Definitions for "Prokaryotes: Single-celled Organisms"

What Is A Prokaryote?

Archaea: One of two prokaryotic domains, the other is Bacteria. Known for their ability to survive in extreme environments, they have now been shown to live in a variety of habitats.

Bacteria: One of two prokaryotic domains, the other is Archaea. They are the most abundant organisms and live in a variety of environments yet cannot be seen by the naked eye.

Cell Wall: A protective layer external to the plasma membrane in plant cells, bacteria, fungi and some protists. In plant cells, the wall is formed of cellulose fibers embedded in a polysaccharide-protein matrix. The primary cell wall is thin and flexible, whereas the secondary cell wall is stronger and more rigid and is the primary constituent of wood.

Cyanobacteria: Photosynthetic, oxygen-producing bacteria that were formerly known as blue-green algae.

Domain: A taxonomic category above the kingdom level. The three domains are Archaea, Bacteria, and Eukarya.

Eukaryotic Cell: A type of cell with a a membrane-enclosed nucleus and membrane enclosed organelles that is present in protists, plants, fungi and animals.

Extreme Thermophile: Also known as Hyperthermophile, it is a prokaryote having an optimum growth temperature of 80 degrees Celcius or higher.

Prokaryotic Cell: A type of cell lacking a membrane-enclosed nucleus and membrane-enclosed organelles; found only in the domains Bacteria and Archaea.

Protist: A unicellular, eukaroytic organism belonging to the former taxonomic kingdom Protista, which has since been split into many kingdoms. They are the most diverse of all eukaryotes.

Protozoa: Protists that live primarily by ingesting food, an animal-like mode of nutrition.

Ribosome: A cell organelle constructed in the nucleolus and functioning as the site of protein synthesis in the cytoplasm; consists of rRNA and protein molecules, which make up two subunits.

Slime Mold: A type of protist that has unicellular amoeboid cells and multicellular reproductive bodies in its life cycle.

Stromatolites: Rock made of banded domes of sediment in which are found the most ancient forms of life; prokaryotes dating back as far as 3.5 billion years.

Prokaryotic Cells: What Structures Are Present in All Cells?

Chemotaxis: An innate behavioral response by an organism to move either towards or away from a stimulus, in this case movements are determined according to certain chemicals in the environment.

Chromosome: A threadlike, gene carrying structure found in the nucleus. Each chromosome consists of one very long DNA molecule and associated proteins.

Cytosol: The semifluid portion of the cytoplasm, that is made up of water, ions, proteins and other organic molecules but does not include the organelles.

Diffusion: The spontaneous tendency of a substance to move down its concentration gradient from a more concentrated to a less concentrated area.

DNA: Deoxyribonucleic acid, a nucleic acid that contains the genetic instructions specifying the biological development of all cellular forms of life. It is usually found in the form of a double helix.

Endoplasmic Reticulum: An extensive membranous network in eukaryotic cells, continuous with the outer nuclear membrane and composed of ribosome-studded (rough) and ribosome-free (smooth) regions.

Enzyme: A protein that serves as a catalyst, a chemical agent that changes the rate of a reaction without being consumed by the reaction.

Gene: A discrete unit of hereditary information consisting of a specific nucleotide sequence in DNA (or RNA, in some viruses).

Genome: The complete gentic composition of a cell or a species.

Hydrophilic: Having an affinity for water.

Hydrophobic: Having an aversion to water; tending to coalesce and form droplets in water.

Lipid: One of a family of compounds including fats, phospholipids, and steroids that are insoluable in water.

Lipid Bilayer: A membrane or zone of a membrane composed of two opposing layers of lipid molecules arranged so that their hydrophobic tails face one another and form an oily core and their hydrophilic heads face the aqueous solutions on either side of the membrane.

Mitochondria: Organelles in eukaryotic cells that serve as the site of cellular respiration.

mRNA: The abbreviation for messenger RNA, which encodes and carries information from DNA during transcription to sites of protein synthesis to undergo translation in order to produce a gene.

