Читать книгу Astrobiology - Charles S. Cockell - Страница 104

To create a cell, we require some type of vessel in which to concentrate molecules. Most aqueous environments on Earth, and elsewhere, such as lakes, rivers, and oceans, have a tendency to dilute molecules. A cell enclosure, or membrane, provides a way to keep molecules together at relatively high concentrations. A membrane will also retain water, allowing for chemical reactions to proceed even when outside conditions are desiccating.

In the previous chapter, we discussed the lipids, some of which contain a hydrophilic head (which is attracted to water) and a hydrophobic tail made from a fatty acid (which is repelled from water). One important class of lipids that are involved in membrane formation is the phospholipids in which the charged polar end is a phosphate group (Figure 5.4). It is worth examining these in more detail to see how they form a cellular membrane. The principles are the same for all membrane lipids.

Figure 5.4 Amphiphilic molecules such as phospholipids that make up some cell membranes. They have a hydrophilic end (attracted to water) and a hydrophobic end (repelled from water).

A remarkable characteristic of these so-called amphiphilic phospholipid molecules is that when added to water, they have a tendency to assemble spontaneously in such a way that the hydrophilic head is oriented into the water, and the hydrophobic tails, which would like to escape the water surrounding the molecules, are attracted toward each other to expel water. The result is a lipid bilayer membrane (Figure 5.5). These bilayers themselves tend to assemble into vesicles, small spherical structures with fluid in the inside. This shape, like a water droplet, is a minimal energy shape.

Figure 5.5 A simplified diagram showing the structure of a lipid bilayer that makes up cell membranes.

The phospholipids can also assemble in a single layer whereby a ball of lipids is formed, the tails pointing toward the center with a layer of hydrophilic heads on the outside of the ball. These are called micelles. They are less interesting than bilayer vesicles because vesicles have a hollow center in which cellular components can collect.

This property, whereby amphiphilic molecules can form a membranous layer, is by no means rare. Even fatty acids on their own, such as the long-chained carboxylic acid, decanoic acid, extracted from meteorites (Chapter 12) tend to form membranous vesicles, suggesting that this is a fundamental property of this class of molecules and that the assembly of the first cell membranes on Earth may not have been a difficult or extraordinary event.

Within the lipid membrane other proteins are incorporated, resulting in a complex system that regulates the interaction of the internal cell environment with the outside environment. Many of these proteins are trans-membrane proteins acting, for example, as thin channels both for ions and molecules to move into the cell and for wastes to be expelled. Some of these proteins act as a line of communication, allowing the cell to sense changes in physical and chemical conditions in the outside environment and respond to them.