Notes to Instructors
Chapter 28 Protists
What is the focus of this activity?
This activity provides students with an overview of the early evolution of eukaryotic,
single-celled organisms. The first eukaryotes appear in the fossil record around 2.1 billion
What is the particular activity designed to do?
Activity 28.1 How has endosymbiosis contributed to the diversity of organisms on
Earth today?
This activity is designed to help students begin to understand the vast diversity of the
Answers
Activity 28.1 How has endosymbiosis contributed to the diversity
of organisms on Earth today?
1. The protists been called a “catch-all group.” What does this mean? Explain.
This is a paraphyletic group, including all eukaryotes except those assigned to the fungi,
animal, or plant kingdoms. Some systematists have indicated that this group may
2. There are more than 100,000 recognized species of protists; in Campbell Biology,
9th edition, they are subdivided into five supergroups and 11 major grades. Even
though the protists are extremely diverse, we can recognize some common themes in
their evolution, including the evolution of complex cell structure, novel genetic
Notes to Instructors 203
recombination strategies, and complex life cycles. In addition, this group as a whole
has great ecological importance. Provide at least two examples of protists from
Chapter 28 in Campbell Biology, 9th edition, that illustrate each theme.
Note: Any number of different examples can be used to answer the following
questions; a few are given here.
Theme Examples
c. Complex life cycles Plasmodium This malaria-causing organism has a two-host
life history. It reproduces asexually in its human host and
sexually in its mosquito host. (See Figure 28.10 for
additional details.)
Cellular slime molds (such as Dictyostelium) Have a life
cycle that includes single-celled feeding amoebas (n). In the
sexual phase, haploid amoebas can fuse to form a zygote,
which immediately undergoes meiosis. Amoebas can also
204 Activity 28.1
3. In the late 1960s (and since), Lynn Margulis provided considerable evidence for the
endosymbiotic theory of the origin of various organelles in eukaryotic cells.
4. The mitochondria of the chromalveolates and the chlorarachinophytes (in the clade
Rhizaria) are both thought to have arisen as a result of secondary endosymbiosis.
a. What is secondary endosymbiosis?
b. What evidence supports the idea of secondary endosymbiosis?
Evidence includes the following:
a. What is the endosymbiotic theory?
The endosymbiotic theory proposes that eukaryotes originated when ancestral
b. Which two eukaryotic
organelles were proposed to
have arisen as endosymbionts?
c. What evidence did Margulis present to support
each organelle as an endosymbiont?
Mitochondria Mitochondria are bounded by two membranes. In
addition, they contain their own circular DNA (with
Activity 28.1 205
5. Structurally, Giardia lamblia lacks complete mitochondria and was once thought to
be an example of what the earliest living eukaryotes may have looked like.
a. If Giardia is similar in structure to the earliest living eukaryotes, what does this
imply about the order of evolution of the various eukaryotic organelles (that is,
nucleus, cytoskeleton, mitochondria, chloroplasts)?
Giardia has several flagella, a primitive cytoskeleton, and a nucleus. However, it
b. More recently, we have discovered that the Diplomonads (e.g., Giardia) and the
Parabasalids (the other group of Excavates) have modified mitochondria.
i. What types of mitochondria do they have?
Diplomonads contain modified mitochondria called mitosomes, which lack
ii. What does this evidence indicate about the evolution of the various
eukaryotic organelles?
Evidence that both groups have modified mitochondria means that it is still
The metabolic pathways of organisms living today evolved over a long period of time—
undoubtedly in a stepwise fashion because of their complexity. Considering everything
you have learned to date about the evolution of life on Earth, put the following in the
order in which they might have evolved, and provide an explanation for your
arrangement.
206 Activity 28.1
28.1 Test Your Understanding
This question is similar to the question at the end of the Chapter 10 activities. However, it
is more detailed in that it requires integrating the endosymbiotic theory with the basic
logic behind which biochemical pathway must have evolved first.
2.5—Second and third, prokaryotic photosynthesis requires electron transport, which
cannot occur in the absence of oxygen. Both are ranked 2.5 (rather than 2 and 3) because
photosynthesis requires electron transport. Similarly, it is difficult to envision an
independent role for electron transport.
Activity 28.1 207