Primordial membranes: more than simple container boundaries

Architects of their own environment: How membrane proteins shape the Gram-negative cell envelope

Gram-negative bacteria are surrounded by a complex multilayered cell envelope, consisting of an inner and an outer membrane, and separated by the aqueous periplasm, which contains a thin peptidoglycan cell wall. These bacteria employ an arsenal of highly specialized membrane protein machineries to ensure the correct assembly and maintenance of the membranes forming the cell envelope. Here, we review the diverse protein systems, which perform these functions in Escherichia coli, such as the folding and insertion of membrane proteins, the transport of lipoproteins and lipopolysaccharide within the cell envelope, the targeting of phospholipids, and the regulation of mistargeted envelope components. Some of these protein machineries have been known for a long time, yet still hold surprises. Others have only recently been described and some are still missing pieces or yet remain to be discovered.

Deoxyribonucleic acid anchored on cell membranes for biomedical application

Engineering cellular membranes with functional molecules provides an attractive strategy to manipulate cellular behaviors and functionalities. Currently, synthetic deoxyribonucleic acid (DNA) has emerged as a promising molecular tool to engineer cellular membranes for biomedical applications due to its molecular recognition and programmable properties. In this review, we summarized the recent advances in anchoring DNA on the cellular membranes and their applications. The strategies for anchoring DNA on cell membranes were summarized. Then their applications, such as immune response activation, receptor oligomerization regulation, membrane structure mimicking, cell-surface biosensing, and construction of cell clusters, were listed. The DNA-enabled intelligent systems which were able to sense stimuli such as DNA strands, light, and metal ions were highlighted. Finally, insights regarding the remaining challenges and possible future directions were provided.

Population-Level Membrane Diversity Triggers Growth and Division of Protocells

To date, multiple mechanisms have been described for the growth and division of model protocells, all of which exploit the lipid dynamics of fatty acids. In some examples, the more heterogeneous aggregate consisting of fatty acid and diacyl phospholipid or fatty acid and peptide grows at the expense of the more homogeneous aggregate containing a restricted set of lipids with similar dynamics. Imbalances between surface area and volume during growth can generate filamentous vesicles, which are typically divided by shear forces. Here, we describe another pathway for growth and division that depends simply on differences in the compositions of fatty acid membranes without additional components. Growth is driven by the thermodynamically favorable mixing of lipids between two populations, i.e., the system as a whole proceeds toward equilibrium. Division is the result of growth-induced curvature. Importantly, growth and division do not require a specific composition of lipids. For example, vesicles made from one type of lipid, e.g., short-chain fatty acids, grow and divide when fed with vesicles consisting of another type of lipid, e.g., long-chain fatty acids, and vice versa. After equilibration, additional rounds of growth and division could potentially proceed by the introduction of compositionally distinct aggregates. Since prebiotic synthesis likely gave rise to mixtures of lipids, the data are consistent with the presence of growing and dividing protocells on the prebiotic Earth.

Tailoring of Peptide Vesicles: A Bottom-Up Chemical Approach

Spherical ordering from small molecules is a subject of intense interest to chemists. The inherent capability of amphiphiles to assemble spontaneously is the unique feature of the evolutionary process of life. Self-assembly is prevalent in biology and has attracted the interest of scientists across several disciplines. This is because scientists have realized that nature has extensively used this inherent organizational power contained in the molecules. Judicious use of the self-assembly principle is the cornerstone of nature's exotic assemblies. These exotic assemblies lead to unimaginable functions in biology that might not have been predicted from the monomer building blocks alone. Recently, a number of chemical systems that self-assemble in aqueous or organic solvents to form vesicles were reported. This account provides advances made from our laboratory toward designing and understanding the mechanism of formation of spherical vesicular assembly. A bottom-up approach for the de novo design of vesicles using nonlipidated molecular architecture will be a paradigm shift in vesicular research. Vesicles act as a protocell model for studying the origin and evolution of cellular life. They could also act as excellent model systems for studying the fusion of cells and membrane transport. Self-assembled vesicles have enormous potential for several applications such as drug and biomolecule delivery to cells and in materials science. These aspects along with the dynamic nature of vesicular assembly have attracted researchers to the study of spherical assemblies. The common belief was that the molecules that form vesicles must have one polar head and two hydrophobic tails. All attempts to synthesize vesicles are by mimicking nature's strategy, which mainly involves the self-assembly of lipid amphiphiles through a bilayer-like arrangement. Pseudopeptide-based molecules with the ability to form vesicles have changed this long-standing notion. In addition to chemical and medical applications, these peptide vesicles could act as models for protocells, membrane fusion, and the study of the vesiculation mechanism. This Account highlights the progress made toward a heuristic approach to the de novo design of vesicles using pseudopeptides as building blocks.A large number of diverse classes of pseudopeptides showed vesicular assembly. Various acyclic and cyclic molecules were designed and synthesized that showed spherical vesicular assembly. Cystine-based macrocyclic peptides showed drug encapsulation and release. Polymersomes with unusual topology, self-assembling tripodal ligands, and molecules containing amino acids such as lysine, leucine, cystine, and serine were synthesized. The incorporation of a wide variety of amino acids in the vesicle-forming peptides could enhance their scope and applications. The mechanism of vesiculation was also investigated using these designer molecules.

