lichen An organism resulting from the symbiotic association of a true fungus and either a cyanobacterium or a unicellular alga.
mutualism The type of symbiosis, such as that exhibited by fungi and algae or cyanobacteria in forming lichens, in which both species profit from the association.
mycorrhiza An association of the root of a plant with the mycelium of a fungus.
parasite An organism that attacks and consumes parts of an organism much larger than itself. Parasites sometimes, but not always, kill their host.
symbiosis The living together of two or more species in a prolonged and intimate ecological relationship.
cell wall A relatively rigid structure that encloses cells of plants, fungi, many protists, and most prokaryotes. Gives these cells their shape and limits their expansion in hypotonic media.
centriole A paired organelle that helps organize the microtubules in animal and protist cells during nuclear division.
chloroplast An organelle bounded by a double membrane containing the enzymes and pigments that perform photosynthesis. Chloroplasts occur only in eukaryotes.
cilium Hairlike organelle used for locomotion by many unicellular organisms and for moving water and mucus by many multicellular organisms. Generally shorter than a flagellum.
collagen A fibrous protein found extensively in bone and connective tissue.
cytoplasm The contents of the cell, excluding the nucleus.
cytoskeleton The network of microtubules and microfilaments that gives a eukaryotic cell its shape and its capacity to arrange its organelles and to move.
cytosol The fluid portion of the cytoplasm, excluding organelles and other solids.
endomembrane system Endoplasmic reticulum plus Golgi apparatus; also lysosomes, when present. A system of membranes that exchange material with one another.
endoplasmic reticulum (ER) A system of membranous tubes and flattened sacs found in the cytoplasm of eukaryotes. Exists in two forms: rough ER, studded with ribosomes; and smooth ER, lacking ribosomes.
endosymbiosis Two species living together, with one living inside the body (or even the cells) of the other.
endosymbiotic theory The theory that the eukaryotic cell evolved via the engulfing of one prokaryotic cell by another.
eukaryotes Organisms made up of one or more complex cells in which the genetic material is contained in nuclei.
extracellular matrix In animal tissues, a material of heterogeneous composition surrounding cells and performing many functions including adhesion of cells.
Golgi apparatus A system of concentrically folded membranes found in the cytoplasm of eukaryotic cells; functions in secretion from cell by exocytosis.
intermediate filaments Cytoskeletal component with diameters between the larger microtubules and smaller microfilaments.
lysosome A membrane-enclosed organelle found in eukaryotic cells (other than plants). Lysosomes contain a mixture of enzymes that can digest most of the macromolecules found in the rest of the cell.
microfilament Minute fibrous structure generally composed of actin found in the cytoplasm of eukaryotic cells. They play a role in the motion of cells.
microtubules Minute tubular structures found in centrioles, spindle apparatus, cilia, flagella, and cytoskeleton of eukaryotic cells. These tubules play roles in the motion and maintenance of shape of eukaryotic cells.
mitochondrion An organelle in eukaryotic cells that contains the enzymes of the citric acid cycle, the respiratory chain, and oxidative phosphorylation.
nuclear envelope The surface, consisting of two layers of membrane, that encloses the nucleus of eukaryotic cells.
nucleoid The region that harbors the chromosomes of a prokaryotic cell. Unlike the eukaryotic nucleus, it is not bounded by a membrane.
nucleolus A small, generally spherical body found within the nucleus of eukaryotic cells. The site of synthesis of ribosomal RNA.
nucleus In cells, the centrally located compartment of eukaryotic cells that is bounded by a double membrane and contains the chromosomes.
organelles Organized structures found in or on eukaryotic cells. Examples include ribosomes, nuclei, mitochrondria, chloroplasts, cilia, and contractile vacuoles.
plasma membrane The membrane that surrounds the cell, regulating the entry and exit of molecules and ions. Every cell has a plasma membrane.
prokaryotes] Organisms whose genetic material is not contained within a nucleus: the bacteria and archaea. Considered an earlier stage in the evolution of life than the eukaryotes.
ribosome A small organelle that is the site of protein synthesis.
vacuole A liquid-filled, membrane-enclosed compartment in cytoplasm; may function as digestive chambers, storage chambers, waste bins.