The cell membrane , and origin of life scenarios

The cell membrane , and origin of life scenarios 1

https://reasonandscience.catsboard.com/t2456-the-cell-membrane-and-origin-of-life-scenarios

Ricard V. Sole Synthetic protocell biology: from reproduction to computation February 16, 2015

Cellular life cannot be described in terms of only DNA (or any other information-carrying molecule) nor as metabolism or as compartment (cell membrane) alone. Cellular life emerges from the coupling among these three components. A container is a prerequisite of biological organization in order to at least confine reactions in a limited space, where interactions are more likely to occur (Deamer et al. 2002). It also provides a spatial location able to effectively facilitate division of labour among reactions. Moreover, in modern cells, the membrane is an active cell component, channelling nutrients in and waste products out of the cell by means of specialized transport catalysts (Pohorille et al. 2005). A metabolism (Smith & Morowitz 2004) provides the source of non-equilibrium and a means of energy storage required in order to build and maintain cellular components. It is also required to allow cell growth to occur and eventually force splitting into two different (but similar) copies. 6

The production of biological membranes under early earth conditions is no trivial task. 2 The lipid bilayer is a simple structure and at first glance may seem as though it is of little consequence, but life could not have arisen without it. 5 Cells all have a membrane constructed from a phospholipid bilayer. From the very beginning, cells used the lipid bilayer to regulate their internal environment. The bilayer blocked or impeded, the passive flow of most molecules into the cell, thus protecting the cell from the external environment. Cells exploited this property by embedding proteins in their membranes that would allow only certain molecules to gain entry. In this way, the cell could fine-tune the selection of what got in and what did not. Other proteins embedded in the membrane acted like sensory antennae, making it possible for cells to gain information about their immediate environment. Some of these proteins were used to detect the presence of food molecules, while others became specialized as transmitters and receivers, allowing the cells to communicate with each other.

Lipids are fundamental to biology as the compounds from which biological membranes are largely constructed. 4 These can broadly be grouped into two classes, which are found in various types of organisms:

the isoprenoid ether lipids and the
fatty acid acyl lipids





The fatty acid ester type (Figure 5.7b) is widely distributed across the eubacterial and eukaryotic domains of life and consists of the union of one or more straight saturated or unsaturated fatty acid chains. These chains typically contain 16–18 carbon atoms and are derived from the malonyl/acetyl-CoA biosynthetic pathways, joined to a polar head group, which is usually some derivative of glycerol, rendered more polar by the attachment of phosphate, choline, or some other low-molecular-weight moiety (represented as R in Figure 5.7a and b). The isoprenoid ether type (Figure 5.7a) is widely distributed among the archaebacteria (Matsumi et al. 2011), particularly in extremophilic species, perhaps rationalizable by their greater resistance to chemical degradation at extremes of salinity, pH, and temperature (Driessen and Albers 2007). These consist of one or more polyisoprenoid chains (derived from the isoprene biosynthetic pathways), attached by ether linkages to polar head groups such as substituted glycerol moieties.

Prebiotic syntheses of the straight-chain fatty acids have been suggested based on Fischer–Tropsch-type synthesis (FTT), for example, by the reactions of CO or CH4 over suitable metal catalysts at high temperature (McCollom et al. 1999, 2010) (Figure 5.8 ).


It is known that straight-chain amphiphiles and various biological lipids will spontaneously self-assemble under the appropriate conditions of pH, temperature, and solute concentration to form aggregates known as micelles or vesicles, which in some cases are able to trap various solutes for extended periods of time (Deamer and Barchfeld 1982). These properties have led to suggestions that such materials may have been important for the self-assembly of the earliest protocells (Szostak 2001). Intriguingly, the straight-chain fatty acids found in carbonaceous chondrites to date are just at the threshold of the types of molecules that are known to display this behavior (Monnard et al. 2002), and even these are present in very low abundance (Yuen and Kvenvolden 1973). For example, the octanoic acid content of 1 g of the Murchison meteorite would need to be dissolved in a few tens of nanoliters of water in order to form vesicles (given a critical vesicle concentration [the concentration above which bilayered or higher aggregated lipid structures form spontaneously] of 130 mM [Apel et al. 2002] and a mean abundance of 0.01 μmol g−1 in a typical meteorite [Lawless and Yuen 1979]). Nevertheless, organic solvent extracts of the Murchison meteorite show the presence of compounds that spontaneously assemble into boundary structures (Deamer 1985) (Figure 5.7c), though it is unlikely these have much compositional similarity to biological membranes.


