Life (at least as we know it) needs a supply of the chemical elements which make up its composition, a source of energy and the presence of liquid water. All the organisms with which we are familiar also maintain a distinct structural integrity and are enclosed by some sort of a membrane that separates their cells from the environment, but which permits and controls the exchange of materials between the organism and its surroundings. Organisms also have nucleic acids (DNA and RNA) that encode the information that allows the production of the proteins and other molecules comprising their structure and functional systems. The nucleic acids also transmit this information to the organism's offspring during reproduction and changes in nucleic acids
222 LIFE AT THE LIMITS
provide the raw material for the operation of the processes of evolution.
The main chemical elements that are involved in the structure of organisms are carbon, hydrogen, oxygen, nitrogen, phosphorus and sulphur. Sodium, potassium, magnesium, calcium and chlorine are also important as are, in trace amounts or in some organisms, manganese, iron, cobalt, copper, zinc, boron, aluminium, vanadium, molybdenum and iodine. These elements are widespread throughout the universe and their absence is unlikely to be a limiting factor for the development of life. During the eighteenth century, chemists developed the distinction between organic compounds which were isolated from organisms, and inorganic compounds which were derived from non-living material. The first artificial synthesis of an organic compound (urea) from inorganic compounds was achieved by the German chemist Frederich Wohler in 1828. The development of life on a planet may require the presence of pre-existing (prebiotic) organic compounds. I will look at the potential sources of such compounds in the next section.
The most familiar source of energy exploited by organisms is sunlight, which is utilised by plants (and some other organisms) via the process of photosynthesis. As we have seen, however, some microorganisms can utilise chemical energy via the oxidation of various inorganic compounds. Geothermal energy and the energy derived from lightning and electrical discharges are other potential sources. Animals gain their energy by consuming plants (or microorganisms or other animals) while some microorganisms gain theirs by decomposing the bodies of other organisms. Sources of energy are likely to be widespread in the universe and will not be the main limiting factor for the development of life.
The requirement for liquid water is the most severe restriction for the development of life. Temperatures in the universe vary from those at the centre of stars (15 million degrees Kelvin [°K] at the core of our sun) to close to absolute zero (0°K or —273°C). Pure water is a liquid over a range of only 100 °C, a small fraction of the range of temperatures found in the universe. The range at which water is liquid may be extended slightly by mixing with other compounds (such as salts), which depress the freezing point, or under high pressures, which elevate the boiling point. Conditions which allow the presence of water as a liquid are, however, likely to be rare in the universe. One of the major roles of water in organisms is as a solvent and a medium in which biological reactions can take place in a controlled fashion. Could other liquids fulfil this role? Some other possibilities include ammonia (which is a liquid between — 33 °C and — 78 °C), methane ( — 164°C to — 182°C), ethane ( — 89°C to — 183°C) and liquid nitrogen (—196 °C to — 210 °C). These are liquid at lower temperatures and over a narrower range of temperatures than water. Their lower temperatures mean that chemical (and biological) reactions would take place at a slower rate than they do in water and the fact that they are liquid at a narrower range of temperatures means that they are even less likely to be found as a liquid than water. They also lack some of the unique properties of water which are important in biological systems.
Water is a polar molecule, meaning that it has a slight negative charge at one end of the molecule (the oxygen end) and a slight positive charge at the other end (the hydrogen end). This polarity means that water molecules are attracted to each other and explains why many of the properties of water seem unusual in comparison with other liquids. The attraction between water molecules means that water is liquid at higher temperatures and for a greater range of temperatures than are comparable molecules. It also means that water has a higher surface tension and cohesion than do most other liquids and this helps organisms to absorb water from their environment and to transport it around their bodies. The ability of plants to absorb water through their roots and to transport it to their leaves via their water-conducting tissues (the xylem) depends on the cohesion of water.
Water has a high capacity to store heat and this has a buffering effect which ameliorates temperature extremes. The climate close to the sea is thus milder than that far inland. Water also absorbs a relatively large amount of heat when it turns into a vapour. This high heat of vaporisation, like the high heat capacity, tends to moderate the Earth's climate and thus improve its habitability to organisms. Terrestrial organisms can take advantage of the high heat of vaporisation of water to cool themselves through evaporative cooling.
The density of ice is lower than that of liquid water and that is why the ice in your gin and tonic floats. This prevents oceans and most lakes freezing solid at low temperatures since ice forming at the surface insulates the liquid water below. If the ice sank, all lakes and even the oceans would eventually freeze solid at low temperatures. Liquid water is so important for life that many consider that the search for life is equivalent to the search for liquid water.
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