Dynamics of Life: Reproduction, Interaction, and Adaptation in Living Organisms
Greeks were the first to investigate the origin and nature of life in a scientific way. The philosopher Aristotle distinguished living beings endowed with a "soul" from non-living objects lacking a soul and therefore "inanimate." Aristotle also proposed the existence of three different types of soul: a "vegetative" soul possessed by plants, an "animal" soul possessed by animals, and a "rational" soul unique to humans, making them different and superior to all other animals.
For many centuries, this theory dominated, although numerous and often conflicting hypotheses were formulated about the "something extra" that living beings possess compared to non-living things: soul, spirit, vital breath, vital force, and so on.
The collection of theories that share the idea that life is something supernatural and cannot be explained by the ordinary laws of physics and chemistry is known as vitalism. According to vitalists, every vital manifestation of an organism is governed by a supernatural force, and all life processes tend toward a predetermined purpose.
Vitalism was soon opposed by mechanicism, which excludes any intervention of supernatural forces in biological phenomena, reducing life to simple chemical-physical processes. Mechanicism is based on the theory of Democritus, who posited that every living being is composed of "atoms," tiny particles that, dispersed throughout the body’s matter, provoke and coordinate its actions.
Starting from the 17th century, the application of the scientific-experimental method to biology made it possible to understand that some parts of animal and plant organisms function in perfect accordance with the laws of chemistry and physics. Many scientists, beginning with Descartes, hypothesized that living organisms could be compared to machines, albeit very complex ones.
The ideas of the mechanists and the application of the scientific-experimental method gave a significant boost to the development of biological sciences.
At Galileo's time, the circulation of blood was still unknown, and physicians relied on the ideas of Galen, who lived in Rome in the 2nd century AD. Galen had intuited that blood transported nourishment, but he believed that the movement of blood was controlled by "vital spirits": the precious fluid was produced in the liver, spread throughout the body, and was slowly consumed. Descartes himself hypothesized that blood was partly composed of "animal spirits" capable of interacting with the thinking substance of the pineal gland (located in the brain) and then flowing along nerve channels to move muscles and other organs.
In the early 1600s, the English physician William Harvey studied blood circulation using experimental scientific methods and provided the correct description of the phenomenon. With the discovery of blood circulation, physiology was born—the science that studies organic functions.
Subsequently, the debate centered on the chemical transformations that occur within living organisms.
The vitalists denied that the chemical reactions typical of living tissues could be experimentally replicated in a laboratory, distinguishing between "chemical" and "vital" reactions. Their new opponents, the reductionists, believed instead that the complex activities of living systems could be reduced to simpler patterns and reproduced experimentally in a laboratory. A confirmation of the reductionists' theories came in 1828, when the German chemist Friedrich Wöhler successfully synthesized an organic substance, urea—a component of human urine—from inorganic substances such as carbon, hydrogen, oxygen, and nitrogen.
In 1833, A. Payen and J.F. Persoz discovered that a substance obtained from germinated barley, which they called diastase, was capable of transforming starch into a sugar, maltose. The two scientists had identified the first molecule with catalytic activity, meaning it could facilitate and direct a specific chemical transformation.
In 1898, German chemists Eduard and Hans Büchner, using a substance extracted from yeast cells, succeeded in performing fermentation—a typical vital reaction—in a laboratory, thereby demonstrating its chemical nature. The substance responsible for fermentation was named "enzyme," derived from the Greek word zýme, meaning "yeast" or "ferment." From that point onward, the vitalists' theory began to lose credibility.
Today, it is generally accepted that living systems obey the laws of physics and chemistry. However, many scientists, inheritors of the vitalist perspective, are convinced that when considering the complexity of living systems, life cannot be fully explained using only the laws of chemistry and physics. They argue that living beings always possess something "extra" compared to non-living things.
Other scientists, on the other hand, relying on their faith in scientific progress, maintain that humanity has not yet achieved sufficient levels of knowledge to fully explain the functioning of living beings, but that this goal will eventually be reached over time.
LIFE CAN BE DEFINED BASED ON THE CHARACTERISTICS OF LIVING BEINGS
Living organisms exhibit certain peculiar and exclusive characteristics that clearly distinguish them from all that is non-living. However, living matter and inert matter do not belong to two entirely different worlds. In fact, scientific research shows us that the difference lies primarily in structure.
Our bodies are made up of the same atoms found in a rock or a helicopter, but these atoms are arranged in just the right way to form a living organism. Furthermore, living organisms operate and evolve in accordance with the fundamental laws of chemistry and physics, particularly the laws governing energy and its transformations.
Organisms Are Made Up of Cells
At first glance, a rhinoceros, a hamster, a whale, an eagle, or a tree might seem to have little in common. However, upon closer inspection, with the help of a microscope, we can discover that they are all composed of countless units called cells. Cells are themselves tiny living units capable of performing all the functions we associate with a living organism. The simplest living organisms, such as bacteria, many species of algae, and yeasts, consist of a single cell and are therefore called unicellular. On the other hand, plants, animals, and almost all fungi are made up of a large number of cells (sometimes billions) and are referred to as multicellular.
In a multicellular organism, cells can be tightly joined together (for example, skin cells) or relatively free and independent (for example, blood cells).