Sources of membrane building blocks
Long-chain hydrocarbons can be formed from carbon monoxide and hydrogen in the presence of certain metals at high temperatures. Deep sea hydrothermal vents have been cited as a potential source of the energy required to synthesize prebiotic molecules, including the building blocks of membranes. Fatty acids and fatty alcohols have been synthesized under these conditions. These fatty acids will combine with ethylene glycol to form ethylene glycolyl alkanoates and bis-alkanoates, or will combine with glycerol to form monoacylglycerols and diacylglycerols.  Others have suggested that the first membranes consisted of highly branched polyprenyl chains, instead of alkyl chains. However, it is unlikely that the starting material would be at sufficient concentrations  and it also unlikely that the required phosphorylating agents would have been available on early Earth.

Vesicle assembly
Even if membrane building blocks were present in sufficient quantities, specific concentrations and other environmental conditions are required for assembly. All possible components other than fatty acids have been eliminated from contention for a lack of plausible synthetic pathways. The assembly of fatty acids into lipid bilayers is dependent upon the chain length, concentration, pH, and temperature. Short chain fatty acids form vesicles at room temperature when the pH is within half a pH unit of the pKa of the acid. Longer chain fatty acids require higher temperatures (30-70°C).15, 16 In addition, the concentration of fatty acids must be quite high (130 to 20 mM) for vesicles to form.17 The presence of such high concentrations of fatty acids would be unlikely on the primordial earth. Some of the exacting conditions can be moderated by the presence of fatty alcohols with the same hydrocarbon chain length as the fatty acid.7, 16 However, the molar ratio must be almost exactly 10:1 (acid:alcohol) in order for any significant effect to be seen.18 In addition to temperature, pH, and concentration requirements, vesicle formation is highly dependent upon ionic strength and the presence of certain ions. Thus, the presence of sodium chloride at levels found in the oceans of primordial earth causes vesicles to aggregate into sheets, and the presence of Ca+2, Mg+2, Fe+2 at primordial concentrations causes fatty acids to precipitate.

The spontaneous formation of bilayer vesicles  is dominant explanation for the appearance of membranes, but is not without challenging problems. It has been shown that fatty acids will spontaneously form phospholipids in the presence of glycerol and phosphates when heated to dryness.  However,  it would be extremely unlikely that nature would produce all three chemicals in the same location and then heat them to dryness. 

Encapsulation and transport
The mere formation of an enclosed vesicle is not sufficient to guarantee functionality. In order to be useful as a mechanism involved in the naturalistic origin of life, membranes must encapsulate materials necessary to initiate life and be able to transport material in and out of the boundary. Modern biological membranes contain protein systems that actively and passively allow the exchange of nutrients and wastes. Since these transport systems would not be available on the primordial earth, other systems must have existed to make the process even remotely feasible. Despite the extremely unlikely appearance of phospholipid membranes under early-earth conditions, most studies that have examined encapsulation have used such membranes. Encapsulation of primitive fatty acid membranes would have to involve repeated rupture and resealing (through agitation) during periods of changing osmotic gradients (increasing and decreasing salt concentrations). Changing the concentrations of solutes would have the additional problem of likely altering pH, which would disrupt the exacting conditions required for fatty acid membrane assembly. The presence of locations where these exact conditions would exist would be very limited on the primordial earth. In addition, in order for some form of life to be created in this manner, both a primitive replicator and metabolic system must be encapsulated at this time. Of course, such systems would both encapsulate and release materials on each cycle, and it is unclear what kind of equilibrium would be eventually achieved.

Once a stable membrane is formed, some kind of transport system for nutrients/wastes would be required to maintain the metabolism of the proto-cell. Passive transport systems would be the easiest to form, but such systems would automatically achieve equilibrium, making further transport impossible. Obviously, due to their complexity, active transport systems would not be expected to be encoded by a primitive replicator.

Energy acquisition
Possible sources of energy for proto-cells are heat energy, chemical energy, and light energy. However, none of these forms of energy harvest are compatible with a primitive fatty acid membrane in the presence of known prebiotic chemicals present in the environment. This is because carboxyl head group of a fatty acid membrane mediates proton permeability, eliminating the possibility of generating a proton gradient. The only way to get around this problem was to use an oleate-arginine system, which slowed the decay of the gradient. However, the unsaturated oleate would not have been present in a prebiotic environment. In addition, the system was inhibited by alkali cations, which would have been present in early earth environments.

The capture of light energy by proto-cells has been hypothesized to occur through encapsulated iron compounds or polycyclic aromatic hydrocarbons (PAH), which absorb light in the near-UV and blue wavelengths.14, 23 While ferrocyanide and PAH may have been present on early earth, these compounds cannot generate a proton gradient when enclosed within fatty acid membranes.

