BIOLOGY 2 AP
Chapter 26 Notes
The origins of eukaryotic diversity
I.
Eukaryotes originated by symbiosis among prokaryotes
there is a greater difference between prokaryotic and eukaryotic cells than between the cells of plants and animals
the cellular structures
and unique process arose during the genesis of the protist.
Characteristics
unique to eukaryotes:
1.
a membrane bound
nucleus
2.
mitochondria,
chloroplasts, and the endomembrane system
3.
a cytoskeleton
4.
9 + 2 flagella
5.
multiple chromosomes
consisting of linear DNA molecules compactly arranged with proteins
6.
diploid life cycle
stages
7.
mitosis
8.
meiosis
9.
sex
the small size and
simpler construction of the prokaryotic cell has many advantages but also
imposes a number of limitations. For example:
·
the number of metabolic
activities that can occur at one time is smaller
·
the smaller size of
the prokaryotic genome limits the number of genes which code for enzymes
controlling these activities.
While prokaryotes are
extremely successful, natural selection resulted in increasing complexity in
some groups. Three trends were:
·
toward multicellular
forms such as the cyanobacteria which have different cells types with special
functions
·
the evolution of
complex bacterial communities where each species benefits from the metabolic
activities of other species.
·
the
compartmentalization of different functions within single cells
n the first eukaryotes resulted from this solution
2 main ides for the evolution of the first
compartmentalization processes:
A. specialization of plasma membrane invaginations resulting in
the nuclear envelope, endoplasmic reticulum, and golgi apparatus
B. endosymbiotis associations of prokaryotes may have resulted
in Mitochondria and chloroplasts and some other organelles evolved from
prokaryotes living within other prokaryotic cells
The endosymiotic
theory proposes that certain prokaryotis species called endosymbionts lived
within larger prokaryotes. developed by Lynn Margulis
four reason we think chloroplasts and
mitochondria arose from endosymbionts:
1.
Have own DNA and circulatory system
2.
there are
endosymbiotic relationships in the modern world today
3.
Reproduce by binary
fusion( like prokaryotes)
4.
RNA most similar to
eubacteria
II. archezoans provide clues to the early evolution of
eukaryotes
archezoa- ancient living things
ancient eukaryotes
from two billion years ago are archezoa.
·
they lack
mitochondria and platids and have simple cytoskeletons
·
their ribosomes more
similar to prokaryotes than eukaryotes
Giardia intestinalis
is a modern rep. of archezoa
·
parasitic in the
human intestine- causes abdomen cramps and severe diarrhea
·
commonly transmitted
through water contaminated with human feces
Giardia’s importance
to evolutionary biologists is its role as a human parasite.
·
diplomonads have two
separate haploid nuclei which many be a result of early eukaryotic evolution.
If the diplomonads diverged from the
eukaryotic lineage before the process of nuclear fusion and meiosis evolved,
their dual nuclei may be a clue to the past. This coupled with the absence of
mitochondria in this group and other archezoana is consistent with an origin
occurring before the endosymbiotic relationships that gave rise to mitochondria
in aerobic species.
III. the
diversity of protists represents different “experiments” in the evolution of
eukaryotic organization
Precambrian rock
dated to about 2.1 billion years of age contained acritarchs, the oldest
commonly accepted fossils of protists.
Adaptive radiation produced a diversity of
protists over the next billion years.
Protists are found in
almost all moist environments:
·
Plankton =
microscopic organisms that drift passively or swim in oceans ponds and lakes.
·
many are bottom
dwellers that attach to rocks or live in the sand and silt
·
also live in damp
soil and leaf litter
·
sybiotio species are
found in the body fluids, tissues, and cells of the host.
Almost all protists
are aerobic and may be photoautotrophic, heterotrophic or mixotrophic.
The different modes
of nutrition separate protists into 3 non-technical categories: photosynthetic
forms are typically referred to as algae, ingestive forms as protozoa and
absorptive protists or fungi.
·
most protists have
flagella or cilia
·
all can reproduce
asexually, sexually or use syngamy to trade genes between asexual reproductive
episodes.
·
some form resistant
cysts when stressed by harsh environments.
·
they are the simplest
eukaryotic organisms because most are unicellular
·
unicellular protist
are complete organisms.
IV. protistan
taxonomy is a state of flux
classifications often change
Eukaryotes =
everything beyond bacteria
·
protozoa
·
fungí
·
animalia
·
plantea
rRNA comparisons
stimulates three main trendsin taxonomy:
1.
reassessment of the
number and membership of protistan phyla
2.
arrangement of the
phyla into a cladogram based largely on what molecular methods and cell
structure comparisons reveal about evolutionary relationships of protists.
