As has been mentioned before, everything seems to point at primitive eucaryotic cells having been amoeboid and able to phagocyte other cells.
Only in rare cases, endosymbiosis or the unification of two cells occurred increasing the probability of a gene exchange, a sexuality. It is considered a well-known fact that a genetic exchange or a recombination of parts of two genomes can only be successful, if both partners are almost or completely identical – the species concept. A large genetic variability has most likely already existed among the primitive eucaryotic cells leading among other things to the existence of many incompatible genomes. The uptake of one cell by another did therefore usually lead to the lysis of the latter causing the necessity of protection. This may have been the reason behind the different strategies that caused the basically different cell types:
Fast and mobile cells, escaping by moving Increase in size and protection by a solid cell wall Settlement in habitats where amoeboid cells cannot survive.
On 1. The first strategy led to the evolution of flagellates and ciliates. In comparison to most amoebas, flagellates are relatively small . Movement is an energy-consuming process, and small cells do therefore have an advantage against large ones. Movement occurs usually by one or two flagella. In ciliates, usually the whole cell surface is studded with short cilia that resemble flagella in both structure and function. The cells are spherical, elongated or spindle-like. Within certain boundaries, the cell shape is flexible, but the degree of variability is far smaller than in amoebae. The position of the cilia marks the cell’s polarity. The cilium does often spring from the front pole of the cell, i.e. anterior, and the opposite pole is called posterior. Many species secrete carbohydrate-containing, jelly-like material at the base of the cilium that can be detected by specific lectines labeled with fluorescence and that do thus constitute a further polarity marker. Some flagellates secrete a building substance like cellulose or chitin at their posterior pole and build thus a specifically structured shell called a lorica that can be glued to solid substances. These flagellates have thus adopted a secondarily sessile life-style (see also point 3).
Euglena and species from other systematic groups react phototactically. They contain a light receptor and a mechanism allowing them to orient themselves towards a source of light. Living in light-extensive habitats increases the rate of photosynthesis and thus the energy turnover and the rate of division.
On 2. A number of primitive, but partly also of highly developed recent algae like Acetabularia, Caulerpa, Vaucheria, etc. are characterized by an extreme increase in size of single cells, some of them are multinucleate. The stability of their shape is guaranteed by a cell wall. The building substance of the cell walls of most green plants is cellulose, while other polysaccharides, too, occur in some primitive, usually unicellular or few-celled species as well as in non-green algae. In simple, flagellate-like species like Chlamydomonas or Volvox, the wall consists of several layers and mostly of proteins with a high percentage of carbohydrate. The building molecules are sometimes organized in fibrillary structures.
Cell walls out of cellulose or out of other polysaccharides have in contrast to the rigid cell walls out of murein the advantage of being elastic, since the linear polymers are interconnected ‘only’ by weak interactions, usually hydrogen bonds, that can easily be broken and reformed. This mechanism is an important precondition for the elongation characteristic for plant cells.
With murein in contrast, the monomers are almost exclusively linked by strong covalent bonds. The wall of a cell consists therefore in principle only of a single, closed molecule, a murein saccule, and its binding pattern can only be altered enzymatically. The synthesis of new wall parts occurs immediately after cell division. A later enlargement of the cell is usually impossible.
The development of a wall serves in primitive eucaryotes on one hand as a protection against feeding by amoebas, on the other hand does it put up a resistance against the osmotic pressure of the cell content.
Protozoa without a cell wall do constantly have to pump out the water seeping into the cell under expense of energy. The production of a cell wall is pretty energy consuming, too, but is most likely still more economic as a once-in-a-lifetime investment than the permanent pumping of pulsating vacuoles.
On 3. Polarity is a characteristic feature of plant and many animal cells. It is, as you have just seen, also typical for flagellates. The polarity of cells seems to be an important prerequisite for specialization and differentiation. Polarity can not only be recognized by the elongated shape of a cell, cytological and especially studies performed with the electron microscope show that numerous cell contents are distributed asymmetrically. The intracellular gradient of distribution causes different physiological activities within the single cell sections.
Polarity can be demonstrated visually with the aid of several biochemical methods. One possibility is to furnish proof of the secretion of certain glycoconjugates, i.e. carbohydrate-containing components like polysaccharides, glycoproteins, glycolipids, etc. by localizing them at the cell surface with the help of fluorescence-labeled lectines. Many of the glycoconjugates that have been detected this way are secreted only at both or even just at one pole of the cell. The excretory products, usually gelatinous substances, are well-suited for anchoring the cells to a solid surface. It does often, too, serve to combine single cells in colonies.
An anchoring to a solid surface is one of the possibilities allowing survival in running waters. Amoebas cannot settle in such biotopes Sessile cells have thus found an ecological niche that was at first protected from predators.
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