Growth and Division
Primitive membranes must have the ability to grow and replicate without the aid of biomolecular machinery in order to function in a hypothesized proto-cell.25 Slow addition of myristoleate micelles to a myristoleic acid/myristoleate vesicle system results in 90% of the added fatty acid was incorporated into the original vesicles, causing them to grow.25 Others have used osmotically swollen oleate vesicles to cause growth through vesicle-vesicle fusion.22 However, since the presence of these unsaturated fatty acids on early earth is unlikely, the relevance of these studies to the origin of life is questionable. When considered in the context of an RNA world scenario, the requirement for the presence of divalent cations by ribozymes would result in the precipitation of fatty acids, disrupting membranes. Division of membranes may occur when they reach a certain size.26 The ability of this to occur is dependent upon the size of the membrane and it composition. However, since the studies were done only with unsaturated fatty acid membranes, it is unclear what relevance there would be to the fission of saturate fatty acid membranes under early earth environments.

Unrealistic studies
Besides the problem that most origin of membranes studies have examined membranes composed of materials that would have never existed on the primordial earth, there is an even more fundamental problem that tends to plague virtually all origin of life research. Once a compound has been declared "prebiotic", researchers immediately begin using the highly purified product at exceptional concentrations. According to Robert Shapiro:

"The observation of a specific organic chemical in any quantity (even as part of a complex mixture) in one of the above sources would justify its classification as "prebiotic," a substance that supposedly had been proved to be present on the early Earth. Once awarded this distinction, the chemical could then be used in pure form, in any quantity, in another prebiotic reaction. The products of such a reaction would also be considered "prebiotic" and employed in the next step in the sequence."27

Conclusion 
The origin of biological membranes, like the origin of replication and metabolism, is fraught with problems and invokes extremely improbable chemistry. Although some of the building blocks of potential membranes might have been synthesized on early earth, the ones used in modern biological membranes (phospholipids) could not have been. Therefore, one must hypothesize some kind of primordial membrane that was later discarded in favor of modern membranes. However, even this scenario suffers from insurmountable problems. The extraterrestrial synthesis and delivery of membrane building blocks remain unproved. Although such materials might have been synthesized near hydrothermal vents in the early seas, the assembly of such materials is quite problematic. Conditions requiring high concentrations, exact pH and temperature, plus the absence of high sodium and small amounts of certain metal ions, prevents the assembly of such components within the earth's early oceans. Conditions that might concentrate fatty acids to sufficient levels to form membranes would also concentrate solutes that disrupt the formation of those membranes. Encapsulation of a proto-cell replicator and metabolic system would be quite problematic, since the conditions that would encourage such activity would likely lead to conditions that would disrupt the primitive membrane completely. Primitive membranes must be able to transport nutrients and wastes, although passive transport systems would readily reach equilibrium and active transport systems would not be expected to be produced immediately upon encapsulation. Energy acquisition is problematic, since fatty acids membranes cannot generate a proton gradient. Membranes composed of unsaturated fatty acids or phospholipids can generate proton gradients, but would not be expected to have existed in early earth environments. Virtually all studies that have examined membrane growth and division have used unsaturated fatty acid membranes, which would not have been present on the early earth. Because of this problem, these studies have questionable relevance to the origin of life on earth.

NUTRIENT UPTAKE BY PROTOCELLS: A LIPOSOME MODEL SYSTEM 1

The fact that all contemporary living cells are defined by lipid-bilayer membranes suggests that protocellular structures could be modeled in the laboratory by dispersing lipid molecules as bilayer vesicles in aqueous phases . However, the use of lipid bilayers for this purpose has a significant drawback, which is their relative impermeability to polar or ionic molecules. To overcome this permeability barrier, complex protein transport systems have evolved in all contemporary cells  which were presumably absent in early cells.

As if cells would function without the protein transport systems.... and just wait their evolution and appearance to take over...... 

Therefore, the latter necessarily relied on simple transport mechanisms such as passive diffusion across bilayer membranes to take up nutrients and sources of chemical energy from the environment.

The just so story is remarkable... why should someone buy to it ? where  is the evidence to back up the claim ? We don't know, therefore lets just make up a story ? And when we add adjectives, like probable, possible, most likely etc.... the story becomes highly compelling......

What solutes might have been important to early cellular life? Several obvious examples include phosphate, simple carbohydrates like glyceraldehyde, and perhaps amino acids. Of these, phosphate and amino acids are ionic, and bilayer membranes have been shown to represent significant permeability barriers to their free diffusion. 

