Nuclear organisms. Superkingdom nuclear, prenuclear organisms. Differences between eukaryotes and prokaryotes
Superkingdom Prenuclear organisms (Procaryota)
Unicellular and multicellular organisms without a separate nucleus. genetic information is concentrated on a single chromosome. The sizes of prokaryotes range from 0.015 to 20 cm. They appeared in the range of 3.7-3.1 billion years. Prokaryotes are divided into two kingdoms: bacteria and cyanobiotes. Their nutrition occurs through the process of chemo- and photosynthesis.
Kingdom of bacteria
Bacteria are microscopic organisms, measuring about 1-5 microns (micromicrons). Single-celled bacteria can have a filamentous, rod-shaped, or spiral shape. Among bacteria there are autotrophic and heterotrophic forms. The former create organic and inorganic substances; the latter use ready-made organic substances. Most bacteria are autotrophic. Their metabolic processes occur without the use of light (chemosynthesis), or only in light (photosynthesis). Bacteria are extremely diverse in their types of metabolism. There are sulfur-forming, ferromanganese, nitrogen, acetate, carbon-forming and other groups of bacteria. The role of bacteria in geological processes is great. Their activity is associated with the formation of various minerals: iron ores (jaspilites, ferruginous nodules), pyrite, sulfur, graphite, phosphorites, oil, gas, etc.
Reliable finds of bacteria are known from siliceous rocks that are 6.5 billion years old. Most likely, the bacteria emerged independently in different habitats. Currently, they inhabit all water basins from the littoral to the abyssal, and also live in the soil, in the air, and inside other organisms. They live in hot springs with temperatures exceeding 100 degrees Celsius and in salt waters with sodium chloride concentrations of up to 32%.
Kingdom Cyanobionts
Solitary and colonial organisms with cells without a separate nucleus. The size of single forms is about 10 microns, and the size of colonies and their metabolic products (stromatolites) is many hundreds of years old. Carbonates accumulate in the body, which subsequently leads to the formation of limestone. Calcareous layered formations are called stromatolites. Stromatolites differ in the shape of the buildings and the type of structure. They can have a sheet, nodular, columnar shape. Oncolites, unlike stromatolites, are represented by small rounded formations with a diameter of up to several centimeters.
Stromatolites are the result of a symbiosis of cyanobionts and bacteria. The formation of stromatolites occurs as follows. Calcium is released in the mucous membrane. After the death of the organism, a carbonate crust remains, which is covered with sediment. Repeated growth cycles of cyanobionts and bacteria lead to the formation of complex carbonate strata up to 1000 m thick. In addition to linear stromatolites, spherical oncolites and patterned ones in the form of irregular stars - catagraphs - are formed. The shape of all stromatolite structures depends on environmental factors and therefore they can be used to restore the physical and geographical conditions of past basins: salinity, temperature, depth, hydrodynamics. Cyanobionts took an active part in the construction of the biostral and...
Cyanobionts appeared about 3.5 billion years ago. Due to the presence of chlorophyll, they are the first photosynthetic organisms to produce molecular oxygen. Modern cyanobionts live in fresh and marine waters, mainly at depths of up to 20 m. They tolerate pollution and sharp fluctuations in physicochemical conditions. Temperatures range from glacial sub-zero to nearly boiling (85 degrees) in the hot springs. In the absence of a nucleus, cyanobionts are similar to bacteria; in the presence of chlorophyll and the ability to photosynthesize, they are similar to algae.
Superkingdom nuclear organisms (Eucaryota)
Single- and multicellular organisms, divided into three subkingdoms: plants, fungi, animals. Unlike prokaryotes, they have a separate nucleus. eukaryotic sizes, from 10 microns (unicellular) to 33 m (the length of a whale) and 100 m (the height of some conifers). Eukaryotes are descendants of prokaryotes. They appeared at the level of 1.7-1.5 billion years (PR1). Plants, unlike animals, are capable of creating organic compounds from inorganic ones through photosynthesis. They have different cells and assimilation processes. A form of existence that is mostly motionless (excluding passively floating plankton).
Plant Kingdom (Phyta)
Various, mostly immobile single- and multicellular, with apical growth. All plants are characterized by photosynthesis: using energy from light absorbed by chlorophyll, they release molecular oxygen and create organic compounds from inorganic ones. A plant cell consists of cytoplasm, which contains a nucleus, vacuoles - voids and organelles - independent intracellular formations. The hard cellulose cell membrane is permeated with vapors, often impregnated with salts and mineralized.
The plant kingdom is divided into two subkingdoms - lower (Thallophyta) and higher (Telomophyta). Lower plants live in bodies of water. This is algae. They live at depths of up to 200 m and among them there are both bottom - benthic and pelagic - planktonic. Higher plants live in terrestrial conditions at almost all latitudes. Plants are preserved in a fossil state in the form of separate parts (stem, leaves, roots, seeds), which makes it difficult to reconstruct their appearance.
Within this superkingdom of plants, the kingdom of fungi and the kingdom of plants are distinguished.
Fungi can form symbiotic relationships with other organisms, such as algae or cyanobacteria, to form lichens. They can also enter into symbiosis with higher plants, enveloping and penetrating the roots of plants with their hyphae and forming structures (root + fungus) called mycorrhizae. This symbiosis with plants ensures the latter’s need for phosphates. For example, 80% of terrestrial plants, including many agricultural plants, form a symbiosis with the fungus Glornus versiforme, which lives on their roots and facilitates their absorption of phosphates and mineral nutrients from the soil.
Among the organisms of this kingdom there are both unicellular (microscopic), or lower, and multicellular (higher) fungi.
Mushrooms are classified into sections: True mushrooms, Oomycetes and Lichens.
Among the True fungi, there are classes Chytrid fungi, Zygomycetes, Ascomycetes (Marsupial fungi), Basidiomycetes and Imperfect fungi (Deuteromycetes).
Ascomycetes are the most numerous group of fungi (more than 30,000 species), differing primarily in size. There are both unicellular and multicellular forms. Their body is represented by haploid mycelium. They form asci (bags) containing ascospores, which is a characteristic feature of these fungi. Among the fungi of this group, the most famous are yeasts (brewer's, wine, kefir and others). For example, the yeast Saccharomices cerevisiae affects the fermentation of glucose (CgH^Og). One molecule of glucose produces two molecules of ethyl alcohol during this enzymatic process.
Basidiomycetes are higher fungi. They are characterized by their large sizes, which can even reach up to half a meter. Their body also consists of mycelium (mycelium), but multicellular, forming mushrooms. The protoplast of fungal cells contains not only nuclei, but also mitochondria, ribosomes, the Golgi apparatus, and even glycogen as a reserve substance. The hyphae intertwine to form fruiting bodies, which in everyday life are called mushrooms, consisting of a stem and a cap.
These fungi reproduce both vegetatively and asexually, as well as sexually. The most famous basidiomycetes are cap mushrooms, among which there are both edible and poisonous.
Oomycetes are mainly aquatic and soil fungi. Among these fungi, species from the genus Phytophtora are very famous, causing diseases of potatoes, tomatoes and other nightshades.
Mushrooms play a significant role in nature. In particular, they are destructive organisms. Being part of many ecological systems, they are responsible for the destruction of organic material of plant origin, because they produce enzymes that act on cellulose, lignin and other substances of plant cells. They are widely used in the cheese industry to produce many popular varieties of cheese. It should be noted that Neurospora crassa plays an outstanding role as an experimental object in the knowledge of many metabolic pathways.
