Encyclopedia of Biology

plasma Inbiology, this term has the following three
meanings:

1. Fluid component of blood in which the blood cells
   and platelets are suspended (blood plasma). Note
   the distinction between plasma, which describes a
   part of the blood (the fluid part of blood, outside
   the blood cells), and serum, which describes a frac-
   tion derived from blood by a manipulation (the fluid
   that separates when blood coagulates).


2. Fluid component of semen produced by the accesso-
   ry glands, the seminal vesicles, the prostate, and the
   bulbourethral gland.


3. Cell substance outside the nucleus (CYTOPLASM).

plasma cell An antibody-producing B cell that has
reached the end of its differentiation pathway. B cells
are white blood cells that develop from B stem cells
into plasma cells that produce immunoglobulins (anti-
bodies).

plasma membrane An interface and permeability-
limiting membrane composed of lipids and proteins
that act as a selective barrier for the cell’s interior cyto-
plasm.

plasmid An extrachromosomal GENETIC element
consisting generally of circular double-stranded DNA,
which can replicate independently of chromosomal
DNA. R plasmids are responsible for the mutual trans-
fer of antibiotic resistance among microbes. Plasmids
are used as vectors for CLONINGDNA in bacteria or
yeast host cells.
See alsoCLONING VECTOR.

plasmodesma Living bridges between cell walls;
small tubes or openings lined with plasma membrane
between cell walls that connect each cell to one another
and arebelieved to allow molecules to pass through.

plasmogamy Aprocess of cytoplasm fusion between
two cells; the first step in syngamy (sexual reproduction).

plasmolysis A process caused by diffusion when the

cell membrane shrinks away from its cell wall, with

the resulting vacuole and cytoplasm shrinking due to

the presence of a foreign material, like salt, becoming

too abundant; water is drawn from the cell into the

extracellular area, and the cell becomes flaccid after

losing its internal turgor.

plastid A type of plant cytoplasmic organelle that

develops from a precursor small and colorless undiffer-

entiated organelle, the proplastid. During cell differentia-

tion, proplastids differentiate into particular plastid

types according to the type of cell in which they are

located in response to the particular metabolic demands.

Plastids are essential components for plant cell function.

Plastids develop into specialized functional types.

They divide by binary fission or budding. Several types

of plastids exist: amyloplast or leucoplast (starch syn-

thesis and storage), chloroplast (photosynthesis), chro-

moplast (plant color), etioplast (night plants, can

develop chloroplasts), proteoplast or proteinoplast

(storage), elaioplast (oil storage).

plastocyanin An ELECTRON TRANSFER PROTEIN, con-

taining a TYPE 1 COPPER site, involved in plant and

cyanobacterial PHOTOSYNTHESIS, which transfers elec-

trons to PHOTOSYSTEM I.

platelet(thrombocyte) Disk-shaped, colorless blood

cells produced by the bone marrow (from megakary-

ocytes); contains numerous proinflammatory mediators

and functions to stop bleeding and allow damaged

areas to clot. A normal platelet count is

150,000–400,000 mm^3 (millimeters cubed).

See alsoBLOOD.

platelet-activating factor(PAF) Acytokine media-

tor of immediate hypersensitivity, perhaps even the

most important, which produces inflammation.

pleated sheet(beta pleated sheet) One type of sever-

al secondary structures, the three-dimensional arrange-

pleated sheet 269