On-Line Biology Book: GLOSSARY

P Q

pacemaker.  See sinoatrial node.

Pacinian corpuscles  Sensory receptors located deep in the epidermis that detect pressure and vibration.

paleontology The study of ancient life by collection and analysis of fossils.

Paleozoic Era  The period of time beginning 570 million years ago ending 245 million years ago; falls between the Proterozoic and Mesozoic Eras and is divided into the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian Periods. PICTURE

palindrome  A sequence that reads the same in either direction; in genetics, refers to an enzyme recognition sequence that reads the same on both strands of DNA.

palisade Layer of mesophyll cells in leaves that are closely placed together under the epidermal layer of the leaf. Palisade parenchyma: Columnar cells located just below the upper epidermis in leaves the cells where most of the light absorbtion in photosynthesis occurs. PICTURE 1 | PICTURE 2

palynology The study of palynomorphs and other acid-resistant microfossils usually produced by plants, protists, and fungi. Medicine
Pharmaceutical
Biochemical
Medical-science

palynomorph Generic term for any object a palynologist studies.secretes the hormones insulin and glucagon into the blood, where they regulate blood glucose levels. A

pancreas  A gland in the abdominal cavity that secretes digestive enzymes into the small intestine and also digestive organ that produces trypsin, chymotrypsin and other enzymes as a pancreatic juice, but which also has endocrine functions in the production of the hormones somatostatin, insulin, and glucagon.

pancreatic islets  Clusters of endocrine cells in the pancreas that secrete insulin and glucagon; also known as islets of Langerhans.

Pangaea The name proposed by German meteorologist Alfred Wegener for a supercontinent that existed at the end of the Paleozoic Era and consisted of all the Earth's landmasses.

parallel evolution  The development of similar characteristics in organisms that are not closely related (not part of a monophyletic group) due to adaptation to similar environments and/or strategies of life.

parasites  Organisms that live in, with, or on another organism. The parasites beneÞt from the association without contributing to the host, usually they cause some harm to the host.

parasitism  A form of symbiosis in which the population of one speci