The temperature range needed for life processes

The input of energy to ecosystems is in the form of the solar photon flux by small portions (quanta). This implies that the exergy (energy that can do work; see Chapter 5) at first can only be utilised at molecular (lowest) levels in the hierarchy. The appropriate atoms or molecules must be transported to the place where order is created. Diffusion processes through a solid are extremely slow, even at room temperature. The diffusion of molecules through a liquid is about three orders of magnitude faster than in a solid at the same temperature. Diffusion coefficients for gases are ordinarily four orders of magnitude greater than for liquids. This implies that the creation of order (and also the inverse process, disordering) is much more rapid in liquid and gaseous phases than in solids. The temperature required for a sufficiently rapid creation of order is consequently considerably above the lower limit mentioned above, 2.726 K. As far as diffusion processes in solids, liquids and gases are concerned, gaseous diffusion allows the most rapid mass transport. However, many molecules on Earth that are necessary for ordinary carbon-based life do not occur in a gaseous phase, and liquid diffusion, even though it occurs at a much slower rate, is of particular importance for biological ordering processes.

The diffusion coefficient increases significantly with temperature. For gases, the diffusion coefficient varies with temperature approximately as T3/2 (Hirschfelder et al., 1954), where T is the absolute temperature. Thus, we should look for systems with the high-order characteristic of life at temperatures considerably higher than 2.726 K. The reaction rates for biochemical anabolic processes on the molecular level are highly temperature-dependent (see Straskraba et al., 1997). The influence of temperature may be reduced by the presence of reaction-specific enzymes, which are proteins formed by anabolic processes. The relationship between the absolute temperature, T, and the reaction rate coefficient, k, for a number of biochemical processes can be expressed by the following general equation (see any textbook in physical chemistry):

where A is the so-called activation energy, b is a constant and R is the gas constant. Enzymes are able to reduce the activation energy (the energy the molecules require to perform the biochemical reaction). Similar dependence of temperature is known for a wide spectrum of biological processes, for instance growth and respiration. Biochemical and biological kinetics point, therefore, towards ecosystem temperatures considerably higher than 2.726 K.

The high efficiency in the use of low-entropy energy at the present "room temperature" on Earth works hand in hand with the chemical stability of the chemical species characteristic of life on Earth. Macromolecules are subject to thermal denaturation. Among the macromolecules, proteins are most sensitive to thermal effects, and the constant breakdown of proteins leads to a substantial turnover of amino acids in organisms. According to biochemistry, an adult man synthesises and degrades approximately 1 g of protein nitrogen per kilogram of body weight per day. This corresponds to a protein turnover of about 7.7% per day for a man with a normal body temperature. A too high temperature of the ecosystem (more than about 340 K) will therefore enhance the breakdown processes too much. A temperature range between 260 and 340 K seems, from these considerations, the most appropriate to create the carbon-based life that we know on Earth. An enzymatic reduction of the activation energy makes it possible to realise basic biochemical reactions in this temperature range, without a too high decomposition rate, which would be the case at a higher temperature. In this temperature range, anabolic and catabolic processes can, in other words, be in proper balance.

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