Metabolic cost

Conduction of the action potential in myelinated axons is called saltatory conduction. Because ion flux only occurs at the nodes of Ranvier, the action potential jumps, in effect, from node to node. This provides two advantages, speeding the rate of conduction and reducing the metabolic cost of an action potential, because energy-dependent ion transporters are not needed along myelinated segments. 

If Km for an endogenous substrate is known, then it establishes an approximate value for the endogenous concentration of that substrate. This is because it would not make sense kinetically or metabolically for a substrate to be present at concentrations significantly below or above its Km value. In the former case, the majority of enzyme protein molecules, synthesized by the cell at significant metabolic cost, would be redundant, and in the latter case, the cell would be unable to respond immediately to fluctuations in substrate concentration because enzymes would approach saturation and v would thus vary only marginally with fluctuations in [S]. 

Normal life may be defined as the conversion of energy to perform meaningful work at an acceptable metabolic cost. Illness and injury may be defined as energy conversion, work requirements or metabolic costs that have new become excessive. Therefore death may be defined as the irreversible loss of the ability to use energy to perform sufficient work in one or more vital organs. 

Metabolic cost High ties up phenylalanine for phenolic portion, plus glucose Lower... 

Illius, A. W. and Jessop, N. S. (1995). Modeling metabolic costs of allelochemical ingestion by foraging hethivotes.JournalofChemicalEcologyZl, 693-719. 

Thomas, D. W., Samson, C., and Bergeron, J. M. (1988). Metabolic costs associated with the ingestion of plant phenolics by Microtus pennsylvaticus. Journal of Mammalogy 69, 512-515. 

In the plant, part of the metabolic cost of producing and maintaining pools of secondary compounds may be reduced by using compounds which contain only carbon, hydrogen and oxygen (which are rarely limiting), by recycling the compounds or by use of inducible systems of defense. 

Fragrance (as well as colorful visual display) would be superfluous in autogamous and wind-pollinated flowers, and, because of its metabolic costs, should be lost entirely. 

The continuous operation of the Ca2+ pumps would be very expensive for a cell (because of the high metabolic cost of generating the necessary ATP) and might also lead to undesirably low cytosolic free Ca2+ levels in the resting steady state. The activities of SERCAs and PMCAs are therefore carefully regulated in vivo and the pumps are kept in a relatively inactive, inhibited state until they are needed. Different ways of regulation of the Ca2+ pumps are discussed in more detail below in the section on their molecular characterization. 

Davies, D.M. 1973. Sixty days in a submarine The pathophysiological and metabolic cost. J.R.Coll. Physicians. Lond. 7(2) 132—144. 

When we subtract the costs of all other metabolic processes by the chemoautotrophs and photoautotrophs, the organic carbon that remains is available for the growth and metabolic costs of heterotrophs. This remaining carbon is called net primary production (NPP) (Lindeman, 1942). From biogeochemical and ecological perspectives, NPP provides an upper bound for all other metabolic demands in an ecosystem. If NPP is greater than all respiratory consumption of the ecosystem, the ecosystem is said to be net autotrophic. Conversely, if NPP is less than all respiratory consumption, the system must either import organic matter from outside its bounds, or it will slowly mn down—it is net heterotrophic. 

Bacteria, as well as fungi, can only transport simple monomers and some dimers across their cell walls and as such must excrete a suite of extracellular enzymes to break down large and complex organic molecules (discussed in detail in Section 8.07.5). The production of extracellular enzymes by an individual microbe can be seen as a metabolically costly and potentially wasteful process. However, many bacteria have developed mechanisms for intercellular communication that allow homogenous populations to coordinate... 

A mechanism which would accomplish extracellular mineralization by single cells may involve the entrance of Ca and HCOj from the medium and their movement through the cell and onto the site of deposition of CaCOs on the crystal surface. The details and metabolic costs of these movements remain obscure. 

It would be instructive to compare the metabolic cost of synthesizing and allocating specific flavonoids, salicylic and hydroxycinnamic acids and their efficacy in filtering biologically active UV radiation. This information could explain their ecological roles in the adaptation or acclimation process. 

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