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To be considered a living being, it must have certain characteristics:
- Be made up of cells;
- Seek energy to survive;
- Respond to environmental stimuli;
- Reproduce itself;
According to the number of cells, can be divided into:
- Unicellulars - Bacteria, cyanophytes, protozoa, unicellular algae and yeast.
- Multicellular - the other living beings.
According to the structural organization, the cells are divided into:
- Prokaryotic Cells
- Eukaryotic Cells
At prokaryotic cells or prokaryotic, also called protocells, are very different from eukaryotes. Its main feature is the absence of library individualizing the cell nucleus, due to the absence of some organelles and the small size believed to be due to the fact that they do not have membranous compartments originated by evagination or invagination. They also have DNA in the form of a non-protein associated ring (as in eukaryotic cells, where DNA is arranged in spiral and histone-associated filaments).
These cells are devoid of mitochondria, plastids, Golgi complex, endoplasmic reticulum and especially karyomembrane which causes DNA to be dispersed in the cytoplasm.
To this group belong unicellular or colonial beings:
- Cyanophytes (cyanophytic algae, blue algae or Cyanobacteria)
- PPLO ("pleuro-pneumonia like organisms") or Mycoplasmas
The bacteria in the rickettsia and chlamydia groups are very small and are called incomplete cells because they do not have the ability to self-duplicate independent of the collaboration of other cells, that is, they only proliferate within other complete cells, thus being intracellular parasites. required
Unlike virus for presenting:
- together DNA and RNA;
- part of the cell synthesis machine needed to reproduce themselves;
- a semipermeable membrane through which they exchange with the surrounding environment.
Note: viruses with DNA, adenoviruses, and RNA, retroviruses, have been found, however are rare, viruses that have DNA and RNA simultaneously.
the cells eukaryotes or eukaryotic, also called eucells, are more complex than prokaryotes. They have individualized nuclear membrane and various types of organelles. Most of the animals and plants we are used to are endowed with these cells.
It is highly probable that these cells arose by a process of continuous enhancement of prokaryotic cells.
It is not possible to accurately assess how long it took the "primitive" cell to undergo improvements in its structure until it gave rise to the model that is repeated today in the vast majority of cells, but it is likely to have taken many millions of years. The "primitive" cell is believed to have been very small and to make its physiology better suited to the relationship size × operation it needed to grow.
The "primitive" cell membrane is believed to have internally emitted prolongations or invaginations of its surface, which multiplied, acquired increasing complexity, clustered around the initial block to the point of forming the intricate mesh of the endoplasmic reticulum. From there it would have undergone other folding processes and originated other intracellular structures such as Golgi complex, vacuoles, lysosomes and others.
As for chloroplasts (and other plastids) and mitochondria, there is currently a stream of scientists who believe that the best theory that explains the existence of these organs is the theory of endosymbiosis, according to which a being with a larger cell had within itself a cell. smaller but with better characteristics, providing a smaller refuge and the ability to photosynthesize or synthesize proteins of interest to the other.
Levels of organization of the eukaryotic cells
In this group are:
- Plant Cells (with chloroplasts and cell wall; usually only a large central vacuole)
- Animal Cells (no chloroplasts and no cell wall; several small vacuoles)