Chapter 6

nucleus

(1) An atom's central core, containing protons and neutrons. (2) The organelle of a eukaryotic cell that contains the genetic material in the form of chromosomes, made up of chromatin. (3) A cluster of neurons.

middle lamella (luh-mel'-uh)

(luh-mel'-uh) In plants, a thin layer of adhesive extracellular material, primarily pectins, found between the primary walls of adjacent young cells.

mitochondrion (mī′-tō-kon'-drē-un)

(plural, mitochondria) An organelle in eukaryotic cells that serves as the site of cellular respiration; uses oxygen to break down organic molecules and synthesize ATP.

Ribosome (rī'-buh-sōm)

A complex of rRNA and protein molecules that functions as a site of protein synthesis in the cytoplasm; consists of a large and a small subunit. In eukaryotic cells, each subunit is assembled in the nucleolus. See also nucleolus.

basal body (bā'-sul)

A eukaryotic cell structure consisting of a " 9+0" arrangement of microtubule triplets. The basal body may organize the microtubule assembly of a cilium or flagellum and is structurally very similar to a centriole.

thylakoid (thī'-luh-koyd)

A flattened, membranous sac inside a chloroplast. Thylakoids often exist in stacks called grana that are interconnected; their membranes contain molecular "machinery" used to convert light energy to chemical energy.

collagen

A glycoprotein in the extracellular matrix of animal cells that forms strong fibers, found extensively in connective tissue and bone; the most abundant protein in the animal kingdom.

microtubule

A hollow rod composed of tubulin proteins that makes up part of the cytoskeleton in all eukaryotic cells and is found in cilia and flagella.

proteoglycan (prō′-tē-ō-glī'-kan)

A large molecule consisting of a small core protein with many carbohydrate chains attached, found in the extracellular matrix of animal cells. A proteoglycan may consist of up to 95% carbohydrate.

flagellum (fluh-jel'-um) (plural, flagella)

A long cellular appendage specialized for locomotion. Like motile cilia, eukaryotic flagella have a core with nine outer doublet microtubules and two inner single microtubules (the " 9+2" arrangement) ensheathed in an extension of the plasma membrane. Prokaryotic flagella have a different structure.

vesicle (ves'-i-kul)

A membrane-bound sac in or outside a cell.

Vacuole (vak'-yū-ōl′)

A membrane-bounded vesicle whose specialized function varies in different kinds of cells.

lysosome (lī'-suh-sōm)

A membrane-enclosed sac of hydrolytic enzymes found in the cytoplasm of animal cells and some protists.

endoplasmic reticulum (ER) (en′-dō-plaz'-mik ruh-tik'-yū-lum)

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

fibronectin

An extracellular glycoprotein secreted by animal cells that helps them attach to the extracellular matrix.

crista (plural, cristae) (kris'-tuh, kris'-tē)

An infolding of the inner membrane of a mitochondrion. The inner membrane houses electron transport chains and molecules of the enzyme catalyzing the synthesis of ATP (ATP synthase).

plasmodesmata (plaz′-mō-dez'-muh) (plural, plasmodesmata)

An open channel through the cell wall that connects the cytoplasm of adjacent plant cells, allowing water, small solutes, and some larger molecules to pass between the cells.

peroxisome (puh-rok∙-suh-sōm′)

An organelle containing enzymes that transfer hydrogen atoms from various substrates to oxygen (O2), producing and then degrading hydrogen peroxide (H2O2).

chloroplast (klōr'-ō-plast)

An organelle found in plants and photosynthetic protists that absorbs sunlight and uses it to drive the synthesis of organic compounds from carbon dioxide and water.

Golgi apparatus (gol'-jē)

An organelle in eukaryotic cells consisting of stacks of flat membranous sacs that modify, store, and route products of the endoplasmic reticulum and synthesize some products, notably noncellulose carbohydrates.

plastids

One of a family of closely related organelles that includes chloroplasts, chromoplasts, and amyloplasts. Plastids are found in cells of photosynthetic eukaryotes.

