphorical way, where the vital process takes place. Following this reasoning, a robot would be an artificially organized, pre-programmed and interactive system, but unlike a living being it is not alive because it is neither organic, nor does it reproduce, adapt, or evolve. A robot or a population of robots cannot “reproduce and evolve” on its own, without the intervention of its "creator" (the human being), it will always need to be built or programmed by an engineer to do so. I do not dispute that the robot can adapt, especially thanks to advances in artificial intelligence, although I am not sure that it can do so in the biological sense of the term. Biological adaptation is a process by which a species eventually adapts to its environment as a result of the action of natural selection on phenotypic characteristics [ 32 ]. A robot may be able to adapt to its environment, but what it cannot do is adapt itself through a selective process (without intervention from its creator) and change into a new type of robot (evolve). On the other hand, regarding the synthetic lifeforms named as xenobots [ 33 ], I think they cannot be considered as pure robots, but as an interface between living beings and artificial robots, as they are made from cells. In the future we will probably build robots so perfect that we can consider them as almost living beings and as the result of the intervention of a creator (their engineer), something that we cannot say about living beings unless we are creationists.

Are viruses alive?

A. Turing, one of the pioneers in the development of computer sciences, wrote: “Can machines think? This should begin with definitions of the meaning of the terms “machine” and “think” [ 34 ]. To paraphrase Turing, we could ask ourselves: can viruses be considered living entities? And the answer to this question, so important for biology and still controversial [ 35 ], is to define what a virus is and what life is. At least from a theoretical point of view, biology should seek a clear and definitive answer to this question instead of adopting a skeptical attitude and assuming what K. Smith wrote in his classic book on viruses, “As to the question asked most frequently of all, are viruses living organisms? that must be left to the questioner himself to answer” [ 36 ].

Viruses are entities that straddle the boundary between living and non-living and therefore their biological status is controversial. A virus can be defined as an acellular infectious agent whose structure consists of a macromolecular complex of proteins and nucleic acids. Viruses are not cells, they do not metabolize substances, nor can they reproduce by themselves, grow, or breathe. Yet, regardless of whether we consider viruses to be living beings or not, they are an inescapable part of life and there is an undeniable biological connection between the virus and the organism it infects. Given the close interconnection between viruses and their hosts, it seems plausible that viruses play essential roles in their hosts [ 37 ]. For example, endogenous retroviral elements have shaped vertebrate genome evolution, not only by acting as genetic parasites, but also by introducing useful genetic novelty [ 38 ]. More recently, it was found in the human genome a gene regulatory network based on endogenous retrovirus that is important for brain development [ 39 ] and a new tamed retroviral envelope that is produced by the fetus and then shed in the blood of the mother during pregnancy [ 40 ].

Viruses are capsid-encoding particles that infect all kind of cells and share hallmark genes with capsidless selfish genetic elements, such as plasmids and transposons [ 41 ]. Traditionally, they have been regarded as lifeless agents because they have no metabolism of their own and need a cell to replicate and generate new viruses [ 42 ]. However, while this is true, I believe that this is not a definitive criterion for excluding them from the tree of life (more on this below). There are scientists in the opposite side that consider viruses as living beings that can evolve [ 43 ] and classify them as capsid-encoding organisms as opposed to the ribosome-encoding organisms that include all cellular life forms [ 37 , 44 ]. Viruses have played a key role in the evolution of species [ 35 ] because they are the most abundant source of genetic material on Earth, are ubiquitous in all environments, and have actively participated in the exchange of genes or DNA fragments with their hosts [ 41 , 45 , 46 ].

