biological system A biological system is a complex network which connects several biologically relevant entities. Biological organization spans several scales and are determined based different structures depending on what the system is. Examples of biological syst ...

s, a double layer is the surface where two different phases of matter are in contact. Biological double layers are much like their interfacial counterparts, but with several notable distinctions. The surface of

biological cell The cell is the basic structural and functional unit of life forms. Every cell consists of a cytoplasm enclosed within a membrane, and contains many biomolecules such as proteins, DNA and RNA, as well as many small molecules of nutrients and ...

s carry many different types of

chemical group In organic chemistry, a functional group is a substituent or moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions regardless of the rest ...

s, each with a different

dissociation constant In chemistry, biochemistry, and pharmacology, a dissociation constant (K_D) is a specific type of equilibrium constant that measures the propensity of a larger object to separate (dissociate) reversibly into smaller components, as when a complex fa ...

, causing them to have varying

electric charge Electric charge is the physical property of matter that causes charged matter to experience a force when placed in an electromagnetic field. Electric charge can be ''positive'' or ''negative'' (commonly carried by protons and electrons respe ...

s at a

physiological pH Physiology (; ) is the scientific study of functions and mechanisms in a living system. As a sub-discipline of biology, physiology focuses on how organisms, organ systems, individual organs, cells, and biomolecules carry out the chemical ...

. This indicates that biosurfaces are chemically

heterogeneous Homogeneity and heterogeneity are concepts often used in the sciences and statistics relating to the uniformity of a substance or organism. A material or image that is homogeneous is uniform in composition or character (i.e. color, shape, siz ...

. This biospecific feature is typical for all biosurfaces, including

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...

macromolecule A macromolecule is a very large molecule important to biophysical processes, such as a protein or nucleic acid. It is composed of thousands of covalently bonded atoms. Many macromolecules are polymers of smaller molecules called monomers. The ...

s and

s. In certain organisms, cells are covered with the

glycocalyx The glycocalyx, also known as the pericellular matrix, is a glycoprotein and glycolipid covering that surrounds the cell membranes of bacteria, epithelial cells, and other cells. In 1970, Martinez-Palomo discovered the cell coating in animal cells ...

layer, which can be modeled as a

polyelectrolyte Polyelectrolytes are polymers whose repeating units bear an electrolyte group. Ion#Anions and cations, Polycations and polyanions are polyelectrolytes. These groups dissociation (chemistry), dissociate in aqueous solutions (water), making the pol ...

layer with a volume spread electric charge. This means that the notion of a

surface charge Surface charge is a two-dimensional surface with non-zero electric charge. These electric charges are constrained on this 2-D surface, and surface charge density, measured in coulombs per square meter (C•m−2), is used to describe the charge di ...

is located on certain flat surfaces. This does not apply; instead, the cell surface is a finite thickness polyelectrolyte layer with a volume charge. At equilibrium, the relationship between these polyelectrolyte layers and a fluid bulk is called the Donnan equilibrium. The polyelectrolyte volume charge creates an equilibrated

electric potential The electric potential (also called the ''electric field potential'', potential drop, the electrostatic potential) is defined as the amount of work energy needed to move a unit of electric charge from a reference point to the specific point in ...

known as the

Donnan potential Donnan potential is the difference in the Galvani potentials which appears as a result of Donnan equilibrium, named after Frederick G. Donnan, which refers to the distribution of ion species between two ionic solutions separated by a semipermea ...

. Part of the Donnan potential is located inside of the polyelectrolyte layer, while the other part is associated with the external double layer located in the

dispersion medium Interface and colloid science is an interdisciplinary intersection of branches of chemistry, physics, nanoscience and other fields dealing with colloids, heterogeneous systems consisting of a mechanical mixture of particles between 1 nm ...

. In another feature, the cells are not in an equilibrium with the fluid bulk. There is a constant

ion exchange Ion exchange is a reversible interchange of one kind of ion present in an insoluble solid with another of like charge present in a solution surrounding the solid with the reaction being used especially for softening or making water demineralised, ...

between living cells and a fluid. Consequently, there is a difference in electric potentials between the cell interior and a fluid bulk, known as the transmembrane potential. This non-equilibrium potential affects the structure of the double layer.Dukhin, A.S. Colloids and Surfaces A, 73, 29-48 (1993)

Notes

General references

*Ohshima. H. ''Theory of Colloid and Interfacial Electric Phenomena'', Elsevier, 2006. *Duval, J.F.L. and oth. ''Langmuir'', 21, 11268-11282 (2005). {{DEFAULTSORT:Double Layer (Biospecific) Physical chemistry Colloidal chemistry