What is the cell theory definition?
The cell theory postulates that the cell is the fundamental unit of living beings, from the simplest (microorganisms) to the most complex higher organisms (animals and plants), both in terms of their structure and their function.
The scientific theory that all living organisms are made of cells, “the smallest functional unit of life”
5 scientists who contributed to the cell theory
- Robert Hooke 1665
- Anton Van Leeuwenhoek 1676
- Matthias Schleiden 1838
- Theodor Schwann 1839
- Rudolph Virchow 1855
History of cell theory
The term cellula or cell was coined in 1665 by the English scientist Robert Hooke when he observed honeycombs like compartments in the cork and other plant tissues under the lens of a rudimentary microscope.
In 1674, Antony van Leeuwenhoek, a Dutch cloth merchant keen on polishing lenses, described that the blood was made up of tiny globules that flowed along thin capillaries and made numerous observations of various “animals” or microscopic organisms, often unicellular, which we know today as microorganisms.
The nineteenth century was, however, the true starting point for the study of the cell and its function, which was developed in parallel with the advances of microscopy and the appearance of the compound microscope. In 1831, Scottish botanist Robert Brown introduced the notion of the cell nucleus and in 1838, botanist Matthias Schleiden and zoologist Theodor Schwann enunciated the basic postulate of cell theory, according to which all living beings, plants and animals, are formed by cells, which they considered the fundamental vital unit of life. In 1839 Jan Evangelista Purkyně called the cell contents “protoplasm.”
Later studies completed the knowledge of the cell. Thus, in 1855, the pathologist Rudolf Virchow established that all cells come from other pre-existing ones ( Omnis cellula e cellula) and, already at the beginning of the 20th century, research on the nervous system structure of the Spanish histologist Santiago Ramón y Cajal, Prize Nobel Prize in Physiology and Medicine in 1906, demonstrated the individuality of neurons and showed the universality of cell theory by also applying it to nerve tissue.
Main points of cell theory
Currently, cell theory is summarized in the following points:
- All living organisms are composed of cells.
- The cell is the structural and physiological unit of living beings.
- The cells constitute the basic units of reproduction: each cell comes from the division of other pre-existing cells, being identical to these genetically, structurally and functionally.
- The cell is the most elementary independent unit of life.
Prokaryotic and Eukaryotic cell / Diversity in Cellular structure
As we have just said, the cell is the basic structural and functional unit of living beings. However, despite sharing a series of essential characteristics in terms of structure and function, not all cells have the same level of complexity, being able to distinguish, as Chatton pointed out in 1925, two different models of cell organization: prokaryotic cells and eukaryotic cells.
All cells have the following common essential components…
- They have a plasma membrane that isolates them from the surrounding environment and constitutes the main “selective barrier” for the exchange of substances with the outside.
- The cellular interior or cytoplasm contains a series of elements (inclusions and, in the case of eukaryotes, organelles) essential for the proper functioning of the cell.
- All cells have genetic information in essential macromolecules ( DNA and RNA ), as well as ribosomes involved in protein synthesis.
All cells, whether prokaryotic or eukaryotic, perform the three vital functions
This is why the cell is defined as the vital unit, that is, the smallest living being that performs vital functions.
- Nutrition is defined as the ability to capture matter and / or energy from the environment and transform it into matter and energy itself.
- The relationship is the ability to capture and respond to stimuli from the environment or other cells.
- Reproduction is the ability to duplicate their ethical material and transmit it to the daughter cells, that is, to form other cells similar to those that transmit the inheritance.
Despite these structures and functions common to all cells, we have said that there are degrees of complexity, being able to establish two levels of organization: prokaryotic and eukaryotic.
Structure of prokaryotic cell
Prokaryotic cells are structurally simpler than eukaryotic cells and are located at the evolutionary basis of living beings. The prokaryotic structure is characteristic and exclusive of bacteria (monetary kingdom).
Most prokaryotic cells are small in size, from less than one micron to a few microns, equal to the size of some organelles of eukaryotic cells.
Basically, a prokaryotic cell has the following structure:
- A plasma membrane that delimits the cellular cytoplasm. Surrounding the membrane there is a rigid cell wall responsible for the shape of the cell. The composition and structure of the wall varies between the main bacterial groups, although it is present in all of them, except in mycoplasmas, the only prokaryotic cells devoid of cell wall
- The cytoplasm, granular in appearance, with 70 S ribosomes and various inclusions surrounded or not with the membrane (mainly with carbon, nitrogen, phosphorus, etc.)
- The nucleoid zone, located in the center of the cell and not separated from the rest of the cytoplasm by any membrane (therefore it is not considered a true nucleus), which contains the densely packed genetic material in the form of DNA
The nucleoid, looking; fibrillar, houses the main chromosome, consisting of a double-stranded circular DNA molecule, and plasmids, also composed of a double circular DNA helix, which carries additional information, such as antibiotic resistance, the mechanism of degradation of hardly biodegradable substances or the ability to bind to other bacteria through conjugative hairs.
