A living being is a creature that is born, takes elements from its environment, multiplies and finally ceases to exist. All these processes are the object of study of biology. The variety of living beings is such that in some way biologists have to define what the characteristics of them are .
We must emphasize that some of these characteristics may not be evident in any organism; however, that an object fulfills only one of the characteristics does not immediately qualify it as a living being. Let’s see what these characteristics are.
1. Cellular organisms
All living organisms are composed of cells. The cell is the basis of life. There are unicellular beings , that is, a single cell fulfills all the functions required to survive. In this group we have bacteria and protozoa.
There are also multicellular organisms, within which we have a great variety: plants, animals and fungi. Pluricellular organisms are classified in the Eukaryadomain .
Also the cells can be:
- according to the presence of nucleus: prokaryotes or eukaryotes,
- according to the kingdom to which they belong and the structures present: vegetables or animals,
- Depending on the function they fulfill, we have a great diversity of cells. For example, neurons are nerve cells, hepatocytes are liver cells, and macrophages are cells that attack invading agents in certain organisms.
The organisms follow an order in their structures. The cells are grouped to form tissues. Different tissues build organs with specific functions. Several organs form systems, such as the esophagus, stomach and intestines in the digestive system. All systems are interrelated in an individual.
Levels of biological organization extend outside of organisms, establishing interactions with other living beings. This is part of the study of ecology.
We could say that one of the most outstanding characteristics of living beings is how complex they are. Starting from the cell, with all its different compartments, its thousands of proteins with varied functions, up to the multicellular organism.
Cell growth is determined by the genetic material. All multicellular organisms use cell division to grow. The growth process also involves living organisms being able to repair themselves when they suffer an injury.
Within a seed concentrates all the necessary material for the growth of a new plant.
Living beings are also characterized by different stages of development. For example, the mycelium of a fungus that lives buried in the ground, under favorable conditions begins to develop structures that are what we see at first glance and identify as a fungus.
The metamorphosis , which involves a change in shape of an animal after the embryonic period, is the process of development characteristic of most insects. Thus, the caterpillar becomes a butterfly, the larva becomes a fly and the nymphs are transformed into dragonflies or “caballitos del diablo”.
Homeostasis is the ability of living beings to maintain their internal environment constant in response to external changes. In order for the cells to work, they need stable temperature, pressure and chemical components. To do this, organisms resort to mechanisms that allow them to restore optimal operating conditions.
For example, when it’s hot, we humans sweat to freshen up and dogs breathe with their mouths open to release heat. On the other hand,
Metabolism is the set of chemical reactions that occur in cells in order to obtain energy, build and repair structures and eliminate waste. These reactions are carried out by proteins that accelerate the reactions, known as enzymes .
We can differentiate the metabolism in anabolism and catabolism.
We have anabolism when new compounds are synthesized or produced from simple molecules. For this type of reactions, energy is required. For example, when proteins are synthesized in ribosomes from amino acids; when starch is produced from glucose for storage in plants.
Living beings also possess enzymes that degrade large molecules to release the energy stored in the chemical bonds. This is catabolism. For example, fermentation is the catabolic transformation that transforms glucose into alcohol or lactic acid and carbon dioxide.
The continuation of life is assured by reproduction, by originating new individuals. There are several forms of reproduction, which are grouped in:
- asexual reproduction : new individuals are formed from a single parent without fertilization. For example, when yeast is placed in water or milk with sugar, these are multiplied by budding.
- Sexual reproduction : individuals are formed from the combination of two different individuals. We can achieve this mainly in animals, but also in plants with flowers and fungi.
One of the strategies to determine if a being is alive or not is to see if it responds to a stimulus, pleasurable or harmful. If an animal that does not move, we puncture it with a needle, if it was alive, it sure jumps or moves or makes a moan. That is reacting to a stimulus.
Sometimes, the response to the stimulus is not as fast as we are waiting for it. For example, sunflowers receive this name because they follow the direction of the sun. Sunlight, in this case, is the stimulus of the plant.
Living organisms have the ability to adapt to survive environmental changes over time and transmit these adaptations to their descendants. This is called evolution.
In Mexico there is an emblematic case of evolution: the topota ( Poecilia mexicana ). This is a small fish that managed to adapt to live in the sulphurous waters of the Cueva del Sulfur (Tabasco, Mexico), appearing a new species: the sulfur top ( Poecilia
The Iranian spider-tail viper Pseudocerastes urarachnoides adapted to the desert environment and developed a deception strategy (the spider-shaped tail) to capture its prey.
One of the less obvious but more important characteristics of living beings is that they can not “live” independently, without establishing interactions with other living beings. For example, photosynthetic dinoflagellates known as zooxanthellae, use sunlight to synthesize organic molecules that feed corals. These, in turn, provide refuge.
The effects of climate change have resulted in the decrease of zooxanthellae, which is known as “coral bleaching,” which will eventually lead to the death of corals.
When a living organism suffers irreparable damage, or does not have at its reach the necessary nutrients for its subsistence, it stops living. In the case of the cells of a multicellular organism there is a programmed death mechanism called apoptosis .
The longevity , or the space of time that an organism lives, depends on each species. For example, it is known as a pine ( Pinus longaeva ) in California, USA, which has lived more than 5,000 years.
13. Capture of environmental material
Living beings need to acquire material from their environment in order to capture the necessary nutrients. So plants capture sunlight through chlorophyll and absorb water and minerals from roots, animals trap their food and digest it to obtain energy.
14. Disposal of waste products
Living beings have various mechanisms to expel material that can no longer be degraded inside, and that would otherwise become toxic. For example, in animals, carbon dioxide, urine and feces are waste products that must be expelled. On the other hand, plants must expel oxygen that is a waste product in photosynthesis.
15. Genetic information
The genetic information of living beings is found in DNA deoxyribonucleic acid. That is, in the DNA are the instructions that the cell needs for the synthesis of proteins. These instructions or genetic code are very similar between different organisms. It should be noted that each species (in fact each individual) has its particular DNA sequence.
Thanks to this, recombinant proteins can be made, which is the production of proteins from one species in another species, usually in bacteria or yeast. Transgenic or genetically modified organisms are possible because pieces of DNA can be introduced from one organism into another through biotechnology.