Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Phospholipids

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In 1905, English pathologist Almroth Wright (1861–

1947) demonstrated that phagocytosis and antibodyfactors in

the blood worked together in the immune response process.

See alsoAntibody and antigen; Antibody-antigen, biochemi-

cal and molecular reactions; Antibody formation and kinetics;

Antibody, monoclonal; Antigenic mimicry; Immune system;

Immunity, active, passive, and delayed; Immunity, cell medi-

ated; Immunity, humoral regulation; Immunization;

Immunogenetics; Immunology; Infection and resistance;

Inflammation

PHAGOCYTE DEFECTS•seeIMMUNODEFICIENCY

DISEASE SYNDROMES

PHENOTYPE AND PHENOTYPIC

VARIATIONPhenotype and phenotypic variation

The word phenotyperefers to the observable characters or

attributes of individual organisms, including their morphol-

ogy, physiology, behavior, and other traits. The phenotype of

an organism is limited by the boundaries of its specific genetic

complement(genotype), but is also influenced by environ-

mental factors that impact the expression of genetic potential.

All organisms have unique genetic information, which

is embodied in the particular nucleotide sequences of their

DNA (deoxyribonucleic acid), the genetic biochemical of

almost all organisms, except for virusesand bacteriathat uti-

lize RNAas their genetic material. The genotype is fixed

within an individual organism but is subject to change (muta-

tions) from one generation to the next due to low rates of nat-

ural or spontaneous mutation. However, there is a certain

degree of developmental flexibility in the phenotype, which is

the actual or outward expression of the genetic information in

terms of anatomy, behavior, and biochemistry. This flexibility

can occur because the expression of genetic potential is

affected by environmental conditions and other circumstances.

Consider, for example, genetically identical bacterial

cells, with a fixed complement of genetic each plated on dif-

ferent gels. If one bacterium is colonized under ideal condi-

tions, it can grow and colonize its full genetic potential.

However, if a genetically identical bacterium is exposed to

improper nutrients or is otherwise grown under adverse con-

ditions, colony formation may be stunted. Such varying

growth patterns of the same genotype are referred to as phe-

notypic plasticity. Some traits of organisms, however, are

fixed genetically, and their expression is not affected by envi-

ronmental conditions. Moreover, the ability of species to

exhibit phenotypically plastic responses to environmental

variations is itself, to a substantial degree, genetically deter-

mined. Therefore, phenotypic plasticity reflects both genetic

capability and varying expression of that capability, depending

on circumstances.

Phenotypic variation is essential for evolution. Without

a discernable difference among individuals in a population

there are no genetic selectionpressures acting to alter the vari-

ety and types of alleles (forms of genes) present in a popula-

tion. Accordingly, genetic mutations that do not result in

phenotypic change are essentially masked from evolutionary

mechanisms.

Phenetic similarity results when phenotypic differences

among individuals are slight. In such cases, it may take a sig-

nificant alteration in environmental conditions to produce sig-

nificant selection pressure that results in more dramatic

phenotypic differences. Phenotypic differences lead to differ-

ences in fitness and affect adaptation.

See alsoDNA (Deoxyribonucleic acid); Molecular biology

and molecular genetics

PHENOTYPE•seeGENOTYPE AND PHENOTYPE

PPhospholipidsHOSPHOLIPIDS

Phospholipids are complex lipids made up of fatty acids, alco-

hols, and phosphate. They are extremely important compo-

nents of living cells, with both structural and metabolic roles.

They are the chief constituents of most biological membranes.

At one end of a phospholipid molecule is a phosphate

group linked to an alcohol. This is a polar part of the molecule—

it has an electric charge and is water-soluble (hydrophilic). At

the other end of the molecule are fatty acids, which are non-

polar, hydrophobic, fat soluble, and water insoluble.

Because of the dual nature of the phospholipid mole-

cules, with a water-soluble group attached to a water-insoluble

group in the same molecule, they are called amphipathic or

polar lipids. The amphipathic nature of phospholipids make

them ideal components of biological membranes, where they

form a lipid bilayer with the polar region of each layer facing

out to interact with water, and the non-polar fatty acid “tail”

portions pointing inward toward each other in the interior of

the bilayer. The lipid bilayer structure of cell membranes

makes them nearly impermeable to polar molecules such as

ions, but proteins embedded in the membrane are able to carry

many substances through that they could not otherwise pass.

Phosphoglycerides, considered by some as synonymous

for phospholipids, are structurally related to 3-phosphoglycer-

aldehyde (PGA), an intermediate in the catabolic metabolism

of glucose. Phosphoglycerides differ from phospholipids

because they contain an alcohol rather than an aldehyde group

on the 1-carbon. Fatty acids are attached by an ester linkage to

one or both of the free hydroxyl (-OH) groups of the glyceride

on carbons 1 and 2. Except in phosphatidic acid, the simplest

of all phosphoglycerides, the phosphate attached to the 3-car-

bon of the glyceride is also linked to another alcohol. The

nature of this alcohol varies considerably.

See alsoBacteremic; Bacterial growth and division; Bacterial

membranes and cell wall; Bacterial surface layers; Bacterial

ultrastructure; Biochemistry; Cell membrane transport;

Membrane fluidity

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