Nutrient uptake, specific rate

As mentioned before and in Chaps. 4 and 6, the concentration of rhizode-position decreases as the distance from the rhizoplane increases, whereas the opposite generally occurs for the concentration of any plant nutrient in soil. In this context, the role of rhizospheric soil, rather than that of the bulk soil, is crucial for plant nutrition. It has also to be considered that very different situations can occur depending on the type of nutrient (24) and the nutritional status of plants (see Chap. 3) furthermore, different portions of the root system are characterized by differential nutrient-specific rates of uptake (25). All the above statements point to the necessity of reconsidering the concept of plant nutrient availability giving more importance to the situation occurring in the soil surrounding the root. 

Peckol, P, DeMeo-Anderson, B., Rivers, J., Valiela, I., Maldonado, M., and Yates, J. (1994). Growth, nutrient uptake capacities and tissue constituents of the macroalgae, Cladophora vagabunda and Gracilaria tikvahiae, related to site-specific nitrogen loading rates. Mar. Biol. 121, 175-185. 

The specific rate of nutrient uptake, v, is defined as the number of millimoles of nutrient consumed per unit (e.g. billion) of living cells present in the culture medium and per unit time (e.g. hour). 

Evaluate the values of the parameters in the rate expressions of the specific rates of nutrient uptake and metabolite or protein productions... 

The model was adapted from Kontoravdi et al. (2005) for cell growth/death, nutrient uptake, and major metabolism. The model was further developed to include description of cell cycle sub-populations. The cell cycle representation was based on the yeast model of Uchiyama Shioya (1999) and the tumour cell model of Basse et al. (2003). Eq.(l)-(4) express viable cell concentration(Xv[cell L" ]) in terms of cells in Gq/Gi, S, and G2/M phases. As a simplification in notation, Gq/Gi cells will be indicated as G unless otherwise stated. Xoi, Xs, X02/M [cell L" ] are concentrations of viable cells in Gq/Gi, S, and G2/M phase, respectively, whereas Fo ,[L h" ] is the outlet flowrate. F[L] is the cell culture volume b, ki, k [h" ] are the transition rates of cells from Gi to S, S to G2, and M to Gi respectively and /[Pg.110]

Eq. 5.12 shows that the biomass-specific metabolic rate of the diffusion-limited cell varies inversely with the square of its size. This means that the cell could potentially increase its specific rate of metabolism 4-fold if the cell diameter were only half as large. The smaller the cell, the less likely it is that its substrate uptake will reach diffusion limitation. Thus, at the low substrate concentrations normally found in marine environments, microorganisms avoid substrate limitation by forming small cells of <1 pm size. Thereby, the bacteria become limited by their transport efficiency of molecules across the cell membrane rather than by diffusion from their surroundings (Fig. 5.6). In the nutrient-poor seawater, where substrates are available only in sub-micromolar and even nanomolar concentrations, free-living... 

No carrier is completely specific for a given trace metal metals of similar ionic radii and coordination geometry are also susceptible to being adsorbed at the same site. The binding of a competing metal to an uptake site will inhibit adsorption as a function of the respective concentrations and equilibrium constants (or kinetic rate constants, see below) of the metals. Indeed, this is one of the possible mechanisms by which toxic trace metals may enter cells using transport systems meant for nutrient metals. The reduced flux of a nutrient metal or the displacement of a nutrient metal from a metabolic site can often explain biological effects [92]. 

As active transport uses a carrier system, it is normally specific for a particular substance or group of substances. Thus, the chemical structure of the compound and possibly even the spatial orientation are important. This type of transport is normally reserved for endogenous molecules such as amino acids, required nutrients, precursors, or analogues. For example, the anticancer drug 5-fluorouracil (Fig. 3.6), an analogue of uracil, is carried by the pyrimidine transport system. The toxic metal lead is actively absorbed from the gut via the calcium transport system. Active uptake of the toxic herbicide paraquat into the lung is a crucial part of its toxicity to that organ (see chap. 7). Polar and nonionized molecules as well as lipophilic molecules may be transported. As active transport may be saturated, it is a zero-order rate process in contrast to passive diffusion (Fig. 3.3). 

Dugdale, R. C., and Wilkerson, F. P. (1991). Low specific nitrate uptake rate A common feature of high-nutrient low-chlorophyU marine ecosystems. Limnol. Oceanogr. 36, 1678—1688. 

Point Conception, California was obtained during an extreme El Nino year (1983) when nutrients (and uptake) were lower than more normal years (Wilkerson et al., 1987). Accompanying the high transport rates are high specific uptake rates (FNO3 based upon PON) up to 0.1 h [pmol 1 h (pmol that can... 

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