Protocell Membranes Research Articles

A Model Protometabolic Pathway Across Protocell Membranes Assisted by Photocatalytic Minerals.

Protocell analogs (lipid vesicles) to modern cell membranes have been postulated as compartments that may have been involved in primordial metabolism during the transition from geochemistry to biochemistry on early Earth. The transduction of light energy into chemical energy for metabolism was a key step in the transition from the earliest metabolisms to phototrophy. Photocatalytic minerals may have served the role of enzymes during these transitional stages. Here, we demonstrate a simple photoheterotrophic protometabolism promoted by photocatalytic minerals across a model protocell (vesicle) membrane. These minerals in the extra-vesicular medium utilized light energy to drive a coupled, multi-step transmembrane electron transfer reaction (TMETR), while simultaneously generating a transmembrane pH gradient and reducing nicotinamide adenine dinucleotide (NAD+) to NADH within the vesicle. The proton gradient or chemiosmotic potential could have provided a basis for adenosine triphosphate (ATP) synthesis and NADH could potentially have driven further metabolic chemistry inside the protocells.

A Model Protometabolic Pathway across Protocell Membranes Assisted by Photocatalytic Minerals

Protocell analogs (lipid vesicles) to modern cell membranes have been postulated as compartments that may have been involved in primordial metabolism during the transition from geochemistry to biochemistry on early Earth. The transduction of light energy into chemical energy for metabolism was a key step in the transition from the earliest metabolisms to phototrophy. Photocatalytic minerals may have served the role of enzymes during these transitional stages. Here, we demonstrate a simple photoheterotrophic protometabolism promoted by photocatalytic minerals across a model protocell (vesicle) membrane. These minerals in the extra-vesicular medium utilized light energy to drive a coupled, multistep transmembrane electron transfer reaction (TMETR), while simultaneously generating a transmembrane pH gradient and reducing nicotinamide adenine dinucleotide (NAD+) to NADH within the vesicle. The proton gradient or chemiosmotic potential could have provided a basis for adenosine triphosphate (ATP) synthesis and ...

Lipid constituents of model protocell membranes.

Primitive life must have possessed the essential features of modern cellular life, but without highly evolved proteins to perform dynamic functions such as nutrient transport and membrane remodeling. Here, we consider the membrane properties of protocells - minimal cells with hereditary material, capable of growth and division - and how these properties place restrictions on the components of the membrane. For example, the lipids of modern membranes are diacyl amphiphilic molecules containing well-over 20 carbons in total. Without proteins, these membranes are very stable and kinetically trapped. This inertness, combined with the need for enzymes to synthesize them, makes modern diacyl amphiphiles unsuitable candidates for the earliest membranes on Earth. We, therefore, discuss the progress made thus far with single-chained amphiphiles, including fatty acids and mixtures of fatty acids with related molecules, and the membrane-related research that must be undertaken to gain more insight into the origins of cellular life.

Prebiotic Protocell Model Based on Dynamic Protein Membranes Accommodating Anabolic Reactions.

The nature of the first prebiotic compartments and their possible minimal molecular composition is of great importance in the origin of life scenarios. Current protocell model membranes are proposed to be lipid-based. This paradigm has several shortcomings such as limited membrane stability of monoacyl lipid-based membranes (e.g., fatty acids), missing pathways to synthesize protocell membrane components (e.g., phospholipids) under early earth conditions, and the requirement for different classes of molecules for the formation of compartments and the catalysis of reactions. Amino acids on the other hand are known to arise and persist with remarkable abundance under early earth conditions since the fundamental Miller-Urey experiments. They were also postulated early to form protocellular structures, for example, proteinoid capsules. Here, we present a protocell model constituted by membranes assembled from amphiphilic proteins based on prebiotic amino acids. Self-assembled dynamic protein membrane-based compartments (PMBCs) are impressively stable and compatible with prevalent cellular membrane constituents forming protein-only or protein-lipid hybrid membranes. They can embed processes essential for extant living cells, such as enclosure of molecules, membrane fusion, phase separation, and complex biosynthetic elements from modern cells demonstrating "upward" compatibility. Our findings suggest that prebiotic PMBCs represent a new type of protocell as a possible ancestor of current lipid-based cells. The presented prebiotic PMBC model can be used to design artificial cells, important for the study of structural, catalytic, and evolutionary pathways related to the emergence of life.

