Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Oxidation-reduction reaction

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tion is a means of identifying the invading particle to the

phagocyte. Without the opsonization process the recognition

and destruction of invading agents such as bacteria would be

inefficient.

The process of opsonization begins when the immune

systemrecognizes a particle (e.g., a bacterium) as an invader.

The recognition stimulates the production of antibodies that

are specific for the antigenic target. Certain antibodymole-

cules are stimulated to bind to the surface of the particle.

Typically, the binding molecules are a type of antibody classi-

fied as IgG. As well, proteins involved in the complement-

mediated clearance of foreign material, specifically a protein

designated C3b, can bind to the surface of the foreign object.

Proteins such as IgG and C3b, which can promote opsoniza-

tion, are designated as opsonins.

When the IgG antibodies bind to the invading bac-

terium, the binding is in a specific orientation. An antibody is

somewhat “Y” shaped. The binding of IgG to the bacterium is

via the branching arms of the “Y.” The stalk of the molecule,

which is termed the Fc region, then protrudes from the surface.

The Fc region is recognized by a receptor on the surface of an

immune cell called a phagocyte. When the Fc region is bound

to the phagocytic receptor the invading particle is taken into

the phagocyte and enzymatically digested.

The Cb3 complementprotein can bind in a nonspecific

manner to an invading particle. Phagocytes also contain sur-

face receptors that recognize and bind Cb3. As with IgG, the

binding of Cb3 to the phagocytes triggers a process whereby

the invading particle is engulfed, surrounded, and taken inside

the phagocytic cell for destruction.

Examples of phagocytic cells that can participate in

opsonization are neutrophils and monocytes.

Bacteria that are associated with the development of

infections typically possess a capsule, which is a layer of car-

bohydrate material. The capsular material encases the bacter-

ial cell. The carbohydrate is not recognized as readily by the

immune machinery of the body as is protein. As well, the pen-

etration of antibodies through the capsule network to the sur-

face of the bacterium is impeded. Thus, possession of a

capsule can dampen the opsonization response.

See alsoComplement; Immunoglobulins and immunoglobulin

deficiency syndromes; Immunity, active, passive and delayed

OPTIC INFECTIONS, CHRONIC•seeEYE

INFECTIONS

OPTICAL DENSITY AND MEASUREMENTS

OF•seeLABORATORY TECHNIQUES IN MICROBIOLOGY

ORIGIN OF LIFE•seeLIFE, ORIGIN OF

OTIC INFECTIONS, CHRONIC•seeEAR INFEC-

TIONS, CHRONIC

OOxidation-reduction reactionXIDATION-REDUCTION REACTION

Oxidation-reduction reactions are significant to physiological

reactions and biochemical pathways important to microorgan-

ismsand immune processes.

The term oxidation was originally used to describe reac-

tions in which an element combines with oxygen. In contrast,

reduction meant the removal of oxygen. By the turn of this

century, it became apparent that oxidation always seemed to

involve the loss of electrons and did not always involve oxy-

gen. In general, oxidation- reduction reactions involve the

exchange of electrons between two species.

An oxidation reaction is defined as the loss of electrons,

while a reduction reaction is defined as the gain of electrons.

The two reactions always occur together and in chemically

equivalent quantities. Thus, the number of electrons lost by one

species is always equal to the number of electrons gained by

another species. The combination of the two reactions is known

as a redox reaction. Species that participate in redox reactions

are described as either reducing or oxidizing agents. An oxi-

dizing agent is a species that causes the oxidation of another

species. The oxidizing agent accomplishes this by accepting

electrons in a reaction. A reducing agent causes the reduction

of another species by donating electrons to the reaction.

In general, a strong oxidizing agent is a species that has

an attraction for electrons and can oxidize another species.

The standard voltage reduction of an oxidizing agent is a

measure of the strength of the oxidizing agent. The more pos-

itive the species’ standard reduction potential, the stronger the

species is as an oxidizing agent.

In reactions where the reactants and products are not

ionic, there is still a transfer of electrons between species.

Chemists have devised a way to keep track of electrons during

chemical reactions where the charge on the atoms is not read-

ily apparent. Charges on atoms within compounds are

assigned oxidation states (or oxidation numbers). An oxida-

tion number is defined by a set of rules that describes how to

divide up electrons shared within compounds. Oxidation is

defined as an increase in oxidation state, while reduction is

defined as a decrease in oxidation state. Because an oxidizing

agent accepts electrons from another species, a component

atom of the oxidizing agent will decrease in oxidation number

during the redox reaction.

There are many examples of oxidation-reduction reac-

tions in the world. Important processes that involve oxidation-

reduction reactions include combustion reactions that convert

energy stored in fuels into thermal energy, the corrosion of

metals, and metabolic reactions.

Oxidation-reduction reaction occur in both physical and

biological settings (where carbon-containing compounds such

as carbohydrates are oxidized). The burning of natural gas is

an oxidation-reduction reaction that releases energy [CH 4 (g) +

2O 2 (g) →CO 2 (g) + 2H 2 O(g) + energy]. Redox reactions burn

carbohydrates that provide energy [C 6 H 12 O 6 (aq) + 6O 2 (g) →

6CO 2 (g) + 6H 2 O(l)]. In both examples, the carbon-containing

compound is oxidized, and the oxygen is reduced.

See alsoBiochemistry

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