- Some bacteria also contain other elements, whose presence or not varies from one group to another:
- Hairs and fimbriae: rigid appendages that participate in the exchange of genetic information (conjugation) or in the adhesion to the host.
- capsules and mucous layers: mucosal sheaths external to the cell wall.
Internal membrane systems: Although rare among bacteria, some, like many autotrophic bacteria, have internal membrane systems, connected or not with the cell membrane, and generally associated with certain metabolic processes.
PHYSIOLOGY OF THE PROKARYOTIC CELL: NUTRITION AND BACTERIAL REPRODUCTION
Nutrition. Bacteria are a very large group of individuals of different species. They have a great diversity of types of nutrition, there are autotrophs: photosynthetic or chemosynthetic or heterotrophs: with catabolism type cellular or fermentative respiration. They can be aerobic or anaerobic (strict or optional). The majority are heterotrophs, being able to be: saprophytes, diners, symbionts or parasites. Some can directly fix atmospheric nitrogen, although they usually incorporate it as salts. Cyanobacteria are all photosynthetic autotrophs and parasitic heterotrophic mycoplasmas.
Reproduction. Its normal form of reproduction is simple division by binary fission. Their reproductive capacity is enormous, in favorable conditions they can double their number every half hour. The bacterial chromosome, attached to the mesosome, duplicates, separating the two-child chromosomes as the membrane grows between the anchor points of these. Subsequently, the plasma membrane is invaded and a separation wall is produced, which results in two daughter cells, each with an exact replica of the stem cell chromosome.
With this type of asexual reproduction, the daughter cells are identical and the only form of genetic variability in the offspring would be by mutation of their DNA. It has been proven that bacteria can receive or transmit genetic information to other bacteria, within the same generation. This way of transmitting genetic information is called: parasexual mechanisms. These can be of several types:
- -Transformation (free DNA fragments in the cytoplasm, plasmids, pass through the membrane of a donor bacterium to the recipient).
- Conjugation (a donor bacteria transmits a replica of its own chromosome to another recipient bacteria).
- Transduction (in which a virus acts as a vehicle for the DNA molecule that is transferred between bacteria).
The bacteria become resistant to heat, cold, desiccation and chemical substances when entering dormancy forming cysts (surrounded by a thick membrane) or forming spores (a thick membrane is formed inside the cell surrounding the nucleus and with a small portion of cytoplasm). At the end of the unfavorable conditions, the bacteria break the covers and germinates.
THE ENDOSYMBITIC THEORY
Carl Woese (1980) called progenote or protobiont the common ancestor of all organisms and, therefore, would represent the most primitive living unit, already endowed with mechanisms of transcription and genetic translation. From this common trunk, three prokaryotic cells would arise in the evolution: archaea, Eukaryotes, and bacteria (also called eubacteria).
The next step in cellular evolution was the appearance of eukaryotes about 1500 million years ago. Lynn Margulis, in his endosymbiotic theory, proposes that they originated from a primitive Eukaryotic cell (host cell), which at one time would encompass prokaryotic cells or organisms, establishing an endosymbiont relationship between them.
These prokaryotic cells would be the precursors of peroxisomes (due to their ability to eliminate toxic substances), mitochondria (which would come from aerobic bacteria) and chloroplasts (which would be old photosynthetic bacteria). In fact, mitochondria and chloroplasts are similar to bacteria in size and, like them, reproduce by division. But the most important thing is that both mitochondria and chloroplasts have their own DNA, which encodes the synthesis of some of its components. In addition, both organelles have their own ribosomes with ribosomal RNA closer to those of bacteria than to those of eukaryotic cells. According to this theory, part of the mitochondrial DNA and chloroplast genes would become incorporated into the genes of the host cell’s DNA.
The intracellular incorporation of these prokaryotic organisms into the primitive eukaryotic cell provided two fundamental characteristics that were initially lacking:
– The capacity of oxidative metabolism, with which the anaerobic cell could become an anaerobic cell.
– The ability to perform photosynthesis and therefore be an autotrophic organism capable of using as a carbon source the CO 2 to produce organic molecules.
Likewise, the primitive cell provided prokaryotic symbionts with a safe environment and food for their survival.
It would be, therefore, a highly advantageous endosymbiosis for the organisms involved, since all of them would have acquired metabolic peculiarities that they did not possess separately and, consequently, would be selected in the course of evolution.
EUCARIOTE CELL STRUCTURE.
Except for the bacteria, the rest of the living beings (protoctist kingdoms, fungi, plants, and animals), from the unicellular protoctists (protists) to the complex multicellular organisms with differentiated tissues, present a eukaryotic cell organization.
The structure of a typical eukaryotic cell consists of the following elements:
– The plasma membrane, which constitutes the outer limit of the cell and whose primary function is to regulate the transport and exchange of substances with the external environment.
– Sometimes, surrounding the plasma membrane, there is a rigid cell wall, main cellulose in plant cells and chitin in the case of some fungi.
– The cellular cytoplasm contains the cellular organelles and is occupied by a network of protein filaments that make up the cellular skeleton or cytoskeleton, also involved in the formation of cilia and flagella, intracellular movements and cell division.
