Cell uptake and intracellular transport of nutrients

To describe adequately the main structures of a eukaryotic unicellular phytoplankter that are involved in the uptake, transport and assembly of inorganic components, it is helpful to refer to the simplified and stylised diagram in Fig. 4.1. Inside the multiple-layered plasmalemma (shown as a single line), there is a nucleus containing the genomic proteins (marked 'DNA'); the ribosomal centres of protein synthesis are represented by 'RNA' and part of the structure of the chloroplast and the thylakoid membranes are also sketched. Superimposed upon the cell is a series of arrows that provides a fragmentary indication of the key pathways located within the protoplast. The arrows refer, in part, to the dynamics of photosynthetic reduction of inorganic carbon dioxide and, in part, to the uptake and intracellular delivery of key nutrients to the

Cell Uptake

Figure 4.1

Diagram of a phytoplankton cell to show the essential pathways for the gathering and deployment of the key resources. Based on an illustration of Harris (1986) and reproduced with permission from Reynolds (1997a).

Figure 4.1

Diagram of a phytoplankton cell to show the essential pathways for the gathering and deployment of the key resources. Based on an illustration of Harris (1986) and reproduced with permission from Reynolds (1997a).

sites of their anabolism into proteins and, eventually, into organelles. The cell is ordered, with relative compositional homeostasis based on balanced resource deployment and controlled composition. Outside the cell, the external medium is chaotic: besides signalling irregular and rapid fluctuations in the photon flux, the solutes to which the cell is exposed are often patchily distributed, even at the scale of a few millimetres.

Some initial calculations illustrate the magnitude of the uptake requirement. Starting from the premise that the ash-free dry mass of the cytoplasm accounts for between 0.41 and 0.47 pg | m-3 of live volume and that between 46% and 56% of the ash-free dry mass is carbon, then it follows that the carbon concentration in the replete, healthy, live cell is in the range 0.19 to 0.26 pg C |im-3, or 225 ± 35 g C L-1. This is equivalent to 18.8 mols C L-1. Against the air-equilibrium concentration of carbon dioxide in water (0.5-1 mg L-1, or between 11 and 23 |mol L-1), the growing cell is literally accumulating carbon atoms against a concentration gradient in the order of 1 000 000 to 1. Moreover, in order to accomplish a doubling of cell material, it has to acquire another 1 mol carbon for every 1 mol of carbon in the newly isolated daughter tissue. The corresponding calculations for the average cell concentrations of nitrogen (~2.8 mols N L-1) and phosphorus (~0.18 mols P L-1) are of a similar magnitude greater than they might typically occur in natural waters (2-20 |mol N L-1; 0.1-5 |mol P L-1). In relation to the carbon requirement, each cell has to draw on the equivalent of ~151 mmol N and 9.4 mmol P for each mol of C required to replicate the cell mass.

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