Functional Properties of Amino Acid Side Chains as Biomarkers of Extraterrestrial Life.

The present study proposes to search our solar system (Mars, Enceladus, Europa) for patterns of organic molecules that are universally associated with biological functions and structures. The functions are primarily catalytic because life could only have originated within volume/space-constrained compartments containing chemical reactions catalyzed by certain polymers. The proposed molecular structures are specific groups in the side chains of amino acids with the highest catalytic propensities related to life on Earth, that is, those that most frequently participate as key catalytic groups in the active sites of enzymes such as imidazole, thiol, guanidinium, amide, and carboxyl. Alternatively, these or other catalytic groups can be searched for on non-amino-acid organic molecules, which can be tested for certain hydrolytic catalytic activities. The first scenario assumes that life may have originated in a similar manner as the terrestrial set of α-amino acids, while the second scenario does not set such a requirement. From the catalytic propensity perspective proposed in the first scenario, life must have invented amino acids with high catalytic propensity (His, Cys, Arg) in order to overcome, and be complemented by, the low catalytic propensity of the initially available abiogenic amino acids. The abiogenic and the metabolically invented amino acids with the lowest catalytic propensity can also serve as markers of extraterrestrial life when searching for patterns on the basis of the following functional propensities related to protein secondary/quaternary structure: (1) amino acids that are able to form α-helical intramembrane peptide domains, which can serve as primitive transporters in protocell membrane bilayers and catalysts of simple biochemical reactions; (2) amino acids that tend to accumulate in extremophile proteins of Earth and possibly extraterrestrial life. The catalytic/structural functional propensity approach offers a new perspective in the search for extraterrestrial life and could help unify previous amino acid–based approaches.

Modification of fatty acid vesicle using an imidazolium-based surface active ionic liquid: a detailed study on its modified properties using spectroscopy and microscopy techniques $$^{\S }$$ §

Fatty acid vesicles have attracted views as model protocell membranes in understanding the emergence of life, but their properties can be further modified in the presence of some external molecules. In this work, we have investigated the spontaneous formation of large unilamellar vesicles (LUVs) of oleic acid in aqueous medium in presence of a popular imidazolium-based cationic surface active ionic liquid (SAIL) $$[\hbox {C}_{16}\hbox {mim}]\hbox {Cl}$$ and studied the micelle–vesicle transition of aqueous $$[\hbox {C}_{16}\hbox {mim}]\hbox {Cl}$$ solution in presence of different molar fractions (f) of oleic acid. This newly formed oleic $$\hbox {acid}/[\hbox {C}_{16}\hbox {mim}]\hbox {Cl}$$ vesicles exhibit some modified properties compared to the pure fatty acid vesicles. Unlike pure fatty acid vesicles, these vesicles are stable in the pH range of 2 to 11.2. We have observed the fusion process of these oleic acid/SAIL vesicles to form giant unilamellar vesicles (GUVs) in presence of low concentration of NaCl solution. To investigate the dynamics of different oleic $$\hbox {acid}/[\hbox {C}_{16}\hbox {mim}]\hbox {Cl}$$ self-assemblies, we have used fluorescence correlation spectroscopy (FCS). The translational diffusion behavior of three different dyes, Rhodamine 6G, DCM and Pyrromethene 597, which are non-covalently bound to the different regions of the oleic acid/SAIL self-assemblies, have been determined using FCS during the micelle–vesicle transition and upon varying the pH of the vesicular solution. Oleic acid vesicles prepared in presence of a surface active ionic liquid $$[\hbox {C}_{16}\hbox {mim}]\hbox {Cl}$$ have been characterized. Modified properties of these vesicles like stability towards a greater pH range as compared to pure oleic acid vesicles and fusion of vesicles in presence of low concentration of NaCl solution have been studied.

Protocells realize their potential

How the first metabolic network was organized to power a cell remains an enigma. Now, simple iron–sulfur peptides have been used to generate a pH-gradient across a protocell membrane by catalysing hydrogen peroxide reduction. This indicates that short peptides could have fulfilled the role of redox active metalloproteins in early life.

