This results in swelling of the cell and potential hemolysis bursting of the cell. In an isotonic solution, the flow of water in and out of the cell is happening at the same rate. Osmosis is the diffusion of water molecules across a semipermeable membrane from an area of lower concentration solution i. Water moves into and out of cells by osmosis.
A red blood cell will swell and undergo hemolysis burst when placed in a hypotonic solution. When placed in a hypertonic solution, a red blood cell will lose water and undergo crenation shrivel.
Animal cells tend to do best in an isotonic environment, where the flow of water in and out of the cell is occurring at equal rates. Passive transport is a way that small molecules or ions move across the cell membrane without input of energy by the cell. The three main kinds of passive transport are diffusion or simple diffusion , osmosis, and facilitated diffusion.
Simple diffusion and osmosis do not involve transport proteins. Facilitated diffusion requires the assistance of proteins. Diffusion is the movement of molecules from an area of high concentration of the molecules to an area with a lower concentration. For cell transport, diffusion is the movement of small molecules across the cell membrane. The difference in the concentrations of the molecules in the two areas is called the concentration gradient.
The kinetic energy of the molecules results in random motion, causing diffusion. In simple diffusion, this process proceeds without the aid of a transport protein. It is the random motion of the molecules that causes them to move from an area of high concentration to an area with a lower concentration.
Diffusion will continue until the concentration gradient has been eliminated. Since diffusion moves materials from an area of higher concentration to the lower, it is described as moving solutes "down the concentration gradient".
The end result is an equal concentration, or equilibrium , of molecules on both sides of the membrane. At equilibrium, movement of molecules does not stop. At equilibrium, there is equal movement of materials in both directions. Not everything can make it into your cells. The osmotic gradient acts on solutions having a semi permeable membrane between them; allowing water to diffuse between the two solutions toward the solution with the higher concentration.
Eventually, water with higher concentration will be equally diffused to the side of a lesser concentration. It creates equilibrium for water continues to flow equally both ways, resulting in a stabilized solution. Reverse osmosis is a separation process using pressure to force a solvent to pass through a semi permeable membrane that keeps the solute on one side and directs the pure solvent to the other side.
In other words, this is the process where osmotic pressure is applied to force a solvent from an area of high solute concentration towards an area of low solute concentration.
Examples: As a solution of water shortage, rain water is purified as drinking water. Big industries use reverse osmosis to remove minerals from their boiler water to be recycled. Reverse osmosis is the technique used in liver dialysis. A dialysis machine mimics the function of the kidneys.
Forward osmosis uses osmosis to directly separate water from a feed solution with unwanted solutes. In the FO process, the solvent water transport is driven solely by osmotic pressure difference without the need of any external hydrostatic pressure, allowing for lower energy consumption compared to RO. To extract water from the feed solution, the osmotic pressure at the opposite side of the membrane must be higher, which requires a highly concentrated solution; this concentrated solution is typically referred to as the draw solution.
Draw solutes need to be inert and easily removable. A semipermeable membrane separates the feed solution and the concentrated draw solution where the chemical potential difference allows the water to flow through the membrane while leaving behind the solutes in the feed stream. Regions of high and low solute concentrations refer to those of low and high solvent chemical potentials, respectively.
As the semipermeable membrane restricts the solute transport and maintains chemical potential differences of both solute and solvent, water migrates from its high solvent chemical-potential region i.
Such a water transport leads to dilution of the draw solution where the diluted draw solution can be further recycled such that the initial solute concentration is recovered. Particularly for desalination applications, the solutes in the draw solution osmotic agent or draw solutes are chosen to be inert, nontoxic, and easily removed to obtain the desalinated water with ease.
The water flux across the membrane results in concentration of the feed solution and dilution of the draw solution since the membrane mainly allows passage of water molecules. This phenomenon, referred to as concentration polarization CP , has an adverse impact on the efficacy of the FO process since such an effect reduces the effective osmotic pressure difference across the membrane, thus hindering water transport. CP is highly influenced by the morphology of the membranes. The membranes used in the FO process consist of a thin, dense layer that rejects the solutes active layer followed by a coarse, thick porous layer support layer or porous substrate to reinforce the mechanical stability against fluid pressure and shear.
This configuration makes the membrane asymmetric in which the orientation of the membrane with respect to the direction of the water flux i.
Typically in the FO process, the active layer is placed against the feed stream in order to minimize fouling since the support layer is more susceptible to colloidal fouling due to the large pores. This configuration is called FO mode, as shown in Figure 1 a. However, the downside of placing it in this way is that there is a significant dilutive internal concentration polarization ICP in the thick porous substrate.
This is because the support layer is in contact with the concentrated draw solution hindering the solute diffusion, which significantly reduces the water flux Figure 1 a. Influence of CP on the osmotic pressure distribution in the FO process. In contrast, when the active layer is placed against the draw stream, one can expect a higher water flux since this configuration can avoid the dilutive ICP at the expense of accelerated membrane fouling.
This configuration is called the pressure-retarded osmosis PRO mode, as shown in Figure 1 b , typically realized in standard PRO systems. To avoid any confusion, we will refer to the membrane configuration in which the active layer is placed against the feed solution as the FO mode , whereas the opposite case is the PRO mode during FO processes. The first quantitative experiments on temperature-dependent osmosis go back almost a century ago [ 21 ].
