Stomatal guard cells play a key role in gas exchange for photosynthesis while minimizing transpirational water loss from plants by opening and closing the stomatal pore. Foliar gas exchange has long been incorporated into mathematical models, several of which are robust enough to recapitulate transpirational characteristics at the whole-plant and community levels. Few models of stomata have been developed from the bottom up, however, and none are sufficiently generalized to be widely applicable in predicting stomatal behavior at a cellular level. We describe here the construction of computational models for the guard cell, building on the wealth of biophysical and kinetic knowledge available for guard cell transport, signaling, and homeostasis. The OnGuard software was constructed with the HoTSig library to incorporate explicitly all of the fundamental properties for transporters at the plasma membrane and tonoplast, the salient features of osmolite metabolism, and the major controls of cytosolic-free Ca 2+ concentration and pH. The library engenders a structured approach to tier and interrelate computational elements, and the OnGuard software allows ready access to parameters and equations 'on the fly' while enabling the network of components within each model to interact computationally. We show that an OnGuard model readily achieves stability in a set of physiologically sensible baseline or Reference States; we also show the robustness of these Reference States in adjusting to changes in environmental parameters and the activities of major groups of transporters both at the tonoplast and plasma membrane. The following article addresses the predictive power of the OnGuard model to generate unexpected and counterintuitive outputs.Stomatal guard cells surround pores in the epidermis of plant leaves and regulate the pore aperture. They open the pore in response to low CO 2 and light to facilitate CO 2 access for photosynthesis, and they close the pore in the dark, under drought stress, and in the presence of the water-stress hormone abscisic acid to minimize water loss through transpiration. Stomata have a profound impact on the water and carbon cycles of the world (Gedney et al., 2006;Betts et al., 2007). Their dynamics have been incorporated into models for transpiration and water use efficiency (Farquhar and Wong, 1984;Ball, 1987;Williams et al., 1996;Eamus and Shanahan, 2002;West et al., 2005), successfully reproducing the gas exchange, CO 2 , and transpirational characteristics of experiments at the plant and community levels. To date, these models have taken a top-down approach. They subsume stomatal movements within a few empirical parameters of linear hydraulic pathways and conductances without reference to the molecular mechanics of the guard cell. No generalized guard cell model has yet to be developed from the bottom up, drawing on the wealth of knowledge available for guard cell transport, signaling, and homeostasis. It is clear that such a model is now needed. The depth and breadth of information ava...

The dynamics of stomatal movements and their consequences for photosynthesis and transpirational water loss have long been incorporated into mathematical models, but none have been developed from the bottom up that are widely applicable in predicting stomatal behavior at a cellular level. We previously established a systems dynamic model incorporating explicitly the wealth of biophysical and kinetic knowledge available for guard cell transport, signaling, and homeostasis. Here we describe the behavior of the model in response to experimentally documented changes in primary pump activities and malate (Mal) synthesis imposed over a diurnal cycle. We show that the model successfully recapitulates the cyclic variations in H + , K + , Cl 2, and Mal concentrations in the cytosol and vacuole known for guard cells. It also yields a number of unexpected and counterintuitive outputs. Among these, we report a diurnal elevation in cytosolic-free Ca 2+ concentration and an exchange of vacuolar Cl 2 with Mal, both of which find substantiation in the literature but had previously been suggested to require additional and complex levels of regulation. These findings highlight the true predictive power of the OnGuard model in providing a framework for systems analysis of stomatal guard cells, and they demonstrate the utility of the OnGuard software and HoTSig library in exploring fundamental problems in cellular physiology and homeostasis.

Stomata account for much of the 70% of global water usage associated with agriculture and have a profound impact on the water and carbon cycles of the world. Stomata have long been modeled mathematically, but until now, no systems analysis of a plant cell has yielded detail sufficient to guide phenotypic and mutational analysis. Here, we demonstrate the predictive power of a systems dynamic model in Arabidopsis (Arabidopsis thaliana) to explain the paradoxical suppression of channels that facilitate K + uptake, slowing stomatal opening, by mutation of the SLAC1 anion channel, which mediates solute loss for closure. ] i is sufficient to recover K + channel activities and accelerate stomatal opening in the slac1 mutant. Thus, we uncover a previously unrecognized signaling network that ameliorates the effects of the slac1 mutant on transpiration by regulating the K + channels. Additionally, these findings underscore the importance of H + -coupled anion transport for pH i homeostasis.

