Table of Contents
Living Thing Definition
A living thing is any creature or life form that possesses or shows the attributes of life or being alive. The following are the basic characteristics of life: an ordered structure, the need for energy, the ability to respond to stimuli and adjust to environmental changes, and the ability to reproduce, grow, move, metabolize, and die.
Living organisms are currently divided into three domains: (Eu) bacteria (real bacteria), Archaea (archaebacteria), and Eucarya (archaebacteria) (eukaryotes).
Living Thing Etymology
The term “living” comes from the Old English word “lifende,” which means “to live” or “to have a life.” The word thing comes from the Old English word ing, which means “entity,” “being,” “body,” or “stuff.” Organisms, living forms, and creatures are all synonyms.
Living Thing History
While the Earth is estimated to be 4.54 billion years old, life on Earth began much later, most likely about 3.5 billion years ago, while some scientists believe life may have begun much earlier.
Abiogenesis, or the natural process of life arising from non-living materials, is referred to as the origin of life. Scientists are still debating how this might have happened. There has never been an agreement about how life on Earth originated.
The term “primordial soup” refers to a hypothesized model of the early Earth in which biological material and water collected in a soup-like state. This soup functioned as a laboratory for the creation of organic substances. The Miller–Urey experiment is a generally recognized scientific finding. Chemical synthesis of the cell membrane’s basic structure (e.g., phospholipids creating lipid bilayers) and organic molecules from inorganic sources appear to have favored the simulated-primitive Earth. The primordial soup is also declared by Alexander Oparin and John Burdon Sanderson Haldane’s heterotrophic hypothesis of life’s beginnings.
RNA World Hypothesis
It’s possible that the shift from non-living to living beings happened gradually. The RNA universe theory, which argues that primordial life was based on RNA since it can function as both a genetic material and a catalyst, is one of the most prevalent hypotheses today. This RNA-based life might have been the forerunners of today’s life on Earth.
The building components of RNA and DNA may have started and developed in asteroids from outer space before being transported to Earth through meteorites. RNA and DNA nucleobases such as adenine and guanine were discovered in meteorites, according to NASA. These nucleobases might have resulted in the formation of RNA and DNA on their own.
The early living forms may have utilized these organic compounds to exist and reproduce. Single-celled creatures that appeared around the end of the Hadean Eon or early in the Archean Eon might be the first living forms. This is based on the finding of biogenic graphite in Western Greenland, which is thought to be 3.7 billion years old. Organisms without membrane-bound organelles were most likely the first to rule the planet. Prokaryotes are a category of bacteria and archaea that includes bacteria and archaea.
Endosymbiosis between a bigger cell and a prokaryote is thought to have resulted in the first photosynthetic eukaryote, according to the endosymbiotic theory. According to this idea, the bigger eukaryote may have absorbed prokaryotes that evolved into semi-autonomous organelles inside the cell, such as chloroplasts and mitochondria.
Multicellular life is thought to have first appeared 600 million years ago and has recurred numerous times throughout biological history. Haeckel’s Gastraea Explanation is the most widely accepted theory on the genesis of multicellularity. As a result, multicellularity began when cells from the same species gathered in a blastula-like colony. Certain cells in the colony begin to differentiate over time. This idea, however, falls short of understanding the origins of multicellularity.
During the Ediacaran epoch, about 600 million years ago, a biota of single-celled and multicellular creatures thrived. The first creatures initially emerged in this biota. They resemble sponges and range in size from 1 cm to less than 1 meter.
In the Cambrian era, approximately 541 million years ago, a rapid explosion of life erupted. The Cambrian Explosion is the name given to this phenomenon. Plants and animals of all kinds arose. Animals began to go onto land in the late Cambrian or early Ordovician periods. Animals developed and diversified in tandem with the emergence of terrestrial plants. They eventually invaded terrestrial environments, including inland areas.
In May 2019, the total number of species on Earth was predicted to be about 7.674 billion. According to the Census of Marine Life, there are around 8.7 million eukaryote species on the planet. Unfortunately, many of the world’s living organisms (estimated to be over five billion species) have gone extinct.
Classification of Living Things
Originally, living things were divided into two categories: plants and animals.
Animals and plants are both eukaryotes, but they are characterized by their distinguishing traits, such as motility, manner of feeding, and cellular structures. Animals are non-motile, photosynthetic, and have a cell wall, whereas plants are non-motile, photosynthetic, and do not have a cell wall. Bacteria and archaea, on the other hand, are neither plants nor animals since they are prokaryotes.
The RNA polymerase is one of the distinctions between bacteria and archaea in terms of bacterial and archaeal classification. It has 10 subunits in archaea. It contains four types of bacteria. The composition of the cell wall is another example. The peptidoglycan in archaeal cell walls is absent, but peptidoglycan is present in bacterial cell walls.
The categorization of living organisms into three domains is now used in modern biological taxonomy: (1) domain Eukarya, (2) domain Bacteria, and (3) domain Archaea. According to Carl Woese’s 3-domain system of taxonomy, a biological domain is the highest taxonomic rank of organisms. There are seven primary taxonomic levels below the domain.
Domain » Kingdom » Phylum » Class » Order » Family » Genus » Species
All eukaryotic living entities are included in the domain of Eukarya. Animals, plants, fungus, algae, and protists are among them. Within their cells, they have membrane-bound organelles.
