GLOSSARY 
Amino Acid — A building block of 
proteins. There are 20 different kinds of 
amino acids; a protein consists of a 
specific sequence of amino acids. 
Angstrom — A unit of length, one 
hundred-millionth of a centimeter (ap- 
proximately 0.000000004 inch); used 
for describing atomic dimensions. 
Antibody — A protein produced by 
animals in response to an antigen (a 
foreign, often disease-causing, particle), 
which binds uniquely to the antigen. 
ATP (adenosine triphosphate) — The 
compound that serves as a source of en- 
ergy for the physiological reactions in 
cells. 
Bacterium — A one-celled microorgan- 
ism that contains no nucleus. 
Base — The basic subunit of DNA or 
RNA. Paired bases — adenine with ' 
thymine and guanine With cytosine (uracil 
replaces thymine in RNA) — make up 
each "rung" of the "ladder" of the DNA 
molecule. See nucleotide. 
Biochemistry — The study of the 
chemical reactions that occur in living 
organisms. 
Cell — The basic subunit of any living 
organism; the simplest unit that can exist 
as an independent living system. 
Cell Division — The doubling in mass 
and splitting of one "mother" cell into 
two "daughter" cells. 
Cell Surface Membrane — A complex 
film of lipids interspersed with proteins. 
It covers the cell, maintains its integrity, 
and controls what goes in and what 
comes out. 
Centrifuge — A machine that separates 
particles according to their size and,, 
density by spinning them at varying 
speeds. 
Chloroplast — The chlorophyll-contain- 
ing organelle in green plants in which 
light energy is converted into sugars. 
Cholesterol — A waxy lipid produced 
by animal cells that is a prominent com- 
ponent of cell membranes. 
Chromosome — A rod-shaped structure 
containing genes that is found in the cell 
nucleus. It is composed of DNA and 
proteins, and can be seen in a light 
microscope during some stages of cell 
division. 
Codon — A sequence of three consecu- 
tive nucleotides in a DNA or RNA mole- 
cule that codes for 1 of the 20 amino 
acids in proteins or for a signal to start or 
stop protein production. 
Cristae — The inward folds of a mito- 
chondrion's inner membrane. 
Cytoplasm — All the substance inside a 
cell, excluding the nucleus. 
Cytoskeleton — A group of non-mem- 
brane-bound organelles that supports the 
cell. Some serve as conduits for the 
transport of various cell components. 
Differentiation — The series of biochemi- 
cal and structural changes that groups of 
cells undergo in order to form a special- 
ized tissue. 
DNA (deoxyribonucleic acid)— The 
substance of heredity; a large molecule 
that carries the genetic information neces- 
sary for all cellular functions, including 
the building of proteins. DNA is com- 
posed of the sugar deoxyribose, phos- 
phate, and the bases adenine, thymine, 
guanine, and cytosine. 
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