We cannot say whether a virus is a living thing or not without defining what is life and what is a living thing. Obviously, if we take the cell as the minimum vital unit, we cannot consider viruses as living entities, and any discussion of this is superfluous. As far as I am concerned, considering viruses as non-living creatures because they need a cell to reproduce is not a very strong argument for two reasons. First, viruses are obligate intracellular parasites, and, like all parasites, they use the host for their own benefit, and this is their survival strategy. Viruses need nothing else to pursue the same goal as all species on this planet, which is to generate more viruses better adapted to infect new organisms. They apply the “law of least effort” to achieve this goal and may even decide to remain inside the host cell in a lysogenic manner, as in the case of bacteriophage lambda [ 47 ], or by establishing latency as herpesviruses do [ 48 ]. Second, as I said before no cell or organism is self-sufficient, as it needs at least a supply of food/energy to survive and reproduce. We know that life is absolutely interdependent. For example, we depend directly on our intestinal bacterial flora for our survival, and indirectly on nitrogen-fixing bacteria or photosynthesis. We could take to absurdity the argument that because viruses need a cell to reproduce, they are not alive and say that a man or a woman is not a living being because they cannot reproduce by themselves. The argument that a virus is not a living thing because it is an inert entity outside the cell is also not valid because such a virus could still have the ability to infect cells. Similarly, a spore or a seed cannot be considered lifeless because it is inert, as it is only waiting for the right environmental conditions to germinate, and that wait can last for thousands of years.

To answer the question of whether viruses are alive or not, I base my argument in support of considering viruses as living entities obviously on my own definition of life (this paper), as well as on what we know about the biology of viruses. First, viruses, like all cellular entities in nature, are composed of organic molecules; a virus consists of a nucleic acid (DNA or RNA), which is its genetic material as in all living things, and a protein capsid encoded by the viral genome that protects the viral genetic material and participates in the propagation of the virus in the host; viral capsids show fascinating dynamics during the viral life cycle [ 49 ]. Secondly, viruses are highly organized structures. There is an astonishing diversity of organization and geometric design of viruses, requiring only a few different structural subunits of the capsid to construct an infectious particle. Many viruses have developed very successful self-assembly systems; so much so that the viral capsid can self-assemble even outside the host cell [ 50 ]. The third feature common to all living things is that they are pre-programmed, and viruses also fulfill this characteristic because in their genetic material are written the instructions to make new viruses capable of infecting new cells or organisms. Viruses in their genome have the necessary (though not sufficient because they need elements provided by the host cell) instructions to make new viruses, and in this they are the same as any other living thing. In addition, the process of self-assembly to generate new viruses occurs spontaneously because the instructions to do it autonomously are both in the capsid-forming molecules themselves and in the nucleic acid, either DNA or RNA [ 49 ].

Two other characteristics of living organisms are the ability to interact with other living organisms (interaction) and to adapt genetically to new circumstances (adaptation). Viruses interact with their host in multiple ways: during infection, when their genes are expressed and their genome replicated, when virions are formed, when they integrate into the genome of the host cell, or when they engage in horizontal gene transfer processes. Viruses not only interact with their host, but also adapt by generating new variants that increase their ability to infect other cells, or by taking control of cell metabolism for their own benefit, or even to escape the immune response [ 51 ]. In terms of reproduction and evolution, which are two closely related processes, viruses reproduce in the host cell and evolve through changes in their genome. Viral evolution, like that of all living things, refers to the heritable genetic changes that a virus accumulates during its life cycle, which may arise from adaptations in response to environmental changes or host immune response. Because of their short generation times and large population sizes, viruses can evolve rapidly [ 52 ].

Microbiologist and Nobel laureate J. Lederberg said that “The very essence of the virus is its fundamental entanglement with the genetic and metabolic machinery of the host”. As far as I am concerned, this statement is essentially true and its profound meaning is, at least for me, further proof that viruses are living things. Viruses form part of many integrated biological systems, and they played an important role in the evolution of species [ 53 ]. They can exchange genetic material and participate in horizontal gene transfer [ 43 ] even between individuals from different species [ 54 ]. Due to their high frequency of mutation [ 55 ], viruses are so abundant in nature and present such a high degree of diversity that they constitute by themselves the virosphere [ 46 ]. This great viral biodiversity is proof that these living entities perform fundamental evolutionary and ecological functions [ 56 , 57 ]. In conclusion, I believe that viruses should be considered as living entities that can participate in events as diverse as causing pandemics, destroying bacteria, causing cancer, or participating in horizontal gene transfer.

