Basic provisions cell theory, its meaning
All living organisms are made up of cells - either one cell (unicellular organisms) or many (multicellular organisms). The cell is one of the main structural, functional and reproductive elements of living matter; it is an elementary living system. There are non-cellular organisms (viruses), but they can only reproduce in cells. There are organisms that have secondarily lost cellular structure(some algae). The history of the study of cells is associated with the names of a number of scientists. R. Hooke was the first to use a microscope to study tissues and on a section of the cork and core of an elderberry he saw cells, which he called cells. Antoni van Leeuwenhoek first saw cells under 270x magnification. M. Schleiden and T. Schwann were the creators of the cell theory. They mistakenly believed that the cells in the body arise from a primary non-cellular substance. Later, R. Virchow formulated one of the most important provisions of cell theory: “Every cell comes from another cell...” The importance of cell theory in the development of science is great. It became obvious that the cell is the most important component of all living organisms. It is their main component morphologically; the cell is the embryonic basis of a multicellular organism, because the development of an organism begins with one cell - a zygote; The cell is the basis of physiological and biochemical processes in the body. Cell theory made it possible to come to the conclusion that the chemical composition of all cells is similar and once again confirmed the unity of everything organic world.
Modern cell theory includes the following provisions:
The cell is the basic unit of structure and development of all living organisms, the smallest unit of a living thing;
The cells of all unicellular and multicellular organisms are similar (homologous) in structure, chemical composition, basic manifestations of vital activity and metabolism;
Cells reproduce by dividing them, and each new cell formed as a result of division of the original (mother) cell;
In difficult multicellular organisms cells are specialized in the function they perform and form tissues; tissues consist of organs that are closely interconnected and subordinate to nervous and humoral regulatory systems.
Importance of Cell Theory in the development of science is that thanks to it it became clear that the cell is the most important component of all living organisms. It is their main “building” component; the cell is the embryonic basis of a multicellular organism, because The development of an organism begins with one cell - a zygote. The cell is the basis of physiological and biochemical processes in the body, because Ultimately, all physiological and biochemical processes occur at the cellular level. The cellular theory made it possible to come to the conclusion that the chemical composition of all cells is similar and once again confirmed the unity of the entire organic world. All living organisms are made up of cells—one cell (protozoa) or many (multicellular organisms). The cell is one of the main structural, functional and reproductive elements of living matter; it is an elementary living system. There are evolutionarily non-cellular organisms (viruses), but they can only reproduce in cells. Different cells differ from each other in structure, in size (cell sizes range from 1 micron to several centimeters - these are the eggs of fish and birds), and in shape (they can be round like red blood cells, tree-shaped like neurons), and in biochemical characteristics ( for example, in cells containing chlorophall or bacteriochlorophyll, photosynthesis processes take place, which are impossible in the absence of these pigments), and by function (sex cells are distinguished - gametes and somatic cells - body cells, which in turn are divided into many different types).
8. Hypotheses of the origin of eukaryotic cells: symbiotic, invaginative, cloning. Most popular currently symbiotic hypothesis origin eukaryotic cells, according to which the basis, or host cell, in the evolution of a cell of the eukaryotic type was an anaerobic prokaryote, capable only of amoeboid movement. The transition to aerobic respiration is associated with the presence of mitochondria in the cell, which occurred through changes in symbionts - aerobic bacteria that penetrated the host cell and coexisted with it.
A similar origin is suggested for flagella, the ancestors of which were symbiont bacteria that had a flagellum and resembled modern spirochetes. The acquisition of flagella by a cell, along with the development of an active mode of movement, had an important general consequence. It is assumed that the basal bodies with which the flagella are equipped could evolve into centrioles during the emergence of the mitotic mechanism.
The ability of green plants to photosynthesize is due to the presence of chloroplasts in their cells. Proponents of the symbiotic hypothesis believe that the symbionts of the host cell, which gave rise to chloroplasts, were prokaryotic blue-green algae.
A serious argument in favor symbiotic The origin of mitochondria, centrioles and chloroplasts is that these organelles have their own DNA. At the same time, the proteins bacillin and tubulin, which make up the flagella and cilia of modern prokaryotes and eukaryotes, respectively, have different structures.
