How does solute concentration affect osmotic pressure?

The rate of osmosis always depends on the concentration of solute. The process is illustrated by comparing an environmental or external solution to the internal concentration found in the body. A hypertonic solution is any external solution that has a high solute concentration and low water concentration compared to body fluids. In a hypertonic solution, the net movement of water will be out of the body and into the solution. A cell placed into a hypertonic solution will shrivel and die by a process known as plasmolysis. An isotonic solution is any external solution that has the same solute concentration and water concentration compared to body fluids. In an isotonic solution, no net movement of water will take place. A hypotonic tonic solution is any external solution that has a low solute concentration and high water concentration compared to body fluids. In hypotonic solutions, there is a net movement of water from the solution into the body. A cell placed into a hypotonic solution will swell and expand until it eventually burst through a process known as cytolysis.  These three examples of different solute concentrations provide an illustration of the spectrum of water movement based on solute concentration through the process of osmosis. The body, therefore, must regulate solute concentrations to prevent cell damage and control the movement of water where needed.

Summary of Red Blood Cell Placed into Hypertonic, Isotonic, and Hypotonic Solutions

Hypertonic

A hypertonic solution has a higher solute concentration compared to the intracellular solute concentration. When placing a red blood cell in any hypertonic solution, there will be a movement of free water out of the cell and into the solution. This movement occurs through osmosis because the cell has more free water than the solution. After the solutions are allowed to equilibrate, the result will be a cell with a lower overall volume. The remaining volume inside the cell will have a higher solute concentration, and the cell will appear shriveled under the microscope. The solution will be more dilute than originally. The overall process is known as plasmolysis.  

Isotonic

An isotonic solution has the same solute concentration compared to the intracellular solute concentration. When a red blood cell is placed in an isotonic solution, there will be no net movement of water. Both the concentration of solute and water are equal both intracellularly and extracellularly; therefore, there will be no net movement of water towards the solution or the cell. The cell and the environment around it are in equilibrium, and the cell should remain unchanged under the microscope. 

Hypotonic

A hypotonic solution has a lower solute concentration compared to the intracellular solute concentration. When a red blood cell is placed in a hypotonic solution, there will be a net movement of free water into the cell. This situation will result in an increased intracellular volume with a lower intracellular solute concentration. The solution will end up with a higher overall solute concentration. Under the microscope, the cell may appear engorged, and the cell membrane may eventually rupture. This overall process is known as cytolysis. 

Note that osmosis is a dynamic equilibrium, so at any given moment, water molecular can momentarily flow toward any direction across the semipermeable membrane, but the overall net movement of all water molecules will be from an area of high free water concentration to an area of low free water concentration.[5][6]

Osmotic pressure is a concept that has been used to explain the hypersensitivity of dentin. The change in pressure in carious, exposed dentin from contact with saliva or concentrated solutions causes diffusion throughout the structure that increases or decreases the pressure on the sensory system.

Just as diffusion through membranes is important, so also is the diffusion from a substance of a given concentration to that of another concentration important in many materials in dentistry. Salts and dyes diffuse through human dentin. Stains and discoloring agents diffuse through polymeric restorative materials. Diffusion of salts and acids through some cavity liners is a potential problem.

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Microbes Culture Methods

Md Latiful Bari, Sabina Yeasmin, in Encyclopedia of Infection and Immunity, 2022

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Determination and Survey of Osmolality in Culture Media

Charity Waymouth, in Tissue Culture, 1973

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Methods in Tau Cell Biology

Peter J. Chung, ... Cyrus R. Safinya, in Methods in Cell Biology, 2017

5.4 SAXS Osmotic Pressure Samples

One of the unique advantages of in situ sample measurements is the ability to add noninteracting polymeric depletants to samples to induce osmotic pressure between colloidal objects (in our case, microtubules) (Fig. 4). As the force applied can be calculated via osmotic depletants added and the change in distance between microtubules measured by SAXS analysis, a force–distance diagram can be obtained. This reveals valuable information with regard to the nature and magnitude of interactions between microtubules while also replicating the macromolecular crowded conditions of the cell.

Fig. 4. Paclitaxel-stabilized microtubules with Tau may still be bundled under the influence of osmotic pressure, with structure deducible via the SAXS. (A–C) While paclitaxel-stabilized microtubules do not have a higher-order structure and are, instead, nematically aligned (A), under osmotic pressure microtubules can transition to a buckled rectangular phase (B) and a hexagonal phase (C). (D,E) While microtubules coated with Tau isoforms with the shortest projection domain (4RS Tau to tubulin ratio, Φ4RS = 1/10) exhibit a standard transition from nematic to buckled rectangular to hexagonally bundled microtubules, microtubules coated with Tau isoforms with the longest projection domain (4RL Tau to tubulin ratio Φ4RL = 1/10) require a much higher osmotic pressure for the columnar phase to begin and, even then, only exhibit the hexagonally bundled phase.

Adapted from Chung, P. J., Choi, M. C., Miller, H. P., Feinstein, H. E., Raviv, U., Li, Y., et al. (2015). Direct force measurements reveal protein Tau confers short-range attractions and isoform-dependent steric stabilization to microtubules. Proceedings of the National Academy of Sciences of the United States of America, 112(47), E6416–E6425. http://dx.doi.org/10.1073/pnas.1513172112.