Prebiotic iron–sulfur peptide catalysts generate a pH gradient across model membranes of late protocells

Prebiotic chemistry was likely mediated by metals, but how such prebiotic chemistry progressed into the metabolic-like networks needed to sustain life remains unclear. Here we experimentally delineate a potential path from prebiotically plausible iron–sulfur peptide catalysts to the types of pH gradients exploited by all known living organisms. Iron–sulfur peptides cooperatively accept electrons from NADH in a manner that is only partially mediated by ionic interactions. The electrons are then either passed to a terminal electron acceptor, such as hydrogen peroxide, or to an intermediate carrier, such as ubiquinone. The reduction of hydrogen peroxide leads to the production of hydroxide, which then contributes to the formation of a pH gradient across late protocell membranes. The data are consistent with the activity of prebiotic iron–sulfur peptide catalysts providing a selective advantage by equipping protocells with a pathway that connects catabolism to anabolism.

Compartmentalized Assembly of Motor Protein Reconstituted on Protocell Membrane toward Highly Efficient Photophosphorylation.

Molecule assembly and functionalization of protocells have achieved a great success. However, the yield efficiency of photophosphorylation in the present cell-like systems is limited. Herein, inspired by natural photobacteria, we construct a protocell membrane reconstituting motor protein for highly efficient light-mediated adenosine triphosphate (ATP) synthesis through a layer-by-layer technique. The assembled membrane, compartmentally integrating photoacid generator, proton conductor, and ATP synthase, possesses excellent transparency, fast proton production, and quick proton transportation. Remarkably, these favorable features permit the formation of a large proton gradient in a confined region to drive ATP synthase to produce ATP with high efficiency (873 ATP s-1). It is the highest among the existing artificial photophosphorylation systems. Such a biomimetic system provides a bioenergy-supplying scenario for early photosynthetic life and holds promise in remotely controlled ATP-consumed biosensors, biocatalysts, and biodevices.

Mineral Surface Chemistry and Nanoparticle-aggregation Control Membrane Self-Assembly

The self-assembly of lipid bilayer membranes to enclose functional biomolecules, thus defining a “protocell,” was a seminal moment in the emergence of life on Earth and likely occurred at the micro-environment of the mineral-water interface. Mineral-lipid interactions are also relevant in biomedical, industrial and technological processes. Yet, no structure-activity relationships (SARs) have been identified to predict lipid self-assembly at mineral surfaces. Here we examined the influence of minerals on the self-assembly and survival of vesicles composed of single chain amphiphiles as model protocell membranes. The apparent critical vesicle concentration (CVC) increased in the presence of positively-charged nanoparticulate minerals at high loadings (mg/mL) suggesting unfavorable membrane self-assembly in such situations. Above the CVC, initial vesicle formation rates were faster in the presence of minerals. Rates were correlated with the mineral’s isoelectric point (IEP) and reactive surface area. The IEP depends on the crystal structure, chemical composition and surface hydration. Thus, membrane self-assembly showed rational dependence on fundamental mineral properties. Once formed, membrane permeability (integrity) was unaffected by minerals. Suggesting that, protocells could have survived on rock surfaces. These SARs may help predict the formation and survival of protocell membranes on early Earth and other rocky planets, and amphiphile-mineral interactions in diverse other phenomena.

The evolution and sustenance of life

An experiment with liquid nitrogen gave anomalous results, I proposed proton-ordered ice had contracted to accommodate water molecules' irregular tetrahedral shape. Recent reports of ice XIc at 72 K, below the ferroelectric transition temperature, confirmed it. Temperature fluctuations on primordial Earth’s poles released ice XIc's latent energy as ~4 m infrared laser light. After reflection by ice in clouds and on Earth's surface, it energized deoxynucleotides, turning Darwin's warm tropical waters to DNA noodle soup. tRNA analogues, transport DNAs lined proto-cell membrane pores with H-bonds. Laser light drove a ratchet pump mechanism, concentrating substrates and promoting synthetic reactions within. Replicate t-DNAs marked life's origin. They elude detection, chromosomal DNA masks ~2,000/cell. A report of DNA particles around the entry of sperm to ovum might be verified. Differentiation DNAs selecting tDNAs determine tissue cell diet and metabolism, c.f. mRNAs selecting tRNAs for protein synthesis. Adequate trace elements tDNAs employ govern survival. Twenty-one flat protein hairpins bind 189 base-pairs forming a ring, nine rings complete a minion. Minions pack chromosomes and ensure error-free replication. Their roles as biological clocks and chips in the brain explain psychology. Computers modeled on them promise human-friendly intelligence. They control nine metabolic pathways, predating enzyme synthesis. Adequate trace element nutrition promises better mental and physical health and longevity, public assurance of their safety is vital. ATP's Pi~Pi bond stores~4 m wavelength energy, e.g. sarcomeres form ½-wave resonant cavities for muscle contraction. Oscillating H-bond chains accelerate protons along adjacent minion tunnels fast enough to fuse with obstructing nuclei. Trapping the g-rays released could provide clean power, resolving global warming. My proposals restore the simplicity and public trust in science prevailing c 1900 AD, reinterpreting quantum weirdness and cosmology would redirect funds to conservation, humanitarian projects and diplomacy, ensuring a happy future.

