How does osmosis affect osmotic pressure?

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The correct osmotic pressure in the culture medium is essential for the survival of the cells.

LEARNING OBJECTIVES

Describe osmotic effects

KEY TAKEAWAYS

Key Points

  • Osmosis is the net movement of solvent molecules through a partially permeable membrane into a region of higher solute concentration in order to equalize the solute concentrations on the two sides.
  • Osmosis provides the primary means by which water is transported into and out of cells.
  • Osmoregulation is the homeostasis mechanism of an organism to reach balance in osmotic pressure.
  • If the medium is hypotonic, the cells will gain water through osmosis.
  • If the medium is hypertonic, the cells will lose water through osmosis.

Key Terms

  • osmosis: the net movement of solvent molecules from a region of high solvent potential to a region of lower solvent potential through a partially permeable membrane
  • hypotonic: Having a lower osmotic pressure than another.
  • isotonic: Having the same osmotic pressure.
  • hypertonic: Having a greater osmotic pressure than another.
  • halophile: Organisms that thrive in high salt concentrations.

Osmotic pressure is an important factor that affects cells. Osmosis is the net movement of solvent molecules through a partially permeable membrane into a region of higher solute concentration. The intent of osmosis is to equalize the solute concentrations on the two sides. Osmosis is essential in biological systems because biological membranes are semi permeable. In general, these membranes are impermeable to large and polar molecules such as ions, proteins, and polysaccharides. However, they are permeable to non-polar and/or hydrophobic molecules like lipids as well as to small molecules like oxygen, carbon dioxide, nitrogen, nitric oxide, etc. Osmosis provides the primary means by which water is transported into and out of cells. Osmoregulation is the homeostasis mechanism of an organism to reach balance in osmotic pressure.

Having the correct osmotic pressure in the culture medium is essential. A cell can be influenced by a solution in three ways. Suppose a cell is placed in a solution of sugar or salt water. If the medium is hypotonic — a diluted solution with a higher water concentration than the cell — the cell will gain water through osmosis. If the medium is isotonic — a solution with exactly the same water concentration as the cell — there will be no net movement of water across the cell membrane. If the medium is hypertonic — a concentrated solution with a lower water concentration than the cell — the cell will lose water by osmosis.

Figure: Osmotic Pressure on Red Blood Cells: Effect of different solutions on blood cells.

Essentially, this means that if a cell is put in a solution that has a solute concentration higher than its own, then it will shrivel up. If it is put in a solution with a lower solute concentration than its own, the cell will expand and burst.

Obligate and Facultative Halophiles

A halophile is a microorganism that can survive and replicate in a high salt concentration environment (high osmotic pressure).

Obligate halophiles are microorganisms that can only survive in high salt concentration environments. Facultative halophiles are able to survive in bothhigh and normal salt concentration environments.


This page titled 6.10.2: Osmotic Pressure is shared under a CC BY-SA license and was authored, remixed, and/or curated by Boundless.

Large quantities of water molecules constantly move across cell membranes by simple diffusion, often facilitated by movement through membrane proteins, including aquaporins. In general, net movement of water into or out of cells is negligible. For example, it has been estimated that an amount of water equivalent to roughly 100 times the volume of the cell diffuses across the red blood cell membrane every second; the cell doesn't lose or gain water because equal amounts go in and out.

There are, however, many cases in which net flow of water occurs across cell membranes and sheets of cells. An example of great importance to you is the secretion of and absorption of water in your small intestine. In such situations, water still moves across membranes by simple diffusion, but the process is important enough to warrant a distinct name - osmosis.

Osmosis and Net Movement of Water

Osmosis is the net movement of water across a selectively permeable membrane driven by a difference in solute concentrations on the two sides of the membrane. A selectively permiable membrane is one that allows unrestricted passage of water, but not solute molecules or ions.

Different concentrations of solute molecules leads to different concentrations of molecules on either side of the membrane. On the side of the membrane with higher free water concentration (i.e. a lower concentration of solute), more water molecules will strike the pores in the membrane in a give interval of time. More strikes equates to more molecules passing through the pores, which in turn results in net diffusion of water from the compartment with high concentration of free water to that with low concentration of free water.

The key to remember about osmosis is that water flows from the solution with the lower solute concentration into the solution with higher solute concentration. This means that water flows in response to differences in molarity across a membrane. The size of the solute particles does not influence osmosis. Equilibrium is reached once sufficient water has moved to equalize the solute concentration on both sides of the membrane, and at that point, net flow of water ceases. Here is a simple example to illustrate these principles:

Two containers of equal volume are separated by a membrane that allows free passage of water, but totally restricts passage of solute molecules. Solution A has 3 molecules of the protein albumin (molecular weight 66,000) and Solution B contains 15 molecules of glucose (molecular weight 180). Into which compartment will water flow, or will there be no net movement of water? [ ]
How does osmosis affect osmotic pressure?