He showed that as the temperature is increased, the transport of pyridine across the membrane is also increased Figure 2 b. The first quantitative experiments reported on the effect of temperature on the osmosis phenomenon. Reprinted with permission from Ref. However, temperature not only influences the osmotic pressure but also impacts many other key properties that are important to the transport process such as viscosity, diffusivity, solubility, density, and so forth.
In this section, we provide a summary of how the system temperature influences the water transport, solute rejection, and membrane fouling. The most direct consequence of raising the system temperature is the increased water flux across the membrane due to lowered water viscosity and increased water diffusivity, which effectively increases the water permeability across the membrane. However, we also note a counterexample where Petrotos et al. On the basis of our survey, the available literature related to the temperature-dependent FO reported increased water flux with temperature.
Table 1 provides a summary of experimental conditions and resulting water flux from the available literature [ 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 ]. Here, we define a new quantity to indicate how much solvent flux increases with respect to the system temperature, as indicated in the last column of Table 1 :. FO mode: active layer placed against feed solution; PRO mode: active layer placed against draw solution.
The survey shows that raising the temperature does increase the water flux, but the extent of such an increase varies across the literature, especially depending on the membrane orientation. This observation implies that the CP phenomena are uniquely influenced by the temperature, leading to variations in the water flux. McCutcheon and Elimelech were the first to study the influence of temperature on the CP phenomena [ 29 ]. At the same time, however, the higher flux also increases both the ICP and ECP, which essentially limit the water flux as a feedback hindrance.
This self-hindering effect of the solvent flux is unavoidable in most membrane separation processes. It is similar to the fact that, in RO, applying high pressure initiates increasing permeate flux, which will eventually bring more solutes from the bulk phase to the membrane surface, enhancing the CP. Therefore, additional gain of the RO permeate flux is not as much as anticipated when the pressure is increased.
The change in the temperature influences the CP phenomena in different ways depending on the orientation of the membrane. This is because the formation of the ICP, which is the most critical factor that limits the driving force, is dependent on the membrane configurations.
By reducing the ICP using deionized water as the feed, the water flux was shown to be highly dependent on the temperature, confirming the impact of ICP on the FO process [ 29 ].
Although the increased solute diffusion at higher temperature mitigates the concentrative ICP in the support layer so that the water flux can be increased, such an increased water flux carries more solutes from the feed bulk phase to the vicinity of the support layer surface and enhances the dilutive ECP, thereby reducing the osmotic driving force. Therefore, the two opposing effects on the water transport effectively limit the enhancement of the water flux such that the temperature has a marginal effect on the overall water flux.
If both water and solute diffusivities increase in a similar behavior, the net diffusive transport must be more or less the same. In the FO mode, however, the water flux was shown to be significantly influenced by the temperature. This was proven mathematically using the method of proof by contradiction [ 32 ].
Such a low water flux effectively suppresses the extent of concentrative ECP in the feed side. Also, the influence of concentrative ECP on the water flux is less important than the dilutive ECP in the draw solution side because the initial solute concentration in the bulk phase is much lower at the feed solution than the draw solution.
Therefore, when the membrane is placed in the FO mode, the water flux is significantly influenced by the temperature since the ICP is the only major factor that determines the driving force. One assumption McCutcheon and Elimelech had made while analyzing their data were the insignificant solute diffusion across the membrane [ 29 ], which otherwise leads to further ICP.
Obviously, commercially available membranes are known to permit diffusion of the solutes, which can impact the formation of the CP effect. Since the solute diffusion is also sensitive to the temperature, the transmembrane solute flux should also lead to a change in the water flux.
We discuss the effect of temperature on the solute diffusion and rejection in the following section. It is of general consensus that the transmembrane solute diffusion increases with temperature. A number of groups have recently investigated experimentally the temperature effect on the transmembrane solute diffusion and the solute rejection [ 26 , 27 , 28 ].
Xie et al. Hydration of charged organic solutes results in an increase in the effective solute size, which directly influences the solute diffusion and rejection rate, as it was well understood that the rejection of the charged organic solutes would be much higher than the neutral organic solutes.
In this regard, neutral solutes were more likely to diffuse across the pores than the charged solutes in both the cellulose triacetate membranes and polyamide membranes. This implies that increasing the temperature leads to higher solute diffusion due to the increased solute diffusivity. Moreover, increasing the temperature leads to faster dissolution of the solutes into the membrane such that even hydrophobic neutral solutes absorb into the membrane at an order of magnitude higher rate at elevated temperatures.
Notably, the ratio between the water flux J w and the solute flux J s was shown to be more or less constant regardless of the system temperature [ 27 ]. However, it is more reasonable to say that the structural properties of FO membranes change with temperature in a way that the ratio between solvent and solute fluxes remain almost constant.
In a solution-diffusion model, permeabilities of solvent and solutes, A and B , respectively, are believed to increase with membrane temperature. This is because although higher T increases both the solute and solvent fluxes, it is only the ratio that influences the concentration of solutes passing through the membrane. Cells can gain or lose water by the process of osmosis. This depends on the water concentration of the solution inside the cell compared to water concentration of the solution outside the cell.
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