Lichens are complex organisms formed as a result of symbiosis between fungi, green algae, or cyanobacteria, and Azotobacter (Fig. 4). Consequently, a lichen is a combined organism, i.e., a fungus + an algae + azotobacter, the existence of which is ensured by the fact that the hyphae of the fungus are responsible for the absorption of water and minerals, the algae for photosynthesis, and the azotobacter for the fixation of atmospheric nitrogen. Lichens are inhabitants of all botanical and geographical zones. They reproduce vegetatively, asexually and sexually.
The importance of lichens in nature is great. Due to their high sensitivity to environmental pollutants, lichens are used as indicators of atmospheric cleanliness. In the north they are the main food for deer. They are also used in pharmacy and perfumery.
Mushrooms have ancient origins. Their fossil remains are recorded in the Silurian and Devonian. Some botanists suggest that they originated from green algae that lost chlorophyll. The more common view is that fungi evolved from flagellates (protozoans).
Fossils of lichens are also found in the Devonian, which puts their age at approximately 400 million years. It is believed that the formation of lichens was the first case of the establishment of symbiotic relationships between organisms. This provided the possibility of their wide distribution in different ecological niches.
Plant Kingdom (Plantae or Vegetabilia). This kingdom is represented by organisms whose cells have dense cell walls and are capable of photosynthesis. The plants of this kingdom are classified into three subkingdoms, namely: purple algae (Phycobionta), true algae (Phycobionta) and higher plants (Embryophyta).
The body of purple algae and true algae is not divided into tissues and organs. For this reason, they are often called lower or layer plants. On the contrary, the remaining plants are known as higher plants, since they are characterized by the presence of different tissues and the division of the body into organs. These plants are adapted to life in terrestrial conditions.
Sub-kingdom of the Purple (Rhodophyta). Plants of this subkingdom are multicellular organisms (Fig. 5). The body of the purple flowers is represented by a thallus. There are about 4,000 species of scarlet, among which the most famous are porphyra, ne-malyon, coralline and others. Their crimson color depends on the content of chlorophyll, carotenoids, red phycoerythrins, blue phycocyanins and other pigments. They are inhabitants of great depths of seas and oceans. They are often called red algae. The Red Sea is especially rich in them.
They reproduce both asexually and sexually with alternating sexual and asexual generations.
They are of economic importance. Certain species serve as the raw material from which agar-agar is extracted. In a number of countries they are used as livestock feed.
Scarlet beetles are ancient organisms, but their origin and phylogenetic relationships between individual species remain unclear.
Subkingdom True algae (Phycobionta). True algae are plants whose body is represented by a thallus. About 30,000 species of these organisms are known. Both unicellular and multicellular algae are found. They are inhabitants mainly of freshwater reservoirs and seas, but soil algae and even snow and ice algae are found. Unicellular algae reproduce by fission; multicellular forms reproduce both asexually and sexually. Virgil once wrote - “nigilvilor algo” (there is nothing worse than algae). In our time, algae have acquired other assessments.
Algologists classify algae into several divisions.
Department Green algae (Chlorophyta). This section is represented by mobile and immobile unicellular and multicellular organisms, which have a rather thick cell wall and are shaped like threads and tubes (Fig. 6). Some species form mobile and immobile colonies. There are over 13,000 species of these algae, most of which are inhabitants of fresh water bodies. But marine forms are also known.
Unicellular and multicellular green algae are capable of photosynthesis, because they contain chloroplasts, in which chlorophyll is concentrated and from the presence of which they have a green color. They also contain xanthophyll and carotene.
Typical representatives of unicellular green algae are chlamydomonas (from the genus Chlamidomonas), which live in puddles and other small fresh water bodies, and chlorella from the genus of the same name (Chlorella), which lives in fresh and salty waters, on the surface of damp earth, on the bark of trees. Chlorella has exceptional photosynthetic activity, being able to capture and use 10-12% of light energy. Contains a number of valuable proteins, vitamins B, C and K.
An example of multicellular green algae is the pond dweller Volvox. Forming a colony, this organism consists of 500-60,000 cells, each of which is equipped with two flagella, and also contains an ocellus, a differentiated nucleus and a chloroplast. A thick pulpous membrane surrounds each cell and separates it from neighboring cells. If one cell in a colony dies, the rest continue to live. The arrangement of cells in a colony ensures the movement of this organism.
They reproduce by fission or the formation of motile zoospores, which are separated from the mother's body, attached to some substrate, and then develop into a new organism. In Spirogyra the sexual process takes place in the form of conjugation.
The economic importance of these algae is small, except that due to the rich content of proteins and vitamins, chlorella is used as animal feed. Being a component of phytoplankton, it serves as food for fish.
It is believed that green algae arose as a result of aromorphoses, which turned out to be the formation of a nucleus, the appearance of multicellularity and the sexual process. It is also believed that they gave rise to primitive land plants, which became the ancestral forms of bryophytes.
Department Diatoms algae, or diatoms (Chrysophyta) is represented mainly by multicellular organisms, and sometimes even by colonial forms (Fig. 7). Single-celled forms are also found. There are 5,700 known species. They are characterized by a clear differentiation of the body into cytoplasm and nucleus. The cell wall is “impregnated” with silica, as a result of which it is called the shell. They are inhabitants of fresh water bodies, seas and oceans and are part of phytoplankton.
The cells of these algae contain chloroplasts in the form of grains or plates, which are colored in different colors due to the content of different pigments (carotene, xanthophyll and its variant diatomine). For this reason, diatoms are often called golden brown.
Reproduction occurs by dividing cells in half. Some species have sexual reproduction. Diatoms are diploid organisms.
Layers of dead diatoms gave rise to diatomite, which consists of 50-80% of their shells and is used as absorbents in the chemistry and food industry.
The importance of diatoms in nature is very great. They occupy an extremely important place in the cycle of substances, being the main food for fish. Their nutritional value is very high.
Evolutionarily, diatoms are closest to green algae, but their origin is unclear.
Department Brown algae (Phaeophyta). These algae are multicellular organisms. Each cell contains only one nucleus. In size they are the largest (longest) algae, reaching a length of several tens of meters (Fig. 8). About 900 species are known. They are inhabitants of seas and oceans, including northern ones. Their pigmentation is determined by the fact that they contain chloroplasts, colored brown due to the content of chlorophyll, as well as brown pigments (carotene, xanthophyll and fucoxanthin).
The most famous are algae from the genera Laminaria and Fucus.
They reproduce vegetatively, asexually and sexually. Vegetative reproduction occurs in parts of the thallus, asexual (spore-based) - with the help of haploid spores developing into a gametophyte, sexual - through isogamy, heterogamy or oogashi. Characteristic is the alternation of haploid and diploid generations. Sex cells are equipped with flagella.
The economic importance of these algae, especially kelp, is very great. Iodine, potassium salts, and agar-like substances used in the food industry are extracted from them. Kelp, known as seaweed, is used for human food. Some algae are used as fertilizer.
Brown algae are the oldest aquatic plants. It is believed that they gave rise to fern-like plants.
To conclude this summary of the algae data, it should be noted that, in general, algae are important in many ecological systems. In fact, they are the main source of organic matter in water bodies. It is estimated that algae are responsible for the annual synthesis of 550 billion tons of organic matter in the World Ocean, which constitutes a significant part of the productivity of the entire biosphere. Further, they play a very significant role in enriching the atmosphere with oxygen. Finally, algae participate in the self-purification of water bodies and in soil formation.
Subkingdom Higher plants (Embryophyta or Embryobionta). The plants that make up this subkingdom are often called deciduous, since their body is divided into stem, leaf and root. In addition, they are also called germinal, because they contain an embryo. Finally, they are called vascular plants (except bryophytes), since the organs of their sporophytes contain vessels and tracheids.
