The origin of protocells

Abstract

Cellularization has the following main aspects that we have to explain: • The need for active (self-generated) compartmentation when metabolism is liberated from the surface. • The origin of membranogenic molecules and membranes. • The origin and mechanism of spontaneous protocell fission. • The transportation problem. Simple membranes are not ‘leaky’ enough to permit important nutrients to pass through. • Were the first protocells autotrophs or heterotrophs? The evolution of the first autocatalytic metabolic cycle. • The iron-sulphur world and the RNA world: are they mutually exclusive or complementary? • The problem of the origin of the two membranes of negibacteria, the most ancient existing group of organisms. • The origin of chromosomes and DNA synthesis. We shall discuss these problems in turn. As we discussed before, the prebiotic pizza has the ability to localize metabolites and genes. This is advantageous for two reasons: Reactants are kept in each other’s proximity, which ensures that reaction rates will be high enough and that important compounds do not drift away. Genes will interact, directly (e.g. by influencing each other’s replication) or indirectly (by catalysing steps of metabolism), only with their neighbours: selection will thus be able to ensure cooperation among genes that would otherwise compete against each other. Life liberated itself from surfaces a long time ago. Somehow, passive localization must have been replaced by an active process of membrane generation, maintenance and fission. The basic structure of contemporary biomembranes is as follows. There is a molecular bilayer of lipids, to which proteins are attached in various ways. The bilayer is formed because the membrane constituents are so-called amphipathic molecules: they have a hydrophilic head and a hydrophobic tail. Since the binding interaction of water with itself is much stronger than that between water and hydrophobic compounds, the latter are expelled by water as much as possible; this results in tails coming together. A simple sheet of bilayer would still be not at the energy minimum because its edges would be exposed to water. An energetically favourable solution is the formation of a lipid vesicle.