During their asexual reproduction cycle (about 48 hours) in human red cells, Plasmodium falciparum parasites consume most of the host cell hemoglobin, far more than they require for protein biosynthesis. They also induce a large increase in the permeability of the host cell plasma membrane to allow for an increased traffic of nutrients and waste products. IntroductionDuring their intraerythrocytic phase, Plasmodium falciparum parasites grow and divide within the red blood cells to occupy about 16-to 20-fold the volume of the invading merozoite. The parasite ingests and digests about 70% of the host cell hemoglobin (Hb) 1 but uses only up to 16% of the released amino acids for protein biosynthesis. 2 The excess is discharged out of the infected red blood cells (IRBCs) to the surrounding plasma 3 mainly through new permeation pathways (NPPs) of broad solute selectivity induced by the parasite in the host cell membrane. [4][5][6] The reason why parasites expend so much energy ingesting and digesting excess hemoglobin [7][8][9][10] and detoxifying the cell from toxic ferriprotoporphyrin IX 11-13 remains puzzling.Another unresolved puzzle concerns the mechanism by which parasitized red cells are able to retain their osmotic stability for the approximately 48-hour reproductive cycle of the parasite despite the rapid NPP-mediated dissipation of the Na ϩ and K ϩ gradients. 14 A recent study by Staines et al 15 demonstrated that if NPPs were induced in uninfected cells as they are in infected cells, the uninfected cells would hemolyse by approximately 44 hours. Since the additional volume of the parasite was excluded from their computations, their estimates make the lysis resistance of IRBCs with large internal parasites even harder to comprehend.To investigate these issues we developed a mathematical model of the homeostasis of a parasitized red cell, formulated critical predictions on the stage-related volume changes of IRBCs, and carried out experimental tests to determine the validity of the model. The experimental results confirmed the predicted stagerelated volume changes, and validated the model-based suggestion that NPP-mediated permeability and excess Hb consumption are fine-tuned to ensure the osmotic stability and integrity of the parasitized cell for the duration of its asexual cycle. Materials and methods Preparation of cells and determination of the osmotic fragility distribution of infected red cell populationsRed cells infected with P falciparum A4 clone (kind gift from B. C. Elford, Institute of Molecular Medicine, Oxford, United Kingdom), derived from the ITO4 line 16 were cultured under a low oxygen atmosphere by standard methods. 17 The culture medium, changed daily, was RPMI 1640 supplemented with 40 mM HEPES (N-2-hydroxyethylpiperazine-NЈ-2-ethanesulfonic acid), 25 mg/L gentamicin sulfate, 10 mM D-glucose, 2 mM glutamine, and 8.5% vol/vol pooled human serum. There were 9 experiments carried out, 6 with IRBCs harvested from nonsynchronized cultures and 3 from synchronized cultures. Synchronization w...

A basic mathematical model of human red cells is presented which integrates the charge and nonideal osmotic behavior of hemoglobin and of other impermeant cell solutes with the ion transport properties of the red cell membrane. The computing strategy was designed to predict the behavior of all measurable variables in time in ways that optimize comparison with experimentally determined behavior. The need and applications of such a model are illustrated in three separate examples covering different areas of experimentation in the physiology and pathophysiology of red cells.

Polymers of deoxyhemoglobin S deform sickle cell anemia red blood cells into sickle shapes, leading to the formation of dense, dehydrated red blood cells with a markedly shortened life-span. Nearly four decades of intense research in many laboratories has led to a mechanistic understanding of the complex events leading from sickling-induced permeabilization of the red cell membrane to small cations, to the generation of the heterogeneity of age and hydration condition of circulating sickle cells. This review follows chronologically the major experimental findings and the evolution of guiding ideas for research in this field. Predictions derived from mathematical models of red cell and reticulocyte homeostasis led to the formulation of an alternative to prevailing gradualist views: a multitrack dehydration model based on interactive influences between the red cell anion exchanger and two K(+) transporters, the Gardos channel (hSK4, hIK1) and the K-Cl cotransporter (KCC), with differential effects dependent on red cell age and variability of KCC expression among reticulocytes. The experimental tests of the model predictions and the amply supportive results are discussed. The review concludes with a brief survey of the therapeutic strategies aimed at preventing sickle cell dehydration and with an analysis of the main open questions in the field.

SUMMARY1. A method was developed for measuring the cytoplasmic magnesium buffering of intact red cells using the divalent cation selective ionophore A23187. Addition of A23187 to a suspension of red cells induces rapid equilibration of ionized magnesium across the cell membrane.2. Entry of magnesium into red cells is associated with cell swelling and depolarization of the membrane potential.3. At an external ionized magnesium concentration of about 0-15 mm corresponding to an internal ionized concentration of 0-4 mm the addition of A23187 did not produce a change in the magnesium content of the cells. This indicates that the normal ionized magnesium concentration inside the oxygenated red cell is about 0-4 mM.4. The magnesium buffering curve for oxygenated, inosine-fed human red blood cells is adequately described by the existence of three buffer systems of increasing capacity and decreasing affinity. These are 0-15 mm with a Km < 10-7 M, probably structural magnesium bound within the cell proteins; 1-6 mm with a Km 0-08 mM, mainly ATP and other nucleotides; and about 21-25 mm with a Km 3-6 mM, a

No abstract