In the course of historical development, higher plants have adapted to life in terrestrial conditions. These plants have an alternation of sexual (gametophyte) and asexual (sporophyte) generations. The gametophyte produces gametes and protects the embryo, while the sporophyte produces spores that support the next generation of gametophyte. In higher plants, the diploid sporophyte dominates, which determines the appearance of the plant.
In the subkingdom Higher plants, a distinction is made between higher spore plants and higher seed plants. Higher spores are characterized by the separation of sexual and asexual reproduction. In the first case, reproduction occurs by unicellular spores formed in the sporangia of sporophytes, in the second - by gametes formed in the genital organs of gametophytes. Higher seed plants are characterized by the presence of a multicellular formation - a seed, which is formed during the process of reproduction and gives seed plants the most important evolutionary advantage over spore plants.
Subkingdom Higher plants are classified into several divisions. In particular, higher spore plants are classified into the divisions Rhyniophyta and Zosterophyllophyta, the organisms of which are completely extinct, as well as into the existing divisions Bryophyta, Lycopodiophyta, Psilotophyta, Eguisetophyta ), Ferns (Polypodiophyta). Higher seed plants are classified into the divisions Gymnospermae (Gymnospermae) and Angiospermae (Angiospermae, or Magnoliophyta). Gymnosperms and Angiosperms are seed plants, while all others are higher spore plants. In some higher spores, all spores are the same (unisporous plants), and in some, the spores have different sizes (heterosporous plants).
Of the plants of modern divisions, only a few of them will be considered below.
Department Bryophytes(Bryophyta). This department is represented by low-growing, perennial plants. In some of them the body is represented by a thallus, but in most it is divided into stems and leaves (Fig. 9). There are about 25,000 species of bryophytes. They are inhabitants of damp places in all geographical zones. They are attached to the soil using hair-like projections called rhizoids. Through these structures they provide soil nutrition. The most famous representatives of this type are cuckoo flax, diverse Marchantia, and sphagnum mosses (300 species).
The development of mosses is characterized by alternation of sexual (gametophyte) and asexual (sporophyte) generations. On plants of the sexual generation, spores of different sizes are formed. After fertilization of female germ cells by male ones, a sporophyte (sporangium with spores) develops, the cells of which have a diploid set of chromosomes. Spores formed as a result of meiosis in sporangia have a haploid set of chromosomes. Having spilled onto the soil, the spores germinate, giving rise to a plant, a gametophyte, which has a haploid set of chromosomes in cells that reproduce by mitosis. The haploid gametophyte dominates the development cycle. Sex cells are formed again on the gametophyte, and the process is repeated. A specific feature of these plants is not only the dominance of the haploid gametophyte, but also the fact that the gametophyte (sexual generation) and sporophyte (asexual generation) are one plant.
The importance of bryophytes in nature lies in the fact that, being in ecosystems, they affect the habitat of many species of other plants, as well as animals. Intensive reproduction of mosses contributes to the deterioration of the soil. As sphagnum mosses die, they become peat-forming and form peat deposits. Some types are used in the medical industry.
It is believed that plants of this group were among the first land plants and grew widely 450-500 million years ago, and that their evolution consisted of a regressive development of the sporophyte. Bryophytes are believed to be a blind evolutionary branch.
Department Ferns(Palypodiophyta). Within this division, herbaceous plants that also live in damp places are classified (Fig. 10). Some Ferns living in the tropics are represented by arboreal forms, some of which reach 25 meters in height. There are more than 10,000 species of these plants. Typical representatives of pteridophytes are ferns.
Ferns are also characterized by an alternation of sexual and asexual generations, however, unlike bryophytes, in organisms belonging to this department, the sporophyte, which is characterized by diploidity, is predominant. The sporophyte has the main organs - stem, leaves, root. On the contrary, the gametophyte is characterized by a very small size, representing a small plate attached to the soil with the help of rhizoids.
Ferns are characterized by a complex development cycle. The cycle begins with the development of gametophyte isospores (prothallus), on which reproductive organs are formed in the form of antheridia and archegonia. In the latter, germ cells develop. After their fertilization, a sporophyte is formed from the zygote, on which spores are formed, giving rise to the gametophyte. Most ferns are represented by heterosporous plants.
The importance of ferns in nature is great, since they are part of many ecosystems. The economic importance of modern fern-like plants is small, except for the fact that plants of certain species serve as medicinal raw materials.
Ferns are classified into 7 divisions, most of which are represented by extinct species.
Ferns are the most ancient spore plants. They were already present in the Devonian, and in the Carboniferous they formed forests of plants, the height of which reached up to 30 m. The remains of these plants took part in the formation of coal.
Department Gymnosperms(Gymnospermae). Plants of this department produce seeds, which are essentially ready-made embryos of future plants. The main organs of the seed are the embryonic root, the embryonic stalk, and the germ layers. However, in gymnosperms the seed is not covered with carpels. For this reason they are called gymnosperms.
Gymnosperms are represented by trees, shrubs and vines. The number of species is about 700. Distributed throughout the globe. In the northern hemisphere they occupy vast areas, forming coniferous forests.
Gymnosperms are characterized by alternation of generations associated with the change of haploid and diploid states, but they have a decrease in the gametophyte. Juniper, cycad, thuja, spruce, pine, larch are sporophytes. Like all seed plants, gymnosperms are heterosporous. The reproductive organs are female and male cones, which are formed on the same tree and contain the gametophyte.
Seed formation is the first stage in sporophyte development. Female cones are built from large scales called megasporophylls, each of which bears two megasporangia on the inner surface, and each megasporangium in turn contains a megaspore, which develops into a multicellular gametophyte containing two or three archegonia. Each archegonia consists of a single large egg cell and several small elongated cells. The megasporangium is covered with the so-called integument. Megasporangium with integument is called ovule.
Male cones bear on the inner surface of their scales (on microsporophylls) two microsporangia containing microspores, each of which develops into haploid pollen. Pollen granules (grains) make up the male gametophyte.
Megasporophylls and microsporophylls are collected into mega- and microstrobils (respectively) on a shortened spore-bearing shoot, which is a stem with spore-bearing leaves.
When pollen lands on female cones, it passes into the ovule, with each pollen granule developing into a stamen tube and two sperm nuclei, and when the stamen tube penetrates the egg, the sperm nucleus fuses with the nucleus of the egg. This is fertilization. The diploid zygote becomes a diploid embryo. Over time, the outer integument of the ovule turns into the seed coat, and the endosperm is formed from the remains of the megasporangium. Consequently, the ovule turns into a seed. After ripening, the seeds fall out of the cones.
Gymnosperms are a very ancient group of higher plants. Having appeared in the Devonian (about 350 million years ago), gymnosperms at the end of the Paleozoic - beginning of the Mesozoic took the place of pteridophytes, since they turned out to be more adapted to life in terrestrial conditions. One of their hypotheses is that gymnosperms evolved from the most ancient pteridophytes.
Department Angiosperms, or Flowering(Angiospermae, or Magnoliophyta). Plants of this department are found almost everywhere. They account for 250,000-300,000 species, i.e., almost two-thirds of the species of the plant kingdom. They are currently the most prosperous group of plants.
Within this department, monocotyledonous and dicotyledonous plants are distinguished, which are both herbaceous and shrubby species, as well as trees. Typical representatives of this department are rye, wheat, rose, birch, aspen and others. There are monocotyledonous and dicotyledonous angiosperms.
These plants are also characterized by alternation of generations, but they have had a significant decrease in the gametophyte.