Membrane self-assembly processes: Steps toward the first cellular life   3

Some form of compartmentation is a necessary prerequisite for maintaining the integrity of interdependent molecular systems that are associated with metabolism, and for permitting variations required for speciation. The fact that lipid-bilayer membranes define boundaries of all contemporary living cells suggests that protocellular compartments were likely to have required similar, self-assembled boundaries. Amphiphiles such as short-chain fatty acids, which were presumably available on the early Earth, can self-assemble into stable vesicles that encapsulate hydrophilic solutes with catalytic activity. Their suspensions in aqueous media have therefore been used to investigate nutrient uptake across simple membranes and encapsulated catalyzed reactions, both of which would be essential processes in protocellular life forms.

The emergence of life on the early Earth required the presence of at least three different substances and related physical properties: liquid water, a source of free energy, and organic compounds capable of self-assembly. Liquid water is essential for all life today, and it is highly implausible that life can exist in its absence. Possible energy sources include sunlight (if life began on the Earth's surface) and energy arising from chemical disequilibria in submarine or subterranean sites. Self-assembling compounds must have been available to provide building blocks for polymer synthesis and formation of boundary structures.

In contemporary cells, a fundamental role of membrane boundaries is to provide a selective permeability barrier that is necessary for separating the cytoplasm from the external environment. The transmembrane transport of nutrients and ionic solutes is mediated by a variety of membrane-associated proteins that act as channels, carriers, and active transporters (pumps). Membrane receptors provide a sensor mechanism that permits communication between the intracellular milieu and the outside world. Membranes also capture light energy and redox energy by using pigment systems and electron transport to generate electrochemical proton gradients as a source of free energy.

As we learned more about the role of membranes in defining cell structures, it became clear that all membranes incorporated lipid bilayers as the primary permeability barrier, and that phospholipid is a nearly ubiquitous amphiphilic component of the bilayer.  Lipid bilayer vesicles are commonly referred to as liposomes, and such self-assembled membrane structures can be used as models of the earliest cell membranes. Several early papers demonstrated that phospholipids could be synthesized under simulated prebiotic conditions from mixtures of fatty acids, glycerol, and phosphate.   However, the simultaneous presence of all three components on the early Earth is highly speculative, and we have therefore turned our attention to simpler membranogenic amphiphiles.  

Stability and Permeability of Amphiphile Vesicles
Although the ability of phospholipids to self-assemble into membranous vesicles is common knowledge, it is less well known that a variety of membranous structures can also be prepared from single-chain amphiphiles such as fatty acids fatty alcohols, and monoglycerides. We will argue that such vesicles are plausible models for the formation of early cellular compartments.

Although the composition of the membrane-forming amphiphiles present in the Murchison organic mixture has not yet been established in detail, it is clear that substantial amounts of monocarboxylic acids are present . For this reason we have begun to investigate the physical properties of self-assembled structures produced by monocarboxylic acids of various chain lengths, and of mixtures with other simple amphiphilic compounds. Gebicki and Hicks (1973, 1976) first established that oleic acid, a fatty acid, forms vesicular structures. Since this discovery, the bilayer-forming potential of fatty acids with shorter hydrocarbon chains (C8-C11) has been investigated . The length and the degree of unsaturation of the hydrocarbon chains play an important role in determining the bilayer-membrane properties, such as permeability and stability, which would have been essential for the primitive life forms.

1. http://complex.upf.es/~andreea/2006/Bib/MonnardDeamer.NutrientUptakeByProtocells.pdf
2. http://www.godandscience.org/evolution/origin_membranes.html
3. http://onlinelibrary.wiley.com/doi/10.1002/ar.10154/full
4. ASTROBIOLOGY An Evolutionary Approach,  page 97
5. The Cell, Panno, page 18
6. https://sci-hub.ee/10.1098/rstb.2007.2065

How are lipids synthesized?

Lipids play a life essential role, and so, in many biochemical and cell biological processes. They are involved in the formation of biological membranes and therefore are important elements of organelle biogenesis and function. 25 This is not only true for bulk lipids such as the major classes of glycerophospholipids, sterols, and sphingolipids, but also for less abundant lipid species. Lipids with only minor concentrations in cells have more subtle functions. For example, polyphosphoinositides, lysophospholipids, or ceramides are essential for signaling processes. Certain classes of lipids, such as sphingolipids, are also sensors of cellular stress (heat shock) and important components for signal transduction. These lipids provide the appropriate environment for membrane proteins to function as enzymes or transporters, or to act as positive regulators of enzymes to gain optimum activity in membranes.