Sodium Chloride Triggered the Fusion of Vesicle Composed of Fatty Acid Modified Protic Ionic Liquid: A New Insight into the Membrane Fusion Monitored through Fluorescence Lifetime Imaging Microscopy.

The development of stable vesicular assemblies and the understanding of their interaction and dynamics in aqueous solution are long-standing topics in the research of chemistry and biology. Fatty acids are known to form vesicle structure in aqueous solution depending on the pH of the medium. Protic ionic liquid of fatty acid with ethyl amine (oleate ethyl amine, OEA) as a component spontaneously forms a vesicle in aqueous solution. The general comparison of dynamics and interaction of these two vesicles have been drawn using fluorescence correlation spectroscopy (FCS) and fluorescence lifetime imaging microscopy (FLIM) measurements. Further, FLIM images of a single vesicle are taken at multiple wavelengths, and the solvation of the probe molecules has been observed from the multiwavelength FLIM images. The lifetime of the probe molecule in OEA vesicle is higher than that in simple fatty acid vesicles. Therefore, it suggests that the membrane of the OEA vesicle is more dehydrated compared to that of fatty acid vesicles, and it facilitates OEA vesicles to fuse themselves in the presence of electrolyte, sodium chloride (NaCl). However, under the same conditions, only fatty acid vesicles do not fuse. The fusion of OEA vesicles is successfully demonstrated by the time scan FLIM measurements. The different events in the fusion process are analyzed in the light of the reported model of vesicle fusion. Finally, the local viscosity of the water pool of the vesicle is determined using kiton red, as a molecular rotor. With addition of NaCl, the fluidity in the interior of the vesicle is increased which leads to disassembly of vesicle. The rich dynamic properties of this vesicular assembly and the FLIM based approach of vesicle fusion will provide better insight into the growth of a protocell membrane.

N-Carboxyanhydride-Mediated Fatty Acylation of Amino Acids and Peptides for Functionalization of Protocell Membranes.

Early protocells are likely to have arisen from the self-assembly of RNA, peptide, and lipid molecules that were generated and concentrated within geologically favorable environments on the early Earth. The reactivity of these components in a prebiotic environment that supplied sources of chemical energy could have produced additional species with properties favorable to the emergence of protocells. The geochemically plausible activation of amino acids by carbonyl sulfide has been shown to generate short peptides via the formation of cyclic amino acid N-carboxyanhydrides (NCAs). Here, we show that the polymerization of valine-NCA in the presence of fatty acids yields acylated amino acids and peptides via a mixed anhydride intermediate. Notably, Nα-oleoylarginine, a product of the reaction between arginine and oleic acid in the presence of valine-NCA, partitions spontaneously into vesicle membranes and mediates the association of RNA with the vesicles. Our results suggest a potential mechanism by which activated amino acids could diversify the chemical functionality of fatty acid membranes and colocalize RNA with vesicles during the formation of early protocells.

Insights Arising When Quantum Mechanics is Reappraised From a Biological Perspective

