Valinomycin Superficially valinomycin resembles a cyclic peptide Fig. Crown Ethers Crown ethers are a family of synthetic ionophores that are generally similar in function to the natural product valinomycin [36]. Nystatin Nystatin Fig. Gap Junctions Gap junctions are a common structural feature of many animal plasma membranes [39] , [40]. Other Ways to Cross the Membrane There are several other ways that solutes, including large macromolecules, can cross membranes.
Pinocytosis Pinocytosis is a form of endocytosis involving fluids containing many solutes. Phagocytosis, a type of endocytosis that involves uptake of large solid particles.
Exocytosis Exocytosis is the process by which cells excrete waste and other large molecules from the cytoplasm to the cell exterior [49] and therefore is the opposite of endocytosis.
Blebbing Blebbing of the plasma membrane is a morphological feature of cells undergoing late stage apoptosis programmed cell death, see Chapter 24 [53]. Summary Carefully controlled solute movement into and out of cells is an essential feature of life. References 1. Alberts B. Molecular biology of the cell. Garland Science; New York: Principles of membrane transport. Blok M. Effect of the gel to liquid crystalline phase transition on the osmotic behaviour of phosphatidylcholine liposomes.
Biochim Biophys Acta. Miller F. Fick's laws of diffusion; p. Water systems: aqua technology for the 21st century. Introductory information on reverse osmosis. Bangham A. Osmotic properties and water permeability of phospholipids liquid crystals. Chem Phys Lipids. De Gier J.
Osmotic behaviour and permeability properties of liposomes review Chem Phys Lipids. Baldwin S. Membrane transport: a practical approach; p. Essential cell biology. Garland Publishing: Taylor Francis Group; Comparison of passive and active transport. Huang S. The GLUT4 glucose transporter. Cell Metab. Biologia Medica. Lippiat J. Potassium channels. Series in methods in molecular biology. Doyle D. Goldin A. Resurgence of sodium channel research.
Ann Rev Physiol. Penzotti J. Biophys J. Noda Y. Aquaporins in kidney pathophysiology. Nat Rev Nephrol. Hill A. What are aquaporins for? J Membr Biol. Nelson D. Lehninger: principles of biochemistry. Worth Publishers; Chapter Biological membranes and transport.
Solute transport across membranes. Lopina O. Membr Cell Biol. Wright E. Am J Physiol Renal Physiol. Jakubowski H. Chapter 9 — signal transduction. Energy transduction: uses of ATP. Martin S. Nutrient transport by ruminal bacteria: a review. J Anim Sci. Mitchell P. Group-translocation: a consequence of enzyme-catalysed group-transfer. Coupling of metabolism and transport by enzymic translocation of substrates through membranes.
Proc R Phys Soc Edinb. Chemiosmotic hypothesis of oxidative phosphorylation. Proton current flow in mitochondrial systems. Herzberg O. Proteopedia, Weizmann Institute of Science in Israel; Enzyme I of the phosphoenolpyruvate: sugar phosphotransferase system. Pressman B. Biological applications of ionophores. Annu Rev Biochem.
Szabo G. Structural aspects of ionophore function. Fed Proc. Garland Scientific; New York: Moore C. Mechanism of action of valinomycin on mitochondria. Biochem Biophys Res Comm. Induced active transport of ions in mitochondria.
Page S. Avcare Limited; Canberry Australia : The role of enteric antibiotics in livestock production. Cheng Y. Tainter M. Use of dinitrophenol in obesity and related conditions: a progress report. J Am Med Assoc. Huszthy P.
Synthesis and molecular recognition studies of crown ethers. Period Polytech. Borgos S. J Med Chem. Lopes S. Revealing the orientation of nystatin and amphotericin B in lipidic multilayers by UV-Vis linear dichroism.
Phys Chem B. Revel J. Hexagonal array of subunits in intracellular junctions of the mouse heart and liver. J Cell Biol. In: Peracchia C, editor. Gap junctions: molecular basis of cell communication in health and disease. Benos D, series editor. Current topics in membranes and transport, series ed. New York: Elsevier Publishing; Simpson I. Size limit of molecules permeating the junctional membrane channels.
