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 differential control of transport may be important for changing the local concentrations of defense compounds Gao et al. The interplay between passive transport and active transport remains to be elucidated, also whether different concentrations of molecules that cannot pass through plasmodesmata may induce osmotic flows between cells.
The space between the cytoplasmic sleeve and the desmotubule of plasmodesmata can prevent some molecules from passing into the channel Faulkner, Molecules that are larger than the so-called size-exclusion limit SEL; typically measured in molecular weight cannot traverse plasmodesmata passively.
If some molecules water, ions, small metabolites, small macromolecules can pass but others not large metabolites, macromolecules , then we have an instantiation of a semipermeable membrane. This may potentially mean that any changes to the population of molecules above the SEL in connected cells might induce an osmotic flow between the cells. Furthermore, recent observations suggest that neighboring cells can be under different turgor pressure and this was recapitulated in a model that showed this pressure differential to depend on their tissue connectivity Long et al.
With different turgor pressures and potentially different osmotic pressures between neighboring cells, we can expect pressure-driven flows advection to occur Anisimov and Egorov, , yet increases in turgor pressure are known to reduce cell-to-cell movement Oparka and Prior, This pressure-induced reduction in permeability is rapid and difficult to reconcile with a transport model of static geometry and the time-scales of callose deposition, suggesting a role for plasmodesmal dynamics that operate on faster timescales.
Based on this premise, Park et al. Both these pressure and concentration values depend on the geometrical and mechanical parameters in the model. A model based on the underlying physics of advection and diffusion with realistic geometries was recently used to investigate phloem loading of sugars through plasmodesmata Comtet et al.
Shear-enhanced diffusion has been suggested to be important for xylem transport Blyth and Morris, ; Box 3. Similar mechanisms may be active in aquaporins and plasmodesmata. The extent to which advection and diffusion interact could potentially depend on the mechanical properties of the conduit and membrane dynamics. Recent work on artificial tubes has uncovered mechanisms that may be relevant for flows in biological systems.
Marbach et al. These modulations to the membrane surface could be thermal or induced by other mechanisms. Transport could in principle be tuned by adjusting the geometrical and dynamical properties of the channel-defining structures. How important such processes may be for plasmodesmata remains to be investigated and will depend on the interplay between osmosis, pressure flows, diffusion, membrane, and desmotubule dynamics.
Intercellular diffusion is likely to occur for small molecules and depending on the presence of concentration gradients this may or may not result in net fluxes. Advection requires a pressure difference or a temperature difference which could be caused by different turgor pressures between cells and osmosis.
Some molecules above the SEL typically proteins, transcription factors, or messenger RNA are also transported between cells through plasmodesmata Guenoune-Gelbart et al. Varying reports on the importance of diffusion over advection for plasmodesmal transport exist, suggesting this may be different depending on the species, tissue type, developmental stage, and type of plasmodesmata Comtet et al.
Furthermore, modification of the aperture by callose deposition changes the SEL, implying that molecules which could previously move from cell to cell are no longer able to do so Faulkner, This in turn potentially increases the number of chemical species that could be considered to be osmolytes; thus, changing the osmotic pressure but also modifying the geometry that affects advection and diffusion. For instance, if the sugar concentration was higher in one cell relative to a neighboring cell and if sugars were not blocked from passing through plasmodesmata, then the cell with higher sugar concentration would lead to an osmotic flow into that cell from the extracellular space as sugars would act as an osmolyte between inside and outside that cell but not to influx from the neighboring cell as plasmodesmata with a large enough SEL would not act as a semipermeable membrane for sugar.
However, the resulting increase in turgor pressure in the cell with sugars might then lead to mass flow to its neighboring cell or pressure-induced plasmodesmata closure Park et al. The uncertainties and potential confusion of this paragraph demonstrate that there is still much that we the authors do not understand about the regulation and mode of plasmodesmal transport Figure 4.
Related points are discussed in excellent recent reviews Nicolas et al. Advanced computational models with smooth i. Membrane transport keeps cells alive. This tutorial will be more or less a quick review of the various principles of water motion in reference to plants.
The kidneys are responsible for the regulation of water and inorganic ions. Read this tutorial to learn about the different parts of the kidneys and its role in homeostasis Muscle cells are specialized to generate force and movement. Learn about the different types of muscle tissues in this tutorial and the molecular mechanisms of contraction The circulatory system is key to the transport of vital biomolecules and nutrients throughout the body. Learn about the different components and functions of the human circulatory system dealt with in detail in this tutorial.
The lymphatic system is also elucidated elaborately here Cell Biology. Skip to content Main Navigation Search. Dictionary Articles Tutorials Biology Forum. Table of Contents. An illustration to show how passive transport occurs.
Water-soluble molecules move down the concentration gradient across the membrane via a channel protein an example of facilitated diffusion. In this example, the solute cannot diffuse through the membrane, but the water can. Water has a concentration gradient in this system. Thus, water will diffuse down its concentration gradient, crossing the membrane to the side where it is less concentrated. Osmosis proceeds constantly in living systems. A solution with low osmolarity has a greater number of water molecules relative to the number of solute particles.
