- introduction -
The cell, together with the nucleus, is the fundamental unit of life and living systems grow by cellular multiplication; it was the basis of every living organism, both animal and vegetable.
The organism, based on the number of cells of which it is composed, can be monocellular (bacterium, protozoa, amoebae, etc.), or multicellular (metazoa, metaphites etc.). The cells present uniform morphological characters only in the lowest species, therefore in the simplest animals; in the others, among the different cells, differences in shape, size, relationships are established, following a process that leads to the formation of various organs with different functions: this process is called morphological and functional differentiation.
The shape of the cell is linked to the state of aggregation and its function: it is thus possible to have c. spheroidal, which are generally those that are found free in a liquid medium (white blood cells, egg cells); but the greatest part of the cells takes the most varied form following the mechanical thrusts and the pressures of the contiguous cells: thus there are pyramid, cube, prism, and polyhedron cells. The size is very variable, generally of microscopic order; in humans the smallest cells are the granules of the cerebellum (4-6 microns), the largest are the pyrenophores of some c. nerve (130 micron). We have tried to establish whether the cellular quantity depended on the somatic body of the organism, that is, if the body volume was a consequence of a greater number of cells or greater individual sizes. Following Levi's observations it was found that cells of the same type, in individuals of different sizes, have the same size, from which the important Driesch's law or constant cellular quantity derived, which states that not the quantity but primarily the number of cells conditions the different body size.
CONSTITUTIVE AND ESSENTIAL PARTS OF THE CELL
The protoplasm is the main constituent of the cell and is divided into two parts: cytoplasm and nucleus. Between these two parts (that is, between the nuclear size and the total cellular size) there is a relationship called the core-plasmatic index: it is obtained by dividing the volume of the nucleus by the volume of the cell, to which the previous one was subtracted, and it is expresses in hundredths. This index is very important because it can reveal metabolic and functional changes; for example, during growth the index tends to shift in favor of the cytoplasm. In the latter two constituents are always shown: the one called fundamental part, or hyaloplasm, and the other said chondromal, consisting of small bodies in the form of granules or filaments called mitochondria. Also in the ialoplasma there are structures detectable by the electron microscope: ergastoplasma, endoplasmic reticulum, Golgi apparatus, centriole apparatus and plasma membrane.
Click on the names of the various organelles to read the in-depth analysis
Image taken from www.progettogea.com
THE PROCARIOTS
The prokaryotes have a much simpler organization than the eukaryotes: they lack in fact organized nuclei included in a nuclear membrane; they do not have complex chromosomes, nor an endoplasmic reticulum and mitochondria. They also lack chloroplasts or plastids. Almost all prokaryotes have a rigid cell wall.
Iprocariotics lack a primitive nucleus; in fact, they do not have a nucleus that can be isolated, but the "nuclear chromatin", that is the nuclear DNA, in a single ring chromosome, immersed in the cytoplasm. The prokaryotes are the point of origin both for the animal kingdom and for the vegetable kingdom.
Prokaryotes can be divided into two basic classes: blue algae and bacteria (schizomiceti).
The current prokaryotes, represented by blue bacteria and algae, do not present particular differences from their fossil ancestors. Fossil bacterial cells differ from fossil algae cells in that the unicellular algae, like their current descendants, were photosynthetic. In other words, they were able to synthesize nutrient substances with a high energy content, starting from simple elements (in this case carbon dioxide and water) using sunlight as a source of energy.
The blue algae, having the structures and enzymes necessary for photosynthesis, are called autotrophic organisms (that is, they feed on their own). Bacteria, on the other hand, are heterotrophic organisms, since they assimilate from the external environment the nutrients necessary for their energy metabolism.
One of the most famous direct reports of bacteria with humans is that of the intestinal bacterial flora; another is that of infectious bacterial diseases.
The prokaryotes date back to about four to five billion years ago and represent the primitive forms of life ; with the passage of time we have reached the most complex organisms, up to man. Consequently prokaryotes are the simplest and oldest organisms.
During the evolution of the species, up to the higher forms, the primitive forms did not become extinct, but they also maintained a specific task in the life balance. The blue algae are an example of this, which are still today among the major synthesizers of organic material in water (eg spirulina algae).
Eukaryotes
Eukaryotes are characterized by the presence of specialized structures (organelles), absent in prokaryotes. The cells that make up the somatic tissues of plants and animals are all eukaryotic, as well as those of many single-celled organisms.
UNICELLULAR AND PLURICELLULAR ORGANISMS
The main differences between prokaryotes and eukaryotes can be summarized as follows:
a) the former do not have a very distinct nucleus, unlike eukaryotes, which, instead, have an evident and well-defined nucleus.
b) prokaryotes are always single-celled organisms and, even in the event of adhesion, the latter only affects the external envelope. Eukaryotes, on the other hand, are distinguished in unicellular and multicellular. However, their multicellularity begins with a still primitive organization, as can be seen from the so-called coenobia; these, in fact, are nothing but colonies of unicellular similar organisms, joined together. Each cell has a life of its own, which does not depend on the others, and the coenobium can survive serious accidents. In the most differentiated cenobias we then discover that sometimes the cells are joined by very thin filaments (plasmodesmata) and that some cells are thicker than the others.
Unlike single-celled organisms and primitive cenobia, in which cells are equal and have all functions, specific cells with a particular function appear in Volvox. In fact we note a flagellate part, suitable for movement, and a part composed of larger cells destined for reproduction. Ultimately, each cell tends to have its own primary structures, which are fundamental for the life of the cell itself, and secondary (for specific tasks).
A single-celled organism has a moment of pause during reproduction, in which all its structures fulfill a single task; the cells that are produced will have to restore normal specialization in order to survive. Any damage to one's structures would mean death. Multicellular organisms, on the other hand, continue to live, being able to regenerate individual cells.
Ultimately, it can be said that every cell has its own structure, which can be similar to the type structures, or it can move away from generality, lacking some cellular constituent.
Edited by: Lorenzo Boscariol