3. reevaluation of the five kingdom system and debate about the
addition of new kingdoms.
The
proposal of additional kingdoms challenges the current view of biological
diversity at the highest taxonomic levels
V. diverse modes of locomotion and feeding evolved among
protozoa
protozoa
= divers group of heterotrophic protists “first animals”
detritus
= dead organic mater
A. Rhizopoda (amoebas)
the phylum Rhizopoda (rootlike feet)
includes amoebas
·
simplest of protists
·
unicellular
·
no flagellated stages
of life
·
move by Pseudopodia (p.524)
·
are detritus (eat
dead things)
·
reproduce asexually
·
inhabit freshwater,
marine and soil habitats
·
most are free-living
although some are parasitic
B. Actinopoda (heliozoans and Radiozoans) p.524
the phylum Actinopoda (=ray feet) possesses
axopodia (projections reinforced by bundles of microtubules thinly covered by
cytoplasm) which help the organism float and function in feeding.
The two main groups
of the Actinopoda are the heliozoans and the radiozoans
·
most are planktonic
·
heliozoans live
primarily in fresh water
·
radiozoans are
primarily marine organisms with shells made of silica
C. Foraminífera (Forams)
p.525
forams
have porous shells hardened by calcium carbonate
·
exclusively marine
·
cytoplasmic strands
extend through the shells pores and function in swimming, feeding and shell
formation
·
many have symbiotic
algae living beneath the shell
·
90% of the decribes
species are fossils
·
shells help form
sedimentary rock
D. Apicomplexa (Sporozoans)
all are parasites of animals
·
infectious cell
produced called sporozoites
·
both asexual and
sexual reproduction ; often requiring two or more different host species.
Several species of
plasmodium cause malaria
·
anopheles mosquitoes
are the intermediate host and humans are final host
·
results in about two
million deaths each year
·
lifetime spent in
blood or liver cells
·
had ability to alter
surface proteins
E. Zoomastigophora
(Zooflagellates) p. 527
heterotrophs
that absorb or phagocytize prey
·
Use whip like
flagella to move
· Many are free living although a large number are symbiotic
Symbiotic forms which live in the gut of termites digest the cellulose in wood eaten by the host
Species of Trypanosoma cause African sleeping
sickness and are spread by the bite of the tsetes fly.
·
There is a lage
natural reservoir for the disease since many tropical African mammals harbor
the parasite
· Trypanosomes can evade the host’s immune system by rearranging genes coding for the molecular composition of cat proteins
Molecular systematic has recently confirmed that the
zooflagellates are closely related to the group of flagellated protists which
includes photosynthetic forms such as Euglena.
F. Ciliophora (climates)
Species within the phylm Ciliophora use cilia to move and feed.
·
-most ciliates exist as solitary cells in fresh water
· cilia are relatively short and beat in synchrony.
· The cilia are associated with a submembranous system that coordinates the movement of thousands of cilia.
· Cilia maybe dispersed over surface, or clustered in fewer rows or tufts.
· Some species move on leg-like cirri (many cilia bonded together).
· Other species have rows of tightly packed cilia that function together as locomotor membranelles
Ciliates possess two types of nuclei: one large macronucleus and from one to several small micronuclei.
Characteristics of
the macronucleus:
-it is large and ha over 50 copies of the genome.
-Genes are packaged in a large number of small units, each with hundreds of copies of just a few genes.
-It controls everyday functions of the cell by synthesizing RNA.
- It is also necessary for asexual reproduction during binary fusion. The macronucleus elongates and splits instead of undergoing mitosis
Characteristics of
the micronucleus:
-it is small and may number from 1 to 80 micronuclei, depending on the species
-it does not function in growth, maintenance or asexual reproduction.
-functions in conjugation, a sexual process which produces genetic variation.
Note: meiosis and syngamy are separate from reproduction
VI. fungus-like protists have morphological
adaptations and life cycles that enhance their ecological role as decomposers.
The resemblance of slime and water molds to true fungi in result of convergent evolution of filamentous body structure.
- A filamentous body structure increases exposure to the environment and enhances their roles as decomposers
- slime molds differ from true fungi in their cellular organization, reproduction, and life cycles.
Slime molds are more closely related to amoeboid protists that to true fungi
- Molecular comparisons indicate water molds are related to certain algae although they lack chloroplasts.
a.
Myxomycota
(plasodial slime molds)
The phylum Myxomycota consists of the plasmodial slime molds which are all heterotrophs and many are brightly pigmented.