Following the metaphor of the “container” as the vessel or system (the living being) in which the life process takes place, the fact that viruses are obligated intracellular parasites and do not have a cellular structure and metabolism of their own does not seem to fit this metaphor. It is obvious that the virus cannot be the “container” where the life process takes place, since the virus, when outside the cell, is in a “dormant” state waiting to find a suitable host to infect and complete its life cycle; we could say that it is inert but not yet dead. Therefore, in the special case of viruses, the “container” is the cell. Once the virus finds its specific “container”, it can then reproduce, or integrate into the genome of the host cell, or remain as an episome, or intervene in the evolutionary process through exchange of genetic material. From genomic and metagenomic data, we know that co-evolution between viral and host genomes involves frequent horizontal gene transfer and the occasional co-option of novel functions over evolutionary time. We can say that viruses and their cellular hosts are ecologically and evolutionary intertwined [ 58 ].

I would like to refer to an interesting reflection on the defining characteristics of life and how viruses fit into this conceptual framework [ 59 ]. Thus, Dupré and O'Malley consider collaboration as a common criterion of life and I can only agree with this assessment; in this sense, in a previous paper on the principles that govern life [ 6 ], I use the expression “cooperative thrust” to refer to the importance of collaboration in the origin and evolution of living beings. Without considering collaboration or cooperation as a key interaction, we could not explain endosymbiosis, eukaryogenesis, metabolism, multicellularity, etc. In the present paper, collaboration is implicit in what I call interaction as a common and fundamental feature of all living things. Interestingly, these authors point out that “leaving viruses out of evolutionary, ecological, physiological or conceptual studies of living entities, would allow only an incomplete understanding of life at any level” [ 59 ]. Considering this emphasis on collaboration as a sine qua non condition for life, how does the world of viruses fit in? Dupré and O'Malley propose, and I agree, that viruses can be understood as alive when they actively collaborate (I mean when they are infecting the target cell) and when they do not collaborate (I would say they are inactive), they have at most a potential for life.

Finally, I would like to add that I am aware that there are many scientists who consider that viruses are not living beings basically because they do not have a cellular structure with all that this means. Therefore, this biological dilemma will probably be with us for a long time to come. I think it will only be resolved when we reach a consensus on what life is because only then will we be able to say categorically whether something is alive or not. This is what I have modestly tried to do in this paper.

What would life be like elsewhere in the universe?

The massive number of exoplanets strongly suggests that there is a high probability that life evolved elsewhere in the universe. Astrobiologists are committed to the search for life in the cosmos and for that purpose it is very convenient to have a criterion about what life is [ 16 ]. How can we be sure that there is life on a distant planet? To do so, we need to define some biosignatures that can establish the possible existence of living things elsewhere in the universe [ 60 ], otherwise what are we looking for? In addition to this, it would also help a lot in this search for life on other planets, finding out how life began on Earth.

Some scientists and philosophers of science think that this preconception of what life is may be a problem rather than a solution in the search for life in other planets. C. Cleland in her book about the nature of life states, “Life is not the sort of thing that can be successfully defined. In truth, a definition of life is more likely to hinder than facilitate the discovery of novel forms of life” [ 5 ]. I do not entirely agree with this double statement because although we must be open-minded in the search for life outside our galactic home, at the same time I think it is a good idea to have a hypothesis based on the only certainty we have about vital phenomena, which is life on Earth, that will help in the design of the search for extraterrestrial life.

Is there life elsewhere in the universe? We don't know yet and it is probably only a matter of time before we find life on other planets or aliens find us. In my opinion if there is life elsewhere in the universe, it will most likely be similar to what existed, exists or will exist on our planet. Let us see why. First of all, the laws of physics and chemistry are universal and these laws, directly or indirectly, govern everything that happens with the matter of the universe. According to the cosmological principle, the same physical laws and models that applies here on Earth also works in all parts of the universe [ 61 ]; it is also assumed that physical constants (gravitational constant, speed of light, etc.) remain the same everywhere in the universe. Second, the elements that make up the matter of the stars are the same everywhere in the universe although in different proportions; the “periodic table” is the same for the whole universe. Whether life exists elsewhere in the universe based on a chemistry other than carbon we do not know and can only speculate, but what we do know for sure is that life on Earth is based on carbon chemistry, perhaps because it cannot be otherwise. Third, there is the aforementioned principle of inexorability [ 6 ]. In this context, what does this principle mean? It means that if the environmental conditions are suitable, glucose will be converted into pyruvate in an aqueous medium, chemiosmotic processes will be an important mechanism for generating chemical energy, flying organisms will have wings, or genetic information will be encoded in a language analogous or identical to what we know on Earth. According to this, the differences between the Earth living forms and the “space creatures” could be attributed to a different evolutionary stage or to specific environmental conditions. This hypothetical premise could be very important when developing projects that seek life elsewhere in the universe.