Characteristics of Living Thing
Organisms that exhibit the qualities of life are referred to as living entities. The following features distinguish living things from non-living things:
i. An Organised Structure
Living things are a well-organized system. It might be single-celled, like a bacterial cell, or multicellular, like animals and plants, which have several cells. A cell is an organism’s most basic biological unit. The cell orchestrates and systematizes a variety of cellular functions. Protoplasm and a plasma membrane make up a cell. Organelles and other cytoplasmic structures are hung in the cytosol of the cell, each having their own tasks and functions.
The survival of a living organism involves the use of energy. Energy is necessary since it fuels a cell’s myriad metabolic operations. Photosynthesis, in which light energy is transformed into chemical energy, is one way that organisms generate energy. Another method is cellular respiration, which involves extracting biochemical energy from an organic substance (such as glucose) and storing it in an energy-carrying biomolecule like ATP for later use.
iii. Reproductive Capability
A live thing has the ability to reproduce. Living creatures can reproduce copies of themselves in two ways: sexual reproduction and asexual reproduction. In sexual reproduction, the two parents’ sex cells combine to form a zygote, which will eventually develop into a creature of their own species. Asexual reproduction, on the other hand, is a method of reproduction that does not require the use of sex cells. Only one parent gave birth to the children. Binary fission, budding, vegetative propagation, sporogenesis, fragmentation, parthenogenesis, apomixis, and nucellar embryony are examples of these processes.
A living being develops. At the cellular level, growth can be defined as an increase in quantity or size. Cell division is the process by which the number of cells grows. Animal stem cells and plant meristematic cells divide in order to produce new cells. The rise in cytoplasmic bulk is frequently blamed for the increase in cell size.
In the cell cycle, the cell passes through a succession of stages. Interphase is the stage in which the new cell created by mitosis spends the majority of its time. It is the stage of the cell cycle during which the cell expands in size. The cell could copy its DNA to prepare for the next cell division if it is not fully differentiated. New cells in plants expand their capacity by absorbing and storing water in a vacuole.
Between the primary cell wall and the plasma membrane, some plant cells develop a secondary cell wall. There are two forms of growth in vascular plants at the tissue level: primary and secondary. Primary growth is characterized by vertical growth as primary xylem emerges from the procambium, whereas secondary growth is characterized by lateral growth as secondary xylem emerges from the vascular cambium.
The growth of tissues in higher animals follows a pattern that is genetically set. Plants have infinite regeneration capacities, while humans don’t. The amount of regeneration varies depending on the species. Salamanders, for example, can regenerate new eyes or limbs, whereas humans cannot. Humans, on the other hand, can regenerate specific sections of their bodies, such as skin and liver tissue.
Metabolize is the process by which a living thing breaks down its food. Metabolism refers to the different processes involved in a cell’s or organism’s ability to maintain its alive state. Cell development, respiration, reproduction, reaction to stimuli, nourishment, biomolecular syntheses, waste disposal, and other homeostatic activities are examples of those involved.
Catabolism and anabolism are the types of metabolism. Catabolism is a process in which living things carry out degradative chemical reactions that result in the breakdown of complex molecules into smaller components and the release of energy. Energy-driven chemical reactions in anabolism create compounds from smaller units.
vi. Responsive to Stimuli
Living things respond to stimuli and adapt to changes in their environment. It can detect changes in the environment, particularly through receptor cells. Humans, for example, have five basic senses: sight, hearing, smell, touch, and taste. Vestibular sense (detects body movement, direction, and acceleration), thermoception sense, kinesthetic sense (detects body component positions), internal sense (interoception), and so on are some of the other senses. It is capable of not only detecting but also adapting to changes in its environment.
The living thing is in motion. A living organism can respond to stimuli in its environment because it can detect them. Animals, for example, travel to forage, avoid predators, and find a suitable partner. Plants have a restricted form of mobility known as nastic movement, which allows them to move at will (e.g., thigmonasty, nyctinasty).
Living things perish. A living creature has a life, and that existence will come to an end at some point. Biological ageing is referred to as senescence. It occurs when the health of a living organism deteriorates over time. The organism’s ability to function and deal with stimuli deteriorates over time. As a result, it is more susceptible to illnesses and malfunctions.
The cell no longer divides at the cellular level, but it may still be metabolically active. The shortening of telomeres, which leads to DNA damage, is one of the natural causes of cellular senescence. Some living organisms, on the other hand, are thought to be eternal because they appear to be immune to death. Turritopsis doohmii, an age-reversing jellyfish, regenerating flatworms, and seemingly invincible tardigrades are all examples.
Is it possible for viruses to live?
For a long time, biologists have been divided on this issue. Because viruses appear to be alive when they are within their host, some people consider them to be living things. They have genetic material, multiply, and evolve through natural selection. Others, on the other hand, do not regard them as living creatures because they are essentially dead when they are not in contact with their hosts. Viruses are incapable of self-replication.
They rely on the machinery of the host cell to do so. Viruses are thus neither alive nor non-living. Viruses are passive and appear to be lifeless when they leave their host. They become active once inside their host, able to use the host cell’s structures and proliferate. Viroids are another category of organisms that appear to exist outside of cells. Short strands of circular, single-stranded RNA are infectious and harmful.
Living Things Citations
- From Animaculum to single molecules: 300 years of the light microscope. Open Biol . 2015 Apr;5(4):150019.
- Viruses in the sea. Nature . 2005 Sep 15;437(7057):356-61.
- The disparity mutagenesis model predicts rescue of living things from catastrophic errors. Front Genet . 2014 Dec 4;5:421.
- Endosymbiosis and Eukaryotic Cell Evolution. Curr Biol . 2015 Oct 5;25(19):R911-21.