Plasmodium = feeding stage of life cycle consisting of an amoeboid, Coenocytic (multinucleated cytoplasms undivided by membranes) mass
· In most species, the nuclei of plamodia are diploid and exhibit synchronous mitotic divisions
· Cytoplamic streaming within the plasmodium helps distribute nutrients and oxygen.
· Engulfs food by phagocytosis as it grows by extending pseudopodia
· Live in moist soil, leaf mulch and rotting logs.
· When stressed by drying or lack of food, the plasmodium ceases growth and forms sexually reproductive structures called fruiting bodies, or sporangia.
B. Acrasiomycota (cellular slime molds)
The phylum acrasiomycota includes the cellular slime molds
· Feeding stage of life cycle consists of individual, solitary haploid cells.
· When the food supply is depleted, cells aggregate to form a mass similar to those of myxomycota but cells remain separate (not coenocytic).
·
Fruiting bodies
function in asexual reproduction (unlike plasmodial slime molds)
·
Only a few have
flagellated stages.
The phylum Oomycota
includes water molds, white rusts, and downy mildews.
·
Have coenocytic
Hyphae (fine, branching filaments) that are analogous to fungal hyphae
·
Cell walls are made
of cellulose rather than the chitin found in true fungi.
·
Diploid condition in
the life cycle prevails in most species
·
Biflagellated cells
are present in the life cycles, while fungi lack flagellated cells.
In water molds:
·
A large egg is
fertilized by a smaller sperm cell to form a resistant zygote.
·
These organisms are
usually decomposers which grow on dead algae and animals in fresh water.
·
Some are parasitic,
growing on injured tissues but may also grow on the skin and gills of fish
White rusts and downy mold: in their life cycle
· Usually are parasitic on terrestrial plants.
· Disperse by windblown spores but also from flagellated zoospores at some point in their life cycle.
· Some of the most important plant pathogens are members of this phylum
VII. Eukaryotic algae are key producers in most aquatic ecosystems
A majority of the eukaryotic algae are aquatic photosynthetic with only a few of the phyla having heterotrophic or mixotrophic members
Algae = relatively simple photoautrophic aquatic organisms
· These organisms were classified in the kingdom protista in the five-kingdom system.
· Algae are of great ecological significance:
-account for amount 50% of the global photosynthetic production of organic material.
-forms include fresh water plankton marine plankton and intertidal saeweeds that form the basis of aquatic food webs.
All algae have chlorophyll a (like plants) the phylogenetic relationships among algal phyla have been determined by differences in:
· Accessory pigments such as carotenoids, xanthophylls, phycobilins, and other forms of chlorophyll
· Chloroplast structure
· Cell wall chemistry
· Number, type and position of flagella
· Food storage product
A. Dinoflagellata (dinoflagellates)
Dinoflagellates are components of phytoplankton which provides the foundation of most marine food chains
· May cause red tides by explosive growth (bloom)
-these dinoflagellates produce a toxin that is concentrated by invertebrates including shellfish
-the toxin is dangerous to humans consuming shellfish and cuases the condition known as paralytic shellfish poisoning.
· Most are unicellular, some are colonial.
· Cell surface is reinforces by cellulose plates with flagella in perpendicular grooves, creating its whirling movement and resulting in a characteristic shape.
· Some live as photosynthetic symbionts of the cnidarians that build coral reefs.
· Some lack chloroplasts and live as parasites; a few carnivorous species are known.
· Have brownish plastids containing chlorphyll a, chlorophyll c and mix of carotenoids, including peridinin (found only in this phylum)
· Food is stored as starch
· Chromosomes lack histones and are always condensed.
· Has no mitotic stages
· Kinetochores are attached to the nuclear envelope and chromosomes distributed to daughter cells by the splitting of the nucleus.
Dinoflagellates are probably more closely related to zooflagellates than to any phylum of the algae.
B. bacillariophyta (diatoms)
the phylum Bacillariophyta includes the diatoms which are yellow or brown in color due to the presence of brown plastids.
· Many have a gliding movement produced by chemical secretions.
· Usually reproduce asexually; sexual stages (eggs and sperm production) are rare.
· Some produce resistant cysts.
· Mostly unicellular organisms with overlapping glasslike walls of hydrated silica in an organic matrix
· Have the same photosynthetic pigments as in Chrysophyta.
· Components of freshwater and marine plankton.
· Store food in a form of oil which also makes cells buoyant
diatomiceous earth is formed from accumulated of fossilized diatoms- which are used for filtering medium.