A remarkable feature of these plants is the presence of a flower, which is a modified shoot and is a derivative of a sporophyte (Fig. 11). It is for this reason that plants that produce flowers are called flowering plants. As a rule, flowers are bisexual, but sometimes dioecious. A flower has a pistil and stamens, which are its main parts. Seeds develop in the lower part of the pistil (ovary). For this reason, these plants are called angiosperms. The lower part of the pistil is represented by the ovary, a narrow style and stigma. As for the stamens, each of them consists of a filament and an anther.
In bisexual plants, which make up the majority among angiosperms, flowers have both pistils and stamens, that is, these plants have pistillate (female) and staminate (male) flowers. But in many species, some flowers have only pistils, while others have only stamens. Such plants are called dioecious. Pollination results from the transfer of pollen from the stamens to the stigma.
General diagram of angiosperm reproduction in Fig. 12.
The female gametophyte of flowering plants consists of 8 embryo sac cells, one of which is an egg cell. This microscopic structure develops from a single megaspore. The male gametophyte develops from a microspore, or pollen granule, located in the microsporangium of the anther. Once on the stigma of the pistil, the pollen granule, as a result of division, gives rise to a generative cell and a cell that develops into a pollen tube. Next, the pollen tube grows into the cavity of the ovary. The nucleus of the generative cell tube migrates to the bottom of the pollen tube, where the generative cell divides to produce two sperm cells. One of these sperm fuses with the egg to form a diploid zygote, while the second sperm fuses with the nucleus (in the center of the embryo sac, in the ovule), giving a triploid nucleus, which then develops into the endosperm. Ultimately, both structures end up in the seed, and the seed ends up in the ovary, which develops into a fruit. The latter may contain from one to several seeds. This type of fertilization is called double fertilization (Fig. 13). It was discovered in 1898 by S. G. Navashin (1857-1950). The biological meaning of double fertilization is that the development of triploid endosperm in combination with a huge number of generations ensures savings in plant plastic and energy resources.
It was discovered in 1898 by S. G. Navashin (1857-1950). The biological meaning of double fertilization is that the development of triploid endosperm in combination with a huge number of generations ensures savings in plant plastic and energy resources.
The stem is the plant organ to which leaves, roots, and flowers are attached. (The structure of the stem of a woody plant is shown in Fig. 14.)
Leaves are the most important organ of plants. They are characterized by different shapes and are built from several layers of cells containing a large number of chloroplasts. Serve as an organ of gas exchange between plants and the environment. Due to the presence of chlorophyll in the leaves, photosynthesis occurs, which is based on two reactions - photolysis of water and fixation of COg.
The root is a plant organ that absorbs water and minerals from the soil and carries them to the stem. In angiosperms, like gymnosperms, water and nutrients from the soil are adsorbed by root hairs and carried into the xylem as a result of osmotic pressure in the root system, the action of capillaries, negative pressure in the xylem, sometimes reaching up to 100 bar in some tree forms, and transpiration, t i.e. evaporation of water from the leaves (Fig. 15).
It is very difficult to overestimate the economic importance of angiosperms, since they are extremely widely used in human life (source of food, raw materials for industry, animal feed, etc.).
Angiosperms are the dominant plants of our planet. Therefore, the explanation of their origin has long been one of the most important tasks in the doctrine of evolution. Since Charles Darwin, several hypotheses have been put forward to explain angiosperms. According to one of them, it is assumed that angiosperms descended from some gymnosperms, and monocots descended from some ancient dicotyledons. However, this and other hypotheses are not exhaustive. There are also disagreements in determining the time of the appearance of angiosperms. According to the latest ideas, the main diversification of flowering plants, including the division into monocots and dicotyledons, occurred 130-90 million years ago, and this then gave rise to changes in the earth's ecosystems.
Issues for discussion
1. How do you understand the differences between prenuclear and nuclear organisms?
2. Name the subkingdoms of prenuclear organisms.
3. What do you know about archaebacteria and their properties that other prenuclear organisms do not have?
4. What is the role of bacteria in nature and in human life? What morphological forms of bacteria do you know?
5. List the main properties of mushrooms. How do mushrooms differ from lichens?
6. What are the similarities and differences between plant cells and animal cells?
7. How do green algae differ from cyanobacteria?
8. Do algae have any characteristics of economic importance?
9. What properties are characteristic of higher plants?
10. What does alternation of generations mean in plants and what is its biological role?
11. Are there differences between bryophytes and fern-like plants? Is there a commonality in their origins?
13. Why do angiosperms have this name?
14. What is the meaning of the flower?
16. What is double fertilization in angiosperms?
16. What importance do angiosperms have in human life?
17. What do you know about the origin of angiosperms?
Literature
Green N., Stout W.. Taylor D. Biology. M.: Mir. 1996. 368 pp.
Nidon K., Peterman I., Scheffel P., Shayba B. Plants and animals. M.: Mir. 1991. 260 pp.
Starostin B. A. Botany. In the book. "History of Biology". M.: Science. 1975. 52-77.
Yakovlev G. P., Chelombitko V. A. Botany. M.: Higher school. 1990. 367 pp.
Rosemweig M. L. Species Diversity in Space and Time. Cambridge University Press. 1995. 436 pp.
10. Vacuole 11. Hyaloplasm 12. Lysosome 13. Centrosome (Centriole)
Eukaryotes, or Nuclear(lat. Eucaryota from Greek εύ- - good and κάρυον - nucleus) - a superkingdom of living organisms whose cells contain nuclei. All organisms except bacteria and archaea are nuclear.
