Turnover time defined

Average turnover time (defined as storage volume divided by annual inflow or outflow volume, assuming steady state) is a measure of... 

The cycling of elements in bioturbated surface sediments can also be expressed in terms of turnover times defined as period of time required for a complete oxidation - reduction cycle of the reactive fraction. Additional consideration of the bioturbation depth and the sedimentation rate then reveals the number of redox-cycles before ultimate burial. In Tab. 7.2 representative results for estuarine (coastal) and slope sediments are given. 

If material is removed from the reservoir by two or more separate processes, each with a flux S then turnover times with respect to each process can be defined as ... 

Before deriving the rate equations, we first need to think about the dimensions of the rates. As heterogeneous catalysis involves reactants and products in the three-dimensional space of gases or liquids, but with intermediates on a two-dimensional surface we cannot simply use concentrations as in the case of uncatalyzed reactions. Our choice throughout this book will be to express the macroscopic rate of a catalytic reaction in moles per unit of time. In addition, we will use the microscopic concept of turnover frequency, defined as the number of molecules converted per active site and per unit of time. The macroscopic rate can be seen as a characteristic activity per weight or per volume unit of catalyst in all its complexity with regard to shape, composition, etc., whereas the turnover frequency is a measure of the intrinsic activity of a catalytic site. 

A model based on the assumption that a metabolite is present within a single compartment with defined rate constants for absorption and elimination of the metabolite. The rate of appearance of a tracee and the infusion of tracer are assumed to take place in a single pool that is instantly well-mixed. Wolfe has described in detail how the constant tracer infusion method allows one to calculate half-life, pool size, turnover time, mean residence time, and clearance time. 

There are different time scales associated with the various emissions and uptake processes. Two terms that are frequently used are turnover time and response or adjustment) time. The turnover time is defined as the ratio of the mass of the gas in the atmosphere to its total rate of removal from the atmosphere. The response or adjustment time, on the other hand, is the decay time for a compound emitted into the atmosphere as an instantaneous pulse. If the removal can be described as a first-order process, i.e., the rate of removal is proportional to the concentration and the constant of proportionality remains the same, the turnover and the response times are approximately equal. However, this is not the case if the parameter relating the removal rate and the concentration is not constant. They are also not equal if the gas exchanges between several different reservoirs, as is the case for C02. For example, the turnover time for C02 in the atmosphere is about 4 years because of the rapid uptake by the oceans and terrestrial biosphere, but the response time is about 100 years because of the time it takes for C02 in the ocean surface layer to be taken up into the deep ocean. A pulse of C02 emitted into the atmosphere is expected to decay more rapidly over the first decade or so and then more gradually over the next century. 

The turnover frequency, N, (commonly called the turnover number) defined, as in enzyme catalysis, as molecules reacting per active site in unit time, can be a useful concept if employed with care. In view of the problems in measuring the number of active sites discussed in 1.2.4, it is important to specify exactly the means used to express Q in terms of active sites. A realistic measure of such sites may be the number of surface metal atoms on a supported catalyst but in other cases estimation on the basis of a BET surface area may be the only readily available method. Of course, turnover numbers (like rates) must be reported at specified conditions of temperature, initial concentration or initial partial pressures, and extent of reaction. 

For a steady-state reservoir, we have kt = IJCi. Since turnover time (x) is defined as 1 Iki, at steady state, x equals the inventory of carbon divided by the input rate, CJI,... 

The concept of atmospheric lifetime is useful in discussions of the atmospheric degradation of anthropogenic molecules [5]. It can be defined in several ways. Most simply put it can be expressed as the turnover time, which is the atmospheric burden of a given species divided by its rate of emission, assuming a constant emission rate and steady state condition. Alternatively, it can be stated as the reciprocal of the pseudo first order rate constant (k ) for its removal ... 

Regardless of the size cutoff used to define the colloid fraction (1 kDa or 10 kDa), turnover times of colloidal material were found to be short (ranging from about a day to a few weeks). In studies where COM was further divided into size fractions of >1 kDa and >10 kDa, the larger size fraction was found to have a shorter turnover time (about a week in the Mid-Atlantic Bight and in the Gulf of Mexico) than the smaller size fraction (about a month Guo et al., 1997). These residence times represent lower limits, because desorption of thorium and disaggregation of particles was not considered. Nevertheless, even allowing for substantial errors due to these... 

As a much simpler example, let us consider a system consisting of two connected reservoirs (Fig. 4-8). Steady-state is assumed to prevail. Material is introduced at a constant rate Q in reservoir 1. Some of this material is removed (Si) and the rest (T) is transferred to reservoir 2, from which it is removed at a rate S2. The turnover times (average residence times) of the two reservoirs and of the combined reservoir (defined as the sum of the two reservoirs)... 

Whereas some characteristics and properties of CETP have been defined, many questions remain to be answered. We do not yet know the origin of the protein, the nature of factors that regulate its concentration in the plasma, its turnover time, the origin and metabolism of its inhibitor, or even the fundamental question of its physiological role in the plasma. A great deal more investigation into CETP is clearly warranted. 

Catalysis is a kinetic phenomenon we would like to carry out the same reaction with an optimum rate over and over again using the same catalyst surface. Therefore, in the sequence of elementary reactions leading to the formation of the product molecule, the rate of each step must be of steady state. Let us define the catalytic reaction turnover frequency, as the number of product molecules formed per second. Its inverse, 1 /, yields the turnover time, the time necessary to form a product molecule. By dividing the turnover frequency by the catalyst surface area, (i, we obtain the specific turnover rate, (R (molecules/cm /sec) = /Ct ((R often called the turnover frequency also, in the literature). This type of analysis assumes that every surface site is active. Although the number of catalytically active sites could be much smaller (usually uncertain) than the total number of available surface sites, the specific rate defined this way gives a conservative lower limit of the catalytic turnover rate. If we multiply (R by the total reaction time, bt, we obtain the turnover number, the number of product molecules formed per surface site. A turnover number of one corresponds to a stoichiometric reaction. Because of the experimental uncertainties, the turnover number must be on the order of 10 or larger for the reaction to qualify as catalytic. 

A useful concept in box modeling is that of turnover time. The turnover time of an element in a given reservoir is defined as the ratio between its reservoir content and its total output flux (eqn [3]). 

The turnover number of an enzyme, is a measure of its maximal catalytic activity, is defined as the number of substrate molecules converted into product per enzyme molecule per unit time when the enzyme is saturated with substrate. The turnover number is also referred to as the molecular activity of the enzyme. For the simple Michaelis-Menten reaction (14.9) under conditions of initial velocity measurements, Provided the concentration of... 

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