Central and difficult to answer is the question of the origin of the nucleus. It is believed that it could also be formed from a prokaryotic symbiont. The increase in the amount of nuclear DNA, many times higher than in a modern eukaryotic cell, its amount in mitochondria or chloroplasts, apparently occurred gradually by moving groups of genes from the genomes of symbionts. It cannot be excluded, however, that the nuclear genome was formed by expanding the genome of the host cell (without the participation of symbionts).
According to intussusception hypothesis, the ancestral form of the eukaryotic cell was an aerobic prokaryote. Inside such a host cell there were simultaneously several genomes, initially attached to the cell membrane. Organelles with DNA, as well as a nucleus, arose by invagination and unlacing of sections of the shell, followed by functional specialization into the nucleus, mitochondria, and chloroplasts. In the process of further evolution, the nuclear genome became more complex and a system of cytoplasmic membranes appeared.
Intussusception hypothesis explains well the presence of a nucleus, mitochondria, chloroplasts, and two membranes in the shells. However, it cannot answer the question why protein biosynthesis in chloroplasts and mitochondria corresponds in detail to that in modern prokaryotic cells, but differs from protein biosynthesis in the cytoplasm of a eukaryotic cell.
Cloning. In biology, a method of producing several identical organisms through asexual (including vegetative) reproduction. This is exactly how many species of plants and some animals reproduce in nature over millions of years. However, now the term "cloning" is usually used in a narrower sense and means copying cells, genes, antibodies and even multicellular organisms in the laboratory. Specimens that appear as a result of asexual reproduction are, by definition, genetically identical, however, hereditary variability can be observed in them, caused by random mutations or created artificially by laboratory methods. The term “clone” as such comes from the Greek word “klon”, which means twig, shoot, cutting, and relates primarily to vegetative propagation. Cloning plants from cuttings, buds or tubers into agriculture has been known for thousands of years. During vegetative propagation and cloning, genes are not distributed among descendants, as in the case of sexual reproduction, but are preserved in their entirety. Only in animals everything happens differently. As animal cells grow, their specialization occurs, that is, the cells lose the ability to implement all the genetic information embedded in the nucleus of many generations.
The German zoologist T. Schwann (1810-1882) in 1839 published the work “Microscopic studies on the correspondence in the structure and growth of animals and plants.” This classic work laid the foundations of cell theory. Based on the works of M. Schleiden, T. Schwann found the correct principle for comparing the cells of plant and animal organisms. He found that animal cells are too diverse and differ significantly from plant cells, but the nuclei in all cells are similar. If a certain formation, which can be seen under a microscope, has a nucleus, then this formation, according to Schwann, can be considered a cell. Based on this assumption, T. Schwann put forward the main provisions of the cell theory: 1) the cell is the main structural unit of all organisms (plants and animals); 2) the process of cell formation determines the growth, development and differentiation of plant and animal tissues.
Rice. 1.15. Evolution of living things.
Development of cell theory by R. Virkhovim (Fig. 1.16). In 1858, the main work of the German pathologist R. Virchow, “Celular Pathology,” was published. This work, which became a classic, influenced further development The doctrine of the cell was of great and progressive significance for its time. Before G. Virchow, the basis of all pathological processes was seen in changes in the composition of the fluid and the struggle of the intangible forces of the body. G. Virho approached the explanation of pathological processes in connection with morphological structures, with certain changes in the structure of cells. This research generated new science- pathology, which is the basis of theoretical and clinical medicine. G. Virho introduced into science a number of new ideas about the role of cellular structures in the body.
Rice. 1.16. G. L. K. Virho(Rudolf Ludwig Karl Virchow) (1821 -1902).
G. Virchow’s position “every cell is from a cell” was brilliantly confirmed by the further development of biology and is the third position of the cell theory. At present, there are no other ways of creating new cells other than dividing existing ones. However, this thesis does not deny that at the dawn of life, cells developed from precellular structures.
G. Virkhov’s position on volume, that there is no life outside cells has also not lost its meaning. For example, in a rich-cell organism there are non-cellular structures, but they are derived cells. Primitive forms - viruses - become capable of active life processes and reproduction only after penetration into the cell.
An important generalization was the statement that highest value In the life of cells, it is not the membrane, but its contents: the cytoplasm and the nucleus.
