Extracellular Osmolarity and Cell Volume

We can now apply the principles learned about osmosis to cells, which meet all the criteria necessary to produce an osmotic flow of water across a membrane. Both the intracellular and extracellular fluids contain water, and cells are surrounded by a membrane that is very permeable to water but impermeable to many substances (nonpenetrating solutes).

About 85 percent of the extracellular solute particles are sodium and chloride ions, which can diffuse into the cell through protein channels in the plasma membrane or enter the cell during secondary active transport. As we have seen, however, the plasma membrane contains Na,K-ATPase pumps that actively move sodium ions out of the cell. Thus, sodium moves into cells and is pumped back out, behaving as if it never entered in the first place; that is, extracellular sodium behaves like a nonpenetrating solute. Also, secondary active-transport pumps and the membrane potential move chloride ions out of cells as rapidly as they enter, with the result that extracellular chloride ions also behave as if they were nonpenetrating solutes.

Inside the cell, the major solute particles are potassium ions and a number of organic solutes. Most of the latter are large polar molecules unable to diffuse through the plasma membrane. Although potassium ions can diffuse out of a cell through potassium channels, they are actively transported back by the Na,K-ATPase pump. The net effect, as with extracellular sodium and chloride, is that potassium behaves as if it were a nonpenetrating solute, but in this case one confined to the intracellular fluid. Thus, sodium and chloride outside the cell and potassium and organic solutes inside the cell behave as nonpenetrating solutes on the two sides of the plasma membrane.

The osmolarity of the extracellular fluid is normally about 300 mOsm. Since water can diffuse across plasma membranes, the water in the intracellular and extracellular fluids will come to diffusion equilibrium. At equilibrium, therefore, the osmolarities of the

Vander et al.: Human I I. Basic Cell Functions I 6. Movement of Molecules I I © The McGraw-Hill

Physiology: The Across Cell Membranes Companies, 2001

Mechanism of Body Function, Eighth Edition

Movement of Molecules Across Cell Membranes CHAPTER SIX

intracellular and extracellular fluids are the same— 300 mOsm. Changes in extracellular osmolarity can cause cells to shrink or swell as a result of the movements of water across the plasma membrane.

If cells are placed in a solution of nonpenetrating solutes having an osmolarity of 300 mOsm, they will neither swell nor shrink since the water concentrations in the intra- and extracellular fluid are the same, and the solutes cannot leave or enter. Such solutions are said to be isotonic (Figure 6-20), defined as having the same concentration of nonpenetrating solutes as normal extracellular fluid. Solutions containing less than 300 mOsm of nonpenetrating solutes (hypotonic solutions) cause cells to swell because water diffuses into the cell from its higher concentration in the extracellular fluid. Solutions containing greater than 300 mOsm of nonpenetrating solutes (hypertonic solutions) cause cells to shrink as water diffuses out of the cell into the fluid with the lower water concentration. Note that the concentration of nonpenetrating solutes in a solution, not the total osmolarity, determines its tonicity—hypotonic, isotonic, or hypertonic. Penetrating solutes do not contribute to the tonicity of a solution.

In contrast, another set of terms—isoosmotic, hyperosmotic, and hypoosmotic—denotes simply the osmolarity of a solution relative to that of normal extracellular fluid without regard to whether the solute is penetrating or nonpenetrating. The two sets of terms are therefore not synonymous. For example, a 1-L solution containing 300 mOsmol of nonpenetrating NaCl and 100 mOsmol of urea, which can cross plasma membranes, would have a total osmolarity of 400 mOsm and would be hyperosmotic. It would, however, also be an isotonic solution, producing no change in the equilibrium volume of cells immersed in it. The reason is that urea will diffuse into the cells and reach the same concentration as the urea in the extracellular solution, and thus both the intracellular and extracellular solutions will have the same osmolarity (400 mOsm). Therefore, there will be no difference in the water concentration across the membrane and thus no change in cell volume.

Table 6-3 provides a comparison of the various terms used to describe the osmolarity and tonicity of solutions. All hypoosmotic solutions are also hypotonic, whereas a hyperosmotic solution can be hypertonic, isotonic, or hypotonic.

Hyperosmotic SolutionCell Volume Hypotonic Solution

Hypertonic solution Isotonic solution Hypotonic solution

Cell shrinks No change in cell volume Cell swells

FIGURE 6-20

Changes in cell volume produced by hypertonic, isotonic, and hypotonic solutions.

Hypertonic solution Isotonic solution Hypotonic solution

Cell shrinks No change in cell volume Cell swells

FIGURE 6-20

Changes in cell volume produced by hypertonic, isotonic, and hypotonic solutions.

