The Role of Lipid Membranes in Life's Origin

Abstract

At some point in early evolution, life became cellular. Assuming that this step was required for the origin of life, there would necessarily be a pre-existing source of amphihilic compounds capable of assembling into membranous compartments. It is possible to make informed guesses about the properties of such compounds and the conditions most conducive to their self-assembly into boundary structures. The membranes were likely to incorporate mixtures of hydrocarbon derivatives between 10 and 20 carbons in length with carboxylate or hydroxyl head groups. Such compounds can be synthesized by chemical reactions and small amounts were almost certainly present in the prebiotic environment. Membrane assembly occurs most readily in low ionic strength solutions with minimal content of salt and divalent cations, which suggests that cellular life began in fresh water pools associated with volcanic islands rather than submarine hydrothermal vents.

Keywords: encapsulation; membranes; protocells.

Figure 1

A hydrothermal field on Mount Lassen in northern California.

Figure 2

Ionic composition of seawater and fresh water hydrothermal fields.

Figure 3

Effect of seawater on self-assembly of lipid bilayer membranes. (A) Vesicles composed of decanoic acid readily form in dilute ionic solutions at pH 7.2; (B) In seawater at the same pH, crystals of a calcium-magnesium decanoate form instead of vesicles, thereby preventing the self-assembly of bilayer membranes. Bar shows 20 μm in both images.

Figure 4

Vesicular membranes 1–10 μm in diameter self-assemble from amphiphilic molecules like phospholipids, as shown in the freeze-fracture image (left panel). When dried, the vesicles fuse and flatten to form multilamellar structures (right panel). Each of the layers is the dimension of a single lipid bilayer ~5 nm thick. In the freeze-fracture method, lipid bilayers are split within the plane of the hydrocarbon chains, not along the head groups.

Figure 5

Membranous vesicles self-assemble from mixtures of fatty acids and alcohols such as decanoic acid and 1-decanol [61]. The vesicles shown here (left panel) were stained with rhodamine 6G and photographed by fluorescence microscopy (400× original magnification) [68]. Typical vesicles shown in the micrograph are ~1–10 μm in diameter. After a single dehydration cycle (right panel), the vesicles readily encapsulated macromolecules such as ~600 nt duplex DNA. The DNA was stained with acridine orange, an intercalating fluorescent dye.

Figure 6

X-ray diffraction structure of a lamellar lipid matrix that has imposed order on a solute, in this case 5′-adenosine monophosphate [76].

Figure 7

Proposed mechanism for phosphoester bond formation in a condensation reaction driven by simulated hydrothermal conditions [79].