Topics 11&12
The Protists & the Origin of
Eukaryotes
Biology 1001
October 24-28, 2005
11.1 Introduction to the Protists
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Protists is the informal name given to a
diverse collection of mostly unicellular
(some colonial or multicellular) eukaryotes
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Protists are exceedingly complex at the
cellular level, and exhibit more structural
and functional diversity than any other
group of organisms
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Formerly the Kingdom Protista in the 5K
system of classification, now about 20
different kingdoms in the Domain Eukarya
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The different lineages of protists are not a
monophyletic group; in fact some protists
are more closely related to plants, animals
or fungi than they are to other protists
A Tentative Phylogeny of the Eukaryotes
A Sampling of Protistan Diversity
Giardia intestinalis, a diplomonad
Euglena acus, a euglenid
Trichomonas vaginalis, a parabasalid
Trypanosoma sp., a kinetoplastid
Pfiesteria shumwayae,
a dinoflagellate
Paramecium sp., a ciliate
Diatoms
A radiolarian
Kelp, giant brown algae seaweeds
Amoeba sp., a gymnamoeba
Physarum polychalum,
a slime mold
The Functional & Structural Diversity of Protists
Nutrition
 Protists can be photoautotrophic, chemoheterotrophic, or mixotrophic
 Mode of nutrition not phylogenetically informative but ecologically useful
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Ingestive heterotrophs (animal-like) = protozoa
Absorptive heterotrophs (fungus-like) = no general name
Autotrophs (plant-like) = algae
Habitat
 Most are aquatic, preferring moist environments
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Are important constituents of plankton
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Seas, ponds, lakes, moist soil, the human body…
Phytoplankton contains algae and cyanobacteria
Many protists are symbiotic and some are parasitic/pathogenic
Life Cycle
 Some are strictly asexual
 Others can also reproduce sexually (meiosis & fertilization)
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All three basic types of sexual life cycle are employed1
12.0 Examples of Autotrophic Protists
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12.1 Euglena sp.
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Members of the Euglenid group of the clade Euglenozoa
Characterized by an anterior pocket from which one or two flagella
emerge, and the storage polysaccharide paramylon
The eyespot functions
as a light shield
allowing only certain
light rays to strike the
light detector
The pellicle is
constructed of protein
bands beneath the
plasma membrane and
provides strength and
flexibility
Figure 28.8
More about Euglena sp.
Nutrition
Euglena are mixotrophic –
Perform photosynthesis in the light
Lose chlorophyll in the dark & absorb organic molecules via the
plasma membrane
Locomotion
Locomotion is either swimming (flagellar motion), gliding, or
euglenoid movement1
Euglena exhibit positive phototaxis – the light detector senses
light, the flagellum propels the Euglena toward it
Osmoregulation
Euglena are hypertonic to their freshwater environment
Water enters by osmosis and needs to be removed
The contractile vacuole fills with water and then fuses with the
gullet to release it
12.0 Examples of Autotrophic Protists
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12.2 Laminaria sp.
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A brown algae species of the Stramenopila clade, characterized by
“hairy flagella” (only flagellated stage is a motile reproductive cell)
A seaweed - a large, complex, multicellular, marine alga
The thallus body consists of a rootlike holdfast, a stemlike stipe, and
leaflike blades
More about Laminaria sp.

Exhibits a life cycle called
alternation of generations1

Two multicellular stages that differ
in ploidy
 The sporophyte is diploid; the
gametophyte is haploid
 The gametophyte produces haploid
gametes by mitosis
 The gametes unite by fertilization to
form a zygote that develops into a
sporophyte
 The sporophyte produces haploid
spores by meiosis
 The spores grow up into male or
female gametophytes
 The main form is the sporophyte,
the gametophytes are short,
branched filaments – the two
generations are heteromorphic
Figure 28.21
Eukaryotic CellS Topics 11.2-11.6
 The cells of all protists, plants,
animals, and fungi
 Eukaryotic cells are structurally
more complex and larger than
prokaryotic cells
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They have a true membrane-bound
nucleus and other membrane bound
organelles
Metabolic requirements impose
theoretical lower and upper limits on
cell size1
Eukaryotic cells are 10-100 µm and
the smallest bacteria are 0.1-1 µm
Fig. 6.7
Exploring Plant and Animal Cells
Figure 6.9
• Eukaryote cells have extensively and elaborately arranged
internal membranes that divide the cell into compartments and
house enzymes for various metabolic functions
Components of the Eukaryotic Cell