All cells are small in size; the cells of higher animals are typically 20-30 microns in diameter. The largest cells are the eggs of birds and sharks, but these cells are almost entirely composed of nutrient deposits necessary for the development of the embryo.
Cells themselves are composed of tens of thousands of different chemical substances distributed among numerous ultramicroscopic structures known as cellular organelles. The chemicals that make up these organelles are formed by molecules, which in turn are made up of atoms.
Each cell is capable of performing all the functions of life: it feeds, grows, reproduces, and dies. The functions of a cell are made possible because each organelle participates in a specific cellular function. All the properties and characteristics of an organism can be traced back to the functions and activities of certain groups of cells or combinations of multiple groups.
The function of an organism as a unified whole is the result of the sum of the activities and interactions of individual cellular units.
Organisms Have Different Levels of Organization
Each cellular organelle is composed of molecules, which in turn are formed by atoms, and atoms are made up of subatomic particles.
Cells with a similar structure, specialized in a specific function necessary for the survival of the organism, form associations called tissues. Tissues, in turn, form organs dedicated to various functions. Multiple organs that work together to perform the same function make up systems and apparatuses, which integrate to form the organism.
A group of similar individuals living in a specific area constitutes a population. Different populations living in the same place and interacting with each other form a community. The sum of all the communities of living beings on Earth constitutes the biosphere. A community, along with the environment in which it lives, forms an ecosystem.
Organisms undergo continuous transformations
To grow, maintain their complex structure, and adapt to their environment, every organism undergoes constant transformations. These transformations result from a variety of chemical reactions collectively known as metabolism. Processes such as feeding, respiration, and waste elimination are examples of metabolic activities within an organism. Through metabolism, a living being continuously changes its own atoms: every day, by breathing, eating, and drinking, it takes in billions of atoms from the environment and releases billions of others through sweating, respiration, and waste elimination.
In carrying out this extensive work, every living organism functions like a biochemical machine that, like any other machine, requires energy to operate. The energy used in vital processes comes from the Sun. Plants and other organisms capable of photosynthesis can directly capture this energy, use part of it for their needs, and store the remainder in substances they produce, which then become food and sources of matter and energy for other organisms.
Organisms reproduce and pass on hereditary traits to their descendants
Living systems can reproduce themselves and generate offspring that, in turn, can reproduce. Reproduction results in the formation of identical or nearly identical copies of the complex structure of an organism. Transmitting this complexity from one generation to the next requires a substantial amount of information, even for the simplest living organisms. This collection of information is called the genetic heritage or genome and is present in every cell, encoded chemically in DNA molecules, the substance that forms the foundation of life on Earth.
DNA determines the nature of every living organism, from the simplest unicellular beings to humans. Each organism inherits a copy of DNA from its parents and creates numerous duplicates so that every cell has its own. DNA constitutes the "code of life," the book in which all the rules that cells must follow are written.
DNA is composed of many segments, each of which forms a gene, and the collection of all genes constitutes the genome. Every structure and activity of the organism is encoded within the genes. Each gene contains information about a specific structure or activity of the organism and can be modulated and coordinated with the activity of other genes.
The transmission of genetic information from parents to offspring occurs through reproduction, made possible because DNA can duplicate itself, producing copies of its structure.
A unicellular organism (e.g., a bacterium or a protozoan) reproduces by duplicating its DNA and then dividing into two daughter cells, each inheriting one copy of the DNA. In multicellular organisms, reproduction typically occurs through the fusion of two specialized cells (gametes) produced by individuals of opposite sexes, each containing only 50% of the DNA of a normal cell. This fusion results in a new cell called a zygote, which, through numerous divisions, develops into a new individual whose DNA is derived half from one parent and half from the other.
Organisms Interact with Their Environment and Each Other
All living organisms interact with their environment and with one another. A plant, for example, absorbs water, mineral salts, carbon dioxide, light, and oxygen from its physical environment to perform photosynthesis, grow, and reproduce. Plant life is also influenced by temperature, rainfall, wind, latitude, and altitude.
Plants form the base of the food chain for herbivorous animals, which obtain matter and energy from them. These herbivores, in turn, produce matter and energy for carnivorous animals. Decomposers then recycle the waste produced by plants and animals, transforming it into inorganic substances that plants reuse.
Individuals and Populations Adapt to Their Environment and Evolve
Living organisms have the ability to adjust their structures and functions to the characteristics of their environment.
Different types of adaptation can be observed: evolutionary, physiological, behavioral, and sensory adaptation.
- Evolutionary adaptation occurs over multiple generations, resulting in structural and functional changes that affect entire species rather than just individual organisms. This process is called biological evolution.
- Physiological adaptation consists of normally reversible processes implemented in response to changes in environmental conditions that may disrupt an organism's internal balance. This ability of living beings to maintain internal stability despite external changes is known as homeostasis. For example, a mammal reacts to temperature variations by increasing or decreasing sweating and respiratory rates.
- Behavioral adaptation occurs through learning processes or instinctive behaviors. For instance, an organism learns to recognize its food, distinguish predators, and identify members of its species.
- Sensory adaptation is the process by which a sensory organ adjusts to variations in the intensity of the stimulus to which it is sensitive. An example is the eye adapting to environmental brightness by dilating or constricting the pupil.