Structure of a eukaryotic cellEukaryotic cells are on average much larger than prokaryotic cells, the difference in volume reaches thousands of times. Eukaryotic cells include about a dozen types of different structures known as organelles (or organelles, which, however, somewhat distorts the original meaning of this term), many of which are separated from the cytoplasm by one or more membranes. Prokaryotic cells always contain a cell membrane, ribosomes (significantly different from eukaryotic ribosomes) and genetic material - a bacterial chromosome, or genophore, but internal organelles surrounded by a membrane are rare. The nucleus is a part of the cell, surrounded in eukaryotes by a double membrane (two elementary membranes) and containing genetic material: DNA molecules, “packed” into chromosomes. There is usually one nucleus, but there are also multinucleated cells. Division into kingdomsThere are several options for dividing the eukaryotic superkingdom into kingdoms. The plant and animal kingdoms were the first to be distinguished. Then the kingdom of fungi was identified, which, due to their biochemical characteristics, according to most biologists, cannot be classified as one of these kingdoms. Also, some authors distinguish the kingdoms of protozoa, myxomycetes, and chromists. Some systems have up to 20 kingdoms. Differences between eukaryotes and prokaryotesThe most important, fundamental feature of eukaryotic cells is associated with the location of the genetic apparatus in the cell. The genetic apparatus of all eukaryotes is located in the nucleus and is protected by the nuclear envelope (in Greek, “eukaryote” means having a nucleus). Eukaryotic DNA is linear (in prokaryotes, DNA is circular and floats freely in the cytoplasm). It is associated with histone proteins and other chromosomal proteins that bacteria do not have. In the life cycle of eukaryotes, there are usually two nuclear phases (haplophase and diplophase). The first phase is characterized by a haploid (single) set of chromosomes, then, merging, two haploid cells (or two nuclei) form a diploid cell (nucleus) containing a double (diploid) set of chromosomes. After several divisions, the cell again becomes haploid. Such a life cycle and, in general, diploidity are not typical for prokaryotes. The third, perhaps the most interesting difference, is the presence in eukaryotic cells of special organelles that have their own genetic apparatus, reproduce by division and are surrounded by a membrane. These organelles are mitochondria and plastids. In their structure and life activity they are strikingly similar to bacteria. This circumstance has prompted modern scientists to believe that such organisms are descendants of bacteria that entered into a symbiotic relationship with eukaryotes. Prokaryotes are characterized by a small number of organelles, and none of them are surrounded by a double membrane. Prokaryotic cells do not have an endoplasmic reticulum, Golgi apparatus, or lysosomes. It is equally important, when describing the differences between prokaryotes and eukaryotes, to talk about such a phenomenon in eukaryotic cells as phagocytosis. Phagocytosis (literally “eating”) refers to the ability of eukaryotic cells to capture and digest a wide variety of solid particles. This process provides an important protective function in the body. It was first discovered by I.I. Mechnikov at the starfish. The appearance of phagocytosis in eukaryotes is most likely associated with average size (more about size differences is written below). The sizes of prokaryotic cells are disproportionately smaller and therefore, in the process of evolutionary development, eukaryotes faced the problem of supplying the body with large amounts of food, as a result, the first predators appeared in the group of eukaryotes. Most bacteria have a cell wall that is different from the eukaryotic one (not all eukaryotes have it). In prokaryotes, it is a durable structure consisting mainly of murein. The structure of murein is such that each cell is surrounded by a special mesh sac, which is one huge molecule. Among eukaryotes, fungi and plants have a cell wall. In fungi it consists of chitin and glucans, in lower plants from cellulose and glycoproteins, in diatoms they synthesize a cell wall from silicic acids, in higher plants from cellulose, hemicellulose and pectin. Apparently for larger eukaryotic cells it has become impossible to create a cell wall of high strength from a single molecule. This circumstance could force eukaryotes to use different material for the cell wall. The metabolism of bacteria is also diverse. In general, there are four types of nutrition, and all are found among bacteria. These are photoautotrophic, photoheterotrophic, chemoautotrophic, chemoheterotrophic (phototrophic use the energy of sunlight, chemotrophic use chemical energy). Eukaryotes either synthesize energy from sunlight themselves or use ready-made energy of this origin. This may be due to the emergence of predators among eukaryotes, for which the need to synthesize energy has disappeared. Another difference is the structure of the flagella. In bacteria they are thin - only 15-20 nm in diameter. These are hollow filaments made from the protein flagellin. The structure of eukaryotic flagella is much more complex. They are a cell outgrowth surrounded by a membrane and contain a cytoskeleton (axoneme) of nine pairs of peripheral microtubules and two microtubules in the center. Unlike rotating prokaryotic flagella, eukaryotic flagella bend or wriggle. The two groups of organisms we are considering, as already mentioned, are very different in their average sizes. The diameter of a prokaryotic cell is usually 0.5-10 microns, while the same figure for eukaryotes is 10-100 microns. The volume of such a cell is 1000-10000 times greater than that of a prokaryotic cell. Prokaryotes have small ribosomes (70S type). Eukaryotes have larger ribosomes (80S type). Apparently, the time of emergence of these groups also differs. The first prokaryotes arose in the process of evolution about 3.5 billion years ago, from them about 1.2 billion years ago eukaryotic organisms evolved. |
Biosphere prerequisites for the emergence of eukaryotic organisms
Vladimir Malakhov
A hundred years ago, Russian biologist K.S. Merezhkovsky suggested that the eukaryotic cell arose as a result of the symbiosis of several independent organisms. This idea has become one of the main paradigms of modern biology.
All living organisms inhabiting our planet are divided into two large groups: prokaryotes (non-nuclear) and eukaryotes (nuclear). Prokaryotes are bacteria whose hereditary material is a simple circular DNA molecule. Nuclear are various unicellular and multicellular organisms (protozoa, plants, animals and fungi), the cells of which have a formed nucleus with chromosomes in which linear DNA molecules are associated with special nuclear proteins - histones. In addition to the nucleus, the cells of eukaryotic organisms have other organelles: mitochondria, flagella, chloroplasts. When and how did the eukaryotic organisms that dominate the modern biosphere arise?
According to modern ideas, our planet was formed about 4.5 billion years ago. Initially, the Earth was dry, water appeared as a result of degassing of the subsoil - the release of water vapor and gases that made up the ancient atmosphere into the atmosphere. As the water vapor condensed, small puddles first appeared, which gradually became larger and larger. However, it took 500-700 million years for more or less large bodies of water to appear on Earth, which gradually formed the hydrosphere - the liquid shell of our planet, which currently occupies about 70% of its surface. Then, as a result of various particles settling to the bottom of reservoirs, sedimentary rocks were formed.
The oldest sedimentary rocks are considered to be graphitized shales from the Isua formation in Greenland - their age is about 3.8 billion years. It is surprising that in these rocks undoubted signs of once existing life were found - traces of the activity of organisms that carried out the process of photosynthesis. The fact is that in organic matter created during the process of photosynthesis, the ratio of carbon isotopes 12C and 13C changes in favor of the lighter isotope 12C. And no matter what happens to this substance in the future, this ratio in it will be maintained. The carbon in the shales of the Isua Formation is clearly of organic origin. This means that already 3.8 billion years ago, organisms capable of photosynthesis lived in the primary reservoirs of the planet (most likely the World Ocean did not yet exist at that time). Fossilized cells similar to modern cyanobacteria were found in rocks 3.5 billion years old (Warrawoona Formation in Australia). In slightly younger sediments (more than 3.1 billion years old), remains of chlorophyll were found - phytan and pristane, as well as specific pigments of cyanobacteria - phycobilins.
Of course, among the organisms of that time there were not only photosynthetics, using the energy of sunlight, but also chemosynthetics, obtaining energy through various chemical reactions. In the first billions of years of the existence of the biosphere, due to the activity of chemosynthetic bacteria, many (if not most) of the ore deposits that are still used by humanity were formed, so fossilized remains of bacteria are often found in ore bodies. For example, such a large iron ore deposit as the Kursk Magnetic Anomaly, according to modern data, was formed as a result of the activity of bacteria.
There is no doubt that for a significant part of its history (at least 2 billion years) the biosphere was prokaryotic, that is, it included only organisms similar to modern bacteria. Eukaryotic organisms - various unicellular protozoa, and later (600-800 million years ago) multicellular organisms - took their place in the biosphere only about 1 billion years ago.
Prokaryotes and eukaryotes are the two main types of living beings on our planet. Biologists and doctors, however, are actively studying another group of biological objects - viruses, but they exhibit the properties of a living organism only inside the cells of their “hosts”. The sizes of prokaryotic cells in most cases range from 0.5 to 3 microns, and the smallest (mycoplasmas) do not exceed 0.10-0.15 microns. The giant cells of some sulfur bacteria reach 100 microns in length, and spirochete cells sometimes grow up to 250 microns. The main feature of prokaryotes is the absence of a nucleus. Their genetic material (genophore) is represented by a single circular double-stranded DNA molecule attached to the cytoplasmic membrane that covers the cell. Prokaryotes do not have a nuclear membrane, endoplasmic reticulum (sometimes there are invaginations of the surface membrane - the so-called mesosomes), mitochondria, plastids and other cytoplasmic organelles characteristic of eukaryotes. They also lack microtubules, so they have neither centrioles nor spindles. Prokaryotic ribosomes lack one type of ribosomal RNA (the so-called 5.8S RNA) and have a smaller mass than those of eukaryotes. Typically, the mass of ribosomes is estimated by the so-called sedimentation constant (an indicator of the sedimentation rate during centrifugation). For prokaryotic ribosomes it is 70S, and for eukaryotes it is 80S.