Many regulatory phenomena caused by lipids, which have not been recognized before, became evident through the advance of molecular biological research. Intracellular lipid transport and distribution play an important role in the maintenance of cellular structure and function. The lipid composition of vesicles involved in membrane traffic, recently, turned out to be a most important parameter for protein targeting. For example, the presence of sterols and sphingolipids in secretory and endocytotic vesicles appears to be essential. Membrane contact, as a mechanism of translocation of components between subcellular compartments, may also depend on the lipid patterns of donor and acceptor membranes.

Lipid metabolism must not be regarded only as an isolated process, but rather as a part of total cellular metabolism, which is highly linked to other metabolic and cell biological pathways


The first step in lipid metabolism is the hydrolysis b of the lipid c in the cytoplasm to produce glycerol and fatty acids. 5

Complex lipids consist of backbone structures to which fatty acids d are covalently bound. Principal classes include the glycerolipids, for which glycerol is the backbone, and sphingolipids, which are built on a sphingosine backbone. The two major classes of glycerolipids are glycerophospholipids and triacylglycerols. 

Glycerophospholipids or phosphoglycerides are glycerol-based phospholipids. They are the main component of biological membranes.

The phospholipids, which include both glycerophospholipids and sphingomyelins, are crucial components of membrane structure. Different organisms possess greatly different complements of lipids and therefore invoke somewhat different lipid biosynthetic pathways. For example, sphingolipids and triacylglycerols are produced only in eukaryotes. In contrast, bacteria usually have rather simple lipid compositions. 


Glycerolipid metabolism 12

Glycerol, essential for life
Glycerol a also called glycerine or glycerin is a simple polyol j compound. It is a colorless, odorless, viscous liquid that is sweet-tasting and non-toxic. The glycerol backbone is found in all lipids known as triglycerides. 8 Glycerin is a trihydroxy sugar alcohol that is an intermediate in carbohydrate and lipid metabolism. 9

The phospholipids characteristic of contemporary membranes are composed of two fatty acids esterified to a glycerol phosphate molecule, with any of several groups also attached to the phosphate through an ester bond, such as
choline, ethanolamine, serine, and glycerol. 7 Although hydrocarbons are abundant on today’s Earth, they are primarily products of biological processes, so there must have been supposedly abiotic sources of the amphiphiles required for assembly into first membrane structures if natural membrane assembly mechanisms are presupposed. Two possibilities are geochemical synthesis under volcanic conditions and delivery during the accretion of asteroids and comets in the late stage of planet formation.

Of course, that raises the question, why would biological processes have evolved the ultracomplex protein-machine-based processes to make lipids, if they were supposedly readily available on early earth? 

Phospholipids found in cell membranes show a wide range of chemical variability. Their head groups typically consist of a phosphate group bound to a glycerol (glycerin) backbone. 1
Cell membranes basically consist of phospholipids, which are amphiphile ( water-loving) molecules, that is to say that they possess a hydrophilic ( attracted to water ) head (consisting of glycerol-phosphate) and a hydrophobic ( repelling water )  tail (which may belong fatty acids or isoprenoid derivatives) 2 The fundamental distinction (with no known exceptions) between the phospholipids of the bacteria and eukaryotes, and those of the archaea rests in the type of stereoisomer e of glycerol used  ; 

glycerol-3-phosphate in the bacteria and eukaryotes, 
glycerol-1-phosphate in the archaea. 

The pathways by which these two stereoisomers are synthesized are so different that, for certain authors, the cenancestor did not have membranes and was an acellular organism or, according to yet others, it had mineral membranes consisting of iron monosulfide.

Here we have one reason amongst many others, why common ancestry of all three domains of life is extremely unlikely. 

The idea of an ancestor without lipid membranes, however, comes up against one piece of evidence: there are membrane proteins that are universally conserved, like the ATPase. A less radical hypothesis, which had recently been supported by molecular phylogeny analyses, would be to imagine a cenancestor with a heterochiral membrane, that is to say possessing a mixture of phospholipids constructed with glycerol-1-phosphate and glycerol-3-phosphate. Additional molecular phylogenetic studies further support the idea that the cenancestor had a complete toolkit to make phospholipids of either fatty acids or isoprenoid chains. Evolution would have subsequently led to opposite stereospecificity and choice of lateral chains in the bacteria and in the archaea.

Another explanation is that common ancestry is an unlikely hypothesis, and the origin of the three domains of life is not due to a last common universal ancestor, but distinct creation events. 