Fast bilayer-micelle fusion mediated by hydrophobic dipeptides

To understand the transition from inanimate matter to life, we studied a process that directly couples simple metabolism to evolution via natural selection, demonstrated experimentally by Adamala and Szostak. In this process, dipeptides synthesized inside precursors of cells promote absorption of fatty acid micelles to vesicles, inducing their preferential growth and division at the expense of other vesicles. The process is explained on the basis of coarse-grained molecular dynamics simulations, each extending for tens of microseconds, carried out to model fusion between a micelle and a membrane, both made of fatty acids in the absence and presence of hydrophobic dipeptides. In all systems with dipeptides, but not in their absence, fusion events were observed. They involve the formation of a stalk made by hydrophobic chains from the micelle and the membrane, similar to that postulated for vesicle-vesicle fusion. The emergence of a stalk is facilitated by transient clusters of dipeptides, side chains of which form hydrophobic patches at the membrane surface. Committor probability calculations indicate that the size of a patch is a suitable reaction coordinate and allows for identifying the transition state for fusion. Free-energy barrier to fusion is greatly reduced in the presence of dipeptides to only 4-5 kcal/mol, depending on the hydrophobicity of side chains. The mechanism of mediated fusion, which is expected to apply to other small peptides and hydrophobic molecules, provides a robust means by which a nascent metabolism can confer evolutionary advantage to precursors of cells.

The requirement of cellularity for abiogenesis

The history of modern biochemistry started with the cellular theory of life. By putting aside the holistic protoplasmic theory, scientists of the XX century were able to advance the functional classification of cellular components significantly. The cell became the unit of the living. Current theories on the abiogenesis of life must account for a moment in evolution (chemical or biological) when this was not the case. Investigating the role of compartments and membranes along chemical and biotic evolution can lead a more generalised idea of living organisms that is fundamental to advance our efforts in astrobiology, origin of life and artificial life studies. Furthermore, it may provide insights in unexplained evolutionary features such as the lipid divide between Archaea and Eubacteria. By surveying our current understanding of the involvement of compartments in abiogenesis and evolution, the idea of cells as atomistic units of a general theory of biology will be discussed. The aim is not to undermine the validity of the cellular theory of life, but rather to elucidate possible biases with regards to cellularity and the origin of life. An open discussion in these regards could show the inherent limitations of non-cellular compartmentalization that may lead to the necessity of cellular structures to support complex life.

Cysteine, glutathione and a new genetic code: biochemical adaptations of the primordial cells that spread into open water and survived biospheric oxygenation

Life most likely developed under hyperthermic and anaerobic conditions in close vicinity to a stable geochemical source of energy. Epitomizing this conception, the first cells may have arisen in submarine hydrothermal vents in the middle of a gradient established by the hot and alkaline hydrothermal fluid and the cooler and more acidic water of the ocean. To enable their escape from this energy-providing gradient layer, the early cells must have overcome a whole series of obstacles. Beyond the loss of their energy source, the early cells had to adapt to a loss of external iron-sulfur catalysis as well as to a formidable temperature drop. The developed solutions to these two problems seem to have followed the principle of maximum parsimony: Cysteine was introduced into the genetic code to anchor iron-sulfur clusters, and fatty acid unsaturation was installed to maintain lipid bilayer viscosity. Unfortunately, both solutions turned out to be detrimental when the biosphere became more oxidizing after the evolution of oxygenic photosynthesis. To render cysteine thiol groups and fatty acid unsaturation compatible with life under oxygen, numerous counter-adaptations were required including the advent of glutathione and the addition of the four latest amino acids (methionine, tyrosine, tryptophan, selenocysteine) to the genetic code. In view of the continued diversification of derived antioxidant mechanisms, it appears that modern life still struggles with the initially developed strategies to escape from its hydrothermal birthplace. Only archaea may have found a more durable solution by entirely exchanging their lipid bilayer components and rigorously restricting cysteine usage.