Imanaga I. Cell-to-cell diffusion of fluorescent dyes in paired ventricular cells. Echevarria W. Chapter: channels and transporters. If you needed to remove glucose, the cell would require energy. Letting Concentration Do the Work Sometimes cells are in an area where there is a large concentration difference. For example, oxygen molecule concentrations could be very high outside of the cell and very low inside.
Those oxygen molecules are so small that they are able to cross the lipid bilayer and enter the cell. There is no energy needed for this process. In this case, it's good for the cell because cells need oxygen to survive. It can also happen with other molecules that can kill a cell. Osmosis Another big example of passive transport is osmosis.
This is a water specific process. There are cases where a maximum entropy configuration corresponds to our perceived sense of disorder and low entropy configurations appear ordered and there are cases where a maximum entropy configuration is perceived as a highly ordered state and low entropy configurations appear disordered; Denbigh, ; Lambert, The classic examples of water—oil layer, supercooled water, or sphere packing are cases where the disorder analogy breaks down but maximum entropy arguments deliver the correct solutions Jaynes, Diffusion within plants provides an effective transport mechanism over short distances.
Diffusion was first described by botanist Thomas Brown in based on his observations of the erratic movement of pollen in water for a recent historical account of Brownian motion, see Genthon, Molecules in an environment with an absolute temperature above 0 have kinetic energy and are constantly in motion, colliding with other molecules in their vicinity Figure 2A.
When concentration or thermal gradients are present, the resulting higher number of trajectories leads to directed passive transport. Diffusion is driven by thermal motion. Frequent collisions between particles results in a short mean free path length, viscosity and drag, leading to any movement being dampened. Crowded environments, such as the cytosol, lead to nonlinear behavior and subdiffusion.
Superdiffusion is associated with an active transport process. C The erratic behavior of a single particle Brownian motion can be reproduced well by a random walk and Monte-Carlo simulation is a popular approach for finding particle trajectories. D Entropy considerations lead to correct predictions for reproducible macroscopic states. Shown here is the problem of distributing numbers of particles n i in different locations i.
There is only one configuration in which all particles are in one defined location, whereas a uniform distribution can be achieved in many more ways. Particles fleas all have the same jump probability from one side to the other from dog to dog. The number of trajectories from the side with a high number of particles is greater than the reverse movement, providing a simple explanation for why a net flux of particles down a concentration gradient emerges from undirected thermal motion.
Einstein showed that the diffusion constant for a molecule in a fluid depends on the viscosity of the fluid, the temperature of the system, and the radius of the molecule Einstein, We can thus assign the cause of diffusion to the kinetic energy of the solute molecules.
Maximum entropy arguments Jaynes, ; Box 2 correctly predict the most probable state in which we would expect to find any given number of particles, subject to experimental constraints, and can also be used to analyze diffusion Ghosh et al.
Using this framework might lead to the inference that diffusion is driven by the number of accessible configurations entropy. While these interpretations can be reconciled Grandy, , it is well known that macroscopic descriptions do not determine the underlying microscopic laws and likewise that the microscopic laws by themselves do not lead to macroscopic states. BOX 2. The principle of maximum entropy and transport phenomena.
Although living systems operate far from thermodynamic equilibrium, this framework and extensions thereof can nevertheless represent a plausible approximation and be fruitful for developing insights. Thermodynamic equilibrium is the state of maximum entropy subject to any experimental constraints macroscopic variables by which we can control a system Jaynes, If we change any of the macroscopic variables change in information and allow the system to reach a new thermodynamic equilibrium then that new state will be defined by maximum entropy under the constraints of those new conditions Jaynes, ; Grandy, Building these ideas and the work of Gibbs and Shannon, Jaynes put forward the principle of maximum information entropy as a means assigning probabilities Jaynes, Constraints can be added through the use of Lagrange multipliers.