Prokaryotes have an enormous (compared to eukaryotes) variety of metabolic processes. They are capable of fixing carbon dioxide, nitrogen, various types of fermentation, and oxidizing all kinds of inorganic substrates (compounds of sulfur, iron, manganese, nitrites, ammonia, hydrogen, etc.). Among prokaryotes there are many photosynthetic forms, primarily cyanobacteria, which are often found in the modern biosphere, which are also called blue-green algae. They (or organisms related to them) were widespread in the distant past. Geological structures created by ancient cyanobacteria (probably together with other photosynthetic prokaryotes) - stromatolites - are often found in the oldest layers of the earth's crust, corresponding to the Archean and early Proterozoic. The activity of photosynthetic and other autotrophic prokaryotes, which began about 4 billion years ago, had several important consequences.
The first is due to changes in the Earth's atmosphere. The fact is that in ancient times it was practically oxygen-free. As a result of photosynthesis, molecular oxygen began to be released into the atmosphere, but quickly associated with the unoxidized components of the lithosphere - iron and other metals. Therefore, despite the presence of a constant source of free oxygen, the biosphere remained predominantly anaerobic. Living organisms during this period were also represented mainly by anaerobes. Meanwhile, banded iron ores (so-called jaspilites) were deposited in the lithosphere, in which oxidized iron alternated with underoxidized iron. In anoxic conditions, pyrites (FeS2 type ores) were deposited, which could not form in the presence of free oxygen. Findings of such fossils make it possible to establish that, despite the abundance of photosynthetics, the anaerobic period in the development of the biosphere lasted almost 2 billion years.
However, about 2 billion years ago, the oxygen content in the atmosphere reached 1% and continued to rise, since by that time most of the iron and other metals on the surface had become oxidized. At the same time, the amount of iron and other metals rising from the depths of the Earth gradually decreased. During the formation of the planet, heavy and light components were mixed randomly. Subsequently, in the process of gravitational differentiation, the metals gradually sank towards the center of the planet, forming its iron core, and the light components - silicates - rose upward, forming the mantle.
For anaerobic organisms, an increase in oxygen concentration was a disaster, since oxygen is a very aggressive element; it quickly oxidizes and destroys organic compounds. If in the anaerobic biosphere, in the thickness of the stromatolites, aerobic pockets remained, from where oxygen accumulated as a result of photosynthesis diffused into the atmosphere, now the biosphere, in the apt expression of Academician G.A. Zavarzin, “turned inside out” - it turned into an oxygen one with a few oxygen-free pockets where anaerobic microorganisms found refuge. In the new aerobic atmosphere, only those few prokaryotes (oxybacteria) could survive, which had previously adapted to high oxygen concentrations in oxygen pockets in the thickness of stromatolites.
The second important consequence of the activity of autotrophic prokaryotes is the accumulation of deposits of organic matter. The biotic cycle of substances in the biosphere, consisting exclusively of prokaryotes, was very imperfect. The biomass created by autotrophic bacteria was decomposed mainly under the influence of abiotic physical and chemical processes in the external environment. Without a doubt, heterotrophic bacteria also played a significant role in the decomposition of biomass created by prokaryotic autotrophs, but their capabilities were limited due to the peculiarities of the organization of prokaryotic cells. As is known, prokaryotes are fundamentally incapable of swallowing their victims. Predation in bacteria is very rare and looks quite unusual. The predatory bacterium Bdellovibrio is much smaller in size than its victims; it penetrates the cell wall of the bacterium and multiplies inside the unfortunate organism. 
Cells of fossil prokaryotic organisms close to cyanobacteria in thin sections of Archean sedimentary rocks (left and center). On the right is a photograph of fossil stromatolites formed by ancient photosynthetic bacteria.
Why are prokaryotes incapable of ingesting food? The fact is that they lack actin and myosin - proteins that ensure cytoplasmic mobility in eukaryotes. Thanks to them, during the capture of food particles (phagocytosis) and the formation of digestive vacuoles, pseudopodia (temporary cytoplasmic projections that serve for movement and capture of food) are formed. Prokaryotes cannot do this. Heterotrophic bacteria release enzymes into the external environment, a kind of “external digestion” (exofermentation) occurs, and low molecular weight products are absorbed through the cytoplasmic membrane. All this determined the low rate of decomposition of biomass created by autotrophic prokaryotes. Therefore, in the early stages of the evolution of the biosphere, huge masses of organic carbon were removed from the biological cycle, preserved in sediment, and underwent chemical transformation, turning into oil shale, oil and gas, which humanity still actively uses.
Only the appearance of microscopic aerobic predators that would swallow bacteria, digest them and return carbon (preferably in the form of CO2), nitrogen (in the form of ammonium compounds), phosphorus and others to the biosphere could improve the biological cycle and accelerate the return of carbon and other biogenic elements into it. biogenic elements. The first eukaryotic organisms became such predators.
Predators
Eukaryotes have two universal proteins - actin and myosin, which provide various types of cellular motility: amoeboid activity, movement of organelles within the cell, and in higher organisms - muscle contractions. The actin-myosin system allows the formation of pseudopodia, capturing the victim with them and forming digestive vacuoles (even viruses penetrate into the eukaryotic cell by provoking the so-called “endocytosis” - the cell takes them for something useful, “swallows” them, and the virus, once in the cytoplasm, begins its destructive work). The acquisition of the actin-myosin system allowed eukaryotes to feed by phagocytosis, actively capturing large food particles.
The appearance of such organisms unusually accelerated the biotic cycle, since they became consumers of bacterial biomass. By digesting bacterial cells, phagotrophic eukaryotes quickly returned elements to the cycle of substances that previously could only enter it again through slow decomposition. It can be assumed that the emergence of eukaryotes entailed a sharp decrease in “bacterial fossils,” that is, deposits of organic and inorganic substances resulting from the activity of bacteria.
The ability of eukaryotes to capture food particles meant that the predator had to be larger than the prey. Indeed, the linear dimensions of small soil amoebae or flagellates are approximately 10 times greater than the size of the bacteria on which they feed. Thus, the volume of the cytoplasm of eukaryotes is approximately 1000 times greater than that of prokaryotes, which also requires a large number of gene copies to supply the cytoplasm with transcription products. One way to solve this problem is to increase the number of genophores, that is, circular DNA molecules. Large (so-called “polyploid”) bacteria and the ancestors of eukaryotes with a large volume of cytoplasm followed this path. Multiple genophores (initially identical) became the rudiments of chromosomes, in which differences gradually accumulated. 
Cyanobacteria also thrive in the modern biosphere (left photo). In some places they form structures reminiscent of ancient stromatolites. The right photo shows modern stromatolites from Shark Bay in northern Australia. With amoeboid movement and nutrition by phagocytosis, the cell cytoplasm (especially peripheral) becomes very mobile. Genophores attached to the surface membrane of the cell found themselves in the zone of strong cytoplasmic currents, so a membrane-protected area appeared in the central cytoplasm where the genophores were stored. The process could occur in different ways, but one of the possible ways is deep invaginations of sections of the cytoplasmic membrane with genophores attached to them (after all, the nuclear envelope is part of the endoplasmic reticulum of a eukaryotic cell, which can be connected to the external environment).
Primary eukaryotes, thus, had a nucleus bounded by a double nuclear envelope - a derivative of the endoplasmic reticulum, but also had a ring structure of genophores and were deprived of specific nuclear proteins - histones. Surprisingly, a similar nuclear structure has been preserved in some modern eukaryotes, for example, dinoflagellates. In these protozoa, the nucleus is surrounded by a double nuclear membrane, but the chromosomes contain circular DNA molecules lacking histones. Apparently, the core of dinoflagellates is a relict structure that has retained the structure characteristic of primary eukaryotic organisms.
