Читать книгу Astrobiology - Charles S. Cockell - Страница 84

Another class of compounds is the lipids, which encompass a wide diversity of chained and ring-containing molecules. They include long-chained carboxylic acids (also called fatty acids) which are chains of carbon compounds joined together, mainly through single bonds (e.g. saturated fatty acids) and some containing double bonds (unsaturated fatty acids; Figure 4.12). Fatty acids in life typically contain between 12 and 22 carbon atoms. One important group of lipids is the fats, or triglycerides (Figure 4.12), which are a combination of glycerol and three fatty acids and are found widely in animal fats and plant oils. The energy stored in the many bonds of fatty acids and triglycerides has made them useful as energy-storage molecules in life. Lipids have a variety of functional groups, which allow for important molecular interactions.

Figure 4.12 The molecular structure of some lipids. Free fatty acids are found as energy-storage molecules. Triglycerides are found in vegetable oils and animal fats. Phospholipids form cell membranes in microbes and other organisms. Also shown are two types of molecules found as structural components of membranes. Bacteriohopanetetrol is a hopanoid and is found in bacterial lipid membranes. Cholesterol is a steroid and a structural component of animal cell membranes.

Many lipids have the important characteristic that one end is charged. This end tends to be attracted to water, and the charge helps it to dissolve. We call it the hydrophilic end from the Greek hydro (water) and philos (love; it loves water). The other end, which is non-polar, does not dissolve readily, as it is uncharged. It is called the hydrophobic end because it dislikes water (from the Greek phobos, or fear). This property of having an uncharged and charged end means that these molecules are also referred to as amphiphilic. The phospholipids are one important example of lipids with a hydrophilic phosphate at one end (Figure 4.12). They are involved in cell membrane assembly. In the next chapter, we look at how their amphiphilic properties play a central role in cell membrane assembly, leading to compartmentalization, a fundamental characteristic of cellular life.

Like sugars and proteins, lipids also come in a vast variety, allowing for their complex and diverse functional roles in life. Some contain ring structures such as cholesterol (Figure 4.12), a steroid. Cholesterol is a component of animal cell membranes that is involved in maintaining membrane fluidity and integrity.

The lipids are immensely important to astrobiologists for a number of reasons. The tendency for life to link carbon atoms together to form well-defined chains makes them a strong signature of life. Non-biological processes can produce a wide range of chains, but they tend to have random variable numbers of carbon atoms, and in general, complex long chains are rarer than shorter chains. Life, however, tends to deliberately make long- chained lipids to construct membranes, so a preponderance of long-chained lipids in a sample with well-defined carbon numbers (e.g. 12, 14, 16) tends to suggest the presence of life. An example of such lipids is the hopanoids, lipids that have a five-ringed (pentacyclic) chemical structure (Figure 4.12). They have various roles, including membrane stabilization in bacteria. When bacteria are fossilized in the rock record, the bacterial shapes themselves may be destroyed, but the hopanoids can be preserved for potentially hundreds of millions of years. These compounds, which are discussed in more detail in Chapter 14, can be extremely resilient to heating and pressure. Looking for complex ordered carbon molecules such as these in extraterrestrial environments, such as in the plumes of water being ejected from Saturn's icy moon Enceladus or in sediments, such as on Mars, is one approach to life detection.