The three important steps from the very primitive life towards the prokaryote cell are as follows:
1. DNA was formed from RNA. DNA covers the information storage and replication while RNA utilizes the information protein synthesis. This division made it possible to regroup the genes and the replication of the genes could be carried out in a synchronous fashion, coordinated with cell division.
2. A cell membrane was formed to protect the life processes, the RNAs, DNA and proteins (enzymes). Physically, biological membranes are not very different from soap bubbles. They are very thin, highly flexible, self-sealing films, which consists of mainly carbon and hydrogen. Their constituent molecules are phospholipids.
Formation of the cell membrane was a major progress. It became possible to protect the biochemical components making up the life processes and to maintain more easily another composition inside the cell than outside, but still mediate exchanges with the outside.
3. Introduction of ATP to facilitate the exchange of energy between different parts of the cell and between different biochemical processes. ATP is relatively easy to produce from other organic molecules by input of energy. It can therefore not be excluded that ATP participated in the biochemical processes of the very first primitive life consisting of small proteins and RNAs as described above.
The development corresponding to these three points has probably taken in the order of 100-200 million years. The oldest fossils of cells are about 3.8 billion years old and were found on Greenland (Haugaard Nielsen, 1999). The minimal cell has (see Table 1.1 and the comments to the minimal cell, Chapter 1) a fi-value of about 5.0. The exergy density is now as high as 18.7 x 4.88 = 91 kJ/g. The free energy flow density of synthesizing organic polymer in primitive cells (Geigy, 1990) is in the order of 0.02 J/s kg. The eco-exergy flow density becomes therefore as it includes the accumulation of information 5 x 0.02 = 0.1 J/s kg. It is of course higher than for the Earth in average, as the microorganisms represent an intensive energy flow.
The primeval world was oxygen-free and remained so until about 2.3 billion years ago. The level of atmospheric oxygen started rising at that time due to introduction of the photosynthesis and reached values compatible with aerobic life a few hundred million years later. The organism responsible for this significant change in the composition of the atmosphere was cyanobacteria. Oxygen is deadly toxic to anaerobic organisms, but new life forms—aerobic microorganisms were able to cope with the new challenge of oxygen.
The time from about 3.5 to 2.0 billion years ago gave rise to a wide spectrum of different prokaryote cells using different biochemical processes, particularly to oxidize the organic matter, which is the energy source of these cells. Furthermore, the biochemical processes included in the metabolic processes became more refined. Some scientists talk about bacteria as superstars of the living world, because of the high reproduction rate. A fi-value of 8.5 (see Table 1.1 in Chapter 1) represents the prokaryote cell in the most developed stage, which was approximately reached at the latest 2 billion years ago. The corresponding exergy density is 18.7 x 8.5 = 159 kJ/g. The eco-exergy flow density is about 0.02 x 8.5J/kgs = 0.17J/skg, presuming the same turnover rate as for the primitive cell. The turnover rate is the same as measured today for some bacteria that can be found close to the so-called vents or black smokers.
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