Commensalism
A symbiotic relationship in which the symbiont benefits but the host is neither helped nor
harmed.
Community
All the organisms that inhabit a particular area; an assemblage of populations of different
species living close enough together for potential interaction.
Companion cell
A type of plant cell that is connected to a sieve-tube member by many plasmodesmata and
whose nucleus and ribosomes may serve one or more adjacent sieve-tube members.
Competition
Interaction between members of the same population or of two or more populations using
the same resource, often present in limited supply.
Competitive exclusion principle
The concept that when the populations of two species compete for the same limited
resources, one population will use the resources more efficiently and have a reproductive
advantage that will eventually lead to the elimination of the other population.
Competitive inhibitor
A substance that reduces the activity of an enzyme by entering the active site in place of the
substrate whose structure it mimics.
Complement fixation
An immune response in which antigen-antibody complexes activate complement proteins.
Complement system
A group of at least 20 blood proteins that cooperate with other defense mechanisms; may
amplify the inflammatory response, enhance phagocytosis, or directly lyse pathogens;
activated by the onset of the immune response or by surface antigens on microorganisms or
other foreign cells.
Complementary DNA (cDNA)
A DNA molecule made in vitro using mRNA as a template and the enzyme reverse
transcriptase. A cDNA molecule therefore corresponds to a gene, but lacks the introns
present in the DNA of the genome.
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