DNA (deoxyribonucleic acid) The substance of 
heredity; a large molecule that carries the 
genetic information necessary for all cellular 
functions, including the building of proteins. 
DNA is composed of the sugar deoxyribose, 
phosphate, and the bases adenine, thymine, 
guanine, and cytosine. 
Electron Microscope A powerful microscope that 
uses beams of fast-moving electrons instead of 
light waves to enable objects to be observed. 
Endoplasmic Reticulum An organelle made up of 
membranes that form a system of tubes and 
flattened sacs. Some of the membranes are 
smooth (the smooth endoplasmic reticulum); 
others are rough (the rough endoplasmic 
reticulum) 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 sur- 
rounded by a membrane. This group includes all 
animal and plant cells, except cyanobacteria. 
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 
signal transduction within the cell. 
Gel Electrophoresis A technique used to separate 
molecules according to their sizes and charges. 
Gene A unit of heredity; a segment of the DNA 
molecule containing the code for a specific 
protein product or function. 
Genetic Engineering See recombinant DNA 
technology. 
Glycolipid A molecule composed of sugar and 
fat that forms an important component of cell 
membranes. 
Golgi Apparatus An organelle composed of 
membranous sacs that packages proteins into 
vesicles and sends them to the cell’s surface or 
to lysosomes. 
Intermediate Filament A component of the 
cytoskeleton that acts to strengthen the cell. 
Ion Any atom or molecule that contains an unequal 
number of electrons and protons and, therefore, 
carries a net positive or negative electrical charge. 
Ligand Any molecule that hinds to a specific site 
on a protein or other molecule. 
Light Microscope An instrument that magnifies 
objects using curved lenses and white light as a 
source of illumination. 
Lipid A fat or fat-like compound. 
Lysosoine A small organelle containing 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; 10,000 angstroms; 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. 
Mitochondrion The cell organelle that converts the 
energy in sugars into ATP, thereby fueling the cell. 
Molecule The smallest physical unit of an element 
or compound. A molecule of an element consists 
of one or more identical atoms. A molecule of a 
compound consists of two or more different atoms. 
Multicellular Made up of many cells. 
Nanometer One one-thousandth of a micrometer. 