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Acknowledgements

I would like to thank my colleagues M. L. González Caamaño and R. Anadón for their useful discussions. This work is dedicated to my parents.

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Gómez-Márquez, J. What is life?. Mol Biol Rep 48 , 6223–6230 (2021). https://doi.org/10.1007/s11033-021-06594-5

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2.2: Shared Traits of All Living Things

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The Thinker

You've probably seen this famous statue created by the French sculptor Auguste Rodin. Rodin's skill as a sculptor is evident because the statue looks so lifelike. In fact, the statue is made of rock so its only resemblance to life is how it appears. How does a statue made of rock differ from a living, breathing human being or other living organisms? What is life? What does it mean to be alive? Science has answers to these questions.

Characteristics of Living Things

To be classified as a living thing, most scientists agree that an object must have all seven of the following traits. These are traits that human beings share with other living things.

• homeostasis
• organization
• response to stimuli
• reproduction

Homeostasis

All living things are able to maintain a more-or-less constant internal environment. They keep things relatively stable on the inside regardless of the conditions around them. The condition in which a system is maintained in a more-or-less steady state is called homeostasis. Human beings, for example, maintain stable internal body temperature. If you go outside when the air temperature is below freezing, your body doesn't freeze. Instead, by shivering and other means, it maintains a stable internal temperature.

Organization

Living things have multiple levels of organization. Their molecules are organized into one or more cells. A cell is the basic unit of the structure and function of living things. Cells are the building blocks of living organisms. An average adult human being, for example, consists of trillions of cells. Living things may appear very different from one another on the outside, but their cells are very similar. Compare the human cells and onion cells in the figure below. What similarities do you see?

All living things can use energy. Their cells have the "machinery" of metabolism, which is the building up and breaking down of chemical compounds. Living things can transform energy by converting chemicals and energy into cellular components. This form of metabolism is called anabolism . They can also break down, or decompose, organic matter, which is called catabolism . Living things require energy to maintain internal conditions (homeostasis), for growth, and other life processes.

All living things have the capacity for growth. Growth is an increase in size that occurs when there is a higher rate of anabolism than catabolism. For example, a human infant has changed dramatically in size by the time it reaches adulthood, as is apparent from the image below. In what other ways do we change as we grow from infancy to adulthood?

Adaptations and Evolution

An adaptation is a characteristic of populations. Individuals of a population carry a variety of genes. When the environment changes, some individuals of the population can withstand the changed conditions and reproduce more than the individuals who cannot live in the given environment. A change in the allele frequencies and makeup of the populations over time is called evolution. It comes about through the process of natural selection.

Response to Stimuli

All living things detect changes in their environment and respond to them. A response can take many forms, from the movement of a unicellular organism in response to external chemicals (called chemotaxis), to complex reactions involving all the senses of a multicellular organism. A response is often expressed by motion; for example, the leaves of a plant turning toward the sun (called phototropism).

Reproduction

All living things are capable of reproduction. Reproduction is the process by which living things give rise to offspring. Reproduction may be as simple as a single cell dividing into two cells. This is how bacteria reproduce. Reproduction in human beings and many other organisms is much more complicated. Nonetheless, whether a living thing is a human being or a bacterium, it is normally capable of reproduction.

Feature: Myth vs. Reality

Myth: Viruses are living things.

