In our article we will consider the structure of prokaryotes and eukaryotes. These organisms differ significantly in the level of organization. And the reason for this is the peculiarities of the structure of genetic information.
Features of the structure of prokaryotic cells
Prokaryotes are all living organisms whose cells do not contain a nucleus. Of the representatives of the five modern kingdoms of living nature, only one belongs to them - Bacteria. The prokaryotes we are considering also include blue-green algae and archaea.
Despite the absence of a formed nucleus in their cells, they contain genetic material. This allows you to store and transmit hereditary information, but limits the variety of methods of reproduction. All prokaryotes reproduce by dividing their cells in two. They are not capable of mitosis and meiosis.
Structure of prokaryotes and eukaryotes
The structural features of prokaryotes and eukaryotes that distinguish them are quite significant. In addition to the structure of the genetic material, this also applies to many organelles. Eukaryotes, which include plants, fungi and animals, contain in the cytoplasmmitochondria, Golgi complex, endoplasmic reticulum, many plastids. Prokaryotes do not have them. The cell wall, which both of them have, differs in chemical composition. In bacteria, it is composed of complex carbohydrates pectin or murein, while in plants it is based on cellulose, and in fungi - chitin.
Discovery history
Features of the structure and life of prokaryotes became known to scientists only in the 17th century. And this despite the fact that these creatures have existed on the planet since its inception. In 1676, they were first examined through an optical microscope by its creator Anthony van Leeuwenhoek. Like all microscopic organisms, the scientist called them "animals". The term "bacteria" appeared only at the beginning of the 19th century. It was proposed by the famous German naturalist Christian Ehrenberg. The concept of "prokaryotes" arose later, in the era of the creation of the electron microscope. And first, scientists established the fact of differences in the structure of the genetic apparatus of the cells of different creatures. E. Chatton in 1937 proposed to combine organisms into two groups according to this feature: pro- and eukaryotes. This division exists to this day. In the second half of the 20th century, a distinction was discovered among the prokaryotes themselves: archaea and bacteria.
Features of the surface apparatus
The surface apparatus of prokaryotes consists of a membrane and a cell wall. Each of these parts has its own characteristics. Their membrane is formed by a double layer of lipids and proteins. prokaryotes,the structure of which is quite primitive, they have two types of structure of the cell wall. Thus, in gram-positive bacteria, it consists mainly of peptidoglycan, has a thickness of up to 80 nm, and is tightly attached to the membrane. A characteristic feature of this structure is the presence of pores in it, through which a number of molecules penetrate. The cell wall of gram-negative bacteria is very thin - up to a maximum of 3 nm. It does not adhere tightly to the membrane. Some representatives of prokaryotes also have a mucous capsule on the outside. It protects organisms from drying out, mechanical damage, and creates an additional osmotic barrier.
Prokaryote organelles
The structure of the cell of prokaryotes and eukaryotes has its own significant differences, which primarily consist in the presence of certain organelles. These permanent structures determine the level of development of organisms as a whole. Most of them are absent in prokaryotes. Protein synthesis in these cells occurs by ribosomes. Aquatic prokaryotes contain aerosomes. These are gas cavities that provide buoyancy and regulate the degree of immersion of organisms. Only prokaryotes contain mesosomes. These folds of the cytoplasmic membrane occur only during the use of chemical fixation methods during the preparation of prokaryotic cells for microscopy. The organelles of movement of bacteria and archaea are cilia or flagella. And attachment to the substrate is carried out by drinking. These structures formed by protein cylinders are also called villi and fimbriae.
What is a nucleoid
But the most significant difference is the structure of the gene of prokaryotes and eukaryotes. All these organisms possess hereditary information. In eukaryotes, it is located inside the formed nucleus. This two-membrane organelle has its own matrix called nucleoplasm, envelope and chromatin. Here, not only the storage of genetic information is carried out, but also the synthesis of RNA molecules. In the nucleoli, they subsequently form subunits of ribosomes - organelles responsible for protein synthesis.
The structure of prokaryotic genes is simpler. Their hereditary material is represented by the nucleoid or nuclear region. DNA in prokaryotes is not packed into chromosomes, but has a circular closed structure. The nucleoid also contains RNA and protein molecules. The latter are similar in function to eukaryotic histones. They are involved in DNA duplication, RNA synthesis, chemical structure repair and nucleic acid breaks.
Features of life activity
Prokaryotes, whose structure is not complex, carry out rather complex life processes. This is nutrition, respiration, reproduction of their own kind, movement, metabolism … And only one microscopic cell is capable of all this, the size of which ranges from up to 250 microns! So one can only talk about primitiveness relatively.