For instance, maximizing entropy subject to the condition that all probabilities must sum to one, leads to a uniform distribution, maximizing entropy subject to the additional constraint of knowing the total energy of the system leads to the Boltzmann distribution, including also a constraint on particle numbers leads to the grand-canonical distribution, as well as the standard terms for free energy and the chemical potential Jaynes, Entropy and probabilistic inference are extremely powerful approaches, yet stating that something is driven by entropy is perhaps a little misleading and provides no insight into what is happening mechanistically or the real driving force.
Osmosis is the key for plant life and contributes to maintaining cell form and function, cell growth, plant movement, and various transport processes.
Osmosis was first described in the 18th century, by Jean-Antoine Nollet, although it was Henri Dutrochet in the s who provided one of the first clear demonstrations of the phenomenon in plants for a more complete history with recent developments, see Marbach and Bocquet, Thermodynamically, osmosis flow can be viewed as an entropic effect.
The mixing of two substances solvent and solute increases the entropy which results in a difference in the chemical potential between the two compartments on either side of a semipermeable membrane Borg, ; Marbach and Bocquet, Differences in chemical potential drive the flux of particles, until the chemical potentials are equal.
The entropy of mixing via the chemical potential thus appears as the thermodynamic driving force for this process but it says nothing about the actual mechanism.
This has led to some imaginative microscopic interpretations and much controversy Borg, ; Alleva et al. We need to go to microscopic descriptions of the system to get to the mechanism and physical driving force Figure 3. Osmotic flow is driven by a pressure difference acting on the solvent between the two sides of a semipermeable membrane and this pressure difference arises from changes in the kinetic and potential energies caused by the addition of the solute Bowler, ; Figure 3A.
This can also be understood by considering that the semipermeable membrane has a repulsive potential energy for the solute but not the solvent Kramer and Myers, ; Bowler, ; Marbach and Bocquet, The virial theorem provides a mechanistic explanation for osmosis. A The virial theorem allows for pressure to be expressed via the kinetic, E kin , and potential energy, E pot , in the system Borg, ; Bowler, The forces on sides 1 and 2 that are required to balance the pressure are denoted by F 1 and F 2 , and the pressures acting on the solvent on either side by p 1A and p 2A these are not partial pressures.
The individual steps 1, 2, 3 and the corresponding energetic changes are given in the figure. Note that the pressure differential on the solvent depends on the interaction term between solvent and solute but that the resulting total pressure difference the osmotic pressure depends only on the kinetic energy of the solute and the usually negligible solute-solute interaction energy. These energetic considerations make clear that osmosis is not a special property of water and indeed osmosis occurs also in gases.
Whilst molecular diffusion will be present it is not a significant contributor. A net diffusive flux would require a solvent concentration gradient which is often not present and can even go the other way depending on solvent—solute interactions. Thus, diffusion or facilitated diffusion are not the drivers of osmotic flow. B In biological systems, pressure-driven water transport occurs through water channels aquaporins.
In plants, plasmodesmata may also act as water channels between cells Figure 4. The power of thermodynamics is that it makes macroscopic predictions without the need for these microscopic details—this can also be a limitation. Although the nature of the semipermeable membrane does not enter into any of the above considerations or the standard theoretical frameworks for describing osmosis, it is clearly important as without the semipermeable membrane there would be no osmosis flux nor osmotic pressure Figure 3B.
The membrane permeabilities for solutes and the solvent are the key parameters. Aquaporins have been shown to be major routes for water transport across membranes, with fluxes exceeding those expected for diffusion across a lipid bilayer by orders of magnitude. Aquaporins can transport 10 9 water molecules per second Jensen and Mouritsen, Estimates of this ratio vary but are typically greater than 10 Jensen and Mouritsen, ; Wambo et al.
Several other regulatory mechanisms of aquaporins in plants have been described Alleva et al. BOX 3. Molecular diffusion can be expected to occur in any fluid.
Advection mass flow is an effective way of enhancing transport. The Reynolds numbers characterizes fluid flow by the ratio of inertia and viscous forces Jensen et al. For instance, Taylor dispersion, or shear-enhanced diffusion, describes the effects of flow on diffusion which can lead to a significant increase in the effective diffusion, as shown for xylem flow Blyth and Morris, A key route for transport and signaling between plant cells is through plasmodesmata Faulkner, ; Li et al.
