All living organisms are divided into sub-kingdoms of multicellular and unicellular creatures. The latter represent a single 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 variety to which the individual belongs. Most are so small that they can only be seen under a microscope. Cells appeared on Earth approximately 3.5 billion years ago.
In our time, all the processes that occur with living organisms are studied by biology. It is this science that deals with the sub-kingdom of multicellular and unicellular.
Single-celled organisms
Unicellularity is determined by the presence in the body of a single cell that performs all vital functions. The well-known amoeba and the ciliate shoe are primitive and, at the same time, the oldest forms of life,which are members of this species. They were the first living beings that lived on Earth. This also includes groups such as sporozoans, sarcodes and bacteria. They are all 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 cell organelles. These are usually bacteria and cyanobacteria such as E. coli, salmonella, nostocs, etc.
Eukaryotes are made up of a series of cells that depend on each other for survival. They have a nucleus and other organelles separated by membranes. They are mostly aquatic parasites or fungi and algae.
All representatives of these groups differ in size. The smallest bacterium is only 300 nanometers long. Unicellular organisms usually have special flagella or cilia that are involved in their locomotion. 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 organelles of the cell.
Single-celled have dominated the life form on Earth for billions of years. However, evolution from the simplest to more complex individuals has changed the entire landscape as it has led to the emergence of biologically advanced relationships. In addition, the emergence of new species led to the formationnew environment with diverse ecological interactions.
Multicellular organisms
The main characteristic of the multicellular subkingdom is the presence of a large number of cells in one individual. 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 instruments. Plants, fish, birds and animals come out of a single cell. All creatures that are part of the multicellular sub-kingdom regenerate new individuals from embryos that 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, the classification clearly separates the functions in which each of the individual particles performs its task. They are engaged in vital processes, thus supporting the existence of the whole organism.
Subkingdom Multicellular in Latin sounds like Metazoa. To form a complex organism, cells must be identified and attached to others. Only about a dozen protozoa can be seen individually with the naked eye. The remaining nearly two million visible individuals are multicellular.
Pluricellular animals are created by the combination of individuals through the formation of colonies, filaments or aggregation. Pluricellular evolved independently, like Volvox and some flagellar greensalgae.
A sign of the sub-kingdom of multicellular, that is, its early primitive species, was the absence of bones, shells and other hard parts of the body. Therefore, their traces have not survived to this day. The exceptions are sponges that still live in the seas and oceans. Perhaps their remains are found in some ancient rocks, such as 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 multicellular sub-kingdom is presented in all its diversity.
Complex relationships arose as a result of the evolution of protozoa and the emergence of the ability of cells to divide into groups and organize tissues and organs. There are many theories explaining the mechanisms by which unicellular organisms may have evolved.
Theories of emergence
Today, there are three main theories of the emergence of the multicellular subkingdom. A summary of the syncytial theory, so as not to go into details, can be described in a few words. Its essence lies in the fact that a primitive organism, which had several nuclei in its cells, could eventually separate each of them with an internal membrane. For example, several nuclei contain a mold fungus, as well as a ciliate shoe, which confirms this theory. However, having multiple nuclei is not enough for science. To confirm the theory of their multiplicity, a visual transformation into a well-developed animal of the simplest eukaryote is necessary.
Colony theory says that symbiosis, consisting of different organisms of the same species, led to their change and the appearance of more perfect creatures. Haeckel is the first scientist to present this theory in 1874. The complexity of organization arises because cells stay together, rather than being pulled apart during division. Examples of this theory can be seen in such protozoan metazoans as green algae called eudorina or volvax. They form colonies that number up to 50,000 cells depending on the species.
Colony theory proposes the fusion of different organisms of the same species. The advantage of this theory is that it has been observed that during food shortages, amoebas cluster into a colony that moves as a unit to a new location. Some of these amoebas are slightly different.