Reality: The traditional scientific view of viruses is that they originated from bits of DNA or RNA that were shed from the cells of living things but that they are not living things themselves. Scientists have long argued that viruses are not living things because they do not have most of the defining traits of living organisms. A single virus called a virion, consists of a set of genes (DNA or RNA) inside a protective protein coat, called a capsid. Viruses have an organization, but they are not cells and do not possess the cellular "machinery" that living things use to carry out life processes. As a result, viruses cannot undertake metabolism, maintain homeostasis, or grow. They do not seem to respond to their environment, and they can reproduce only by invading and using "tools" inside host cells to produce more virions. The only traits viruses seem to share with living things is the ability to evolve adaptations to their environment. In fact, some viruses evolve so quickly that it is difficult to design drugs and vaccines against them. That's why maintaining protection from the viral disease influenza, for example, requires a new flu vaccine each year.

Within the last decade, new discoveries in virology, the study of viruses, suggest that this traditional view about viruses may be incorrect and the "myth" that viruses are living things may be the reality. Researchers have discovered giant viruses that contain more genes than cellular life forms such as bacteria. Some of the genes code for proteins needed to build new viruses, suggesting that these giant viruses may be able — or were once able — to reproduce without a host cell. Some of the strongest evidence that viruses are living things comes from studies of their proteins, which show that viruses and cellular life share a common ancestor in the distant past. Viruses may have once existed as primitive cells but at some point lost their cellular nature to become modern viruses that require host cells to reproduce. This idea is not so far-fetched when you consider that many other species require a host to complete their life cycle.

• Identify seven traits that most scientists agree are shared by all living things.
• What is homeostasis? What is one way humans fulfill this criterion of living things?
• Define reproduction, and describe an example.
• Assume that you found an object that looks like a dead twig. You wonder if it might be a stick insect. How could you determine if it is a living thing?
• Describe viruses and what traits they do and do not share with living things. Do you think viruses should be considered living things? Why or why not?
• People who are biologically unable to reproduce are certainly still considered to be alive! Discuss why this situation does not invalidate the criteria that living things must be capable of reproduction.
• What are the two types of metabolism described here and what are their differences?
• What are some similarities between cells of different organisms? If you are not familiar with the specifics of cells, simply describe the similarities you see in the pictures above.
• What are two processes that use energy in a living thing?
• Give an example of a response to stimuli in humans.
• Do unicellular organisms, such as bacteria, have an internal environment that they maintain through homeostasis?
• Evolution occurs through ___________ ____________ .
• If alien life is found on other planets, do you think they will necessarily have cells? Discuss your answer.
• Movement in response to an external chemical is called ___________, while movement towards light is called ___________ .

Explore More

Attributions.

• The Thinker by innoxiuss , Licensed CC BY 2.0 via Wikimedia Commons
• Onion cells by kaibara87, CC BY 2.0 via Wikimedia Commons
• Baby , public domain via Nappy
• Basic scheme of a virus by DEXi , dedicated CC0 via Wikimedia Commons
• Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0

• Biology Article
• Living And Non Living Things

Characteristics Of Living And Non Living Things

Table of Contents

Introduction

Living things, non-living things, difference between living and non-living things, criteria for differentiating living things from non-living things.

We can find many things around us, from mountains and oceans to plants and animals. The earth in which we live is made up of several things.  These “things” can be categorized into two different types – Living and Non-living Things.

• All living things breathe, eat, grow, move, reproduce and have senses.
• Non-living things do not eat, grow, breathe, move and reproduce. They do not have senses.

Living things have “life,” though some might not show its evident signs. For instance, a tree would probably not react the same way a human would. It would not react when we hit it, and it might not be able to walk around. Though the signs of life displayed by them are not very observable, it does not make them non-living.

Let us have a detailed look at the important characteristics of living and non-living things and the difference between the two.

Living things exist and are alive and are made of microscopic structures called cells. They grow and exhibit movement or locomotion. They experience metabolism, which includes anabolic and catabolic reactions.

Living things are capable of producing a new life which is of their own kind through the process of reproduction. Living things have a particular life span and are not immortal.

Cellular Respiration   enables living organisms to acquire energy which is used by cells to perform their functions. They digest food for energy and also excrete waste from the body. Their life cycle can be summarised as follows – birth, growth, reproduction and death.

