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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.
(sen´ tree ole) A paired organelle that helps organize the microtubules in animal and protist cells during nuclear division.
[Gk. kloros: green + plast: a particle] An organelle bounded by a double membrane containing the enzymes and pigments that perform photosynthesis. Chloroplasts occur only in eukaryotes.
(sil´ ee um) (plural: cilia) [L.: eyelash] Hairlike organelle used for locomotion by many unicellular organisms and for moving water and mucus by many multicellular organisms. Generally shorter than a flagellum.
The network of microtubules and microfilaments that gives a eukaryotic cell its shape and its capacity to arrange its organelles and to move.
Endoplasmic reticulum plus Golgi apparatus; also lysosomes, when present. A system of membranes that exchange material with one another.
(ER) [Gk. endo: within + L. plasma: form + L. reticulum: net] 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.
[Gk. endo: within + sym: together + bios: life] Two species living together, with one living inside the body (or even the cells) of the other.
The theory that the eukaryotic cell evolved via the engulfing of one prokaryotic cell by another.
 Organisms made up of one or more complex cells in which the genetic material is contained in nuclei. Contrast with archaeans  and bacteria .
In animal tissues, a material of heterogeneous composition surrounding cells and performing many functions including adhesion of cells.
(fla jell´ um) (plural: flagella) [L. flagellum: whip] Long, whiplike appendage that propels cells. Prokaryotic flagella differ sharply from those found in eukaryotes.
(gly ox´ ee soam) An organelle found in plants, in which stored lipids are converted to carbohydrates.
(goal´ jee) A system of concentrically folded membranes found in the cytoplasm of eukaryotic cells; functions in secretion from cell by exocytosis.
Cytoskeletal component with diameters between the larger microtubules and smaller microfilaments.
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.
Minute fibrous structure generally composed of actin found in the cytoplasm of eukaryotic cells. They play a role in the motion of cells.
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.
(my´ toe kon´ dree un) [Gk. mitos: thread + chondros: grain] An organelle in eukaryotic cells that contains the enzymes of the citric acid cycle, the respiratory chain, and oxidative phosphorylation.
The surface, consisting of two layers of membrane, that encloses the nucleus of eukaryotic cells.
nuclear pore complex
A protein structure situated in nuclear pores through which RNA and proteins enter and leave the nucleus.
(new´ klee oid) The region that harbors the chromosomes of a prokaryotic cell. Unlike the eukaryotic nucleus, it is not bounded by a membrane.
(new klee´ oh lus) A small, generally spherical body found within the nucleus of eukaryotic cells. The site of synthesis of ribosomal RNA.
(new´ klee us) [L. nux: kernel or nut] (1) In cells, the centrally located compartment of eukaryotic cells that is bounded by a double membrane and contains the chromosomes. (2) In the brain, an identifiable group of neurons that share common characteristics or functions.
(or gan els´) Organized structures found in or on eukaryotic cells. Examples include ribosomes, nuclei, mitochrondria, chloroplasts, cilia, and contractile vacuoles.
An organelle that houses reactions in which toxic peroxides are formed. The peroxisome isolates these peroxides from the rest of the cell.
The membrane that surrounds the cell, regulating the entry and exit of molecules and ions. Every cell has a plasma membrane.
(pro kar´ ry otes) [L. pro: before + Gk. karyon: kernel, nucleus] 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.
A glycoprotein containing a protein core with attached long, linear carbohydrate chains.
Of an optical device such as a microscope, the smallest distance between two lines that allows the lines to be seen as separate from one another.
surface area to volume ratio
For any cell, organism, or geometrical solid, the ratio of surface area to volume; this is an important factor in setting an upper limit on the size a cell or organism can attain.
(thigh la koid) [Gk. thylakos: sack or pouch] A flattened sac within a chloroplast. Thylakoid membranes contain all of the chlorophyll in a plant, in addition to the electron carriers of photophosphorylation. Thylakoids stack to form grana.
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