Features of the structure of prokaryotes determine the mechanisms of their physiology. For example, they are able to receive energy in three ways. The first isfermentation. It is carried out by some bacteria. This process is based on redox reactions, during which ATP molecules are synthesized. This is a chemical compound, during the splitting of which energy is released in several stages. Therefore, it is not in vain called "cell battery". The next way is breathing. The essence of this process is the oxidation of organic substances. Some prokaryotes are capable of photosynthesis. Examples are blue-green algae and purple bacteria, which contain plastids in their cells. But archaea are capable of chlorophyll-free photosynthesis. During this process, carbon dioxide is not fixed, but ATP molecules are directly formed. So, in essence, this is true photophosphorylation.
Meal type
Bacteria and archaea are prokaryotes, the structure of which allows them to carry out different ways of feeding. Some of them are autotrophs. These organisms themselves synthesize organic substances during photosynthesis. The cells of such prokaryotes contain chlorophyll. Some bacteria obtain energy from the breakdown of certain organic compounds. Their type of nutrition is called chemotrophic. Representatives of this group are iron and sulfur bacteria. Others absorb only ready-made compounds. They are called heterotrophs. Most of them lead a parasitic way of life and live only inside the cells of other creatures. A variety of this group are also saprotrophs. They feed on waste products ordecaying organic matter. As you can see, the way prokaryotes feed is quite diverse. This fact contributed to their wide distribution in all habitats.
Reproduction forms
Prokaryotes, the structure of which is represented by one cell, multiply by dividing it into two parts or by budding. This feature is also due to the structure of their genetic apparatus. The process of binary fission is preceded by duplication, or DNA replication. In this case, the nucleic acid molecule is first unwound, after which each strand is duplicated according to the principle of complementarity. The chromosomes formed as a result of this diverge towards the poles. The cells increase in size, a constriction forms between them, and then their final isolation occurs. Some bacteria are also capable of forming asexually reproducing cells - spores.
Bacteria and archaea: distinguishing features
For a long time, archaea, together with bacteria, were representatives of the Kingdom of Drobyanka. Indeed, they have many similar structural features. This is primarily the size and shape of their cells. However, biochemical studies have shown that they share a number of similarities with eukaryotes. This is the nature of enzymes, under the influence of which the processes of synthesis of RNA and protein molecules occur.
According to the way of feeding, most of them are chemotrophs. Moreover, the substances that are broken down in the process of obtaining energy by archaea are more diverse. These are both complex carbohydrates andammonia, and metal compounds. Among archaea there are also autotrophs. Very often they enter into a symbiotic relationship. There are no parasites among the archaea. Most often in nature, commensals and mutualists are found. In the first case, archaea feed on the substances of the host's body, but do not harm it. In contrast to this type of symbiosis, in a mutualistic relationship, both organisms benefit. Some of them are metagenes. Such archaea inhabit the digestive system of humans and ruminant mammals, causing excessive gas formation in the intestines. These organisms reproduce by binary fission, budding or fragmentation.
Archaea have mastered almost all habitats. They are especially diverse in the composition of plankton. Initially, all archaea were classified as extremophiles, since they are able to live in hot springs, water bodies with high salinity, and at depths with significant pressure.
The importance of prokaryotes in nature and human life
The role of prokaryotes in nature is significant. First of all, they are the first living organisms that arose on the planet. Scientists have found that bacteria and archaea originated about 3.5 billion years ago. The theory of symbiogenesis suggests that some eukaryotic cell organelles also originated from them. In particular, we are talking about plastids and mitochondria.
Many prokaryotes find their application in biotechnology in order to obtain medicines, antibiotics, enzymes, hormones, fertilizers, herbicides. Man has long used beneficial propertieslactic acid bacteria for the manufacture of cheese, kefir, yogurt, fermented products. With the help of these organisms, the purification of water bodies and soils, the enrichment of ores of various metals is carried out. Bacteria form the intestinal microflora of humans and many animals. Along with archaea, they cycle many substances: nitrogen, iron, sulfur, hydrogen.
On the other hand, many bacteria are the causative agent of dangerous diseases, regulating the population of many plant and animal species. These include plague, syphilis, cholera, anthrax, diphtheria.
So, prokaryotes are called organisms whose cells are devoid of a formed nucleus. Their genetic material is represented by a nucleoid, consisting of a circular DNA molecule. Of modern organisms, bacteria and archaea belong to prokaryotes.
