cliffhanley203 said:
According to BBC Bitesize the correct word for what I earlier referred to as respiration, i.e, breathing, is ‘ventilation’; is that correct?
Breathing can be called ventilation. In a larger sense ventilation could apply to actions involving moving the environmental medium (air or water) over specialized gas exchange surfaces (lungs, gills, etc. (there are many)).
cliffhanley203 said:
How might a monocellular organism communicate with its environment?
Communication with its environment would be across its cell membrane. In the simplest cells (archaea and bacteria) this would involve molecules diffusing across the membrane (not that efficient) or things like protein receptors in the membrane that either using energy or not move a bound molecule across the membrane.
more complex (
eukaryotic) single cells could also draw in volumes of the surrounding media by sucking vesicles (making little cell membrane lined bubbles containing what was outside at the surface) into the cell.
Cells can also send things out to their environment, like proteins, enzymes and other molecules.
cliffhanley203 said:
Is the nerve net of a cnidarian analogous to our peripheral nervous system (I know that our pns relays impulses to our cns and that a cnidarian hasn’t got a cns but I’m wonderin if the nerve net was the beginnings a pns to which a cns ‘hooked up’ with later)?
Analogous vs. homologous: a structure is homologous to another structure if it is derived in evolution from that earlier structure. Alternatively, two currently existing structure can be homologous if they are both evolved (derived) from the same earlier structure. Analogous can have a variety of meanings where the structures are not derived from a pre-existing structure but have evolved a similarity of function and/or structure. Much looser term.
I would think of the cniderian nerve net (basically its whole nervous system) as a precursor to both the CNS and the PNS. Part of it can be conceived as evolving into the CNS and part into the CNS.
Here are some pictures. There are more nerve cells around the oral end (acts as both a mouth and anus). This is often called a condensation. Some might consider it as an analogue of the CNS. Others might not.
cliffhanley203 said:
And, finally, would the archaea (prokaryotic microbes) that lived in hydrothermal vents (and are the earliest known life forms – according to Wiki) have had, in a sense, a primitive ‘skin’ (cell membrane); a primitive digestion system; a primitive respiration system (in the proper sense of respiration); a way of communicating with their environment; and, in a sense, a reproductive system?
Archaea and bacteria can both live in hydrothermal vents. They are usually considered equivalent with respect to which came first. An unknown and presumed to now be extinct predecessor (their common ancestor) is though to have existed before them. It would most likely have had those things which both archaea and bacteria have in common: a cell membrane, DNA, RNA,
ribosomes, a cellular respiration system: a system to derive energy from environmental molecules that involved moving electrons through electron transport chain that pumps protons (or hydrogen ions, H
+) out of the cells and which then uses the gradient of H
+ to produce ATP (the predominant cellular energy source)).
The cell's membrane's function is to separate the cell from its environment (as well as provide a unit for selections and the site of much of cellular respiration (pumping of protons and the ATP synthatase).
The respiration system would be there to provide cellular chemical energy for the cell to use.
I would call a digestive system something to break down food into molecules that could be taken into the cell for processing by the respiration system. This may not have been necessary, depending on what the cells were living off of. Some primitive cells would just absorb molecules that could be used by respiration without the preliminary digestion step. I would guess this would be the case in the first cells,. Others might not.
Many origin of life scenarios place the origin events in environments where a simple usable food molecule is provided by the environment. There are several proposed sites for this (hydrothermal and geothermal vents (different things) are among those) where suitable molecules are present in the environment for use without digestion. These are favored as hypothetical sites of origins because it makes one less thing that the first life form would not have to achieve before normal biological evolution could take over.
Any living thing has to be able to reproduce. It would have to have genetic system of some kind to provide their progeny with the information to make more of the specific proteins they are largely made of. Some think the first genetic information storage was RNA and the use of DNA evolved later (this would be in an earlier life form than the immediate ancestor of bacteria and archaea). The genetic system would also require the presence of ribosomes, tRNAs in order to make specific proteins. The proteins of the respiration system would require this.
The combination of a genetic system and reproduction would provide the basis for evolution.