Examples of living things are animals, birds, insects, and human beings.

Also Read:   Living things

Characteristics of Living Things

Following are the important characteristics of living things:

• Living things exhibit locomotory motion, they move. Animals are able to move as they possess specialized locomotory organs, for example – Earthworms move through the soil surface through longitudinal and circular muscles.  Plants move in order to catch sunlight for photosynthesis
• Living things respire. Respiration is a chemical reaction, which occurs inside cells to release energy from the food. Transport of gases takes place. The food that is ingested through the process of digestion is broken down to release energy that is utilized by the body to produce water and carbon dioxide as by-products.
• Living things are sensitive to touch (and other stimuli as well) and have the capability to sense changes in their environment.
• They grow. Living things mature and grow through different stages of development.
• One of the striking features is that living things are capable of producing offspring of their own kind through the process of reproduction, wherein genetic information is passed from the parents to the offspring.
• They acquire and fulfil their nutritional requirements to survive through the process of nutrition and digestion, which involves engulfing and digesting the food. Some living organisms are also autotrophic, which means they can harness the sun’s energy to make their food (also known as autotrophs).
• The digested food is eliminated from the body through the process of excretion.

Also Read:   Characteristics Of Organisms

Non-living things are not alive. They do not possess life. They do not have cells and do not grow or show locomotion/movement. They do not undergo metabolism with anabolic and catabolic reactions. They do not reproduce.

Non-living things do not have a life span. They do not respire as they do not require food for energy and hence do not excrete. They do not fall into any cycle of birth, growth or death. They are created and destroyed by external forces.

Examples of non-living things include stones, pens, books, cycles, bottles, etc.

Characteristics Of Non-living Things

The important characteristics of non-living things are mentioned below:

• Non-living things are lifeless. They do not have cells, and there is no protoplasm which forms the basis for life to exist.
• Lack of protoplasm leads means no metabolic activities.
• They do not have a definite and certain size of their own. They take the shape of the substance they are contained in, for example, a liquid takes the shape of its container. Stones, rocks and boulders are moulded by the changing environment and landscape. The change in the state of a non-living thing is due to an external influence.
• Non-living things “grow” by accretion. It occurs through adding materials externally. For example, A snowball may increase in size due to the accumulation of smaller units of its own on its outer surface.
• Non-living things never die as they do not have cells with a definite lifespan. Immortality is a distinguishing factor.
• Fundamental life processes such as reproduction, nutrition, excretion, etc. are absent in non-living things.

Here are some of the major differences between living and non-living things:

For easy differentiation between living things and non-living things, scientists have come up with traits or characteristics that are unique to them.

The criterion for classification is necessary to avoid the wrong grouping . Hence, science developed a basis for classification. Anything that has life is considered a living being.

For example– humans, trees, dogs, etc.

Things which have no life in them are considered non-living.

For example– stone, mountain, watch, etc.

Scientists have discovered a few criteria for differentiating living things from non-living things.

Here are some of them:

• Living beings can grow and develop.
• Living beings obtain and use energy.
• Living beings adapt to their environment.
• All living beings are made of one or more cells.
• Living beings respond to their environment or stimuli.
• All living things excrete to remove waste material from the body.
• Living beings have the ability to give birth to their young ones through the process of reproduction.
• All living beings require energy to perform different metabolic activities, and they gain energy from food/ nutrition.
• All living beings, apart from plants, move from one place to another. This type of movement is called locomotion.

If something obeys a few of the rules, it cannot be categorized as a living thing. It has to follow all the given rules stringently. For example, an icicle, although it grows (increases its mass or length), is still a non-living thing since it cannot reproduce or respond to stimuli.

Non-living things do not have any of the  life processes , unlike living beings.

Also Read: What is Living

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Any inanimate objects such as furniture, books, and doors are examples of non-living things. All life on earth – from ants and spiders to humans, plants and blue whales are classified as living organisms.

do both living and nonliving things contain DNA

Living things have DNA and non-living things do not. However, some viruses contain DNA even though they are not technically living organisms. Read more: Virus

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Living things breathe and eat. Non-living things don’t breathe and don’t eat.

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