The nucleus and its envelope, Figure 6.10
The nucleus contains the
chromosomes, made of
chromatin (DNA & proteins)

The nuclear envelope is a
double membrane (two
phospholipid bilayers with
associated proteins) perforated
by pores

 A prominent
structure in the
nucleus is the nucleolus where
the ribosomal subunits are
assembled
The Endomembrane System
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A network of membranes with diverse functions, connected to
each other physically or by vesicles
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Contains the nuclear envelope, endoplasmic reticulum, Golgi
apparatus, lysosomes, various vacuoles, and the plasma
membrane
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A lysosome is a membranous sac of hydrolytic enzymes that
an animal cell uses to digest all kinds of macromolecules

Mature plant cells have a large central vacuole enclosed by a
membrane called the tonoplast. This vacuole has numerous
functions: it stores nutrients and defense compounds, acts as a
disposal site for metabolic wastes, increases the membrane
available to the cytosol, and enables the cell to increase in size
by taking in water.
The Endoplasmic Reticulum
Figure 6.12

The ER is an extensive network of
membranes that accounts for half of the
total membrane in a cell

A network of membranous tubules and
sacs called cisternae, it has an interior
lumen continuous with the gap between
the two membranes of the nuclear
envelope
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ER can be smooth or rough (studded
with ribosomes). Smooth ER functions
include synthesis of lipids, metabolism of
carbohydrates, and detoxification
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Rough ER is involved in the synthesis of
proteins that are destined for secretion,
and also the synthesis of new membrane
The Golgi Apparatus
Figure 6.13
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A series of flattened membranous
sacs called cisternae
The Golgi receives products from
the ER, modifies and sorts them,
and transports them to other parts of
the cell
It also synthesizes macromolecules
such as polysaccharides
Products are received from the ER
at the cis face of the Golgi, and
transported away from the trans
face, in transport vesicles
Mitochondria & Chloroplasts

Mitochondria and chloroplasts are the energy transformers of
the eukaryotic cell
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Mitochondria, present in all eukaryotes, are the sites of
cellular respiration, where ATP is generated using energy from
the anabolism of macromolecules – C6H12O6 + O2  CO2 +
H2O + energy

Chloroplasts, present only in plants and algae, are the sites of
photosynthesis, where solar energy is converted to chemical
energy in the form of macromolecules - CO2 + H2O + energy
 C6H12O6 + O2
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Although membrane-bound, neither mitochondria nor
chloroplasts are part of the endomembrane system
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Each of these organelles has its own ribosomes and DNA
Figure 6.17 – The
mitochondrion, site
of cellular respiration
Figure 6.18 – The
chloroplast, site of
photosynthesis
Other Differences Between Prokaryotes and
Eukaryotes
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Type of cell division: binary fission in prokaryotes, meiosis or
mitosis in eukaryotes
Many linear chromosomes instead of one circular one
Presence of a cytoskeleton to support the cell, maintain its
shape, transport vesicles and chromosomes around the cell,
and give the cell motility
The cytoskeleton consists of three types of subunits,
microtubules, microfilaments, and intermediate filaments,
each with several functions
11.7 The Origin of Eukaryotes
 The first eukaryotes were predators: the cytoskeleton
allows a eukaryotic cell to engulf other cells
 The endosymbiosis theory explains how complex
eukaryotic cells likely arose from a prokaryotic ancestor
 The theory proposes that mitochondria and plastids
(chloroplasts and related organelles) were formerly
small prokaryotes called endosymbionts living within
larger host cells
 An aerobic heterotrophic alpha proteobacteria was the
mitochondrial ancestor, and a gram negative
cyanobacteria was the chloroplast ancestor
 Serial endosymbiosis accounts for the fact that all
eukaryotes have mitochondria, but only some have
chloroplasts
Figure 26.13
Endosymbiosis in Eukaryote Evolution
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The evidence for endosymbiosis is
overwhelming
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Mitochondria and chloroplasts have
their own DNA and ribosomes, and
their ribosomes are more similar to
prokaryotes
They reproduce by binary fission
They have enzymes similar to
prokaryotes
Secondary endosymbiosis accounts
for some of the diversity of protists

Red and green algae are thought to
have been engulfed in the past by
other eukaryotes, leading to some of
the contemporary protistan forms
Figure 28.3 Secondary Endosymbiosis