Much of past debate have focused on two particular types of mechanisms. One of these assumes energy or resource transport across the "external" surface area of three-dimensional organisms is the key factor driving the relationship between metabolic rate and body size. The surface area in question may be skin, lungs, intestines, or, in the case of unicellular organisms, cell membranes. In general, the surface area (SA) of a three dimensional object scales with its volume (V) as "SA = cV", where c is a proportionality constant. The Dynamic Energy Budget model predicts exponents that vary between – 1, depending on the organism's developmental stage, basic body plan and resource density. DEB has also been proposed as an alternative to metabolic scaling theory, developed before the MTE, DEB also provides a basis for population, community and ecosystem level processes to be studied based on energetics of the constituent organisms. In this theory, the biomass of the organism is separated into structure (what is built during growth) and reserve (a pool of polymers generated by assimilation). DEB is based on the first principles dictated by the kinetics and thermodynamic of energy and material fluxes but is data demanding and rich in free parameters. In many ways DEB shares similar approaches to MTE. However, DEB, unlike MTE, is rich in parameters, and most of them are species specific, which hinders the generation of general prediction. While some of these alternative models make several testable predictions, others are less comprehensive and none of these other proposed models make as many predictions with a minimal set of assumptions as metabolic scaling theory.