Nucleoid: A dense region of DNA in a prokaryotic cell.

Nucleus: The chromosome-containing organelle of a eukaryotic cell.

Passive Transport: The diffusion of a substance across a biological membrane.

Peptidoglycan: A type of polymer in bacterial cell walls consisting of modified sugars cross-linked by short polypeptides.

Periplasmic Space: The space between the plasma membrane and the outer membrane in gram-negative bacteria. This space is involved in various biochemical pathways such as nutrient acquisition, electron transport and alteration of substances toxic to the cell.

Phospholipids: Molecules that are a constituent of the inner bilayer of biological membranes, having a polar, hydrophilic head and a nonpolar, hydrophobic tail.

Plasma Membrane: The membrane at the boundary of every cell that acts as a selective barrier, thereby regulating the cell's chemical composition.

Plasmid: A small ring of DNA that carries accessory genes separate from those of a bacterial chromosome; also found in some eukaryotes, such as yeast.

Polypeptide: A polymer (or chain) of many amino acids linked together by peptide bonds.

Polysome: Free ribosomes engaged in protein synthesis. Also known as polyribosomes.

Protein: A three-dimemsional biological polymer constructed from a set of 20 different monomers called amino acids.

Protein Domain: The region of a protein that has a specific function, such as the active site of an enzyme; the domain is formed by the 3-dimensional shape of the region.

Proteome: All of the types and relative amounts of proteins that are made in a particular cell at a particular time and under specific conditions.

Ribosome: A cell organelle constructed in the nucleolus and functioning as the site of protein synthesis in the cytoplasm; consists of rRNA and protein molecules, which make up two subunits.

RNA: Ribonucleic acid, a nucleic acid polymer consisting of nucleotide monomers. RNA contains the nucleotide uracil instead of thymine which is in DNA.

rRNA: The abbreviation for Ribosomal RNA which provides the three-dimensional structure for ribosomes.

Signal-Transduction Pathway: A mechanism linking a mechanical or chemical stimulus to a specific cellular response.

tRNA: The abbreviation for Transfer RNA which transfers a specific amino acid to a growing polypeptide chain at the ribosomal site of protein synthesis during translation.

Prokaryotic Cells: What Are Some Additional Features of These Cells?

Capsule: A sticky layer that surrounds the cell walls of some bacteria, protecting the cell surface and sometimes helping to glue the cell to environmental surfaces.

Cell wall: A rigid structure surrounding the plasma membrane of a cell.

Endospore: A thick-coated, resistant cell produced within a bacterial cell that has been exposed to harsh conditions.

Extremophile: An organism that grows optimally under one or more chemical or physical extremes, such as high or low temperature or pH.

Extreme Thermophile: A prokaryote having an optimum growth temperature of 80 degrees Celcius or higher.

Flagella: Long cellular appendages specialized for locomotion. The flagella of prokaryotes and eukaryotes differ in both structure and function.

Gram-Negative: The group of bacteria with a double layer cell wall made of a thin layer of peptidoglycan that is covered by a thick outer membrane consisting of lipopolysaccharides. They are more often likely to cause disease than gram-positive bacteria.

Gram-Positive: The group of bacteria with a single layer cell wall with a relatively large amount of peptidoglycan. Gram-positive bacteria are usually less toxic than gram-negative bacteria.

Gram Stain: A staining method developed by Dr. Hans Christian Gram in 1884 that distinguishes between two different kinds of bacterial cell walls.

Hydrothermal Vents: A fissure in a planet's surface from which geothermally heated water spouts. They are commonly found in places that are also volcanically active.

Meteorite: An extraterrestrial body that survives impact with Earth's atmosphere and surface without being destroyed.

Pili: Surface appendages in certain bacteria that function in adherence and the transfer of DNA during conjugation.

Thermophile: An organism with a growth temperature optimum between 45 and 80 degrees Celcius.