Smooth ER

That portion of the endoplasmic reticulum that is free of ribosomes. processes include synthesis of lipids, metabolism of carbohydrates, detoxification of drugs and poisons, and storage of calcium ions.

rough ER

That portion of the endoplasmic reticulum with ribosomes attached.

endomembrane system

The collection of membranes inside and surrounding a eukaryotic cell, related either through direct physical contact or by the transfer of membranous vesicles; includes the plasma membrane, the nuclear envelope, the smooth and rough endoplasmic reticulum, the Golgi apparatus, lysosomes, vesicles, and vacuoles.

mitochondrial matrix

The compartment of the mitochondrion enclosed by the inner membrane and containing enzymes and substrates for the citric acid cycle, as well as ribosomes and DNA.

chromatin (krō'-muh-tin)

The complex of DNA and proteins that makes up eukaryotic chromosomes. When the cell is not dividing, chromatin exists in its dispersed form, as a mass of very long, thin fibers that are not visible with a light microscope.

stroma (strō'-muh)

The dense fluid within the chloroplast surrounding the thylakoid membrane and containing ribosomes and DNA; involved in the synthesis of organic molecules from carbon dioxide and water.

extracellular matrix (ECM)

The meshwork surrounding animal cells, consisting of glycoproteins, polysaccharides, and proteoglycans synthesized and secreted by cells.

chromosomes (krō'-muh-sōm)

A cellular structure consisting of one DNA molecule and associated protein molecules. A duplicated chromosome has two DNA molecules. (In some contexts, such as genome sequencing, the term may refer to the DNA alone.) A eukaryotic cell typically has multiple, linear chromosomes, which are located in the nucleus. A prokaryotic cell often has a single, circular chromosome, which is found in the nucleoid, a region that is not enclosed by a membrane. See also chromatin.

Food vacuole

A membranous sac formed by phagocytosis of microorganisms or particles to be used as food by the cell.

contractile vacuole

A membranous sac that helps move excess water out of certain freshwater protists.

nuclear lamina

A netlike array of protein filaments that lines the inner surface of the nuclear envelope and helps maintain the shape of the nucleus.

cytoskeleton

A network of microtubules, microfilaments, and intermediate filaments that extend throughout the cytoplasm and serve a variety of mechanical, transport, and signaling functions.

cell wall

A protective layer external to the plasma membrane in the cells of plants, prokaryotes, fungi, and some protists. Polysaccharides such as cellulose (in plants and some protists), chitin (in fungi), and peptidoglycan (in bacteria) are important structural components of cell walls.

motor protein

A protein that interacts with cytoskeletal elements and other cell components, producing movement of the whole cell or parts of the cell.

glycoprotein

A protein with one or more covalently attached carbohydrates.

cilium (sil'-ē-um) (plural, cilia)

A short appendage containing microtubules in eukaryotic cells. A motile cilium is specialized for locomotion or moving fluid past the cell; it is formed from a core of nine outer doublet microtubules and two inner single microtubules (the "9+2" arrangement) ensheathed in an extension of the plasma membrane. A primary cilium is usually nonmotile and plays a sensory and signaling role; it lacks the two inner microtubules (the "9+0" arrangement).

transport vesicle

A small membranous sac in a eukaryotic cell's cytoplasm carrying molecules produced by the cell.

nucleolus (nū-klē'-ō-lus) (plural, nucleoli)

A specialized structure in the nucleus, consisting of chromosomal regions containing ribosomal RNA (rRNA) genes along with ribosomal proteins imported from the cytoplasm; site of rRNA synthesis and ribosomal subunit assembly. See also ribosome.

granum (gran'-um) (plural, grana)

A stack of membrane-bounded thylakoids in the chloroplast. Grana function in the light reactions of photosynthesis.

centriole (sen'-trē-ōl)

A structure in the centrosome of an animal cell composed of a cylinder of microtubule triplets arranged in a " 9+0" pattern. A centrosome has a pair of centrioles.

centrosome (sen'-trō-sōm)

A structure present in the cytoplasm of animal cells that functions as a microtubule-organizing center and is important during cell division. A centrosome has two centrioles.

phagocytosis (fag′-ō-sī-tō′-sis)

A type of endocytosis in which large particulate substances or small organisms are taken up by a cell. It is carried out by some protists and by certain immune cells of animals (in mammals, mainly macrophages, neutrophils, and dendritic cells).

nuclear envelope

In a eukaryotic cell, the double membrane that surrounds the nucleus, perforated with pores that regulate traffic with the cytoplasm. The outer membrane is continuous with the endoplasmic reticulum.

central vacuole

In a mature plant cell, a large membranous sac with diverse roles in growth, storage, and sequestration of toxic substances.

integrins (in'-tuh-grin)

In animal cells, a transmembrane receptor protein with two subunits that interconnects the extracellular matrix and the cytoskeleton.

secondary cell wall

In plant cells, a strong and durable matrix that is often deposited in several laminated layers around the plasma membrane and provides protection and support.

primary cell wall

In plants, a relatively thin and flexible layer that surrounds the plasma membrane of a young cell

endosymbiont theory

The theory that mitochondria and plastids originated as prokaryotic cells engulfed by a host cell. The engulfed cell and its host cell then evolved into a single organism. See also endosymbiosis.