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART ONE Basic Cell Functions

I. Basic Cell Functions

6. Movement of Molecules Across Cell Membranes

© The McGraw-Hill Companies, 2001

TABLE 6-3 Terms Referring to Both the Osmolarity and Tonicity of Solutions

Extracellular fluid

TABLE 6-3 Terms Referring to Both the Osmolarity and Tonicity of Solutions

Isotonic

A solution containing 300 mOsmol/L of nonpenetrating solutes, regardless of the concentration of membrane-penetrating solutes that may be present

Hypertonic

A solution containing greater than 300 mOsmol/L of nonpenetrating solutes, regardless of the concentration of membrane-penetrating solutes that may be present

Hypotonic

A solution containing less than 300 mOsmol/L of nonpenetrating solutes, regardless of the concentration of membrane-penetrating solutes that may be present

Isoosmotic

A solution containing 300 mOsmol/L of solute, regardless of its composition of membrane-penetrating and nonpenetrating solutes

Hyperosmotic

A solution containing greater than 300 mOsmol/L of solutes, regardless of the composition of membrane-penetrating and nonpenetrating solutes

Hypoosmotic

A solution containing less than 300 mOsmol/L of solutes, regardless of the composition of membrane-penetrating and nonpenetrating solutes

As we shall see in Chapter 16, one of the major functions of the kidneys is to regulate the excretion of water in the urine so that the osmolarity of the extracellular fluid remains nearly constant in spite of variations in salt and water intake and loss, thereby preventing damage to cells from excessive swelling or shrinkage.

The tonicity of solutions injected into the body is of great importance in medicine. Such solutions usually consist of an isotonic solution of NaCl (150 mM NaCl—isotonic saline) or an isotonic solution of glucose (5% dextrose solution). Injecting a drug dissolved in such solutions does not produce changes in cell volume, whereas injection of the same drug dissolved in pure water, a hypotonic solution, would produce cell swelling, perhaps to the point that plasma membranes would rupture, destroying cells.

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Responses

  • Shukornia Futsum
    What effect does a 300 mosmolar solution nonpeneratting have on cell?
    5 years ago
  • brutus
    What term describes the tonicity of extracellular solution?
    5 years ago
  • duenna
    What is the percentage of solutes inside the cell?
    5 years ago
  • mariam
    What if a cell with osmolarity?
    5 years ago
  • DIETER
    What solute in both intracellular and extracellular fluids?
    5 years ago
  • ellen
    Is 150 mm solution of nacl hypotonic to cell contents?
    5 years ago
  • poppy
    Why is a hyposmotic solution hypotonic?
    5 years ago
  • jessika
    Is the intracellular osmolarity of cells in the human body the same as extracellular osmolarity?
    5 years ago
  • abrha yusef
    What is the osmolarity of extracellular fluid in humans?
    5 years ago
  • Matthias
    Why a solution of 300 mosmol/l of urea isosmotic but not isotonic?
    5 years ago
  • gorbulas
    What is a non penetrating solute?
    5 years ago
  • aldo
    How does osmolarity affect movement of water across cell membrane?
    5 years ago
  • gloriana
    Are sucrose and salt penetrating or nonpenetrating solute?
    5 years ago
  • david
    Is intracellular fluid hyperosmotic?
    5 years ago
  • tom
    What would a hyperosmotic solution do to the body?
    5 years ago
  • tesmi
    Does water change the osmolarity of cells?
    5 years ago
  • robel
    Why does na behave like its nonpenetrating?
    5 years ago
  • TRANQUILLINO
    How to find out which one is non penetrating solutes?
    5 years ago
  • maximilian
    How intake of sodium into the cells is important in secondary active transport?
    5 years ago
  • kane
    Why is urea unable to diffuse?
    5 years ago
  • Abbondanzio
    What effect does 300 mosmol solution of non penetrating solute have on cell?
    5 years ago
  • Erik
    Why urea doesnt change cell volume?
    5 years ago
  • editta
    What effect does a 300 mOsmolar solution of a nonpenetrating solute have on a cell?
    4 years ago
  • ANTONIA BARESE
    What effect does a 300mOsmolar solution of a nonpentrating solute have on a cell?
    4 years ago
  • KAISA
    How important is osmolarity for cell function?
    4 years ago
  • athos
    Is glucose penetrating or nonpenetrating?
    4 years ago
  • Luis
    How does extracluar volume change in response to osmolarity?
    4 years ago
  • joel
    What effect does 300mOsmolar solution of nonpenetrating solute have on a cell?
    4 years ago
  • Gene
    What happens to a cell if the extracelluler is hyperosmotic?
    4 years ago
  • martin
    What are penetrating solutions in osmolarity of cell?
    4 years ago
  • michael bayer
    How does the body store water intracellular and extracellular of the cell?
    4 years ago
  • daniela
    Why are we concerned about whether a solute is penetrating or nonpenetrating?
    4 years ago
  • Chica
    Does a virus effect osmolarity of a cell?
    4 years ago
  • cassidy
    Why is sodium nonpenetrating?
    2 years ago
  • KASEY MCGREGOR
    Is 25mOsmoles lower salt concentration than 300mOsmol?
    2 years ago
  • harry
    What is the extracellular osmolarity compose of?
    1 year ago
  • Paula Warelius
    Does isosmotic urea change cell volume?
    3 months ago

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