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cell theory
The theory that all living things are composed of cells and that all cells come from other cells.
cell wall
A protective layer external to the plasma membrane in plant cells, bacteria, fungi, and some protists; protects the cell and helps maintain its shape.
cellular metabolism
The chemical activities of cells.
central vacuole
A membrane-enclosed sac occupying most of the interior of a mature plant cell, having diverse roles in reproduction, growth, and development.
centriole
A structure in an animal cell composed of cylinders of microtubule triplets arranged in a 9 and 0 pattern. An animal usually has a centrosome with a pair of centrioles involved in cell division.
chloroplast
An organelle found in plants and photosynthetic protists that absorbs sunlight and uses it to drive the synthesis of organic molecules (sugars) from carbon dioxide and water.
chromatin
The complex of DNA and proteins that constitutes eukaryotic chromosomes; often used to refer to the diffuse, very extended form taken by chromosomes when a cell is not dividing.
chromosome
A threadlike, gene-carrying structure found in the nucleus of a eukaryotic cell and most visible during mitosis and meiosis; also, the main gene-carrying structure of a prokaryotic cell. Chromosomes consist of chromatin, a combination of DNA and protein.
crista
An infolding of the inner membrane of a mitochondrion in which is embedded the electron transport chain and the enzyme catalyzing the synthesis of ATP.
cytoplasm
Everything inside a cell between the plasma membrane and the nucleus; consists of a semifluid medium and organelles.
cytoskeleton
A network of protein fibers in the cytoplasm of a eukaryotic cell; includes microfilaments, intermediate filaments, and microtubules.
electron microscope (EM)
An instrument that focuses an electron beam through, or onto the surface of, a specimen. An electron microscope achieves a hundredfold greater resolution than a light microscope.
endomembrane system
A network of membranes inside and around a eukaryotic cell, related either through direct physical contact or by the transfer of membranous vesicles.
endoplasmic reticulum (ER)
An extensive membranous network in a eukaryotic cell, continuous with the outer nuclear membrane and composed of ribosome-studded (rough) and ribosome-free (smooth) regions.
endosymbiosis
A process by which the mitochondria and chloroplasts of eukaryotic cells probably evolved from symbiotic associations between small prokaryotic cells living inside larger cells.
eukaryotic cell
A type of cell that has a membrane-enclosed nucleus and other membrane-enclosed organelles. All organisms except bacteria and archaea are composed of eukaryotic cells.
extracellular matrix (ECM)
A substance in which the cells of an animal tissue are embedded; consists of protein and polysaccharides.
flagellum
A long cellular appendage specialized for locomotion. The flagella of prokaryotes and eukaryotes differ in both structure and function. Like cilia, eukaryotic flagella have a 9 + 2 arrangement of microtubules covered by the cell's plasma membrane.
glycoprotein
A macromolecule consisting of one or more polypeptides linked to short chains of sugars.
Golgi apparatus
An organelle in eukaryotic cells consisting of stacks of membranous sacs that modify, store, and ship products of the endoplasmic reticulum.
granum
A stack of hollow disks formed of thylakoid membrane in a chloroplast. Grana are the sites where light energy is trapped by chlorophyll and converted to chemical energy during the light reactions of photosynthesis.
intermediate filament
An intermediate-sized protein fiber that is one of the three main kinds of fibers making up the cytoskeleton of eukaryotic cells. Intermediate filaments are ropelike, made of fibrous proteins.
light microscope (LM)
An optical instrument with lenses that refract (bend) visible light to magnify images and project them into a viewer's eye or onto photographic film.
lysosome
A digestive organelle in eukaryotic cells; contains hydrolytic enzymes that digest the cell's food and wastes.
microfilament
The thinnest of the three main kinds of protein fibers making up the cytoskeleton of a eukaryotic cell; a solid, helical rod composed of the globular protein actin.
micrograph
A photograph taken through a microscope.
microtubule
The thickest of the three main kinds of fibers making up the cytoskeleton of a eukaryotic cell; a straight, hollow tube made of globular proteins called tubulins. Microtubules form the basis of the structure and movement of cilia and flagella.
mitochondrial matrix
The fluid contained within the inner membrane of a mitochondrion.
mitochondrion
An organelle in eukaryotic cells where cellular respiration occurs. Enclosed by two concentric membranes, it is where most of the cell's ATP is made.
nuclear envelope
A double membrane, perforated with pores, which encloses the nucleus and separates it from the rest of the eukaryotic cell.
nucleoid
A dense region of DNA in a prokaryotic cell.
nucleolus
A structure within the nucleus of a eukaryotic cell where ribosomal RNA is made and assembled with proteins imported from the cytoplasm to make ribosomal subunits.
nucleus
(1) An atom's central core, containing protons and neutrons. (2) The genetic control center of a eukaryotic cell.
organelle
A membrane-enclosed structure with a specialized function within a cell.
peroxisome
An organelle containing enzymes that transfer hydrogen from various substrates to oxygen, producing and then degrading hydrogen peroxide.
plasma membrane
The membrane that sets a cell off from its surroundings and acts as a selective barrier to the passage of ions and molecules into and out of the cell; consists of a phospholipid bilayer in which are embedded molecules of protein and cholesterol.
plasmodesma
An open channel in a plant cell wall through which strands of cytoplasm connect from adjacent cells.
prokaryotic cell
A type of cell lacking a membrane-enclosed nucleus and other membrane-enclosed organelles; found only in the domains Bacteria and Archaea.
ribosome
A cell structure consisting of RNA and protein organized into two subunits and functioning as the site of protein synthesis in the cytoplasm. The ribosomal subunits are constructed in the nucleolus.
rough ER
A network of interconnected membranous sacs in a eukaryotic cell's cytoplasm. Rough ER membranes are studded with ribosomes that make membrane proteins and secretory proteins.
scanning electron microscope (SEM)
A microscope that uses an electron beam to study the surface architecture of a cell or other specimen.
stroma
The fluid of the chloroplast surrounding the thylakoid membrane; involved in the synthesis of organic molecules from carbon dioxide and water; Sugars are made in the stroma by the enzymes of the Calvin cycle.
thylakoid
One of a number of disk-shaped membranous sacs inside a chloroplast. Thylakoid membranes contain chlorophyll and the enzymes of the light reactions of photosynthesis. A stack of thylakoids is called a granum.
transmission electron microscope
A microscope that uses an electron beam to study the internal structure of thinly sectioned specimens.
Transport vesicle
A tiny membranous sac in a cell's cytoplasm carrying molecules produced by the cell. The vesicle buds from the endoplasmic reticulum or Golgi and eventually fuses with another membranous organelle or the plasma membrane, releasing its contents.
Vacuole
A membrane-enclosed sac that is part of the endomembrane system of a eukaryotic cell, having diverse functions.
Vesicle
A sac made of membrane in the cytoplasm of a eukaryotic cell.
Reverse Transcriptase
Makes RNA into DNA