The phospholipids of Bacterial and eukarya plasma membranes consist of fatty acids ester-linked f to glycerol-3-phosphate
Archaea make theirs of isoprenoids h ether-linked g to glycerol- 1-phosphate


The structure of membrane phospholipids in the three domains of life. 
The type of stereo-isomer of glycerol-phosphate distinguishes, with no known exception, between the phospholipids of bacteria and eukaryotes, and those of the archaea: glycerol-3-phosphate in bacteria and eukaryotes, glycerol-1-phosphate in archaea. Other differences are observed in the hydrophobic chains and the bond between the latter and the glycerol-phosphate, but there are exceptions. Some phospholipids in archaea have chains of fatty acids, and some phospholipids in bacteria include ether bonds.


Glycerol 1-phosphate, sometimes called as D-glycerol 3-phosphate, is an enantiomer of glycerol 3-phosphate. Most organisms use 3-phosphate, or L-configuration, for glycerolipid backborn; however, 1-phosphate is specifically used in archeal ether lipids.

Glycerol 3-phosphate is a phosphoric ester of glycerol, which is a component of glycerophospholipids. 11

Glycerol 3-phosphate synthesis and metabolism

There are several routes to synthesize Glycerol 3-phosphate. 

1. Glycerol 3-phosphate is synthesized by reducing dihydroxyacetone phosphate (DHAP), a glycolysis intermediate, with glycerol-3-phosphate dehydrogenase. 11
Glycerol-3-phosphate dehydrogenase is an enzyme that catalyzes the reversible redox conversion of dihydroxyacetone phosphate  to sn-glycerol 3-phosphate. Glycerol-3-phosphate dehydrogenase serves as a major link between carbohydrate metabolism and lipid metabolism. Sn-glycerol-3-phosphate dehydrogenase (GlpD) 4 is an essential membrane enzyme, functioning at the central junction of respiration, glycolysis, and phospholipid biosynthesis. Its critical role is indicated by the multitiered regulatory mechanisms that stringently controls its expression and function. Once expressed, GlpD activity is regulated through lipid-enzyme interactions. Homologs of GlpD are found in practically all organisms, from prokaryotes to humans, with >45% consensus protein sequences, signifying that these structural results on the prokaryotic enzyme may be readily applied to the eukaryotic GlpD enzymes.

Despite the pioneering work of Hargreaves et al. in 1977 who demonstrated that the synthesis of phosphatidic acid and other lipids could be achieved abiotically, it is considered very improbable that fatty acids, glycerol, and phosphate (i.e., the standard molecular components of a phospholipid) could have been present together in high enough concentrations on the primordial Earth. 6





2. Glycerol can be a source for glycerol-3-phosphate, in which case, a phosphate from ATP is transferred to glycerol by glycerol kinase forming glycerol-3-phosphate and ADP. 22 Glycerol kinase 23 catalyzes the phosphorylation i of glycerol to form glycerol-3-phosphate, which is then acylated at both the 1- and 2-positions to yield phosphatidic acid. Glycerol is a precursor for the synthesis of phospholipids. Before glycerol can enter the pathway of glycolysis or gluconeogenesis (depending on physiological conditions), it must be converted to their intermediate glyceraldehyde 3-phosphate in the following steps:




Glycerol kinase, encoded by the gene GK, is a phosphotransferase enzyme involved in triglycerides and glycerophospholipids synthesis. 



a 


b Biological hydrolysis is the cleavage of biomolecules where a water molecule is consumed to effect the separation of a larger molecule into component parts. 13

c In biology, a lipid is a biomolecule that is soluble in nonpolar solvents. Non-polar solvents are typically hydrocarbons used to dissolve other naturally occurring hydrocarbon lipid molecules that do not (or do not easily) dissolve in water, including fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E, and K), monoglycerides, diglycerides, triglycerides, and phospholipids. 14

d  In chemistry, particularly in biochemistry, a fatty acid is a carboxylic acid with a long aliphatic chain, which is either saturated or unsaturated. Most naturally occurring fatty acids have an unbranched chain of an even number of carbon atoms, from 4 to 28. 15

e In stereochemistry, stereoisomers are isomeric molecules that have the same molecular formula and sequence of bonded atoms (constitution), but differ in the three-dimensional orientations of their atoms in space 16

f In chemistry, an ester is a chemical compound derived from an acid (organic or inorganic) in which at least one –OH (hydroxyl) group is replaced by an –O–alkyl (alkoxy) group 17

g In an organic chemistry general sense, an ether lipid implies an ether bridge between an alkyl group (a lipid) and an unspecified alkyl or aryl group, not necessarily glycerol. 18