The symbiosis theory suggests that the first creature from the multicellular sub-kingdom appeared due to the community of dissimilar primitive creatures that performed different tasks. Such relationships are, for example, present between clownfish and sea anemones or vines that parasitize trees in the jungle.
However, the problem with this theory is that it is not known how the DNA of different individuals can be included in 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 host species mitosis.
Two or three symbioticthe individuals that make up the lichen, although dependent on each other for survival, must reproduce separately and then recombine to form a single organism again.
Other theories that also consider the emergence of the multicellular subkingdom:
- GK-PID theory. About 800 million years ago, a slight genetic change in a single molecule called GK-PID may have allowed individuals to move from a single cell to a more complex structure.
- The role of viruses. It has recently been recognized that genes borrowed from viruses play a crucial role in the division of tissues, organs, and even in sexual reproduction, in the fusion of egg and sperm. The first syncytin-1 protein was found, which was transmitted from a virus to a person. It is found 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 nematode roundworms and is part of the entire FF protein family. Dr. Felix Rey at the Institut Pasteur in Paris built a 3D layout of the EFF1 structure and showed that it is what binds the particles together. This experience confirms the fact that all known fusions of the smallest particles into molecules are of viral origin. It also suggests that viruses were vital for the communication of internal structures, and without them it would not have been possible for a colony of the sub-kingdom of the multicellular sponge type.
All these theories, like many others that famous scientists continue to offer, are very interesting. However, none of them can clearly and unambiguously answerto the question: how could such a huge variety of species come from a single 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 biological functions such as protection, circulation, digestion, respiration and sexual reproduction. They are performed by specific organs such as the skin, heart, stomach, lungs and reproductive system. They are made up 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, thanks to which blood moves continuously through the circulatory system. Skin cells, on the other hand, have fewer mitochondria. Instead, they have dense proteins and produce keratin, which protects soft internal tissues from damage and external factors.
Reproduction
While all protozoa, without exception, reproduce asexually, many of the multicellular sub-kingdom 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 cell with a gamete (gametes are special sex cells containing one set of chromosomes) of a spermatozoon leads to the formation of a zygote.
Zygote contains genetic materialboth sperm and eggs. Its division leads to the development of a completely new, separate organism. During the development and division of cells, according to the program laid down in the genes, they begin to differentiate into groups. This will further allow 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 nerves, bones, muscles, tendons, blood - they all arose from one zygote, which appeared due to the fusion of two single gametes.
Metazoan advantage
There are several major advantages of the sub-kingdom of multicellular organisms, thanks to which they dominate our planet.
Because the complex internal structure allows for size increase, it also helps develop higher order structures and tissues with multiple functions.
Large organisms have the best defense against predators. They also have greater mobility, allowing them to migrate to better places to live.
There is one more indisputable advantage of the multicellular sub-kingdom. A common characteristic of all its species is a fairly long lifespan. The cell body 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. The division of particles within the body allows faster growth and repair of damagedfabrics.
During its division, a new cell copies the old one, which allows you to save favorable features in the next generations, as well as improve them over time. In other words, duplication allows for the retention and adaptation of traits that will enhance the survival or fitness of an organism, especially in the animal kingdom, a sub-kingdom of multicellular organisms.
Disadvantages of multicellular organisms
Complex organisms also have disadvantages. For example, they are susceptible to various diseases arising from their complex biological composition and functions. In protozoa, on the contrary, there are not enough developed organ systems. This means that their risks of dangerous diseases are minimized.
It is important to note that, unlike multicellular organisms, primitive individuals have the ability to reproduce asexually. This helps them not to waste resources and energy on finding a partner and sexual activities.
The simplest organisms also have the ability to take in energy by diffusion or osmosis. This frees them from the need to move around to find food. Almost anything can be a potential food source for a single-celled creature.
Vertebrates and invertebrates
Without exception, the classification divides all multicellular creatures included in the sub-kingdom into two types: vertebrates (chordates) and invertebrates.
