BIOL 1002 Chapter : Biology Chapter 21
Biology Chapter 21
21.1 What are protists?
• Bacteria and Archaea only contain prokaryotes
• Eukarya includes all eukaryotic organisms
• The remaining eukaryotes constitute a diverse array of organisms
collectively known as protists
o Protists do not form a clade, a group consisting of all the descendants
of a particular common ancestor
o Systematists do not use the term “protist” as a formal group name,
but a term of convenience
• Protists use diverse modes of reproduction
o Most protists reproduce asexually
▪ An individual divides by mitotic cell division
▪ Two individuals that are genetically identical to the parent cell
result
o Some also reproduce sexually
▪ Two individuals contribute genetic material to an offspring that
is genetically different from either parent
o Sexual reproduction occurs infrequently
o The details of sexual reproduction and the resulting life cycles vary
among protists
• Protists use diverse modes of nutrition
o They may ingest food
o They may absorb nutrients from their surroundings
o They may capture solar energy directly by photosynthesis
o Protists that ingest food are typically predators
▪ They use extensions of the cell membrane called pseudopods
to surround and engulf prey items
▪ Other predatory protists create tiny currents that sweep food
particles into mouth-like openings in the cell
▪ Food is typically contained in a membrane-surrounded food
vacuole for digestion
o Protists that absorb nutrients directly from the surrounding
environment may be free-living
▪ They live in soil and other environments that contain dead
organic matter