A recently reported ferroelectric phase transition at 72 Kelvin in ice XIc corroborates an observation made as a physics student in 1967. I attributed anomalous readings from a silica helium thermometer suspended in liquid nitrogen to distortion by ice crystallizing on its surface, suggesting a proton ordered tetragonal variant, ice It of cubic ice had changed shape during an ordering phase transition releasing latent energy as 'ice light', wavelength λ ~ 4μ infrared laser light. My research into its consequences reinterprets the axiomatic basis of science, affording simple accounts of familiar topics. Ice light emitted from pools of nitrogen on Earth's poles during a primordial ice age was multiply reflected in cloud and surface ice, creating DNA, including tRNA analogue 'transport DNAs', tDNAs by selectively polymerizing nucleotides in equatorial waters. They constituted H-bond lined pores through proto-cell membranes, polarization by ice light created an electric field propelling ionic substrate-trace element complexes constituting life’s molecular vocabulary inside. 64 tDNA variants orchestrate biochemistry, supplementing trace elements prevents mental and physical maladies. ‘Minions’ arose from comparing cybernetics, psychology and traditional wisdom. 189 flat anti-parallel β-sheet hairpins with alternate neutral and basic amino acid residues bind 1,701 DNA base pairs, forming a nine-coil abacus. Minions probably evolved to pack DNA in chromosomes for accurate replication. 189*18 arrays of proton ordered Hbonds connecting amino acid ω-amines to DNA phosphates control metabolism, are biological clocks and 'chips in the brain'. When sarcomeres of striated muscle contract, they form ½-wave resonant cavities for λ, exemplifying biological energy coupling. Protons accelerated along minion tunnels drive molecular scale nuclear fusion, if coupled they could resolve global warming. Artificial intelligence modeled on minions would satisfy Turing's vision, allowing for personality and compensating the relativity between perception and conception, reinterpreting quantum mechanics, uncertainty and relativity.

Insights Arising When Quantum Mechanics is Reappraised from a Biological Perspective

A recently reported ferroelectric phase transition at 72 Kelvin in ice XIc corroborates an observation made as a physics student in 1967. I attributed anomalous readings from a silica helium thermometer suspended in liquid nitrogen to distortion by ice crystallizing on its surface, suggesting a proton ordered tetragonal variant, ice It of cubic ice had changed shape during an ordering phase transition releasing latent energy as 'ice light', wavelength λ ~ 4 μ infrared laser light. My research into its consequences reinterprets the axiomatic basis of science, affording simple accounts of familiar topics. Ice light emitted from pools of nitrogen on Earth's poles during a primordial ice age was multiply reflected in cloud and surface ice, creating DNA, including tRNA analogue 'transport DNAs', tDNAs by selectively polymerizing nucleotides in equatorial waters. They constituted H-bond lined pores through proto-cell membranes, polarization by ice light created an electric field propelling ionic substrate-trace element complexes constituting life’s molecular vocabulary inside. 64 tDNA variants orchestrate biochemistry, supplementing trace elements prevents mental and physical maladies. ‘Minions’ arose from comparing cybernetics, psychology and traditional wisdom. 189 flat anti-parallel β-sheet hairpins with alternate neutral and basic amino acid residues bind 1,701 DNA base pairs, forming a nine-coil abacus. Minions probably evolved to pack DNA in chromosomes for accurate replication. 189*18 arrays of proton ordered H-bonds connecting amino acid ω-amines to DNA phosphates control metabolism, are biological clocks and 'chips in the brain. When sarcomeres of striated muscle contract, they form ½-wave resonant cavities for λ, exemplifying biological energy coupling. Protons accelerated along minion tunnels drive molecular scale nuclear fusion, if coupled they could resolve global warming. Artificial intelligence modeled on minions would satisfy Turing's vision, allowing for personality and compensating the relativity between perception and conception, reinterpreting quantum mechanics, uncertainty and relativity.

Electrostatic Localization of RNA to Protocell Membranes by Cationic Hydrophobic Peptides

AbstractCooperative interactions between RNA and vesicle membranes on the prebiotic earth may have led to the emergence of primitive cells. The membrane surface offers a potential platform for the catalysis of reactions involving RNA, but this scenario relies upon the existence of a simple mechanism by which RNA could become associated with protocell membranes. Here, we show that electrostatic interactions provided by short, basic, amphipathic peptides can be harnessed to drive RNA binding to both zwitterionic phospholipid and anionic fatty acid membranes. We show that the association of cationic molecules with phospholipid vesicles can enhance the local positive charge on a membrane and attract RNA polynucleotides. This phenomenon can be reproduced with amphipathic peptides as short as three amino acids. Finally, we show that peptides can cross bilayer membranes to localize encapsulated RNA. This mechanism of polynucleotide confinement could have been important for primitive cellular evolution.

Electrostatic Localization of RNA to Protocell Membranes by Cationic Hydrophobic Peptides.

