Electron Microscope — A powerful micro- 
scope that uses beams of high-speed 
electrons instead of light waves to illumi- 
nate objects for observation. 
Endocytosis — The uptake of large 
molecules by the cell membrane. During 
this process, bits of the membrane fold 
inward and eventually pinch off to form 
small vesicles that move, into the cytoplasm. 
Endoplasmic Reticulum — An organelle 
made up of membranes that form a system 
of tubes and flattened sacs continuous with 
the nuclear membrane. Some of the mem- 
branes are smooth (the SER); others are 
"rough" (the RER) because they are dotted 
with ribosomes. 
Enzyme — A substance (usually a protein) 
that speeds up, or catalyzes, a chemical 
reaction without being permanently altered 
or consumed. 
Eukaryotic Cell — A cell that has a true 
nucleus surrounded by a membrane. This 
group includes all animal and plant cells, 
except blue-green algae. 
Exocytosis — The movement of substances 
that are packaged in vesicles to the cell 
surface, where they fuse with the cell mem- 
brane and release their contents outside 
the cell. 
Fluid-Mosaic Model — A model of the 
cell surface membrane in which proteins 
move about within a bed of semi-fluid 
lipids. 
G Protein — One of a group of proteins 
involved in the second messenger system 
that bind to phosphate in order to pass 
along an incoming signal. 
Gene — A unit of heredity; a segment of 
the DNA molecule containing the code for 
a specific protein product or function. 
Glycolipid — A molecule composed of 
sugar and fat that forms an important 
component of cell membranes. 
Golgi Apparatus — An organelle com- 
posed of membranous sacs that packages 
proteins into vesicles and sends them to 
the cell's surface or to lysosomes. 
Intermediate FilamenS-A component 
of the cytoskeleton that acts to strengthen 
the cell. 
Ion — Any atom or small molecule that 
contains an unequal number of electrons 
and protons and, therefore, carries a net 
positive or negative electrical charge. 
Light Microscope — An instrument that 
magnifies objects using curved lenses and 
white light as a source of illumination. 
Lipids — Fats and fat-like compounds. 
Liposome — An artificial bubble made . 
up of lipids that can contain substances, 
including drugs, designed to be absorbed 
by specific cells. 
Lysosome — A small organelle contain- 
ing powerful enzymes that can digest a 
variety of materials. 
Microfilament — A threadlike organelle 
involved in cell motion, particularly muscle 
contraction. 
Micrometer (or micron) — One one- 
thousandth of a millimeter; 1 0,000 ang- 
stroms; convenient for describing the 
dimensions of cells and organelles. 
Microtubule — A thin, tubular organelle 
that acts as a structural support for the 
cell. During cell division, microtubules 
form the spindle that directs chromosomes 
to the daughter cells. 
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