All living organisms are divided into sub kingdoms of multicellular and unicellular creatures. The latter are one cell and belong to the simplest, while plants and animals are those structures in which a more complex organization has developed over the centuries. The number of cells varies depending on the species to which the individual belongs. The size of the majority is so small that they can only be seen under a microscope. Cells appeared on Earth about 3.5 billion years ago.
Nowadays, all processes that occur with living organisms are studied by biology. This science is engaged in the subdomain of multicellular and unicellular organisms.
Unicellular organisms
Unicellularity is determined by the presence in the body of a single cell that performs all vital functions. The well-known amoeba and ciliates-slipper are primitive and, at the same time, the most ancient life forms that are representatives of this species. They were the first living creatures that lived on Earth. This also includes groups such as sporozoans, sarcodes, and bacteria. All of them are small and mostly invisible to the naked eye. They are usually divided into two general categories: prokaryotic and eukaryotic.
Prokaryotes are represented by protozoa or fungi of some species. Some of them live in colonies, where all individuals are the same. The whole process of life is carried out in each individual cell in order for it to survive.
Prokaryotic organisms do not have membrane-bound nuclei and cellular organelles. These are usually bacteria and cyanobacteria, such as E. coli, Salmonella, nostoks, etc.
Eukaryotes are composed of a number of cells whose survival depends on each other. They have a core and other organelles separated by membranes. These are mainly aquatic parasites or fungi and algae.
All representatives of these groups vary in size. The smallest bacterium has a length of only 300 nanometers. Unicellular usually have special flagella or cilia, which are involved in their movement. They have a simple body with pronounced basic features. Nutrition, as a rule, occurs in the process of absorption (phagocytosis) of food and is stored in special cell organelles.
Unicellular dominated as a form of life on Earth for billions of years. However, evolution from protozoa to more complex individuals changed the entire landscape, since it led to the emergence of biologically developed relationships. In addition, the emergence of new species has led to the formation of a new environment with diverse environmental interactions.
Multicellular organisms
The main characteristic of the multicellular sub kingdom is the presence in a single individual of a large number of cells. They are fastened together, thereby creating a completely new organization, which consists of many derived parts. Most of them can be seen without any special devices. Plants, fish, birds and animals emerge from a single cage. All creatures that are part of the multicellular kingdom regenerate new individuals from the embryos, which are formed from two opposite gametes.
Any part of an individual or a whole organism, which is determined by a large number of components, is a complex, highly developed structure. In the sub-kingdom of multicellular classification, it clearly separates the functions in which each of the individual particles performs its task. They are engaged in vital processes, thereby supporting the existence of the whole organism.
Subdomination Multicellular in Latin sounds like Metazoa. To form a complex organism, cells need to be identified and attached to others. Only a dozen protozoa can be seen individually with the naked eye. The remaining almost two million visible individuals are multicellular.
Pluricellular animals are created by combining individuals through the formation of colonies, strands, or aggregation. The pluricellular ones developed independently, like volvox and some flagellated green algae.
A sign of the subdomain of multicellular, that is, its early primitive species, was the absence of bones, shells and other solid parts of the body. Therefore, their traces have not survived to the present day. An exception are sponges that still live in the seas and oceans. Perhaps their remains are in some ancient rocks, such as, for example, Grypania spiralis, whose fossils were found in the oldest layers of black shale dating back to the Early Proterozoic era.
In the table below, the sub-kingdom of multicellular is represented in all its diversity.
Complex relationships arose as a result of the evolution of protozoa and the emergence of the ability of cells to separate into groups and organize tissues and organs. There are many theories that explain the mechanisms by which single-celled organisms can evolve.
Theories of occurrence
Today, there are three main theories for the emergence of multicellular subdominance. A summary of syncytial theory, so as not to go into details, can be described in a few words. Its essence is that a primitive organism, which had several nuclei in its cells, could eventually separate each of them with an internal membrane. For example, several kernels contain a mold fungus, as well as a ciliates, which confirms this theory. However, having multiple cores is not enough for science. To confirm the theory of their multiplicity, it is necessary to visually transform a simple eukaryote into a well-developed animal.
The theory of colonies says that symbiosis, consisting of different organisms of the same species, led to their change and the emergence of more advanced creatures. Haeckel is the first scientist to introduce this theory in 1874. The complexity of the organization arises because the cells remain together, and are not disconnected in the process of division. Examples of this theory can be seen in such simplest multicellular organisms as green algae called eudorin or volvax. They form colonies that number up to 50,000 cells depending on the species.
The theory of colonies suggests the fusion of various organisms of the same species. The advantage of this theory is that it was observed that during a shortage of food, amoeba grouped into a colony, which moves like a whole into a new place. Some of these amoebas are slightly different from each other.