h  Isoprenoids are any of a class of organic compounds composed of two or more units of hydrocarbons, with each unit consisting of five carbon atoms arranged in a specific pattern. These compounds are derived from five-carbon isoprene units and are biosynthesized from a common intermediate known as mevalonic acid, which is itself synthesized from acetyl-CoA. These lipids are considered to be the largest group of natural products, playing a wide variety of roles in physiological processes of plants and animals. 19

i  In chemistry, phosphorylation of a molecule is the attachment of a phosphoryl group. Together with its counterpart, dephosphorylation, it is critical for many cellular processes in biology. 21

j A polyol is an organic compound containing multiple hydroxyl groups. 24

1. Origins of life : biblical and evolutionary models face off / Fazale Rana & Hugh Ross., page 102
2. Young Sun, Early Earth and the Origins of Life , page 122
3. https://ipfs.io/ipfs/QmXoypizjW3WknFiJnKLwHCnL72vedxjQkDDP1mXWo6uco/wiki/Glycerol_3-phosphate.html
4. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2265192/
5. http://chemistry.elmhurst.edu/vchembook/622overview.html
6. Prebiotic Systems Chemistry: New Perspectives for the Origins of Life : http://sci-hub.tw/https://pubs.acs.org/doi/10.1021/cr2004844
7. http://sci-hub.tw/10.1016/j.resmic.2009.06.004
8. https://en.wikipedia.org/wiki/Glycerol
9. https://pubchem.ncbi.nlm.nih.gov/compound/glycerol
10. https://en.wikipedia.org/wiki/Glycerophospholipid
11. https://en.wikipedia.org/wiki/Glycerol_3-phosphate
12. https://www.genome.jp/kegg-bin/show_pathway?map00561
13. https://en.wikipedia.org/wiki/Hydrolysis
14. https://en.wikipedia.org/wiki/Lipid
15. https://en.wikipedia.org/wiki/Fatty_acid
16. https://en.wikipedia.org/wiki/Stereoisomerism
17. https://en.wikipedia.org/wiki/Ester
18. https://en.wikipedia.org/wiki/Ether_lipid
19. https://en.wikibooks.org/wiki/Structural_Biochemistry/Lipids/Isoprenoids
20. https://en.wikipedia.org/wiki/Phosphatidic_acid
21. https://en.wikipedia.org/wiki/Phosphorylation
22. http://reactome.org/content/detail/R-HSA-75887
23. https://en.wikipedia.org/wiki/Glycerol_kinase
24. https://en.wikipedia.org/wiki/Polyol
25. Lipid Metabolism and Membrane biogenesis, page 1

https://evolutionnews.org/2022/04/origin-of-life-the-problem-of-cell-membranes/

What came first: Lipid membranes, or membrane proteins?
Eugene V. Koonin (2009): A topologically closed membrane is a ubiquitous feature of all cellular life forms. This membrane is not a simple lipid bilayer enclosing the innards of the cell: far from that, even in the simplest cells, the membrane is a biological device of a staggering complexity that carries diverse protein complexes mediating energy-dependent – and tightly regulated - import and export of metabolites and polymers. Despite the growing understanding of the structural organization of membranes and molecular mechanisms of many membrane proteins, the origin(s) of biological membranes remain obscure. 7

Armen Y. Mulkidjanian (2010) The origins of membrane proteins are inextricably coupled with the origin of lipid membranes. Indeed, membrane proteins, which contain hydrophobic stretches and are generally insoluble in water, could not have evolved in the absence of functional membranes, while purely lipid membranes would be impenetrable and hence useless without membrane proteins. The origins of biological membranes – as complex cellular devices that control the energetics of the cell and its interactions with the surrounding world – remain obscure 8

Eugene V. Koonin: The origin of the cellular membrane itself seems to involve a catch-22: for a membrane to function in a cell, it must be endowed with at least a minimal repertoire of transport systems but it is unclear how such systems could evolve in the absence of a membrane. 6

The challenge to start harvesting energy
Geoffrey Zubay (2000):  Metabolism depends on factors that are external to the organism. The living system must extract nutrients from the environment and convert them to a biochemically useful form. In the next phase of the metabolism, which is internal, small molecules are synthesized and degraded. 19

Jeremy England (2020): EVERY LIFE IS ON FIRE How Thermodynamics Explains the Origins of Living Things: A spring has first to be brought to a compressed state, that is ready to burst apart, forcefully, when properly triggered. When glass and dishes are thrown to the ground, the stored energy is released, but they get smashed, broken, or damaged. Accordingly, people can eat sugar, but not dynamite; plants love sunlight,  but not intense gamma rays. Life needs access to energy, but it has to absorb it in specific ways that are conducive to activating “healthy” motions while avoiding “unhealthy” ones. To get a little bit more technical, it helps to remember that living things are in highly specialized, exceptionally rare configurations of their constituent parts that would not easily be discovered by a random and unbiased search of the space of their possible arrangements. 17