Rationally Designed Multivalent Aptamers Targeting Cell Surface for Biomedical Applications

Specific interactions between ligands and receptors on cell surface play an important role in the cell biological process. Nucleic acid aptamers as commonly used ligands enable specific recognition and tight binding to membrane protein receptors for modulation of cell fate. Therefore, molecular probes with aptamers can be applied for cancer diagnosis and targeted therapy by targeting overexpression membrane proteins of cancer cells. However, because of their fast degradation and rapid glomerulus clearance in vivo, the applications of aptamers in physiological conditions remain challenged. Inspired by natural multivalent interactions, many approaches have been developed to construct multivalent aptamers to improve the performance of aptamers in complex matrices with higher binding affinity, more stability, and longer circulation time. In this review, we first introduce the aptamer generation from purified protein-based SELEX and whole cell-based SELEX for targeting the cell surface. We then highlight the approaches to fabricate multivalent aptamers and discuss their properties. By integrating different materials (including inorganic nanomaterials, diacyllipid, polymeric nanoparticles, and DNA nanostructures) as scaffolds with an interface modification technique, we have summarized four kinds of multivalent aptamers. After that, representative applications in biosensing and targeted therapy are illustrated to show the elevated performance of multivalent aptamers. In addition, we analyze the challenges and opportunities for the clinical practices of multivalent aptamers.

Primitive Compartmentalization for the Sustainable Replication of Genetic Molecules

Sustainable replication and evolution of genetic molecules such as RNA are likely requisites for the emergence of life; however, these processes are easily affected by the appearance of parasitic molecules that replicate by relying on the function of other molecules, while not contributing to their replication. A possible mechanism to repress parasite amplification is compartmentalization that segregates parasitic molecules and limits their access to functional genetic molecules. Although extent cells encapsulate genomes within lipid-based membranes, more primitive materials or simple geological processes could have provided compartmentalization on early Earth. In this review, we summarize the current understanding of the types and roles of primitive compartmentalization regarding sustainable replication of genetic molecules, especially from the perspective of the prevention of parasite replication. In addition, we also describe the ability of several environments to selectively accumulate longer genetic molecules, which could also have helped select functional genetic molecules rather than fast-replicating short parasitic molecules.

Membrane lipids and transporter function

Transport proteins are essential for cells in allowing the exchange of substances between cells and their environment across the lipid bilayer forming a tight barrier. Membrane lipids modulate the function of transmembrane proteins such as transporters in two ways: Lipids are tightly and specifically bound to transport proteins and in addition they modulate from the bulk of the lipid bilayer the function of transport proteins. This overview summarizes currently available information at the ultrastructural level on lipids tightly bound to transport proteins and the impact of altered bulk membrane lipid composition. Human diseases leading to altered lipid homeostasis will lead to altered membrane lipid composition, which in turn affect the function of transporter proteins.

Prebiological Membranes and Their Role in the Emergence of Early Cellular Life

Membrane compartmentalization is a fundamental feature of contemporary cellular life. Given this, it is rational to assume that at some stage in the early origins of life, membrane compartments would have potentially emerged to form a dynamic semipermeable barrier in primitive cells (protocells), protecting them from their surrounding environment. It is thought that such prebiological membranes would likely have played a crucial role in the emergence and evolution of life on the early Earth. Extant biological membranes are highly organized and complex, which is a consequence of a protracted evolutionary history. On the other hand, prebiotic membrane assemblies, which are thought to have preceded sophisticated contemporary membranes, are hypothesized to have been relatively simple and composed of single chain amphiphiles. Recent studies indicate that the evolution of prebiotic membranes potentially resulted from interactions between the membrane and its physicochemical environment. These studies have also speculated on the origin, composition, function and influence of environmental conditions on protocellular membranes as the niche parameters would have directly influenced their composition and biophysical properties. Nonetheless, the evolutionary pathways involved in the transition from prebiological membranes to contemporary membranes are largely unknown. This review critically evaluates existing research on prebiotic membranes in terms of their probable origin, composition, energetics, function and evolution. Notably, we outline new approaches that can further our understanding about how prebiotic membranes might have evolved in response to relevant physicochemical parameters that would have acted as pertinent selection pressures on the early Earth.

