Paramecium

Paramecium (pronounced PARR-ə-MEE-s(ee-)əm or -see-əm; the plural "paramecia" is used only as a common name) is a genus of single-celled, eukaryotic ciliates. These organisms thrive in freshwater, brackish, and ocean environments, and are especially common in stagnant ponds and basins. Because certain species are easy to grow in culture and can be readily induced to undergo conjugation and cell division, they have become standard tools in classrooms and laboratories for studying biological processes. They are frequently used as model organisms for the ciliate group and have been called the "lab rats" of the phylum Ciliophora.

Historically, paramecia were among the first ciliates seen by microscopists in the late 1600s. The Dutch protozoology pioneer Antonie van Leeuwenhoek likely encountered them, and they were clearly described by his contemporary Christiaan Huygens in a 1678 letter. The earliest known illustration of a Paramecium species appeared anonymously in the Philosophical Transactions of the Royal Society in 1703. In 1718, French mathematics teacher and microscopist Louis Joblot described and illustrated a microscopic "fish" he found in an infusion of oak bark in water, naming it "Chausson" (slipper); the term "slipper animalcule" remained a common nickname for Paramecium through the 1700s and 1800s. The name "Paramecium," from the Greek παραμήκης (paramēkēs, meaning "oblong"), was coined in 1752 by English microscopist John Hill for animalcules with no visible limbs or tails and an irregularly oblong shape. In 1773, O. F. Müller, the first to place the genus in the Linnaean system, adopted the name but changed the spelling to Paramæcium. Johann Hermann altered it again in 1783 to Paramœcium. C. G. Ehrenberg, in a major 1838 study of infusoria, restored Hill's original spelling, which most researchers have since followed.

Paramecium species range from 0.06 mm to 0.3 mm in length. Their cells are typically ovoid, elongate, or shaped like a foot or cigar. The cell body is enclosed by a stiff but elastic pellicle, composed of an outer plasma membrane, a layer of flattened membrane-bound sacs called alveoli, and an inner epiplasm. The pellicle is not smooth; it has hexagonal or rectangular depressions, each perforated by a central opening through which a single cilium projects. Between the alveolar sacs, most species have closely spaced, spindle-shaped trichocysts—explosive organelles that discharge thin, non-toxic filaments, often used for defense. An anal pore (cytoproct) is usually located on the ventral surface in the posterior half of the cell. All species have a deep oral groove running from the anterior to the midpoint, lined with inconspicuous cilia that beat continuously to draw food into the cell. Paramecia are primarily heterotrophic, feeding on bacteria and other small organisms; a few species are mixotrophs, obtaining some nutrients from endosymbiotic algae (chlorella) carried in their cytoplasm. Osmoregulation is managed by contractile vacuoles, which actively expel water to offset fluid absorbed by osmosis; the number of vacuoles varies by species.

For movement, a Paramecium propels itself with whip-like motions of its cilia, arranged in tightly spaced rows around the body. Each cilium's beat has two phases: a fast, stiff "effective stroke" followed by a slow "recovery stroke," during which the cilium curls loosely to one side and sweeps forward counter-clockwise. The densely packed cilia move in coordinated waves across the "ciliary carpet," an effect sometimes compared to wind blowing across a field of grain. The organism spirals through the water as it moves. When it encounters an obstacle, it reverses the effective stroke of its cilia and swims backward briefly before resuming forward progress—a behavior called the avoidance reaction. If it hits the same object again, it repeats this process until it gets past. Calculations show that a Paramecium uses more than half its energy just to propel itself through water, and this ciliary method of locomotion is less than 1% efficient—though this is close to the maximum theoretical efficiency for an organism with cilia as short as those of Paramecium.

To gather food, Paramecium feeds on microorganisms like bacteria, algae, and yeasts. It uses ciliary movements to sweep prey and some water through the oral groove (vestibulum) and into the cell. Food then passes from the cilia-lined oral groove into a narrower buccal cavity (gullet), then through a small opening called the cytostome (cell mouth) into the cell interior. Once inside, food is collected into food vacuoles, which periodically close off and are released into the cytoplasm. These vacuoles circulate through the cell body via cyclosis (cytoplasmic streaming). As a food vacuole moves along, enzymes from the cytoplasm enter it to digest the contents.