– Ribosomes have a sedimentation coefficient of 80 S, higher than in prokaryotic cells, and their function, as in these, is protein synthesis.
– Mitochondria and chloroplasts, organelles related to obtaining energy through breathing processes and photosynthesis, respectively. Both organelles are surrounded by a double membrane, although chloroplasts are exclusive to plant cells.
Eukaryotic cells have a complex internal membrane system consisting of the endoplasmic reticulum, connected to the nuclear membrane, and the Golgi complex, organelles related to the biosynthesis of molecules and their distribution within the cell, as well as with the secretion of substances abroad. Other membranous organelles are vacuoles, which reach a great development in plant cells, and lysosomes, related to the Golgi complex, which contains enzymes essential for the degradation of substances inside digestive vacuoles.
Finally, all eukaryotic cells have a nucleus bounded by a double membrane. Inside it is chromatin, consisting of histone-associated DNA and whose structural unit is the nucleosome. The double nuclear membrane has pores that communicate the nucleoplasm and cytoplasm.
The cells have a great variability of forms and, even, some do not have a fixed form. Defined-shaped cells can be rounded, elliptical, fusiform, starry, prismatic, flattened, etc., that is, there is no prototype cell shape. The fact that they are normally represented as a circle, or an ellipse, with a point that represents the nucleus, is a mere simplification of reality.
Many free cells, such as amoebas and leukocytes, which lack a rigid secretion membrane and have an easily deformable plasma membrane, are constantly changing shape by emitting cytoplasmic extensions (pseudopods), to move and phagocyte particles Other similar free cells, but without the ability to emit pseudopods, such as many ciliates, erythrocytes and lymphocytes, have a globose shape. This is due to the cohesion between water molecules. The same cause that explains that the liquid drops are spherical and that, if the cohesion is very high, as in mercury, keep this form even on a solid.
Cells that are attached to others forming tissues, if they lack a rigid cell wall, have a shape that depends, in large part, on the tensions that the joints with adjacent cells generate. For example, the animal epithelial tissue, which serves to cover both the external surface and the internal ducts and cavities, it can be seen that the deep cells have a prismatic shape, while the superficial ones, which do not experience tensions by superior ones, are flattened. In addition, if cells are separated from tissue, by breaking the connections that bind them, and placed in a culture medium, the cells tend to take on the spherical shape.
In all cells lacking a rigid membrane, its shape is also greatly influenced by osmosis phenomena.
Cells with a rigid secretion wall, such as bacteria that have a murein wall, most plant cells that have a cellulose cell wall and bone tissue osteocytes, logically have a very stable shape. Although they are also subject to osmotic phenomena, their form does not vary.
Finally, it should be noted that the shape of the cells is closely related to the function they play. Thus, muscle cells are usually elongated and fusiform, adapted, therefore, to be able to contract and relax; nerve tissue cells are irregular and have numerous extensions, which is related to the ability to capture stimuli and transmit them; the cells of the intestinal epithelium present the free plasma membrane with innumerable folds to increase its absorption surface; etc.
In summary, the shapes of the cells are basically determined by their function and can vary more or less in relation to the absence of a rigid cell wall, tensions of adjoining cell junctions, cytosol viscosity, osmotic phenomena and type of internal cytoskeleton.
The size of the cells is extremely variable. Thus, bacteria usually measure between 1 and 2 µ in length and most human cells between 5 and 20 µ; for example, erythrocytes measure about 7 µ in diameter, liver cells or hepatocytes 20 µ in diameter, etc. Cells above these values are also frequent, particularly those that have special functions that require a high size, such as sperm (for example, human sperm are 53 µ in length), oocytes (for example, the human oocyte measures about 150 microns).
The pollen grains of some plants that reach sizes of 200 to 300 microns, some species of paramecium that can reach more than 500 microns (so they are already visible to the naked eye). The oocytes of the birds (for example, quail egg yolk, which is a single cell whose nucleus is a small white dot on its surface, measures 1 cm., that of the chicken 2.5 cm. and that of the ostrich 7 cm. in diameter). Finally, the cells of greater length are the neurons that, although their body only measures several tens of microns, their axonal extensions can reach, in the great cetaceans, several meters in length.
Cell Theory timeline from 1665 to 1862
- First of all, the ancient Greeks studied the natural world.
- Aristotle gave the idea that plants and animals are related.
- In 1665 Robert Hooke made his microscope and discovered cell. He studied honeycomb-like compartments in cork and called them “cellulae”.
- A few years later Antonie van Leeuwenhoek observed first living cells as tiny organisms in pond water called them “animalcules”.
- In 1809 Jean Baptist de- Lamarck said that “nobody can have a life if it is not composed by cells or cellular tissues”.
- In 1831, British botanist Robert Brown discovered the nucleus in the cell.
- In 1838, German botanist Mathias Schleiden said that all plants are composed of cells.
- In 1839, German zoologist Theodor Schwann said that all Animals are composed of cells.
- In 1855 Rudolf Virchow said that cells all arise from pre-existing cells (“Omnis cellula e cellula”). This idea was confirmed by Louis Pasteur in 1862.