MEMBRANE LIPIDS OF THE PAST AND PRESENT
Modern cells use lipid membranes to selectively control what molecules may enter and exit the cell. The cell membrane is composed mainly of phospholipids, which consist of a hydrophobic (or “water-fearing”) tail and a hydrophilic (or “water-loving”) head group. When phospholipids are placed in water, the molecules spontaneously arrange such that the tails are shielded from the water, resulting in the formation of membrane structures such as bilayers, vesicles, and micelles (illustrated on the right).
Earlier forms of life probably needed a membrane compartment for many of the same reasons that modern cells do: to keep molecules that are important for cellular growth and survival readily accessible, and to keep unneeded or potentially harmful molecules outside of the cell. Rather than being made up of phospholipids, however, early membranes may have formed from fatty acids. Similar to phospholipids, fatty acids have a hydrophobic tail and hydrophilic head, and can thus form the same types of structures, such as bilayers, vesicles and micelles, but are structurally much simpler and may have formed more readily in a prebiotic environment.
WHY LIFE NEEDS A MEMBRANE COMPARTMENT
Why are membranes so important for the RNA World? An early RNA replicase probably would not have a built-in way of differentiating between a replicase or non-replicase sequence, and as a result, will make a copy of any RNA that happens to be close by. Without some means of separating the replicases from the non-replicases, the population of replicases is unlikely to grow and prosper. This issue can be resolved if the replicases are placed within a compartment, such as a vesicle, which can physically separate the replicases from other RNAs. This concept is illustrated in the animation on the left.
In addition, a membrane may have played an important role in the early cell's ability to store energy in the form of a chemical gradient. In modern eukaryotic cells, the mitochondria, often called the "cellular powerhouse" uses an internal chemical gradient to create energy-storing molecules known as ATP.
FORMING FATTY ACIDS ON THE EARLY EARTH
How might fatty acids have formed on the early Earth? Some scientists have proposed that hydrothermal vents may have been sites where prebiotically important molecules, including fatty acids, were formed. The animation on the left shows a theoretical scenario in which fatty acids are formed along the face of a geyser. Research has shown that some minerals can catalyze the stepwise formation of hydrocarbon tails of fatty acids from hydrogen and carbon monoxide gases -- gases that may have been released from hydrothermal vents. Fatty acids of various lengths are eventually released into the surrounding water.
The fatty acids produced in this manner would only be found in low concentrations. Relatively high concentrations of fatty acids are required, however, to form higher order structures such as micelles and vesicles. Pools of water may have slowly accumulated fatty acids through cycles of shinkage by evaporation and growth by the delivery of additional dilute fatty acid solution. It is also possible that droplets of fatty acids may have become aerosolized, as shown in the animation on the left, allowing the dry fatty acid particulate to travel long distances away from its original site of synthesis. Over time, small pools of water may have accumulated high concentrations of fatty acids.
STUDYING FATTY ACID VESICLES IN THE LAB
The Szostak lab at Massachusetts General Hospital has conducted numerous studies to examine how fatty acid vesicles may form, grow and divide.
At relatively low concentrations, fatty acids will form micelles, which can be thought of as tiny spheres of fatty acids, organized such that the tails of the fatty acid point towards the center of the sphere. Research in the Szostak lab has shown that at higher concentrations and under the appropriate pH conditions, fatty acids micelles can form vesicles. The process by which this is thought to occur is shown in the animation on the left.
The Szostak lab has also shown that vesicle formation may also be catalyzed by the clay montmorillonite, which has also been found to catalyze the formation of strands of RNA from single nucleotides (illustrated in the nucleic acids section). Clays such as montmorillonite may very well have been the key to the formation of the first protocells.
Once formed, fatty acid vesicles are highly stable, and appear outwardly unchanging over the course of days or even months. At a molecular level, however, fatty acids are extremely dynamic, and are constantly entering and exiting the vesicle bilayer, as well as flipping between the inner and outer leaflet of the membrane. Phospholipids, on the other hand, do not typically undergo flipping. The dynamic qualities of fatty acids are illustrated in the animation to the left.
Fatty acid flipping may play an important role in the ability for some small molecules, such as RNA nucleotides, to enter the vesicle. This process is illustrated in the animation on the left. If the nucleotides are incorporated into a strand of RNA, they become trapped inside the vesicle, since long polymers of RNA are unlikely to be able to use the same mechanism to pass through the fatty acid membrane.
Phospholipid bilayers, on the other hand, are relatively impermeable to molecules such as nucleotides, and require special transporters to allow their passage through the membrane.
How do fatty acid vesicles grow? Research in the Szostak lab has shown that when fatty acid micelles are added to a solution of pre-formed vesicles, the vesicles grow rapidly. A molecular model of this observation is shown on the left. Vesicle growth is thought occur first through the formation of a micelle shell around a vesicle. Individual fatty acids are transferred from the micelles to the outer leaflet of the vesicle membrane. Fatty acids may then flip from the outer leaflet to the inner leaflet (as illustrated in a previous animation on fatty acid dynamics), which allows the membrane bilayer to grow evenly.
Next: Putting it all together in a protocell.