Cell Communications among Microorganisms, Plants, and Animals: Origin, Evolution, and Interplays

Cellular communications play pivotal roles in multi-cellular species, but they do so also in uni-cellular species. Moreover, cells communicate with each other not only within the same individual, but also with cells in other individuals belonging to the same or other species. These communications occur between two unicellular species, two multicellular species, or between unicellular and multicellular species. The molecular mechanisms involved exhibit diversity and specificity, but they share common basic features, which allow common pathways of communication between different species, often phylogenetically very distant. These interactions are possible by the high degree of conservation of the basic molecular mechanisms of interaction of many ligand-receptor pairs in evolutionary remote species. These inter-species cellular communications played crucial roles during Evolution and must have been positively selected, particularly when collectively beneficial in hostile environments. It is likely that communications between cells did not arise after their emergence, but were part of the very nature of the first cells. Synchronization of populations of non-living protocells through chemical communications may have been a mandatory step towards their emergence as populations of living cells and explain the large commonality of cell communication mechanisms among microorganisms, plants, and animals.

Racemic Phospholipids for Origin of Life Studies

Although prebiotic condensations of glycerol, phosphate and fatty acids produce phospholipid esters with a racemic backbone, most experimental studies on vesicles intended as protocell models have been carried out by employing commercial enantiopure phospholipids. Current experimental research on realistic protocell models urgently requires racemic phospholipids and efficient synthetic routes for their production. Here we propose three synthetic pathways starting from glycerol or from racemic solketal (α,β-isopropylidene-dl-glycerol) for the gram-scale production (up to 4 g) of racemic phospholipid ester precursors. We describe and compare these synthetic pathways with literature data. Racemic phosphatidylcholines and phosphatidylethanolamines were obtained in good yields and high purity from 1,2-diacylglycerols. Racemic POPC (rac-POPC, (R,S)-1-palmitoyl-2-oleoyl-3-phosphocholine), was used as a model compound for the preparation of giant vesicles (GVs). Confocal laser scanning fluorescence microscopy was used to compare GVs prepared from enantiopure (R)-POPC), racemic POPC (rac-POPC) and a scalemic mixture (scal-POPC) of (R)-POPC enriched with rac-POPC. Vesicle morphology and size distribution were similar among the different (R)-POPC, rac-POPC and scal-POPC, while calcein entrapments in (R)-POPC and in scal-POPC were significantly distinct by about 10%.

Cyclophospholipids Increase Protocellular Stability to Metal Ions

Model protocells have long been constructed with fatty acids, because these lipids are prebiotically plausible and can, at least theoretically, support a protocell life cycle. However, fatty acid protocells are stable only within a narrow range of pH and metal ion concentration. This instability is particularly problematic as the early Earth would have had a range of conditions, and extant life is completely reliant on metal ions for catalysis and the folding and activity of biological polymers. Here, prebiotically plausible monoacyl cyclophospholipids are shown to form robust vesicles that survive a broad range of pH and high concentrations of Mg²⁺ , Ca²⁺ , and Na⁺ . Importantly, stability to Mg²⁺ and Ca²⁺ is improved by the presence of environmental concentrations of Na⁺ . These results suggest that cyclophospholipids, or lipids with similar characteristics, may have played a central role during the emergence of Darwinian evolution.

Introduction to virus origins and their role in biological evolution

Viruses are diverse parasites of cells and extremely abundant. They might have arisen during an early phase of the evolution of life on Earth dominated by ribonucleic acid or RNA-like macromolecules, or when a cellular world was already well established. The theories of the origin of life on Earth shed light on the possible origin of primitive viruses or virus-like genetic elements in our biosphere. Some features of present-day viruses, notably error-prone replication, might be a consequence of the selective forces that mediated their ancestral origin. Two views on the role of viruses in our biosphere predominate; viruses considered as opportunistic, selfish elements, and viruses considered as active participants in the construction of the cellular world via the lateral transfer of genes. These two models have a bearing on viruses being considered predominantly as disease agents or predominantly as cooperators in the shaping of differentiated cellular organisms.

