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 identi- 
cal atoms. A molecule of a compound 
consists of two or more different atoms. 
Nanometer — One one-thousandth of a 
micrometer. 
Nucleic Acid — Either of two kinds of 
molecules (DNA and RNA), formed by 
chains of nucleotides, that carry genetic 
information. 
Nucleotide — A subunit of DNA or RNA. 
It includes one base, one phosphate 
molecule, and one sugar molecule (deoxyri- 
bose in DNA, ribose in RNA). See base. 
Nucleus — In eukaryotic cells, the mem- 
brane-bound organelie that contains the 
genetic material. 
Organelle — A specialized structure 
having a definite function in a cell; for 
example, the nucleus, a mitochondrion, a 
ribosome. 
Peroxisome — A membrane-bound 
organelle that both generates and breaks 
down hydrogen peroxide. 
Phospholipid — A fatty compound that 
contains phosphate. Phospholipids make 
up much of the outer membranes of cells 
and organelles. 
Prokaryotic Cell — A cell that does not 
have a membrane around its nuclear 
region; for example, a bacterium. 
Protein — A molecule made up of a 
number of amino acids arranged in a 
specific order determined by the genetic 
code. Proteins are essential for all life 
processes. 
Receptor — A specialized molecule of a 
cell's membrane that receives information 
from the environment and conveys it to 
other parts of the cell. The information is 
transmitted in the form of a specific chemi- 
cal that must fit the receptor like a key in 
a lock. 
Replication — The duplication of heredi- 
tary material prior to cell division. 
Respiration — Within cells, the break- 
down of food molecules to liberate 
metabolically useful energy. 
Ribosome — An organelle that contains 
RNA and protein, and is the site of protein 
synthesis. 
RNA (ribonucleic acid) — A single- 
stranded nucleic acid that contains the 
sugar ribose. There are several forms of 
RNA, including messenger RNA, transfer 
RNA, and ribosomal RNA (all involved in 
protein synthesis), as well as several small 
RNA's whose functions are unclear. 
Certain viruses have RNA, instead of 
DNA, as their genetic material. 
Second Messenger System — A multi-step 
signal amplification process used by the 
cell to transmit, for example, signals from 
many hormones that cannot enter the cell 
directly. 
Transcription — The transfer of informa- 
tion from various parts of the DNA 
molecule to new strands of messenger 
RNA, which then carry this information 
from the nucleus to the cytoplasm. 
Translation — The conversion of the 
genetic instructions for a protein from 
nucleotides of messenger RNA into amino 
acids. 
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