Lipid world

CIPAHs, PBDEs, and PCDD/Fs in soil samples were in ng/g dry weight. For PCDD/Fs in human samples, pg WHO-TEQ1998/g lipid (World Health... 

These successes in the area of precellular structures led Daniel Segre et al. from the Weizmann Institute in Rehovot to publish the lipid world hypothesis together with Deamer. The authors are of the opinion that the (more probable) lipid world determined chemical and molecular evolution on the primeval Earth prior to the (less probable) RNA world . Further development stages (or worlds ) finally led to biogenesis (Segre et al., 2001). 

Attempts have recently been made to link the RNA world with the lipid world. Two groups involved in RNA and ribozyme research joined up with an expert on membrane biophysics (Szostak et al., 2001). They developed a model for the formation of the first protocells which takes into account both the most recent experimental results on replication systems and the self-organisation processes of amphiphilic substances to give supramolecular structures. 

Szostak et al. worked on the basis of a simple cellular system which can replicate itself autonomously and which is subject to Darwinian evolution. This simple protocell consists of an RNA replicase, which replicates in a self-replicating vesicle. If this system can take up small molecules from its environment (a type of feeding ), i.e., precursors which are required for membrane construction and RNA synthesis, the protocells will grow and divide. The result should be the formation of improved replicases. Improved chances of survival are only likely if a sequence, coded by RNA, leads to better growth or replication of membrane components, e.g., by means of a ribozyme which catalyses the synthesis of amphiphilic lipids (Figs. 10.8 and 10.9). We can expect further important advances in the near future from this combination ( RNA + lipid world ). 

Evershed R.P., Biomolecular archaeology and lipids, World Archaeology, 1993, 25, 74 93. 

The Weizmann Institute of Science, Rehovot, Israel Lipid-world composomes as early prebiotic replicators... 

Keywords Autocatalysis Chemoton Evolution Heredity Infrabiological systems Lipid world Metabolism Minimal life Protocell Unit of evolution... 

A similar approach is possible when considering questions in the lipid world the issue is however complicated by the fact that we need to tackle the problem of reflexive auto catalysis. This has also precedence in the literature the reflexively autocatalytic protein networks (e.g. [25]) are perhaps the best known example. We hasten to point out that nobody has seen a real reflexively autocatalytic protein set, apart from very small ones where replication is in fact modular and analogous to the complementary replication of oligonucleotides [26]. Let us see whether one can be more hopeful regarding autocatalytic lipid sets. 

On the experimental side it is worrying [23] that Fendlers compendium cites mostly hydrolytic reactions. It may well be the case that different types of reaction (such as biosynthetic ones) should be taken from a different, unknown distribution. This is an exciting problem that should be tackled, irrespective of the lipid world. 

The demonstration that the polymerase chain reaction can be carried out in liposomes [50] is important because it demonstrates that liposomes can resist the required temperature changes. In the light of the lipid world model it is useful to ask what catalytic functions Luisi s structures show behind direct auto catalysis. Binding of peptides to and polymerisation of amino acids in liposomes was demonstrated in various systems [51]. We are not aware of a similar effect on nucleic acid synthesis. 

The Lipid World hypothesis states that polar hydrocarbons formed in a prebi-otic Earth, or originated from extraterrestrial meteoric sources, and then went on to aggregate into vesicles. These vesicles then capture chemical species at random in some cases the concentrating effect of the vesicle would facilitate chemical reactions and some of these would eventually lead to self sustaining chemical reactions. Eventually protein-based enzymes would emerge that could synthesize lipids and the entire system would then become symbiotic. 

An alternative candidate for a cell membrane is one composed of mostly hydrophobic molecules that would spontaneously aggregate if suspended in water. It is important that the molecule is not entirely hydrophobic, like hexane or benzene, as some part of it must be solvated if it is to aggregate with others to form a structure capable of encapsulating other species. Candidates for this type of molecule are the lipids and related compounds. The formation of protocells by lipids has been discussed previously as part of the Lipid World hypothesis. In the context of compartmentalization it is important to consider how such an aggregate could evolve. 

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