Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Cytoplasm, eukaryotic

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cells after six hours. Such rapid growth rates help to explain

the rapid development of disease, food spoilage, decay, and

the speed at which certain chemical processes used in industry

take place. Once the culture has been grown, a variety of

observation methods can be used to record the strain’s charac-

teristics and chart its growth.

See alsoAgar and agarose; Agar diffusion; American type cul-

ture collection; Antibiotic resistance, tests for; Bacterial

growth and division; Bacterial kingdoms; Epidemiology,

tracking diseases with technology; Laboratory techniques in

microbiology

CYCLOSPORIN•seeANTIBIOTICS

CYTOGENETICS•seeMOLECULAR BIOLOGY AND

MOLECULAR GENETICS

CCytokinesYTOKINES

Cytokines are a family of small proteins that mediate an

organism’s response to injury or infection. Cytokines operate

by transmitting signals between cells in an organism. Minute

quantities of cytokines are secreted, each by a single cell type,

and regulate functions in other cells by binding with specific

receptors. Their interactions with the receptors produce sec-

ondary signals that inhibit or enhance the action of certain

genes within the cell. Unlike endocrine hormones, which can

act throughout the body, most cytokines act locally, near the

cells that produced them.

Cytokines are crucial to an organism’s self-defense.

Cells under attack release a class of cytokines known as

chemokines. Chemokines participate in a process called

chemotaxis, signaling white blood cells to migrate toward the

threatened region. Other cytokines induce the white blood

cells to produce inflammation, emitting toxins to kill

pathogens and enzymesto digest both the invaders and the

injured tissue. If the inflammatory response is not enough to

deal with the problem, additional immune systemcells are

also summoned by cytokines to continue the fight.

In a serious injury or infection, cytokines may call the

hematopoietic, or blood-forming system into play. New white

blood cells are created to augment the immune response, while

additional red blood cells replace any that have been lost.

Ruptured blood vessels emit chemokines to attract platelets,

the element of the blood that fosters clotting. Cytokines are

also responsible for signaling the nervous system to increase

the organism’s metabolic level, bringing on a fever that

inhibits the proliferation of pathogens while boosting the

action of the immune system.

Because of the central role of cytokines in fighting infec-

tion, they are being studied in an effort to find better treatments

for diseases such as AIDS. Some have shown promise as thera-

peutic agents, but their usefulness is limited by the tendency of

cytokines to act locally. This means that their short amino acid

chains are likely either to be destroyed by enzymes in the

bloodstream or tissues before reaching their destination, or to

act on other cells with unintended consequences.

Other approaches to developing therapies based on

research into cytokines involve studying their receptor sites on

target cells. If a molecule could be developed that would bind

to the receptor site of a specific cytokine, it could elicit the

desired action from the cell, and might be more durable in the

bloodstream or have other advantages over the native

cytokine. Alternatively, a drug that blocked receptor sites

could potentially prevent the uncontrolled inflammatory

responses seen in certain autoimmune diseases.

See also Autoimmunity and autoimmune diseases;

Immunochemistry; Immunodeficiency disease syndromes;

Immunodeficiency diseases

CCytoplasm, eukaryoticYTOPLASM, EUKARYOTIC

The cytoplasm, or cytosol of eukaryotic cells is the gel-like,

water-based fluid that occupies the majority of the volume of

the cell. Cytoplasm functions as the site of energy production,

storage, and the manufacture of cellular components. The vari-

ous organelles that are responsible for some of these functions

in the eukaryotic cell are dispersed throughout the cytoplasm, as

are the compounds that provide structural support for the cell.

The cytoplasm is the site of almost all of the chemical

activity occurring in a eukaryotic cell. Indeed, the word cyto-

plasm means “cell substance.”

Despite being comprised mainly of water (about 65%

by volume), the cytoplasm has the consistency of gelatin.

Unlike gelatin, however, the cytoplasm will flow. This enables

eukaryotessuch as the amoeba to adopt different shapes, and

makes possible the formation of pseudopods that are used to

engulf food particles. The consistency of the cytoplasm is the

result of the other constituents of the cell that are floating in

fluid. These constituents include salts, and organic molecules

such as the many enzymesthat catalyze the myriad of chemi-

cal reactions that occur in the cell.

When viewed using the transmission electron micro-

scope, the cytoplasm appears as a three-dimensional lattice-

work of strands. In the early days of electron microscopythere

was doubt as to whether this appearance reflected the true

nature of the cytoplasm, or was an artifact of the removal of

water from the cytoplasm during the preparation steps prior to

electron microscopic examination. However, development of

techniques that do not perturb the natural structure biological

specimens has confirmed that this latticework is real.

The lattice is made of various cytoplasmic proteins.

They are scaffolding structures that assist in the process of cell

division and in the shape of the cell. The shape-determinant is

referred to as the cytoskeleton. It is a network of fibers com-

posed of three types of proteins. The proteins form three fila-

mentous structures known as microtubules, intermediate

filaments, and microfilaments. The filaments are connected to

most of organelles located in the cytoplasm and serve to hold

together the organelles.

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