Cooperative interactions between RNA and vesicle membranes on the prebiotic earth may have led to the emergence of primitive cells. The membrane surface offers a potential platform for the catalysis of reactions involving RNA, but this scenario relies upon the existence of a simple mechanism by which RNA could become associated with protocell membranes. Here, we show that electrostatic interactions provided by short, basic, amphipathic peptides can be harnessed to drive RNA binding to both zwitterionic phospholipid and anionic fatty acid membranes. We show that the association of cationic molecules with phospholipid vesicles can enhance the local positive charge on a membrane and attract RNA polynucleotides. This phenomenon can be reproduced with amphipathic peptides as short as three amino acids. Finally, we show that peptides can cross bilayer membranes to localize encapsulated RNA. This mechanism of polynucleotide confinement could have been important for primitive cellular evolution.

An embryo of protocells: The capsule of graphene with selective ion channels.

The synthesis of artificial cell is a route for searching the origin of protocell. Here, we create a novel cell model of graphene capsules with selective ion channels, indicating that graphene might be an embryo of protocell membrane. Firstly, we found that the highly oxidized graphene and phospholipid-graphene oxide composite would curl into capsules under a strongly acidic saturated solution of heavy metallic salt solution at low temperature. Secondly, L-amino acids exhibited higher reactivity than D-amino acids on graphene oxides to form peptides, and the formed peptides in the influence of graphene would be transformed into a secondary structure, promoting the formation of left-handed proteins. Lastly, monolayer nanoporous graphene, prepared by unfocused 84Kr25+, has a high selectivity for permeation of the monovalent metal ions ( Rb+ > K+ > Cs+ > Na+ > Li+, based on permeation concentration), but does not allow Cl- go through. It is similar to K+ channels, which would cause an influx of K+ into capsule of graphene with the increase of pH in the primitive ocean, creating a suitable inner condition for the origin of life. Therefore, we built a model cell of graphene, which would provide a route for reproducing the origin of life.

Controlled growth of filamentous fatty acid vesicles under flow.

The earliest forms of cellular life would have required a membrane compartment capable of growth and division. Fatty acid vesicles are an attractive model of protocell membranes, as they can grow into filamentous vesicles that readily divide while retaining their contents. In order to study vesicle growth, we have developed a method for immobilizing multilamellar fatty acid vesicles on modified glass surfaces and inducing filamentous membrane growth under flow. Filament formation strictly depended on the presence of freshly neutralized fatty acid micelles in the flow chamber. Using light microscopy, we observed a strong dependence of initial growth velocity on initial vesicle size, suggesting that new fatty acid molecules were incorporated into the membrane over the entire external surface of the vesicle. We examined the influences of flow rate, fatty acid concentration, and salt concentration on filamentous growth and observed drastic shape changes, including membrane pearling, of preexisting membrane tubules in response to osmotic stress. These results illustrate the versatility of flow studies for exploring the process of fatty acid vesicle growth following exposure to free fatty acids.

Chain-Length Heterogeneity Allows for the Assembly of Fatty Acid Vesicles in Dilute Solutions

A requirement for concentrated and chemically homogeneous pools of molecular building blocks would severely restrict plausible scenarios for the origin of life. In the case of membrane self-assembly, models of prebiotic lipid synthesis yield primarily short, single-chain amphiphiles that can form bilayer vesicles only at very high concentrations. These high critical aggregation concentrations (cacs) pose significant obstacles for the self-assembly of single-chain lipid membranes. Here, we examine membrane self-assembly in mixtures of fatty acids with varying chain lengths, an expected feature of any abiotic lipid synthesis. We derive theoretical predictions for the cac of mixtures by adapting thermodynamic models developed for the analogous phenomenon of mixed micelle self-assembly. We then use several complementary methods to characterize aggregation experimentally, and find cac values in close agreement with our theoretical predictions. These measurements establish that the cac of fatty acid mixtures is dramatically lowered by minor fractions of long-chain species, thereby providing a plausible route for protocell membrane assembly. Using an NMR-based approach to monitor aggregation of isotopically labeled samples, we demonstrate the incorporation of individual components into mixed vesicles. These experiments suggest that vesicles assembled in dilute, mixed solutions are depleted of the shorter-chain-length lipid species, a finding that carries implications for the composition of primitive cell membranes.