Current state of cell theory. Since the creation of the cell theory, the doctrine of the cell as an elementary microscopic structure of organisms has acquired new meaning. For T. Schwann and his contemporaries, the cell remained a space limited by a shell. Gradually, this imagination gave way to the understanding that the main part of the cell is the cytoplasm. By the end of the last century, thanks to the successes of microscopic technology, the complex structure of the cell was discovered, the described organelles are the parts of the cell that perform different functions, and the pathways for the formation of new cells (mitosis) were studied. Already at the beginning of the 20th century. the paramount importance became clear cellular structures in the transmission of hereditary properties. Currently, it can be considered generally accepted that the cell is the basic structural and functional unit of organization of living things.
On modern stage Development of cytology cell theory includes the following provisions:
The cell is the elementary unit of structure and development of all living organisms;
The cells of all unicellular and multicellular organisms are similar in origin (homologous), structure, chemical composition, and basic manifestations of life activity;
Each new cell is formed solely as a result of the reproduction of the mother cell through division;
In multicellular organisms that develop from a single cell - zygotes, spores, etc. - various types of cells are formed due to their specialization during the individual development of the individual and form tissues;
Tissues consist of organs that are closely interconnected and subordinate to the neurohumoral and immune regulatory systems.
The importance of cell theory for medicine The cell is the unit of pathology. The cell must be known not only as a unit of structure of the organism, but also as a unit of pathological changes. Almost all diseases are associated with disruption of the structure and function of cells from which all tissues and organs are formed. Violation of the structure and function of some cells is the root cause of the onset and development of the disease, while disruption of others may already be a consequence of unfavorable changes in the body. For example, during myocardial infarction, the functioning is disrupted, and then death of cardiomyocytes occurs due to an acute lack of oxygen. Due to the fact that part of the heart muscle does not take part in contraction, blood supply to the body is disrupted, which leads to hypoxia and changes in the function and structure of cells, primarily neurons of the brain.
Disruption of the normal functioning of cells (pathology) is associated with many different factors (physical, chemical, biological) and is characterized by general or local disturbances in the organization of cell organelles and changes in individual metabolic processes. Unfavorable factors for the cell can be wave radiation, ionizing radiation, low and high temperatures, various chemical compounds, viral, bacterial and fungal infections, deficiency nutrients or individual physiologically active compounds (essential amino acids and fatty acids, vitamins and microelements), lack of oxygen, etc. Internal factors, such as mutations, also have a negative impact hereditary material, leading to congenital defects in the synthesis of proteins, lipids, excess production of hormones, and impaired utilization of toxic metabolites.
Among the pathological changes in the cell, one can note a violation of the structure and permeability of the membranes of mitochondria, lysosomes and other intracellular formations. Due to adverse effects, mitochondria swell and take on the appearance of vesicles, limited only by the outer membrane. Degeneration and edema are accompanied by disruption of redox reactions in mitochondria and insufficient formation of high-energy compounds, which negatively affects the homeostasis of the entire cell. Similar phenomena occur with diabetes and starvation in liver cells, with heart and kidney diseases. Pathological changes in the membrane of the endoplasmic reticulum lead to disruption of the synthesis of cell proteins. These disorders also occur when there is a lack of essential amino acids in the diet.
An increase in the permeability of lysosome membranes, which is observed, for example, with vitamin E deficiency, or under the influence of ionizing radiation, can increase the release of hydrolytic enzymes into the cytoplasm from lysosomes, leading to partial or even complete destruction of cells. With many intoxications, liver or kidney cells are damaged, and disruption of the functioning of these organs causes changes in other cells of the body through metabolic products.
A widespread cause of cell pathology is the penetration and reproduction of viruses in it. At the same time, metabolic processes in the cell are disrupted - the virus forces the cell to work exclusively “for itself.” After mass formation and release of viral particles from the cell, it dies. Some pathogenic viruses do not kill the cell, but cause its degeneration.
If the factor has not completely damaged the cell, then after its action ceases, the cell can restore its structure and functions. This process is called intracellular repair.