Invertebrates do not have a solid skeleton, while chordates have a well-developed internal skeleton of cartilage, bone and a highly developed brain that is protected by a skull. Vertebrateshave well-developed sense organs, a respiratory system with gills or lungs, and a developed nervous system, which further distinguishes them from their more primitive counterparts.
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 seabed mud.
To date, almost two million species of the sub-kingdom of multicellular invertebrates 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. Humans belong to the chordate family.
Vertebrates are divided into fish, amphibians, reptiles, birds and mammals. Animals without backbones represent phyla such as arthropods, echinoderms, worms, coelenterates, and mollusks.
One of the biggest differences between these species is their size. Invertebrates such as insects or coelenterates are small and slow because they cannot develop large bodies and strong muscles. There are a few exceptions, such as the squid, which can reach 15 meters in length. Vertebrates have a universal support system, and therefore can develop faster and become larger than invertebrates.
Chordates also have a highly developed nervous system. With the help of a specialized connection between nerve fibers, they can react very quickly to changes in their environment, which gives thema definite advantage.
Compared to vertebrates, most spineless animals use a simple nervous system and behave almost entirely instinctively. This 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, which are considered among the most intelligent animals in the invertebrate world.
All chordates, as we know, have a backbone. However, a feature of the subkingdom of multicellular invertebrates is the similarity with their relatives. It lies in the fact that at a certain stage of life, vertebrates also have a flexible support rod, the notochord, which later becomes the spine. The first life developed as single cells in water. Invertebrates were the initial link in the evolution of other organisms. Their gradual changes led to the emergence of complex creatures with a well-developed skeleton.
Celiacs
Today there are about eleven thousand species of coelenterates. These are one of the oldest complex animals that appeared on earth. The smallest of the coelenterates cannot be seen without a microscope, and the largest known jellyfish is 2.5 meters in diameter.
So, let's take a closer look at the sub-kingdom of multicellular organisms, the intestinal type. The description of the main characteristics of habitats can be determined by the presence of an aquatic or marine environment. They live alone or in colonies that canmove freely or live in one place.
The body shape of coelenterates is called a "bag". The mouth connects to a blind sac called the "gastrovascular cavity". This sac functions in the process of digestion, gas exchange and acts as a hydrostatic skeleton. The single opening serves as both a mouth and an anus. Tentacles are long, hollow structures used to move and capture food. All coelenterates have tentacles covered with suckers. They are equipped with special cells - nemocysts, which can inject toxins into their prey. Suckers also allow the capture of large prey, which animals place in their mouths by retracting their tentacles. Nematocysts are responsible for the burns some jellyfish inflict on humans.
Animals of the sub-kingdom are multicellular, such as coelenterates, have both intracellular and extracellular digestion. Respiration occurs by simple diffusion. They have a network of nerves that extend 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 random (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-swimming, ciliated larva called a planla.
Typical examples of the sub-kingdom Multicellular type coelenterates are hydras,obelia, portuguese boat, sailboat, aurelia jellyfish, head jellyfish, sea anemones, corals, sea pen, gorgonians, etc.
Plants
In the sub-kingdom Multicellular plants are eukaryotic organisms that can feed on photosynthesis. Algae were originally considered plants, but now they are classified as protists, a special group that is excluded from all known species. The modern definition of plants refers to organisms that live primarily on land (and sometimes in water).
Another distinctive 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.
Alternate generations are the sporophyte generation and the gametophyte generation. In the gametophyte phase, gametes are formed. The haploid gametes fuse to form a zygote, called a diploid cell because it has a complete set of chromosomes. From there, diploid individuals of the sporophyte generation grow.
Sporophytes go through a phase of meiosis (division) and form haploid spores.
So, the multicellular sub-kingdom can be briefly described as the main group of living beings that inhabit the Earth. These include everyone who has a number of cells, different in structure and function and combined into a singleorganism. The simplest of multicellular organisms are coelenterates, and the most complex and developed animal on the planet is man.