Symbiotic origin of mitochondria and flagella
The ability for phagotrophic nutrition predetermined the possibility of the emergence of intracellular symbionts in eukaryotes. Prokaryotes could not do this - deprived of the ability to swallow anyone, they did not acquire intracellular endosymbionts. For eukaryotes, on the contrary, the inclusion of various prokaryotic and eukaryotic organisms as intracellular symbionts is very typical. The eukaryotic cell arose as a result of the symbiosis of a primordial amoeboid organism with various prokaryotic and eukaryotic creatures. This position formed the basis of the so-called concept of symbiogenesis, which has become one of the paradigms of modern biology.
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| "Predation" in modern prokaryotic organisms. Above shows how the "predatory" bacterium Bdellovibrio invades and multiplies within E. coli. Below - the bacteriophage virus injects its DNA into the bacterium, while its protein shell remains outside. Viral DNA ensures the synthesis of new viral particles. | Thanks to the actin-myosin system, eukaryotic organisms can form pseudopodia and phagocytose bacteria and other particles (top). The virus uses this property of eukaryotic organisms and provokes endocytosis - the absorption of the viral particle into the cell itself (below). |
According to current concepts, such important organelles of eukaryotic cells as mitochondria have a symbiotic origin. They ensure the synthesis of the main energy resource of any cell - ATP due to oxidative phosphorylation, which is possible only in the presence of oxygen. Only some protozoa that live in anaerobic conditions (for example, in the intestines of animals or in oxygen-deprived swamp waters) do not have mitochondria. Undoubtedly, their lack of mitochondria is a secondary feature associated with existence in oxygen-free conditions, this is confirmed by the fact that some mitochondrial genes were found in the genome of such protozoa.
As is known, mitochondria are surrounded by two membranes, the inner one (the one that forms the cristae of mitochondria) belongs to the mitochondrion itself, and the outer one belongs to the vacuole in which the symbiont is located. The mitochondrion has its own hereditary material, organized in the same way as in prokaryotic organisms. This is a circular DNA molecule devoid of histones, carrying information about proteins that are synthesized in the mitochondrion itself on its own prokaryotic-type ribosomes with a sedimentation constant of 70S. True, mitochondria have a circular DNA molecule that is approximately a hundred times shorter than that of bacteria that exist independently. The fact is that many mitochondrial proteins are encoded in the nuclear DNA of a eukaryotic cell. Apparently, during the long-term co-evolution of the host cell and the symbiont, a significant part of the genes from the mitochondrial genome passed into the nucleus of the eukaryotic cell. In the mitochondrial genome, only the genes for those proteins that cannot overcome the barrier of two membranes (for example, hydrophilic proteins) remain. However, mitochondria are not born anew in the cell - they divide in the same way as free-living bacteria. What prokaryotes could be the ancestors of mitochondria? Among modern prokaryotes, the purple alpha-proteobacteria are closest to them (this is evidenced, in particular, by new molecular phylogeny data) - aerobic photosynthetic bacteria, the membrane of which forms deep invaginations similar to mitochondrial cristae. The progenitors of such bacteria probably lived in oxygen pockets of the anaerobic biosphere. Having entered into symbiosis with ancient amoeboid eukaryotes, proteobacteria lost the ability to photosynthesize, since they began to receive all the necessary organic substances from the host - an ancient eukaryote, who received his benefit: he ceased to be afraid of high concentrations of oxygen, which the symbionts utilized.Primary aerobic eukaryotes with symbionts initially also populated oxygen pockets, but when, 3 billion years after the formation of the biosphere, the oxygen concentration began to increase, eukaryotes were able to spread widely in the biosphere. In the layers of the earth's crust dating back to this period, so-called acritarchs appear - large spherical cells with a diameter of 50-60 microns. They could not belong to prokaryotes, whose spherical cells do not exceed a few microns in diameter (thread-like forms can reach a much greater length). In layers whose age is about 1.7 billion years, sterols were found - substances synthesized in the nucleus of eukaryotic organisms. Thus, in the period from 1 to 2 billion years ago, the adaptive evolution of eukaryotes began.
Flagella are not separated from the cytoplasm by membranes; there are no obstacles to the transfer of proteins from the cytoplasm to the flagellum, therefore most flagellar proteins are encoded in the cell nucleus. At the same time, inside the basal body of the flagellum there is a small circular DNA molecule that contains several genes that control the formation of the basal body. The fact is that centrioles (basal bodies) do not appear in a cell out of nowhere. Before division, two centrioles separate and a new one is formed next to each of them. Thus, for the synthesis of the next organelle, a “seed” in the form of the old one is necessary.
It is assumed that the ancestors of the flagellum were bacteria reminiscent of modern spirochetes, motile bacteria whose narrow spirally twisted cells move quickly, as if screwing into space. True, they themselves could not possibly be the ancestors of flagella: they do not have microtubules, and their fine structure is completely different. But this does not mean at all that in the distant past there were no other spirochete-like organisms, which became the ancestor of the eukaryotic flagellum. Apparently, its ancestors were at first exosymbionts, that is, they were attached to the cytoplasmic membrane of a primitive eukaryote from the outside. The symbiont used metabolites secreted by the host for its nutrition, and in return, thanks to its locomotor activity, contributed to its rapid (compared to the formation of pseudopodia) movement. It is precisely this interaction that has formed between spirochetes and some large protozoa. Symbiotic spirochetes sit on the surface of the flagellate Myxotricha paradoxa (which also has ordinary flagella), their movements are coordinated, like in real cilia, and locomotor activity ensures smooth and gradual movement of the flagellate, while its own flagella allow it to make only rapid forward movements in a spiral. It is curious that for greater convenience of attachment of spirochetes, the host cell kindly forms special compacted “supports”, from which bundles of fibrils, reminiscent of the roots of real flagella and cilia, extend into the host cytoplasm. This example shows that symbiosis between motile bacteria and eukaryotes can occur repeatedly.
Origin of eukaryotic plants
Primary eukaryotes were single-celled animals. They fed by capturing and digesting other microscopic organisms. One of the main directions of their evolution was the acquisition of photosynthetic symbionts, which turned into organelles that ensured the synthesis of organic substances from carbon dioxide and water using the energy of sunlight. This path led to the emergence of various groups of eukaryotic plants, that is, autotrophic photosynthetic organisms. They are not related to each other and arose as a result of the symbiosis of predatory protists (protozoa or their colonies) with various photosynthetic organisms.
In several cases, cyanobacteria - blue-green algae, the most widespread (at least in the modern biosphere) and perhaps the oldest group of photosynthetic prokaryotes - became symbionts of predatory eukaryotes. Their undoubted descendants are the photosynthetic organelles (chloroplasts) of red algae. They are surrounded by only two membranes, have their own circular DNA and ribosomes of the prokaryotic type and contain chlorophyll “a” typical of cyanobacteria and specific pigments of cyanobacteria - phycobilins. Red algae are currently widespread in the seas of our planet. They are capable of existing at depths of several hundred meters, but also live in the tidal zone, and some species also live in fresh waters. Red algae may be the oldest group of eukaryotic plants. This is evidenced by the complete absence of flagellated stages in their life cycle (even their sperm are flagellaless), which suggests that the ancestors of these algae separated from other eukaryotes even before acquiring flagella.
However, red algae are not the only group that uses descendants of cyanobacteria as symbionts. In unicellular flagellates - glaucophytes (not at all related to red algae) photosynthetic organelles are called cyanelles. They even retain the murein membrane characteristic of cyanobacteria (i.e., a mechanically strong element of the cell wall). However, cyanellas are true symbionts that cannot live separately from their host. Even their genome - circular DNA - is approximately 10 times shorter than that of free-living cyanobacteria. This means that in this case, a significant part of the cyanella proteins is encoded in the nuclear genome of the host.
