Pentose
Any of a class of simple sugars having five carbon atoms per molecule. Ribose and
deoxyribose are pentoses.
Penetrance
In genetics, the proportion of individuals with a particular genotype that show the
phenotype ascribed to that genotype.
Peptide bond
The covalent bond between two amino acid units, formed by condensation synthesis.
Peptidyl tranferase
A ribosome-associated protein that has the peptidyl transferase activity needed to
synthesize peptide bonds during translation has never been isolated. The reason for this lack
of success is now known: the enzyme activity is specified by part of the 23S rRNA.
Peptidoglycan
A type of polymer in bacterial cell walls consisting of modified sugars cross-linked by short
polypeptides.
Perception
The interpretation of sensations by the brain.
Perennial
A plant that lives for many years.
Pericycle
A layer of cells just inside the endodermis of a root that may become meristematic and
begin dividing again.
Periderm
The protective coat that replaces the epidermis in plants during secondary growth, formed
of the cork and cork cambium.
Pericardial
The membrancous sac filled with serous fluid that encloses the heart and the roots of the
aorta and other large blood vessels.
Perigynous
Having sepals, petals, and stamens around the edge of a cuplike receptacle containing the
ovary, as in flowers of the rose or cherry.
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