Isoprenoids enhance the stability of fatty acid membranes at the emergence of life potentially leading to an early lipid divide

Two key problems concern cell membranes during the emergence and early evolution of life: what was their initial composition, and why did the membranes of archaea and bacteria diverge? The composition of the first cell membranes could shed light on the most likely environment for the emergence of life. The opposing stereochemistry of modern lipid glycerol-phosphate headgroups in bacteria and archaea suggests that early membranes were composed of single chain amphiphiles, perhaps both fatty acids and isoprenoids. We investigated the effect of adding isoprenoids to fatty acid membranes using a combination of UV-visible spectroscopy, confocal microscopy and transmission electron microscopy. We tested the stability of these membranes across a pH range and under different concentrations of ionic species relevant to oceanic hydrothermal environments, including Na2+, Cl-, Mg2+, Ca2+, HC O 3 - , Fe3+, Fe2+ and S2-. We also tested the assembly of vesicles in the presence of Fe particles and FeS precipitates. We found that isoprenoids enhance the stability of membranes in the presence of salts but require 30-fold higher concentrations for membrane formation. Intriguingly, isoprenoids strongly inhibit the tendency of vesicles to aggregate together in the presence of either Fe particles or FeS precipitates. These striking physical differences in the stability and aggregation of protocells may have shaped the divergence of bacteria and archaea in early hydrothermal environments.

On the existence of endocytosis driven by membrane phase separations

Large endocytic responses can occur rapidly in diverse cell types without dynamins, clathrin, or actin remodeling. Our experiments suggest that membrane phase separations are crucial with more ordered plasma membrane domains being internalized. Not only do these endocytic processes rely on coalescence of membrane domains, they are promoted by participation of membrane proteins in such domains, one important regulatory influence being palmitoylation. Membrane actin cytoskeleton in general resists membrane phase transitions, and its remodeling may play many roles. Besides membrane 'caging' and 'pinching' roles, typically ascribed to clathrin and dynamins, cytoskeleton remodeling may modify local membrane tension and buckling, as well as the presence and location of actin- and tension-free membrane patches. Endocytosis that depends on membrane phase separations becomes activated in metabolic stress and in response to Ca and PI3 kinase signaling. Internalized membrane traffics normally, and the secretory pathway eventually resupplies membrane to the plasmalemma or directs internalized membrane to other locations, including the extracellular space as exosomes. We describe here that endocytosis driven by membrane phase transitions is regulated by the same signaling mechanisms that regulate macropinocytosis, and it may play diverse roles in cells from nutrient assimilation to membrane recycling, cell migration, and the initiation of quiescent or hibernating cell states. Membrane ordering and phase separations have been shown to promote endocytosis in diverse cell types, including fibroblasts, myocytes, glial cells, and immune cells. We propose that clathrin/dynamin-independent endocytosis represents a continuum of related mechanisms with variable but universal dependence on membrane ordering and actin remodeling. This article is part of a Special Issue entitled: Molecular biophysics of membranes and membrane proteins.