ADDY PROSS (2012) Organized complexity and one of the most fundamental laws of the universe—the Second Law of Thermodynamics—are inherently adversarial. Nature prefers chaos to order, so disorganization is the natural order. Within living systems, however, the highly organized state that is absolutely essential for viable biological function is somehow maintained with remarkable precision. : The living cell is able to maintain its structural integrity and its organization through the continual utilization of energy, which is in fact part of the cell’s modus operandi. . So there is no thermodynamic contradiction in life’s organized high-energy state, just as there is no contradiction in a car being able to drive uphill in opposition to the Earth’s gravitational pull, or a refrigerator in maintaining a cool interior despite the constant flow of heat into that interior from the warmer exterior. Both the car driving uphill and the refrigerator with its cold interior can maintain their energetically unstable state through the continual utilization of energy. In the car’s case the burning of gasoline in the car’s engine is the energy source, while in the case of the refrigerator, the energy source is the electricity supply that operates the refrigerator’s compressor. In an analogous manner, energetically speaking, the body can maintain its highly organized state through the continual utilization of energy from some external source—the chemical energy inherent within the foods we eat, or, in the case of plants, the solar energy that is captured by the chlorophyll pigment found in all plants. But how the initial organization associated with the simplest living system came about originally is a much tougher question. 15

The Hot Spring Hypothesis for an Origin of Life
Bruce Damer and David Deamer

Temperature and Stability: Hot springs are characterized by high temperatures, which can be detrimental to the stability of complex organic molecules like RNA. RNA is prone to degradation at high temperatures, making it challenging for the necessary biomolecules to persist and accumulate in such environments.

Lack of Concentration: Hot springs generally have dilute solutions due to high water flow, which can result in low concentrations of crucial organic molecules. Prebiotic chemistry requires a sufficient concentration of building blocks, such as amino acids or nucleotides, for the emergence of life-like processes. The low concentration of these molecules in hot springs may hinder the formation of complex organic structures.

Harsh Chemical Conditions: Hot springs often contain high levels of minerals and metal ions, which can have inhibitory or destructive effects on prebiotic chemistry. These chemical conditions may interfere with the formation and stability of key biomolecules, potentially hindering the emergence of life.

Narrow Environmental Range: Hot springs represent a specific type of geothermal environment, characterized by high temperatures, mineral-rich water, and unique physical conditions. While these conditions may be suitable for certain life forms, they may not be representative of the broader range of environments on early Earth where life could have originated. Critics argue that the Hot Spring Hypothesis does not adequately address the diversity of potential environments involved in the origin of life.

Lack of Experimental Evidence: Currently, there is a lack of direct experimental evidence supporting the specific mechanisms proposed by the Hot Spring Hypothesis. While the hypothesis is based on logical extrapolations from known properties of hot springs, more experimental work and evidence are needed to validate its claims.

Hot springs, with their high temperatures and unique chemical conditions, present obstacles to the stability of complex organic molecules like RNA. RNA is particularly susceptible to degradation at elevated temperatures, making it difficult for the necessary biomolecules to persist and accumulate in hot spring environments. Additionally, the dilute solutions found in hot springs due to high water flow can result in low concentrations of crucial organic molecules. The emergence of life-like processes requires sufficient concentrations of building blocks like amino acids or nucleotides, which may be hindered in such environments. Furthermore, hot springs often contain high levels of minerals and metal ions, which can have inhibitory or destructive effects on prebiotic chemistry. These harsh chemical conditions may interfere with the formation and stability of key biomolecules, further challenging the origin of life in hot springs. Another aspect to consider is the narrow environmental range of hot springs. While these geothermal environments may provide suitable conditions for certain life forms, they may not represent the diversity of potential environments on early Earth where life could have emerged. Critics argue that the Hot Spring Hypothesis, which suggests that life originated in hot springs, may not adequately account for the broad range of environments involved in the origin of life. The Hot Spring Hypothesis lacks direct experimental evidence to support its specific mechanisms. While the hypothesis is based on logical extrapolations from known properties of hot springs, further experimental work and evidence are necessary to validate its claims. This highlights the ongoing need for research and investigation into the role of hot springs in the origin of life.


https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7133448/#:~:text=We%20present%20a%20testable%20hypothesis,and%20dehydration%20to%20form%20protocells.