Briefly, this “depletion force” is an attractive, entropic force between large colloidal particles as a result of being suspended in a dilute solution of smaller solutes or depletants (Asakura & Oosawa, 1958). Often, depletants are preferentially excluded near the vicinity of colloidal particles owing to an entropically favorable reduction in the colloidal excluded volume, causing the colloids to “attract” as a function of colloidal geometry, depletant size, and depletant concentration. While this technique was first used in biological reconstitutions to measure the forces between lipid bilayers (Parsegian, Fuller, & Rand, 1979) and DNA macroions (Parsegian, Rand, Fuller, & Rau, 1986), it has been extended to measure the forces between elements in cytoskeletal networks, such as neurofilaments (Beck, Deek, Jones, & Safinya, 2010; Deek, Chung, Kayser, Bausch, & Safinya, 2013; Safinya, Deek, Beck, Jones, & Li, 2015) and microtubules (Chung et al., 2015, 2016; Needleman, Ojeda-Lopez, Raviv, Ewert, et al., 2004).

5.4.1 Choice of depletant

The osmotic pressure that can be effected by the depletant will depend on the following variables, some of which must be evaluated in tandem:

1.

The depletant should not directly interact with the colloids being depleted; thus, the choice of PEG/PEO as the depletant of choice for many systems as it is considered to be biologically inert. (As an aside, poly(ethylene glycol), or PEG, and poly(ethylene oxide), or PEO, are chemically equivalent, but PEG is usually used for polymers with molecular mass ≤ 20,000 Da, while PEO is used for polymers with molecular mass > 20,000 Da). Should the depletant have known direct interactions with the colloid, a semi-permeable membrane is required (physically segregating colloids and depletants), which is technically challenging for capillary samples.

2.

The size of the depletant should be equal to or larger than the intercolloidal distances being probed. Tau-mediated microtubule bundles can have intermicrotubule spacings of ~ 40 nm, requiring depletants equal to or larger than ~ 40 nm for direct force measurements. Prior work (Devanand & Selser, 1991) measured the radius of gyration (RG) as a function of PEG molecular mass (MW):

RG=0.215MW0.583

The effective depletant radius (a) can be thus calculated:

a=2RGπ

3.

The prior variable must be balanced against the desired osmotic pressure range to be probed. Normally, smaller depletants (e.g., PEG with lower molecular masses) are ideal for probing higher osmotic pressures as an equivalent w/v% of a lower molecular mass PEG means higher number density of PEG molecules can be achieved, thus increasing the dynamic range of the depletion force. However, if the depletant is not sufficiently large, the depletant will penetrate the intercolloidal space and result in inaccurate force–distance measurements.

4.

Prior osmotic pressure characterization of the depletant chosen is ideal, especially at the desired temperature. It is also possible to calculate the osmotic pressure effect of the depletant, without prior characterization, as a virial expansion with theoretical/calculated virial coefficients, but direct experimental observation is preferred.

5.4.2 Calculating osmotic pressure from depletant concentration

Sometimes (but not often!) the amount of depletant solution will make the overall depletant concentration match a prior measurement. For example, suppose that 10 μL of 25 w/v% stock solution of PEG8k has been added to 40 μL of sample solution set to 35°C. According to available osmotic pressure data (Stanley & Strey, 2003), this 5 w/v% solution of PEG8k (overall) has the equivalent osmotic pressure of 37,800 Pa.

However, one is not usually so fortunate. The oft-used strategy for an arbitrary w/v% is to fit available data at one temperature across multiple concentrations of corresponding depletant, with either a modified exponential fit (Rand, n.d.) or a power law fit (Cohen & Highsmith, 1997), the latter based on the mathematical form of the virial expansion. As long as the desired osmotic pressure for a desired w/v% is within the limits of reported data, either fit performs adequately.

In the case that direct osmotic pressure data are not available, the problem becomes slightly more difficult. As there is more literature available on the second virial coefficient of aqueous solutions of various depletants, it is quite possible to theoretically derive the osmotic pressure on the basis of number concentration alone. We have found that this derivation is reasonable as long as the overall depletant concentration is within the dilute limit (i.e., below the overlap concentration, c*).

What is the relationship between osmotic pressure and solute concentration?

The osmotic pressure of a liquid is determined by the number of solute particles that are present in the solution. The osmotic pressure is directly proportional to the solute particles in the liquid, i.e., the higher the osmotic pressure, the greater the number of solute particles.

How does solute concentration affect osmotic potential?

Osmotic potential is directly proportional to the solute concentration. If the solute concentration of a solution increases, the potential for the water in that solution to undergo osmosis decreases. Therefore, the more solute that is added to a solution, the more negative its osmotic (solute) potential gets.

Does high osmotic pressure means high solute concentration?

Osmotic pressure is determined by solute concentration – water will “try harder” to diffuse into an area with a high concentration of a solute, such as a salt, than into an area with a low concentration.

How does solute concentration affect diffusion osmosis?

Increasing the concentration of the solute has no effect on the rate of diffusion. More particles will travel from one side to the other if the concentration is higher. It is analogous to water flowing from a high level to a low level.