Additional examples are provided on how to determine which direction water will flow in different circumstances.

When thinking about osmosis, we are always comparing solute concentrations between two solutions, and some standard terminology is commonly used to describe these differences:

  • Isotonic: The solutions being compared have equal concentration of solutes.
  • Hypertonic: The solution with the higher concentration of solutes.
  • Hypotonic: The solution with the lower concentration of solutes.
How does osmosis affect osmotic pressure?

In the examples above, Solutions A and B are isotonic (with each other), Solutions A and B are both hypertonic compared to Solution C, and Solution C is hypotonic relative to Solutions A and B.

Diffusion of water across a membrane generates a pressure called osmotic pressure. If the pressure in the compartment into which water is flowing is raised to the equivalent of the osmotic pressure, movement of water will stop. This pressure is often called hydrostatic ('water-stopping') pressure. The term osmolarity is used to describe the number of solute particles in a volume of fluid. Osmoles are used to describe the concentration in terms of number of particles - a 1 osmolar solution contains 1 mole of osmotically-active particles (molecules and ions) per liter.

The classic demonstration of osmosis and osmotic pressure is to immerse red blood cells in solutions of varying osmolarity and watch what happens. Blood serum is isotonic with respect to the cytoplasm, and red cells in that solution assume the shape of a biconcave disk. To prepare the images shown below, red cells from your intrepid author were suspended in three types of solutions:

  • Isotonic - the cells were diluted in serum: Note the beautiful biconcave shape of the cells as they circulate in blood.
  • Hypotonic - the cells in serum were diluted in water: At 200 milliosmols (mOs), the cells are visibly swollen and have lost their biconcave shape, and at 100 mOs, most have swollen so much that they have ruptured, leaving what are called red blood cell ghosts. In a hypotonic solution, water rushes into cells.
  • Hypertonic - A concentrated solution of NaCl was mixed with the cells and serum to increase osmolarity: At 400 mOs and especially at 500 mOs, water has flowed out of the cells, causing them to collapse and assume the spiky appearance you see.
How does osmosis affect osmotic pressure?
How does osmosis affect osmotic pressure?

Predict what would happen if you mixed sufficient water with the 500 mOs sample shown above to reduce its osmolarity to about 300 mOs.

Calculating Osmotic and Hydrostatic Pressure

The flow of water across a membrane in response to differing concentrations of solutes on either side - osmosis - generates a pressure across the membrane called osmotic pressure. Osmotic pressure is defined as the hydrostatic pressure required to stop the flow of water, and thus, osmotic and hydrostatic pressures are, for all intents and purposes, equivalent. The membrane being referred to here can be an artifical lipid bilayer, a plasma membrane or a layer of cells.

The osmotic pressure P of a dilute solution is approximated by the following:

P = RT (C1 + C2 + .. + Cn)

where R is the gas constant (0.082 liter-atmosphere/degree-mole), T is the absolute temperature, and C1 ... Cn are the molar concentrations of all solutes (ions and molecules).

Similarly, the osmotic pressure across of membrane separating two solutions is:

P = RT (ΔC)

where ΔC is the difference in solute concentration between the two solutions. Thus, if the membrane is permeable to water and not solutes, osmotic pressure is proportional to the difference in solute concentration across the membrane (the proportionality factor is RT).

What factors affect osmotic pressure?

The osmotic pressure of a solution varies directly with the concentration of the solute in the solution and is equal to the pressure the solute would exert if it would be a gas in the volume occupied by the solution, if the volume of the solute molecules relative to volume of solvent be negligible. 2.

Is osmosis and osmotic pressure same?

The least pressure required to apply to a solution in order to stop the flow of solvent molecules across a semipermeable membrane is known as osmotic pressure (osmosis). It is a colligative property that is regulated by the concentration of solute particles in the solution.

What causes osmotic pressure to increase or decrease?

loss of electrolytes (salt), the osmotic pressure of the extracellular fluids becomes higher than in the cells. Since water passes from a region of lower to a region of higher osmotic pressure, water flows out of the cells into the extracellular fluid, tending to lower its osmotic pressure and increase…

How does movement of water affect osmotic pressure?

Diffusion of water across a membrane generates a pressure called osmotic pressure. If the pressure in the compartment into which water is flowing is raised to the equivalent of the osmotic pressure, movement of water will stop.