The theory of symbiosis suggests that the first creature from the multicellular kingdom appeared due to a community of dissimilar primitive creatures that performed different tasks. Such relationships, for example, are present between clown fish and sea anemones, or in vines that parasitize on trees in the jungle.
However, the problem with this theory is that it is not known how the DNA of different individuals can be incorporated into a single genome.
For example, mitochondria and chloroplasts can be endosymbionts (organisms in the body). This happens extremely rarely, and even then the genomes of endosymbionts retain differences among themselves. They separately synchronize their DNA during mitosis of host species.
Two or three symbiotic individuals forming a lichen, although they depend on each other for the sake of survival, must separately multiply and then reconnect, again creating a single organism.
Other theories that also consider the emergence of multicellular subdominance:
- Theory of GK-PID. About 800 million years ago, a minor genetic change in one molecule called GK-PID may have allowed individuals to move from one cell to a more complex structure.
- The role of viruses. Recently it was recognized that genes borrowed from viruses play a decisive role in the division of tissues, organs, and even during sexual reproduction, in the fusion of an egg and sperm. The first syncytin-1 protein was found, which was transmitted from the virus to humans. It is located in the intercellular membranes that separate the placenta and the brain. The second protein was identified in 2007 and named EFF1. It helps form the skin of roundworm nematodes and is part of a whole family of FF proteins. Dr. Felix Rey at the Pasteur Institute in Paris built a 3D mock-up of the EFF1 structure and showed that it was he who bound the particles together. This experience confirms the fact that all known fusions of the smallest particles into molecules are of viral origin. This also suggests that viruses were vital for the communication of internal structures, and without them it would be impossible for a colony of a subdomain of multicellular sponges to appear.
All these theories, like many others that continue to be offered by famous scientists, are very interesting. However, none of them can clearly and unequivocally answer the question: how could such a huge variety of species emerge from the only cell that originated on Earth? Or: why did single individuals decide to unite and begin to exist together?
Maybe a few years will pass, and new discoveries will be able to give us answers to each of these questions.
Organs and Tissues
Complex organisms have such biological functions as protection, blood circulation, digestion, respiration and sexual reproduction. They are performed by certain organs, such as the skin, heart, stomach, lungs, and reproductive system. They consist of many different types of cells that work together to perform specific tasks.
For example, the heart muscle has a large number of mitochondria. They produce adenosine triphosphate, due to which blood continuously moves through the circulatory system. In contrast, skin cells have fewer mitochondria. Instead, they have dense proteins and produce keratin, which protects soft internal tissues from damage and external factors.
Breeding
While all, without exception, the simplest organisms reproduce asexually, many of the subcellular kingdoms prefer sexual reproduction. Humans, for example, are a complex structure created by the fusion of two single cells called an egg and a sperm. The fusion of one egg with a gamete (gametes are special germ cells containing one set of chromosomes) of the sperm leads to the formation of a zygote.
The zygote contains the genetic material of both sperm and egg. Its division leads to the development of a completely new, separate organism. During development and division, cells, according to the program laid down in the genes, begin to differentiate into groups. This will further enable them to perform completely different functions, despite the fact that they are genetically identical to each other.
Thus, all the organs and tissues of the body that form the nerves, bones, muscles, tendons, blood, - they all arose from one zygote, which appeared due to the merger of two single gametes.
Multicellular advantage
There are several main advantages of the subdomain of multicellular organisms, due to which they dominate our planet.
Since the complex internal structure allows you to increase the size, it also helps to develop structures and fabrics of a higher order with numerous functions.
Large organisms have better protection against predators. They also have greater mobility, which allows them to migrate to more comfortable places to live.
There is another undeniable advantage of the multicellular kingdom. The general characteristic of all its species is a rather long life span. The body of the cell is exposed to the environment from all sides, and any damage to it can lead to the death of the individual. A multicellular organism will continue to exist even if one cell dies or is damaged. DNA duplication is also an advantage. Particle division within the body allows damaged tissues to grow and recover faster.
During its division, the new cell copies the previous one, which allows preserving favorable features in the next generations, as well as improving them over time. In other words, duplication allows you to save and adapt traits that will improve the survival or fitness of the body, especially in the animal kingdom, the subcellular multicellular.
Disadvantages of multicellular
Complex organisms also have disadvantages. For example, they are susceptible to various diseases that arise due to the complex biological composition and functions. In protozoa, on the contrary, there are not enough developed organ systems. This means that the risks of dangerous diseases are minimized.
It is important to note that, unlike multicellular, primitive individuals have the ability to reproduce asexually. This helps them not to spend resources and energy on finding a partner and sexual activity.
The simplest organisms also have the ability to take energy through diffusion or osmosis. This frees them from having to travel to find food. Almost anything can become a potential food source for a single-celled creature.
Vertebrates and Invertebrates
The classification divides all, without exception, multicellular creatures under the kingdom into two types: vertebrates (chordates) and invertebrates.