Plasmodesmata were first described by Eduard Tangl in For a recent historical overview on plasmodesmata research see van Bel Shape, size, and density of plasmodesmata vary greatly between tissue and cell types Nicolas et al. Modeling has mostly focused on simple, nonbranched plasmodesmata Peters et al. In these types of plasmodesmata, the structure consists of a cytoplasmic sleeve between the plasma membrane that lines the pore and the membrane of the desmotubule part of the endoplasmic reticulum that bridges the cytosol of neighboring cells; Figure 4.
The geometry of the cytoplasmic sleeve is such that small molecules can likely diffuse through it. Various models for diffusion of molecules through the cytoplasmic sleeve have been developed. Recent work by Deinum et al. Restrictions to normal diffusion include steric hindrances and interactions with other molecules for instance, membranes or large protein complexes such as tethers that are present in the cytoplasmic sleeve.
Diffusive hindrance could provide part of the explanation for the drastic reduction in diffusivity observed in similar models where plasmodesmata transport is modeled as diffusion, but requires a modified rate of diffusion for models to recapitulate experimental data. Alternative approaches that lead to modified or effective diffusion, resulting from Brownian particles in a confined volume with small exit areas Holcman and Schuss, ; Grebenkov and Oshanin, , have been suggested that model plasmodesmata as nonreflecting boundaries an escape pore; Calderwood et al.
Plasmodesmata may allow for different modes of transport between cells. Small molecules are likely to be able to diffuse through plasmodesmata. This is usually modeled as normal diffusion with or without geometrical hindrance factors. If pressure differences exist between cells then advection may occur. For larger molecules the mode of transport remains unclear.
Selective molecular transport through plasmodesmata may lead to concentration differences of those molecules that cannot pass between neighboring cells and potentially to osmotic flows through plasmodesmata, giving rise to osmotic and turgor pressure differences. Depending on the transport route, turgor pressure or osmotic pressure may be more important which may give rise to some interesting flows and feedbacks.
The precise modes of transport and their dependence on pressure, flow, dynamics of the plasma membrane, dynamics of the desmotubule, the associated proteins, and interactions between plasma membrane and the desmotubule remain to be characterized.
The importance of plasmodesmal fluxes has recently been demonstrated for auxin flows Gao et al. Mellor et al. Through a cycle of model predictions and experimental validation, they found that without accounting for flux through plasmodesmata observed concentration profiles could not be reproduced. Passive diffusion of auxin through plasmodesmata was found to be an important component of establishing auxin gradients within the root with plasmodesmata density emerging as a key parameter Mellor et al.
Gao et al. Describing transport as a diffusion process, they computed local effective diffusion tensors in different areas of the leaf. Importantly, they show that not only is diffusion asymmetric but that within the same cell, different directions can have different permeabilities. This is passive transport. Passive transport usually occurs down a concentration gradient.
Essentially what this means is that molecules will move from areas where there are more of them to where there are fewer of them. When particles move down their concentration gradient from an area of high concentration to low concentration, this is called simple diffusion. Diffusion is how oxygen gets into your cells. Another type of passive transport, filtration, happens when physical pressure pushes fluid through a selectively permeable membrane. In the body, this takes place when blood pressure pushes fluid through openings in the walls of capillaries.
Specialized channels in the cell membrane, called aquaporins, specifically allow water to flow in and out of cells. The direction in which osmosis takes place in or out of a cell depends on the concentration of the solution the cell is immersed in. Normal blood is isotonic to the RBC's cytoplasm, meaning that the concentration of water is the same inside the cell and outside of it. The intake and output of water is balanced and all is harmonious. If an RBC is immersed in a hypotonic solution, like distilled water, water will rush into the cell and it will swell and burst!
For large or multiple particles and drops of fluid, or when a cell needs to move materials against the concentration gradient, active transport is the way to go.
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