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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.
dynein
In cilia and flagella, a large motor protein extending from one microtubule doublet to the adjacent doublet. ATP hydrolysis drives changes in dynein shape that lead to bending of cilia and flagella.
transmission electron microscope (TEM)
A microscope that passes an electron beam through very thin sections stained with metal atoms and is primarily used to study the internal structure of cells.
glycoprotein
A protein with one or more covalently attached carbohydrates.
central vacuole
In a mature plant cell, a large membranous sac with diverse roles in growth, storage, and sequestration of toxic substances.
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.
plasma membrane
The membrane at the boundary of every cell that acts as a selective barrier, regulating the cell's chemical composition.
centriole
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.
actin
A globular protein that links into chains, two of which twist helically about each other, forming microfilaments (actin filaments) in muscle and other kinds of cells.
integrin
In animal cells, a transmembrane receptor protein with two subunits that interconnects the extracellular matrix and the cytoskeleton.
food vacuole
A membranous sac formed by phagocytosis of microorganisms or particles to be used as food by the cell.
smooth ER
That portion of the endoplasmic reticulum that is free of ribosomes.
thylakoid
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.
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.
stroma
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.
scanning electron microscope (SEM)
A microscope that uses an electron beam to scan the surface of a sample, coated with metal atoms, to study details of its topography.
chromosome
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.
rough ER
That portion of the endoplasmic reticulum with ribosomes attached.
cytoskeleton
A network of microtubules, microfilaments, and intermediate filaments that extend throughout the cytoplasm and serve a variety of mechanical, transport, and signaling functions.
myosin
A type of motor protein that associates into filaments that interact with actin filaments to cause cell contraction.
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.
cytosol
The semifluid portion of the cytoplasm.
nucleolus
(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.
phagocytosis
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).
vesicle
A membrane-bound sac in or outside a cell.
nucleoid
A non-membrane-enclosed region in a prokaryotic cell where its chromosome is located.
cytoplasm
The contents of the cell bounded by the plasma membrane; in eukaryotes, the portion exclusive of the nucleus.
chromatin
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.
mitochondrion
(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.
granum
(plural, grana) A stack of membrane-bounded thylakoids in the chloroplast. Grana function in the light reactions of photosynthesis.
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.
extracellular matrix (ECM)
The meshwork surrounding animal cells, consisting of glycoproteins, polysaccharides, and proteoglycans synthesized and secreted by cells.
pseudopodium
(plural, pseudopodia) A cellular extension of amoeboid cells used in moving and feeding.
Golgi apparatus
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.
cilium
(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).
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.
primary cell wall
In plants, a relatively thin and flexible layer that surrounds the plasma membrane of a young cell.
fibronectin
An extracellular glycoprotein secreted by animal cells that helps them attach to the extracellular matrix.
basal body
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.
vacuole
A membrane-bounded vesicle whose specialized function varies in different kinds of cells.
cell fractionation
The disruption of a cell and separation of its parts by centrifugation at successively higher speeds.
contractile vacuole
A membranous sac that helps move excess water out of certain freshwater protists.
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.
centrosome
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.
transport vesicle
A small membranous sac in a eukaryotic cell's cytoplasm carrying molecules produced by the cell.
tight junction
A type of intercellular junction between animal cells that prevents the leakage of material through the space between cells.
motor protein
A protein that interacts with cytoskeletal elements and other cell components, producing movement of the whole cell or parts of the cell.
lysosome
A membrane-enclosed sac of hydrolytic enzymes found in the cytoplasm of animal cells and some protists.
light microscope (LM)
An optical instrument with lenses that refract (bend) visible light to magnify images of specimens.
microfilament
A cable composed of actin proteins in the cytoplasm of almost every eukaryotic cell, making up part of the cytoskeleton and acting alone or with myosin to cause cell contraction; also called an actin filament.
electron microscope (EM)
A microscope that uses magnets to focus an electron beam on or through a specimen, resulting in a practical resolution that is 100-fold greater than that of a light microscope using standard techniques. A transmission electron microscope (TEM) is used to study the internal structure of thin sections of cells. A scanning electron microscope (SEM) is used to study the fine details of cell surfaces.
plasmodesma
(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.
cytoplasmic streaming
A circular flow of cytoplasm, involving interactions of myosin and actin filaments, that speeds the distribution of materials within 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.
peroxisome
An organelle containing enzymes that transfer hydrogen atoms from various substrates to oxygen (O2), producing and then degrading hydrogen peroxide (H2O2).
middle lamella
In plants, a thin layer of adhesive extracellular material, primarily pectins, found between the primary walls of adjacent young cells.
ribosome
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.
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.
intermediate filament
A component of the cytoskeleton that includes filaments intermediate in size between microtubules and microfilaments.
flagellum
(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.
cortex
(1) The outer region of cytoplasm in a eukaryotic cell, lying just under the plasma membrane, that has a more gel-like consistency than the inner regions due to the presence of multiple microfilaments. (2) In plants, ground tissue that is between the vascular tissue and dermal tissue in a root or eudicot stem.
plastid
One of a family of closely related organelles that includes chloroplasts, chromoplasts, and amyloplasts. Plastids are found in cells of photosynthetic eukaryotes.
chloroplast
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.
gap junction
A type of intercellular junction in animal cells, consisting of proteins surrounding a pore that allows the passage of materials between cells.
proteoglycan
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.
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