Chloroplasts of green algae (Chlorella, Chlamydomonas, Volvox, etc.) are also descendants of photosynthetic prokaryotes. They are surrounded by two membranes, contain circular DNA and their own prokaryotic-type ribosomes. However, their set of chlorophylls is completely different - they are chlorophylls “a” and “b”, but there are no phycobilins. This means that the ancestors of green algae chloroplasts could not have been cyanobacteria. For a long time, free-living bacteria with chlorophylls “a” and “b” were not known. Only in the last two decades were representatives of a special group of prochlorophytes discovered - Prochloron and Prochlorotrix - with the same set of chlorophylls. Prochloron is a large, spherical bacterium that lives in the tunic of colonial ascidians, while Prochlorothrix is a filamentous freshwater form. Currently, prochlorophytes are a relict group, numbering only a few species, but in the distant past they probably played a significant role in the biosphere. It is quite possible that ancient prochlorophytes participated (along with cyanobacteria) in the construction of stromatolites. Then they entered into symbiosis with the ancestors of green algae. The significance of this union is all the greater because the descendants of green algae - higher plants - inherited chloroplasts with two membranes and chlorophylls “a” and “b”. Thus, in a green pine needle or a shiny ficus leaf, the descendants of ancient prochlorophytes, which turned into chloroplasts, are preserved.The world of eukaryotic plants is by no means limited to red and green algae. Various groups of organisms with golden-brown chloroplasts thrive in the modern biosphere. Unicellular and colonial diatoms, whose cells are protected by a silica shell, dominate the World Ocean, inhabiting fresh waters and moist soil. The coastal zone of the sea is inhabited by brown algae - fucus, kelp and sargassum (the latter can survive in the open ocean - remember the Sargasso Sea). Among the brown algae there are real giants. For example, off the Pacific coast of South America, the largest plant organism on the planet lives - macrocystis, reaching 150 m in length. Photosynthetic flagellates - golden algae and cryptomonads - are common in the plankton of marine and fresh waters.
The chloroplasts of golden algae, diatoms and brown algae contain chlorophylls “a” and “c” and for some reason are surrounded by 4 membranes. Their origin helped to understand the structure of cryptomonads - a small group of flagellates, whose chloroplasts also have chlorophylls “a” and “c”, are surrounded by 4 membranes, and between the second and third there is a small eukaryotic nucleus - a nucleomorph, and inside the space limited by the last, fourth membrane there is circular DNA. This structure suggests that the chloroplasts of cryptomonads arose as a result of double symbiosis. First, a certain predatory protist acquired a golden bacterium with chlorophylls “a” and “c” as a symbiont, and then itself became a symbiont of cryptomonas. In the chloroplasts of brown, diatoms and golden algae there is no longer a nucleomorph, although they are still surrounded by 4 membranes, which indicates a deeper integration of the symbiont and the host.
Chloroplasts were acquired by different groups of eukaryotic plants independently of each other, and the ancestors of chloroplasts were different free-living organisms: in some cases they were bacteria (green or blue-green), and in others they were eukaryotic protozoa.
Instead of a conclusion
Eukaryotic organisms - protozoa, various groups of plants, fungi and multicellular animals - dominate the modern biosphere. However, they all carry symbionts in their cells - descendants of ancient free-living bacteria. Only thanks to them are eukaryotic organisms able to live in an oxygen atmosphere and use the energy of sunlight to synthesize organic substances. So maybe, in fact, eukaryotes do not dominate the biosphere at all, but it only seems so to them? A proponent of the theory of symbiogenesis, the American biologist L. Thomas once said: “Mitochondria are usually looked at as enslaved creatures, taken captive to supply ATP to the cells, and unable to breathe on their own. Reputable biologists also look at the matter from this slave-owning point of view, "who are themselves all eukaryotes. But from the point of view of the mitochondria themselves, they are creatures that long ago found for themselves the best possible shelter where they can live with the least amount of effort and exposure to the least amount of risk."
We must not forget that in every cell of our body live tiny descendants of ancient oxyphilic bacteria, which crept into the body of our distant ancestors 2 billion years ago and continue to exist in us, preserving their own genes and their own special biochemistry. Another quote from L. Thomas: “Here they are moving in my cytoplasm, breathing for the needs of my body, but they are strangers. I wish I could get to know my mitochondria better. When I concentrate, I can imagine that I feel them; It's not that I feel them wriggling, but every now and then I sense a kind of thrill. I can't help but think that if I knew more about how they achieve such harmony, I would understand music differently ". ABOUT THE AUTHOR:
Malakhov Vladimir Vasilievich, Professor of the Department of Invertebrate Zoology of Moscow State University, author of 190 publications. Area of scientific interests: comparative anatomy and embryology of invertebrates. Full member of the Russian Academy of Natural Sciences, corresponding member of the Russian Academy of Sciences.
Which have a core. Almost all organisms are eukaryotes, except bacteria (viruses belong to a separate category, which not all biologists distinguish as a category of living beings). Eukaryotes include plants, animals, mushrooms and such living organisms as slime mold. Eukaryotes are divided into single-celled organisms And multicellular, but the principle of cell structure is the same for all of them.
It is believed that the first eukaryotes appeared about 2 billion years ago and evolved largely due to symbiogenesis- the interaction of eukaryotic cells and bacteria, which these cells absorbed, being capable of phagocytosis.
Eukaryotic cells They are very large in size, especially compared to prokaryotic ones. A eukaryotic cell has about ten organelles, most of which are separated by membranes from the cytoplasm, which is not the case in prokaryotes. Eukaryotes also have a nucleus, which we have already discussed. This is the part of the cell that is fenced off from the cytoplasm by a double membrane. It is in this part of the cell that the DNA contained in the chromosomes is located. The cells are usually mononucleated, but multinucleated cells are sometimes found.
Kingdoms of eukaryotes.
There are several options for dividing eukaryotes. Initially, all living organisms were divided only into plants and animals. Subsequently, the kingdom of mushrooms was identified, which differ significantly from both the first and the second. Even later, slime molds began to be isolated.
Slime mold is a polyphyletic group of organisms that some classify as the simplest, but the final classification of these organisms has not been fully classified. At one stage of development, these organisms have a plasmodic form - this is a slimy substance that does not have clear hard covers. In general, slime molds look like one multinucleate cell, which is visible to the naked eye.
Slime molds are related to fungi by sporulation, which germinate as zoospores, from which plasmodium subsequently develops.
Slime molds are heterotrophs capable of feeding inspectively, that is, absorb nutrients directly through the membrane, or endocytosis - take vesicles with nutrients inside. Slime molds include Acrasiaceae, Myxomycetes, Labyrinthulae and Plasmodiophorae.
Differences between prokaryotes and eukaryotes.
The main difference prokaryote and eukaryotes is that prokaryotes do not have a formed nucleus, separated by a membrane from the cytoplasm. In prokaryotes, circular DNA is found in the cytoplasm, and the place where the DNA is located is called the nucleoid.
Additional differences between eukaryotes.
- Of the organelles, prokaryotes have only ribosomes 70S (small), and eukaryotes have not only large 80S ribosomes, but also many other organelles.
- Since prokaryotes do not have a nucleus, they divide by fission in two - not with the help meiosis/mitosis.
- Eukaryotes have histones that bacteria do not. Chromantin in eukaryotes contains 1/3 DNA and 2/3 protein; in prokaryotes the opposite is true.
- A eukaryotic cell is 1000 times larger in volume and 10 times larger in diameter than a prokaryotic cell.
