Question

What are liposomes, what are they composed of, & how are they possible precursor to plasma membranes and/or the first protocells?

Answer 1

A liposome is a tiny bubble (vesicle), made out of the same material as a cell membrane. Liposomes can be filled with drugs, and used to deliver drugs for cancer and other diseases. Membranes are usually made of phospholipids, which are molecules that have a head group and a tail group....it was first described in the mid-60s... Among several talented new drug delivery systems, liposomes characterize an advanced technology to deliver active molecules to the site of action, and at present, several formulations are in clinical use. Research on liposome technology has progressed from conventional vesicles to ‘second-generation liposomes’, in which long-circulating liposomes are obtained by modulating the lipid composition, size, and charge of the vesicle. Liposomes with modified surfaces have also been developed using several molecules, such as glycolipids or sialic acid..

A stealth liposome is a sphere-shaped vesicle with a membrane composed of phospholipid bilayer used to deliver drugs or genetic material into a cell. A liposome can be composed of naturally derived phospholipids with mixed lipid chains coated or steadied by polymers of PEG and colloidal in nature.They typically form after supplying enough energy to a dispersion of (phospho)lipids in a polar solvent, such as water, to break down multilamellar aggregates into oligo- or unilamellar bilayer vesicles. Liposomes can hence be created by sonicating a dispersion of amphipatic lipids, such as phospholipids, in water.

One biopolymer/lipid interaction, has been observed with surprising morphogenic properties. Cationic liposomes composed of phospholipids (DOPE/DC-Chol) can be aggregated by the addition of plasmid DNA due to the interaction of the positively charged lipid with the negatively charged DNA backbone. A variety of structures can be formed from this interaction, including linear DNA strands surrounded by “bilayer tubules”. Thus, high DNA concentrations cause the fusion of vesicles, although no quantitative study has been performed. This is a prime example of the complexity of DNA/liposome interaction and the creation of a variety of structures, some of which are the results of fusion events between vesicles. Using tRNA, it is also possible to aggregate preformed liposomes (POPC charged with positive surfactant CTAB) This behavior is dependent on the size of the liposomes. Reversible aggregation occurred for vesicles of 160 nm in diameter but not for those with 80 nm diameter. This dissimilarity is likely due to the membrane curvature, allowing greater tRNA interaction on the gentler curve. The emergence of a vesicle selection mechanism based on size could be significant for both origins of life and artificial systems as a passive method for gathering productive protocells or it could allow growing cells to be differentiated from individuals lacking a metabolism.due to the large surface/volume ratio in the micro space, encapsulated biopolymers exhibit unique self-organized structures through interplay with membrane surfaces. The soft-matter characteristics of the membranes play an important role in the association of molecules and the formation of compartment capsule structures.

The introduction of a membrane changes the dynamics of a chemical reaction when compared to bulk reactions, and could have contributed to the emergence of complex reaction networks, and ultimately protocells. Because of their multiphase properties, membranes can change the chemical environment as well as promoting spatial organization, for example by concentrating them on their surface or within their hydrophobic interior. These alterations should facilitate reactions such as polymerization by condensation of monomers, which is the main synthetic pathway to important biopolymers. For both amino and nucleic acids, enhancements of non-enzymatic polymerization, or self-condensation have been reported compared to the same type of reaction in a bulk aqueous medium.