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The released energy is available to build ATP for various oxidation processes of organic matter at pH 7.0 and 25°C.

The released energy is available to build ATP for various oxidation processes of organic matter at pH 7.0 and 25°C.

produces methane; this is a less complete oxidation than the first because methane has a greater exergy content than water.

Numerous experiments have been performed to imitate the formation of organic matter in the primeval atmosphere of Earth four billion years ago (Morowitz, 1968). Energy from various sources was sent through a gas mixture of carbon dioxide, ammonia and methane (compare also with the discussion in Section 3.6). Analyses have shown that a wide spectrum of compounds, including several amino acids contributing to protein synthesis, is formed under these circumstances. There are obviously many pathways to utilise the energy sent through simple gas mixtures, but mainly those forming compounds with rather large free energies (high exergy storage, released when the compounds are oxidised again to carbon dioxide, ammonia and methane) will form an appreciable part of the mixture (Morowitz, 1968).

(3) Photosynthesis. There are three biochemical pathways for photosynthesis: (1) the C3 or Calvin-Benson cycle, (2) the C4 pathway and (3) the crassulacean acid metabolism (CAM) pathway. The latter is least efficient in terms of the amount of plant biomass formed per unit of energy received. Plants using the CAM pathway are, however, able to survive in harsh, arid environments that would be inhospitable to C3 and C4 plants. CAM photosynthesis will generally switch to C3 as soon as sufficient water becomes available (Shugart, 1998). The CAM pathways yield the highest biomass production, reflecting exergy storage under arid conditions, while the other two give highest net production (exergy storage) under other conditions. While it is true that a gram of plant biomass produced by the three pathways has different free energies in each case, in a general way improved biomass production by any of the pathways can be taken to be in a direction that is consistent, under the conditions, with the exergy-storage hypothesis.

(4) Leaf size. Givnish and Vermelj (1976) observed that leaves optimise their size (thus mass) for the conditions. This may be interpreted as meaning that they maximise their free-energy content. The larger the leaves, the higher their respiration and transpiration, and the more solar radiation they can capture. Deciduous forests in moist climates have a LAI of about 6%. Such an index can be predicted from the hypothesis of highest possible leaf size, resulting from the trade-off between having leaves of a given size versus maintaining leaves of a given size (Givnish and Vermelj, 1976). Size of leaves in a given environment depends on the solar radiation and humidity regime, and while, for example, sun and shade leaves on the same plant would not have equal exergy contents, in a general way leaf size and LAI relationships are consistent with the hypothesis of maximum exergy storage.

(5) Biomass packing. The general relationship between animal body weight, W, and population density, N, is N = A/W, where A is a constant (Peters, 1983); see also Eq. (6.6) in Chapter 3. The highest packing of biomass depends only on the aggregate mass, not on the size of individual organisms. This means that it is biomass rather than population size that is maximised in an ecosystem, as density (number per unit area) is inversely proportional to the weight of the organisms. Of course, the relationship is complex. A given mass of mice would not contain the same exergy or number of individuals as an equivalent weight of elephants. Also, genome differences (Example 1) and other factors would figure in. If other proposed goal functions as, for instance, exergy destruction, then biomass packing would follow the relationship N = A/W065-075, because respiration expressing the conversion of exergy to heat is proportional to the weight in the exponent 0.65 -0.75 (Peters, 1983). As this is not the case, biomass packing and the free energy associated with this lend general support for the exergy-storage hypothesis.

(6) Cycling. If a resource (for instance, a limiting nutrient for plant growth) is abundant, it will typically recycle faster. This is a bit strange, because recycling is not needed when a resource is non-limiting. A modelling study (J0rgensen, 2002a) indicated that free-energy storage increases when an abundant resource recycles faster. Fig. 12.2 shows a r

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