Basic principles of cell theory, its significance
All living organisms are made up of cells - either one cell (unicellular organisms) or many (multicellular organisms). The cell is one of the main structural, functional and reproductive elements of living matter; it is an elementary living system. There are non-cellular organisms (viruses), but they can only reproduce in cells. There are organisms that have lost their cellular structure for the second time (some algae). The history of the study of cells is associated with the names of a number of scientists. R. Hooke was the first to use a microscope to study tissues and on a section of the cork and core of an elderberry he saw cells, which he called cells. Antoni van Leeuwenhoek first saw cells under 270x magnification. M. Schleiden and T. Schwann were the creators of the cell theory. They mistakenly believed that the cells in the body arise from a primary non-cellular substance. Later, R. Virchow formulated one of the most important provisions of cell theory: “Every cell comes from another cell...” The importance of cell theory in the development of science is great. It became obvious that the cell is the most important component of all living organisms. It is their main component morphologically; the cell is the embryonic basis of a multicellular organism, because the development of an organism begins with one cell - a zygote; The cell is the basis of physiological and biochemical processes in the body. The cellular theory made it possible to come to the conclusion that the chemical composition of all cells is similar and once again confirmed the unity of the entire organic world.
Modern cell theory includes the following provisions:
The cell is the basic unit of structure and development of all living organisms, the smallest unit of a living thing;
The cells of all unicellular and multicellular organisms are similar (homologous) in their structure, chemical composition, basic manifestations of life activity and metabolism;
Cell reproduction occurs through cell division, and each new cell is formed as a result of the division of the original (mother) cell;
In complex multicellular organisms, cells are specialized in the function they perform and form tissues; tissues consist of organs that are closely interconnected and subordinate to nervous and humoral regulatory systems.
Importance of Cell Theory in the development of science is that thanks to it it became clear that the cell is the most important component of all living organisms. It is their main “building” component; the cell is the embryonic basis of a multicellular organism, because The development of an organism begins with one cell - a zygote. The cell is the basis of physiological and biochemical processes in the body, because Ultimately, all physiological and biochemical processes occur at the cellular level. The cellular theory made it possible to come to the conclusion that the chemical composition of all cells is similar and once again confirmed the unity of the entire organic world. All living organisms are made up of cells—one cell (protozoa) or many (multicellular organisms). The cell is one of the main structural, functional and reproductive elements of living matter; it is an elementary living system. There are evolutionarily non-cellular organisms (viruses), but they can only reproduce in cells. Different cells differ from each other in structure, in size (cell sizes range from 1 micron to several centimeters - these are the eggs of fish and birds), and in shape (they can be round like red blood cells, tree-shaped like neurons), and in biochemical characteristics ( for example, in cells containing chlorophall or bacteriochlorophyll, photosynthesis processes take place, which are impossible in the absence of these pigments), and by function (sex cells are distinguished - gametes and somatic cells - body cells, which in turn are divided into many different types).
8. Hypotheses of the origin of eukaryotic cells: symbiotic, invaginative, cloning. Most popular currently symbiotic hypothesis the origin of eukaryotic cells, according to which the basis, or host cell, in the evolution of a eukaryotic cell type was an anaerobic prokaryote, capable only of amoeboid movement. The transition to aerobic respiration is associated with the presence of mitochondria in the cell, which occurred through changes in symbionts - aerobic bacteria that penetrated the host cell and coexisted with it.
A similar origin is suggested for flagella, the ancestors of which were symbiont bacteria that had a flagellum and resembled modern spirochetes. The acquisition of flagella by a cell, along with the development of an active mode of movement, had an important general consequence. It is assumed that the basal bodies with which the flagella are equipped could evolve into centrioles during the emergence of the mitotic mechanism.
The ability of green plants to photosynthesize is due to the presence of chloroplasts in their cells. Proponents of the symbiotic hypothesis believe that the symbionts of the host cell, which gave rise to chloroplasts, were prokaryotic blue-green algae.
A serious argument in favor symbiotic The origin of mitochondria, centrioles and chloroplasts is that these organelles have their own DNA. At the same time, the proteins bacillin and tubulin, which make up the flagella and cilia of modern prokaryotes and eukaryotes, respectively, have different structures.
Central and difficult to answer is the question of the origin of the nucleus. It is believed that it could also be formed from a prokaryotic symbiont. The increase in the amount of nuclear DNA, many times higher than in a modern eukaryotic cell, its amount in mitochondria or chloroplasts, apparently occurred gradually by moving groups of genes from the genomes of symbionts. It cannot be excluded, however, that the nuclear genome was formed by expanding the genome of the host cell (without the participation of symbionts).
According to intussusception hypothesis, the ancestral form of the eukaryotic cell was an aerobic prokaryote. Inside such a host cell there were simultaneously several genomes, initially attached to the cell membrane. Organelles with DNA, as well as a nucleus, arose by invagination and unlacing of sections of the shell, followed by functional specialization into the nucleus, mitochondria, and chloroplasts. In the process of further evolution, the nuclear genome became more complex and a system of cytoplasmic membranes appeared.