In search of a primitive signaling code

Cells must have preceded by simpler chemical systems (protocells) that had the capacity of a spontaneous self-assembly process and the ability to confine chemical reaction networks together with a form of information. The presence of lipid molecules in the early Earth conditions is sufficient to ensure the occurrence of spontaneous self-assembly processes, not defined by genetic information, but related to their chemical amphiphilic nature. Ribozymes are plausible molecules for early life, being the first small polynucleotides made up of random oligomers or formed by non-enzymatic template copying. Compartmentalization represents a strategy for the evolution of ribozymes; the attachment of ribozymes to surfaces, such as formed by lipid micellar aggregates may be particular relevant if the surface itself catalyzes RNA polymerization.It is conceivable that the transition from pre-biotic molecular aggregates to cellular life required the coevolution of the RNA world, capable of synthesizing specific, instead of statistical proteins, and of the Lipid world, with a transition from micellar aggregates to semipermeable vesicles. Small molecules available in the prebiotic inventory might promote RNA stability and the evolution of hydrophobic micellar aggregates into membrane-delimited vesicles. The transition from ribozymes catalyzing the assembly of statistical polypeptides to the synthesis of proteins, required the appearance of the genetic code; the transition from hydrophobic platforms favoring the stability of ribozymes and of nascent polypeptides to the selective transport of reagents through a membrane, required the appearance of the signal transduction code.A further integration between the RNA and Lipid worlds can be advanced, taking into account the emerging roles of phospholipid aggregates not only in ensuring stability to ribozymes by compartmentalization, but also in a crucial step of evolution through natural selection mechanisms, based on signal transduction pathways that convert environmental changes into biochemical responses that could vary according to the context. Here I present evidences on the presence of traces of the evolution of a signal transduction system in extant cells, which utilize a phosphoinositide signaling system located both at nucleoplasmic level as well as at the plasma membrane, based on the very same molecules but responding to different rules. The model herewith proposed is based on the following assumptions on the biomolecules of extant organisms: i) amphiphils can be converted into structured aggregates by hydrophobic forces thus giving rise to functional platforms for the interaction of other biomolecules and to their compartmentalization; ii) fundamental biochemical pathways, including protein synthesis, can be sustained by natural ribozymes of ancient origin; iii) ribozymes and nucleotide-derived coenzymes could have existed long before protein enzymes emerged; iv) signaling molecules, both derived from phospholipids and from RNAs could have guided the evolution of complex metabolic processes before the emergence of proteins.

Investigating Prebiotic Protocells for A Comprehensive Understanding of the Origins of Life: A Prebiotic Systems Chemistry Perspective

Protocells are supramolecular systems commonly used for numerous applications, such as the formation of self-evolvable systems, in systems chemistry and synthetic biology. Certain types of protocells imitate plausible prebiotic compartments, such as giant vesicles, that are formed with the hydration of thin films of amphiphiles. These constructs can be studied to address the emergence of life from a non-living chemical network. They are useful tools since they offer the possibility to understand the mechanisms underlying any living cellular system: Its formation, its metabolism, its replication and its evolution. Protocells allow the investigation of the synergies occurring in a web of chemical compounds. This cooperation can explain the transition between chemical (inanimate) and biological systems (living) due to the discoveries of emerging properties. The aim of this review is to provide an overview of relevant concept in prebiotic protocell research.

Progress in synthesizing protocells

IMPACT STATEMENT

Advances in the understanding of the biophysics of membranes, the nonenzymatic and enzymatic polymerization of RNA, and in the design of complex chemical reaction networks have led to a new, integrated way of viewing the shared chemistry needed to sustain life. Although a protocell capable of Darwinian evolution has yet to be built, the seemingly disparate pieces are beginning to fit together. At the very least, better cellular mimics are on the horizon that will likely teach us much about the physicochemical underpinnings of cellular life.

In situ fluorescent profiling of living cell membrane proteins at a single-molecule level

A signal amplification method is developed for visualization analysis of membrane proteins on living cells at a single-molecule level.

Vesicle Self-Assembly of Monoalkyl Amphiphiles under the Effects of High Ionic Strength, Extreme pH, and High Temperature Environments

Vesicles and other bilayered membranous structures can self-assemble from single hydrocarbon chain amphiphiles. Their formation and stability are highly dependent on experimental conditions such as ionic strength, pH, and temperature. The addition of divalent cations, for example, often results in the disruption of vesicles made of a single fatty acid species through amphiphile precipitation. However, membranes composed of amphiphile mixtures have been shown to be more resistant to low millimolar concentrations of divalent cations at room temperature. In this report, several mixtures of amphiphiles are examined for their propensity to self-assemble into membranous vesicular structures under extreme environmental conditions of low pH, high ionic strengths, and temperatures. In particular, mixtures of decylamine with polar cosurfactants were found to efficiently form membranes under these conditions far away from those normally supporting vesicle formation. We further examined decanoic acid/decylamine mixtures in detail. At pH 2 in low ionic strength solutions, the amphiphiles formed oily or crystalline structures; however, the introduction of salts or/and strong acids in conjunction with high temperature induced a stable vesiculation. Thus, extreme environments, such as volcanic or vent environments whose environmental conditions are known to support high chemical reactivity, could have harbored and most significantly promoted the formation of simple organic compartments that preceded cells.