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Metabolism
Metabolic pathways consist of complex networks, which are responsible for processing of both energy and material. The metabolic rate of a heterotroph is defined as the rate of respiration in which energy is obtained by oxidation of carbon compound. The rate of photosynthesis on the other hand, indicates the metabolic rate of an autotroph. Both body size and temperature affect the metabolic rate of an organism. Metabolic rate scale as 3/4 power of body size, and its relationship with temperature is described by Van’t Hoff-Arrhenius equation over the range of 0 to 40°C.

Stoichiometry
From the ecological perspective, stoichiometry is concerned with the proportion of elements in both living organisms and their environment. In order to survive and maintain metabolism, an organism must be able to obtain crucial elements and excrete waste products. As a result, the elemental composition of an organism would be different from the exterior environment. Through metabolism, body size can affect stoichiometry. For example, small organism tend to store most of their phosphorous in rRNA due to their high metabolic rate , whereas large organisms mostly invest this element inside the skeletal structure. Thus, concentration of elements to some extent can limit the rate of biological processes. Inside an ecosystem, the rate of flux and turn over of elements by inhabitants, combined with the influence of abiotic factors, determine the concentration of elements.

Regarding density of populations, MTE suggest, carrying capacity of the population scale as M-3/4 and exponentially with temperature. The fact that larger organisms reach their carrying capacity sooner than smaller one is intuitive. Temperature also decreases carrying capacity due to the fact that in warmer environments, higher metabolic rate of organisms demands a higher rate of supply (Allen 2002). Empirical evidence in terrestrial plants, also suggest that density is indeed scale as -3/4 power of the body size (Enquist 1998).

Dynamic energy budget theory
Dynamic energy budget (DEB) theory is another metabolic theory, which is also based on the assumption that population, community and ecosystem level processes can be studied based on energetics of the constituent organisms In this theory energetics of individuals is determined by their body size and reserve density, which is the amount of reserves per volume unit of the individual's body. The assimilated food enters a reserve pool and assimilation rate scale as a 2/3 power of the surface area of the organism (unlike MTE which predict supply rate to be proportional to the body mass: M3/4). DEB theory, consider the growth process to be a balance between maintenance of cells and assimilation. However, since it assumes all cells to be similar, maintenance rate would be independent of the ultimate size. Thus, some species grow larger because their area specific assimilation rate is higher.