Intussusception hypothesis explains well the presence of a nucleus, mitochondria, chloroplasts, and two membranes in the shells. However, it cannot answer the question why protein biosynthesis in chloroplasts and mitochondria corresponds in detail to that in modern prokaryotic cells, but differs from protein biosynthesis in the cytoplasm of a eukaryotic cell.
Cloning. In biology, a method of producing several identical organisms through asexual (including vegetative) reproduction. This is exactly how many species of plants and some animals reproduce in nature over millions of years. However, now the term "cloning" is usually used in a narrower sense and means copying cells, genes, antibodies and even multicellular organisms in the laboratory. Specimens that appear as a result of asexual reproduction are, by definition, genetically identical, however, hereditary variability can be observed in them, caused by random mutations or created artificially by laboratory methods. The term “clone” as such comes from the Greek word “klon”, which means twig, shoot, cutting, and relates primarily to vegetative propagation. Cloning plants from cuttings, buds or tubers in agriculture has been known for thousands of years. During vegetative propagation and cloning, genes are not distributed among descendants, as in the case of sexual reproduction, but are preserved in their entirety. Only in animals everything happens differently. As animal cells grow, their specialization occurs, that is, the cells lose the ability to implement all the genetic information embedded in the nucleus of many generations.
Cell as a biological system Cell theory, its main provisions
Basic terms and concepts tested in exam paper: unity of the organic world, cell, cell theory, provisions of the cell theory.
We have already said that a scientific theory is a generalization of scientific data about the object of research. This fully applies to the cell theory created by two German researchers M. Schleiden and T. Schwann in 1839.
The basis of the cellular theory was the work of many researchers who were looking for the elementary structural unit of living things. The creation and development of cell theory was facilitated by the emergence in the 16th century. and further development of microscopy.
Here are the main events that became the precursors to the creation of the cell theory:
– 1590 – creation of the first microscope (Jansen brothers);
– 1665 Robert Hooke – the first description of the microscopic structure of the elderberry branch plug (in fact, these were cell walls, but Hooke introduced the name “cell”);
– 1695 Publication by Anthony Leeuwenhoek about microbes and other microscopic organisms, which he saw through a microscope;
– 1833 R. Brown described the nucleus of a plant cell;
– 1839 M. Schleiden and T. Schwann discovered the nucleolus.
Basic provisions of modern cell theory:
1. All simple and complex organisms consist of cells capable of exchange with environment substances, energy, biological information.
2. A cell is an elementary structural, functional and genetic unit of a living thing.
3. A cell is an elementary unit of reproduction and development of living things.
4. In multicellular organisms, cells are differentiated by structure and function. They are organized into tissues, organs and organ systems.
5. The cell is an elementary, open living system capable of self-regulation, self-renewal and reproduction.
The cell theory developed due to new discoveries. In 1880, Walter Flemming described chromosomes and the processes occurring in mitosis. Since 1903, genetics began to develop. Since 1930, electron microscopy began to develop rapidly, which allowed scientists to study the finest structure of cellular structures. The 20th century was the century of the flourishing of biology and such sciences as cytology, genetics, embryology, biochemistry, and biophysics. Without the creation of the cell theory, this development would have been impossible.
So, the cell theory states that all living organisms are made up of cells. A cell is the minimal structure of a living thing that has all vital properties– the ability to metabolism, growth, development, transmission of genetic information, self-regulation and self-renewal. The cells of all organisms have similar structural features. However, cells differ from each other in their size, shape and function. An ostrich egg and a frog egg consist of the same cell. Muscle cells have contractility, and nerve cells conduct nerve impulses. Differences in the structure of cells largely depend on the functions they perform in organisms. The more complex an organism is, the more diverse its cells are in their structure and functions. Each type of cell has a specific size and shape. The similarity in the structure of cells of different organisms and the commonality of their basic properties confirm the commonality of their origin and allow us to draw a conclusion about the unity of the organic world.
A cell is a unit of structure, vital activity, growth and development of organisms. Diversity of cells. Comparative characteristics of cells of plants, animals, bacteria, fungi
Basic terms and concepts tested in the exam paper: bacterial cells, fungal cells, plant cells, animal cells, prokaryotic cells, eukaryotic cells.
