GLOSSARY 
Amino Acid A building block of proteins. There 
are 20 different kinds of amino acids; a protein 
consists of a specific secpience of amino acids. 
Angstrom A unit of length, one hundred-millionth 
of a centimeter (approximately 0.000000004 
inch); used for describing atomic dimensions. 
ATP (adenosine triphosphate) The compound that 
serves as a source of energy for the physiological 
reactions in cells. 
Bacterium A one-celled microorganism 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. 
Basic Research Scientific research that seeks to 
discover how systems work and develop a base 
of knowledge that other scientists can use in 
order to achieve practical goals, such as 
treatments or cures for diseases. 
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 Cycle The sequence of events by which the cell 
duplicates its contents and divides into two. 
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 si/a* and density by spinning 
them at varying speeds. 
Chloroplast The chlorophyll-containing organelle 
in green plants in which light energy is converted 
into sugars. 
Cholesterol A waxy lipid produced by animal cells 
that is a prominent component 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 consecutive nucleotides 
in a DNA or RNA molecule that codes for 1 of 
the 20 amino acids in proteins or for a signal to 
start or stop protein production. 
Column Chromatography A technique used to 
separate the components of biologically active 
molecules, which move at different speeds 
through a hollow column that is filled with a 
chemically reactive material. 
Cristae The inward folds of a mitochondrion’s 
inner membrane. 
Cyanobacteria (formerly called blue-green algae) 
Single-celled organisms that perform a type of 
photosynthesis. 
Cytoplasm All the substance inside a cell, excluding 
the nucleus but including the other organelles. 
Cytoskeleton A group of non-membrane-bound 
organelles that supports the cell. Some serve 
as conduits for the transport of various cell 
components. 
Differentiation The series of biochemical and 
structural changes that groups of cells undergo 
in order to form specialized cells and tissues. 
