More MCH Science Software

College Science Software BundleCollege Science Bundle: The three college level courses - General Chemistry, General Physics (calculus) and Organic chemistry, bundled together in one package.College Science Software Bundle

High School Science Bundle: The two high school level courses- Introductory Chemistry and General Physics (non- calculus), bundled together into one package. This bundle is very popular with High School and Home School students.Chemistry and Physics Software for High School and Home School

The Donnan Phenomenon in Living Cell Functions

Donnan Equilibrium

Click to enlarge

The Donnan phenomenon is not only limited to thermodynamic concepts in physical chemistry. It can be used to help us understand how living cells function. This post presents the significance of this phenomenon in every day molecular processes. The chemical aspect of the topic is captured in Chapter 7 of the Physical Chemistry book.

The Donnan equilibrium

The Gibbs-Donnan Equilibrium or Donnan Equilibrium is the basis for electrical charges that are found across the membranes of many cells (e.g. nerve and muscle cells).

It refers to the uneven distribution of charged particles on one side of a semipermeable membrane. These particles are not able to evenly distribute themselves by diffusion across both sides of the membrane.

How does the Donnan Equilibrium work?

When two solutions of differing concentrations are separated by a semipermeable membrane their concentrations will equalize as a result of diffusion.Diffusion occurs when substances move from areas of high concentration to low concentration down a concentration gradient.

However if there is an impermeable solute in one of the solutions, the concentration of the solution does not equalize. The concentration of the solution with impermeable solutes remains high even at equilibrium. This effect is called the Donnan equilibrium (refer to Fig. 7.25 above).

The Donnan Equilibrium in living cells

The Donnan effect can be correlated to living cells. Cell membranes are selectively permeable, which means that they allow some molecules to pass through while keeping others out.

The flow of molecules and ions between a cell and its environment is regulated by the Donnan effect. Living cells contain impermeable anionic colloids, which are mostly made up of proteins and organic phosphates; and these colloidal anions cannot cross the cell membrane.

As a result of this, there is a high concentration of non-diffusible anions across the cell membrane, thus creating the Donnan Equilibrium. This means that there are more ions inside the cell than outside. For ease of explanation let’s call this Donnan Equilibrium 1.

What does this do to cells? Water will continuously move into the cell by the process of osmosis. If this process continues, the cells will inevitably rupture.

What is the mechanism that prevents cells from swelling and rupturing?

The answer is the sodium pump (Na⁺- K⁺ ATPase) in the cell membrane. It is the most ubiquitous system in animal cells. The presence of the ATP-driven Na⁺ and K⁺ pump is nature’s way of preventing cells from rupturing by continuously pushing out excess ions.

The pump together with the membrane’s low permeability to sodium effectively prevents sodium from entering the cell. The sodium pump renders the membrane impermeable to sodium, setting up a second Donnan Equilibrium (let’s call this Donnan Equilibrium 2).

The earlier Donnan effect (Donnan Equilibrium 1) with respect to impermeable anionic colloids balances the latter Donnan effect (Donnan Equilibrium 2) of impermeable extracellular sodium. The balancing act between these two effects is by way of allowing cells to maintain and regulate normal cell volume in living functions.

Written by: chemmum

Shaliza Dewa has authored 11 more articles.

I hold a Ph.D. in Chemistry and a B.Sc. in Biochemistry from the University of Sussex, England. I am a professional stay-at-home mom to three kids. I love the wonderful world of Chemistry and its practicality. Prior experiences in research and industry will be the stimulus for helping others in their understanding of Chemistry.

One Response to The Donnan Phenomenon in Living Cell Functions

Leave a Reply

Your email address will not be published. Required fields are marked *

Try our bestselling undergraduate Physical Chemistry courseware

Thermodynamics Module - Physical Chemistry
Thermodynamics module
Chapters 1 to 6 of Physical Chemistry - Laidler, Meiser, Sanctuary

Includes multimedia that opens on relevant pages and allows the student to visualize many of the concepts by varying parameters and plotting different graphs. Things students often have difficulty with, such as isothermal, isobaric, isochoric and adiabatic process, are clearly visualized.

Get it from: Thermodynamics Module - Physical Chemistry

Physical Chemistry - Laidler, Meiser, Sanctuary
Physical Chemistry textbook
by Laidler, Meiser, Sanctuary

This popular Physical Chemistry text book is now available in electronic format. We have preserved much of the material of the former hard copy editions, making changes to improve understanding of the concepts in addition to including some of the recent discoveries in physical chemistry. Many chapters have new sections and the coverage of several chapters has been greatly expanded.

Get it from: Physical Chemistry - Laidler, Meiser, Sanctuary

PChemistry Tips by Email

Don't miss out on Physical Chemistry tips & special offers sent via email

* indicates required

MCH on Twitter