The science that studies the structure and function of cells is called cytology. We have already said that cells can differ from each other in shape, structure and function, although the basic structural elements of most cells are similar. Biologists distinguish two large systematic groups of cells - prokaryotic and eukaryotic. Prokaryotic cells do not contain a true nucleus and a number of organelles. (See the section “Cell Structure.”) Eukaryotic cells contain a nucleus that houses the organism’s genetic apparatus. Prokaryotic cells are cells of bacteria and blue-green algae. The cells of all other organisms are eukaryotic.
Any organism develops from a cell. This applies to organisms that were born as a result of both asexual and sexual methods of reproduction. That is why the cell is considered the unit of growth and development of the organism.
Modern taxonomy distinguishes the following kingdoms of organisms: Bacteria, Fungi, Plants, Animals. The basis for this division is the feeding methods of these organisms and the structure of cells.
Bacterial cells have the following characteristic structures: a dense cell wall, one circular DNA molecule (nucleotide), and ribosomes. These cells lack many organelles characteristic of eukaryotic plant, animal and fungal cells. Based on their feeding method, bacteria are divided into autotrophs, chemotrophs and heterotrophs.
Plant cells contain plastids characteristic only of them - chloroplasts, leucoplasts and chromoplasts; they are surrounded by a dense cell wall of cellulose, and also have vacuoles with cell sap. All green plants are autotrophic organisms.
Animal cells do not have dense cell walls. They are surrounded cell membrane, through which the exchange of substances with the environment occurs.
Fungal cells are covered with a cell wall that differs in chemical composition from the cell walls of plants. It contains chitin, polysaccharides, proteins and fats as its main components. The reserve substance of fungal and animal cells is glycogen.
EXAMPLES OF TASKS
Part A
A1. Which of the following is consistent with the cell theory?
1) the cell is an elementary unit of heredity
2) the cell is a unit of reproduction
3) the cells of all organisms are different in their structure
4) the cells of all organisms have different chemical compositions
A2. Precellular life forms include:
1) yeast 3) bacteria
2) penicillium 4) viruses
A3. plant cell differs from a fungal cell in structure:
1) nucleus 3) cell wall
2) mitochondria 4) ribosomes
A4. One cell consists of:
1) influenza virus and amoeba
2) mucor mushroom and cuckoo flax
3) planaria and volvox
4) green euglena and slipper ciliates
A5. Prokaryotic cells have:
1) nucleus 3) Golgi apparatus
2) mitochondria 4) ribosomes
A6. The species of the cell is indicated by:
1) core shape
2) number of chromosomes
3) membrane structure
4) primary structure squirrel
A7. The role of cell theory in science is
1) opening of the cell nucleus
2) opening the cell
3) generalization of knowledge about the structure of organisms
4) discovery of metabolic mechanisms
Part B
IN 1. Select features characteristic only of plant cells
1) there are mitochondria and ribosomes
2) cell wall from cellulose
3) there are chloroplasts
4) storage substance - glycogen
5) reserve substance – starch
6) the nucleus is surrounded by a double membrane
AT 2. Select the characteristics that distinguish the kingdom of Bacteria from the rest of the kingdoms of the organic world.
1) heterotrophic mode of nutrition
2) autotrophic method of nutrition
3) the presence of a nucleoid
4) absence of mitochondria
5) absence of a core
6) presence of ribosomes
VZ. Find a correspondence between the structural features of the cell and the kingdoms to which these cells belong
Part C
C1. Give examples of eukaryotic cells that do not have a nucleus.
C2. Prove that cell theory generalized a number of biological discoveries and predicted new discoveries.
Answers Part A. A1 – 2. A2 – 4. A3 – 3. A4 – 4. A5 – 4. A6 – 2. A7 – 3.
Part B. B1 – 2, 3.5. B2 – 3, 4, 5. VZ. A – 1; B – 2; IN 1; G – 2; D – 1; E – 2.
Part C. C1 Answer elements: mature human erythrocytes, plant sieve tubes.
C2 Cell theory summarized a number of philosophical and microscopic studies indicating the existence of an elementary unit of life. (Discovery of the cell by Hooke, discovery of single-celled animals by Leeuwenhoek, discovery of the cell nucleus by Brown, etc.)
Subsequent discoveries in the field of cytology, embryology, and genetics confirmed the correctness of the cell theory. More subtle structures were discovered and their role in the life of the body was revealed.