Invertebrates do not have a solid skeleton, while chordates have a well-developed inner skeleton of cartilage, bones and a highly developed brain that is protected by a skull. Vertebrates have well-developed sensory organs, a respiratory system with gills or lungs, and a developed nervous system, which further distinguishes them from more primitive brethren.
Both types of animals live in different habitats, but chordates, thanks to a developed nervous system, can adapt to land, sea and air. However, invertebrates are also found in a wide range, from forests and deserts to caves and mud of the seabed.
To date, almost two million species of the subdomain of multicellular invertebrate animals have been identified. These two million make up about 98% of all living things, that is, 98 out of 100 species of organisms living in the world are invertebrates. Human individuals belong to the chordate family.
Vertebrates are divided into fish, amphibians, reptiles, birds and mammals. Non- spinal animals represent types such as arthropods, echinoderms, worms, intestinal and mollusks.
One of the main differences between these species is their size. Invertebrates, such as insects or intestinal, are small and slow because they cannot develop a large body and strong muscles. There are a few exceptions, such as squid, which can be up to 15 meters long. Vertebrates have a universal support system, and therefore can grow faster and become larger than invertebrates.
Chordates also have a highly developed nervous system. Using specialized communication between nerve fibers, they can respond very quickly to changes in the environment, which gives them a definite advantage.
Compared to vertebrates, most animals without a ridge use the simple nervous system and behave almost completely instinctively. Such a system works well most of the time, although these creatures are often unable to learn from their mistakes. The exceptions are octopuses and their close relatives, who are considered one of the most intelligent animals in the world of invertebrates.
All chordates, as we know, have a spine. However, the peculiarity of the subdomain of multicellular invertebrate animals is the similarity with their relatives. It consists in the fact that at a certain stage of life, vertebrates also have a flexible supporting rod, a notochord, which subsequently becomes the spine. The first life developed in the form of single cells in water. Invertebrates were the initial link in the evolution of other organisms. Their gradual changes led to the appearance of complex creatures with a well-developed skeleton.
Intestinal animals
Today, there are about eleven thousand species of gastrointestinal. This is one of the oldest complex animals that appeared on earth. The smallest of the intestinal can not be seen without a microscope, and the largest known jellyfish - 2.5 meters in diameter.
So, let's get acquainted in more detail with the subdominance of multicellular, intestinal type. . , .
The body shape of the intestinal cavity is called the "bag." The mouth connects to a blind sac called the “gastrovascular cavity”. This bag functions in the process of digestion, gas exchange and acts as a hydrostatic skeleton. A single hole serves both the mouth and the anus. Tentacles are long, hollow structures used to move and capture food. All intestinal cavities have tentacles covered with suction cups. They are equipped with special cells - nemocysts, which can inject toxins into their victim. Suction cups also allow you to capture large prey, which animals place in their mouths by pulling tentacles. Nematocysts are responsible for the burns that some jellyfish inflict on people.
Animal subdomains are multicellular, such as intestinal, possess both intracellular and extracellular digestion. Breathing occurs by simple diffusion. They have a network of nerves that spread throughout the body.
Many forms exhibit polymorphism, that is, a variety of genes in which different types of creatures are present in the colony for different functions. These individuals are called zooids. Reproduction can be called promiscuous (external budding) or sexual (gamete formation).
Jellyfish, for example, produce eggs and sperm, and then release them into the water. When an egg is fertilized, it develops into a free-floating larva with cilia, called the “plana”.
Typical examples of the subdomain Multicellular type of gastrointestinal are hydras, obelisks, Portuguese boat, sailfish, jellyfish-aurelia, head jellyfish, sea anemones, corals, sea feathers, gorgonaria, etc.
Plants
In the kingdom, Multicellular plants are eukaryotic organisms that can feed on photosynthesis. Algae were originally considered plants, but now they belong to protists - a special group that is excluded from all known species. The modern definition of plants refers to organisms that live mainly on land (and sometimes in water).
Another distinguishing feature of plants is the green pigment - chlorophyll. It is used to absorb solar energy during photosynthesis.
Each plant has haploid and diploid phases that characterize its life cycle. It is called alternation of generations, because all phases in it are multicellular.
Alternating generations are the generation of sporophytes and the generation of gametophytes. In the gametophyte phase, gametes are formed. Haploid gametes merge into a zygote called a diploid cell, since it has a complete set of chromosomes. From there, diploid individuals of the sporophyte generation grow.
Sporophytes go through the phase of meiosis (division) and form haploid spores.
So, the sub-kingdom of multicellular organisms can be briefly described as the main group of living creatures that inhabit the Earth. These include all who have a number of cells that are different in their structure and functions and combined into a single organism. The simplest of the multicellular ones are intestinal, and the most complex and developed animal on the planet is man.