A solution with high osmolarity has fewer water molecules with respect to solute particles. This effect makes sense if you remember that the solute cannot move across the membrane, and thus the only component in the system that can move—the water—moves along its own concentration gradient.
An important distinction that concerns living systems is that osmolarity measures the number of particles which may be molecules in a solution.
Therefore, a solution that is cloudy with cells may have a lower osmolarity than a solution that is clear, if the second solution contains more dissolved molecules than there are cells. In a hypotonic situation, the extracellular fluid has lower osmolarity than the fluid inside the cell, and water enters the cell. In living systems, the point of reference is always the cytoplasm, so the prefix hypo — means that the extracellular fluid has a lower solute concentration, or a lower osmolarity, than the cell cytoplasm.
It also means that the extracellular fluid has a higher water concentration in the solution than does the cell. In this situation, water will follow its concentration gradient and enter the cell. Because the cell has a relatively higher water concentration, water will leave the cell. In an isotonic solution, the extracellular fluid has the same osmolarity as the cell. Blood cells and plant cells in hypertonic, isotonic, and hypotonic solutions take on characteristic appearances Figure.
A doctor injects a patient with what the doctor thinks is an isotonic saline solution. The patient dies, and an autopsy reveals that many red blood cells have been destroyed. Do you think the solution the doctor injected was really isotonic?
For a video illustrating the diffusion process in solutions, visit this site. In a hypotonic environment, water enters a cell, and the cell swells. In an isotonic condition, the relative solute and solvent concentrations are equal on both membrane sides. In a hypertonic solution, water leaves a cell and the cell shrinks. Remember, the membrane resembles a mosaic, with discrete spaces between the molecules comprising it.
If the cell swells, and the spaces between the lipids and proteins become too large, the cell will break apart. In contrast, when excessive water amounts leave a red blood cell, the cell shrinks, or crenates. This has the effect of concentrating the solutes left in the cell, making the cytosol denser and interfering with diffusion within the cell. Various living things have ways of controlling the effects of osmosis—a mechanism we call osmoregulation. Some organisms, such as plants, fungi, bacteria, and some protists, have cell walls that surround the plasma membrane and prevent cell lysis in a hypotonic solution.
The cytoplasm in plants is always slightly hypertonic to the cellular environment, and water will always enter a cell if water is available. In nonwoody plants, turgor pressure supports the plant. Conversly, if you do not water the plant, the extracellular fluid will become hypertonic, causing water to leave the cell.
In this condition, the cell does not shrink because the cell wall is not flexible. However, the cell membrane detaches from the wall and constricts the cytoplasm. We call this plasmolysis. Plants lose turgor pressure in this condition and wilt Figure. Tonicity is a concern for all living things. For example, paramecia and amoebas, which are protists that lack cell walls, have contractile vacuoles.
This vesicle collects excess water from the cell and pumps it out, keeping the cell from lysing as it takes on water from its environment Figure. Many marine invertebrates have internal salt levels matched to their environments, making them isotonic with the water in which they live. Fish, however, must spend approximately five percent of their metabolic energy maintaining osmotic homeostasis.
Freshwater fish live in an environment that is hypotonic to their cells. These fish actively take in salt through their gills and excrete diluted urine to rid themselves of excess water.
Saltwater fish live in the reverse environment, which is hypertonic to their cells, and they secrete salt through their gills and excrete highly concentrated urine. In vertebrates, the kidneys regulate the water amount in the body. Osmoreceptors are specialized cells in the brain that monitor solute concentration in the blood.
If the solute levels increase beyond a certain range, a hormone releases that slows water loss through the kidney and dilutes the blood to safer levels. Animals also have high albumin concentrations, which the liver produces, in their blood. This protein is too large to pass easily through plasma membranes and is a major factor in controlling the osmotic pressures applied to tissues.
The passive transport forms, diffusion and osmosis, move materials of small molecular weight across membranes. Substances diffuse from high to lower concentration areas, and this process continues until the substance evenly distributes itself in a system. In solutions containing more than one substance, each molecule type diffuses according to its own concentration gradient, independent of other substances diffusing.
In living systems, the plasma membrane mediates substances diffusing in and out of cells. Some materials diffuse readily through the membrane, but others are hindered and only can pass through due to specialized proteins such as channels and transporters. Some cells require larger amounts of specific substances than other cells; they must have a way of obtaining these materials from extracellular fluids.
This may happen passively, as certain materials move back and forth, or the cell may have special mechanisms that facilitate transport. Some materials are so important to a cell that it spends some of its energy hydrolyzing adenosine triphosphate ATP to obtain these materials.
Red blood cells use some of their energy to do this. All cells spend the majority of their energy to maintain an imbalance of sodium and potassium ions between the interior and exterior of the cell. The most direct forms of membrane transport are passive.
0コメント