C. Chrysophyta (golden algae)
The phylum chrysophyta includes the golden algae
· Plastids have chlorophll a, chlorophyll c, yellow and brown carotenoids, and xanthophyll.
· Live among freshwater plankton; most are colonial
· Have flagellated cells with both flagella attached near one end of the cell
· Store carbohydrates in the form of laminarin, a polysaccharide.
· Survive environmental stress by forming resistant cysts
-Microfossils resembling these cysts have been found in precambrian rocks
D. phaeophyta (brown algae)
· largest and most complex of the algae
· all muticellular and most marine inhabitants
· have chlorophyla, chlorophyll c, and carotenoid pigment fucoxanthin
· store carbohydrate food reserves in the form of laminarin
· cell walls made of cellullose and algin
1.evolutionary adaptations of of seaweed
seaweeds are large , multicellular marine algae which are found in the intertidal and subtidal zones of coastal waters. (cool waters)
· a diverse group of algae including members of the phaeophyta (brown algae) Rhodophyta (red algae), and chlorphyta (green algae)
· The following emphsizes adaptations found in the red algae, however, many of these adaptations also apply to the brown algae and green algae seaweeds
The habitat of seaweeds, particularly the intertidal zone, poses several challenges to the survival of these organisms
· Movement of the water due to wave action and winds produces a physically active habitat
· Tidal rhythms result in the seaweeds being alternantly covered by seawater and exposed to direct sunlight and the drying structural and biochemical adaptations to survive conditions of their habitats
· The body of a seaweed is called a thallus. Plantlike in apperance but no true roots, stem or leaves.
· Floats which help suspend blades near water surface
· Brown algae known as giant kelp
· Have stipes which may reach a length of 100 meters
Biochemical adaptations in some seaweeds reinforce the anatomical adaptations and enhance survival
· Cell walls contain gel-forming polysaccharides (align in brown algae; carageenan in red algae) cushion the thalli against wave action
· Red algae retard grazing by incorporating large amounts of calcium carbonate into their cell walls
Seaweeds are used by humans in a variety of ways:
· Brown and red algae are used as food in many parts on the orient
· Used as nutrient supplements
· Algin agar and carageenan are extracted and used as thinkeners for processed foods and lubricants in oil drilling.
· Agar is also used as a microbiological culture media.
2.Alteration of generations in the life cycles of some algae
alternation of generations= alternation between multicellular haploid forms and
multicellular diploid forms in a life history
-spores released from the sporophyte develop into gametophytes
- Gametophytes produce gametes which fuse fertilization
to form a diploid zygote that develops into a sporpophyte.
· In laminaria the sporophyte and gametophyte generations are said to be heteromorphic because they are morphologically different
In Ulva, a green algae exhibiting alteration of generations, the generations are referred to as isomorphic because they look alike.
E
Rhodophyta (red algae)
Rhodophyta are primarily warm, tropical marine inhabitants, although some are found in fresh water and soil
· Contain chlorophyll a, carotenoids, phycobilins, and chlorophyll d in some.
· Red color of plastids due to the accessory pigment, phycoerythrin
-Phycoerythrin is a phycobilin, a pigment found only in red algae and cyanobacteria
· Color of the thallus may vary (even in a single species) with depth as pigmentation changes to optimize photosynthesis.
-deep water forms are almost black, moderate depth forms are red, and shallow water forms are green.
-one species has been discovered near the Bahamas at a depth of 260 meters.
Some tropical species lack pigmentation and survive as parasites on other red algae
· Carbohydrate food reserves stored as floridean starch (similar to glycogen)
· Cell walls are cellulose with agar and carageenan.
· Most red algae are multicellular and are known as seaweeds
· Most thalli are filamentous and are often branched forming an interwoven lacy network
All red algae reproduce sexually:
· Have no flagellated stages, unlike other algal protists.
· Alteration of generations is common.
F. Chlorophyta (green algae)
Members of the Chlorphyta (green algae) contain plant like chloroplasts and are believed to be related to the ancestors of the plant kingdom.
· At least 700 species are known with most being freshwater; fewer are marine.
· Many unicellular with protozoa types live as plankton, inhabit damp soil, coat snow surfaces, or are symbionts with protozoa or invertebrates.
· When living mutualisticlly with fungi they form the association known as lichens
· Colonial forms are often filamentous (“pound scum”)
· Multicellular forms may have large, complex structures resembling true plants and comprise a group of seaweeds
Evolutionary trends that probably produced colonial and multicellular forms from flagellated unicellular ancestors are:
· Formation of colonies of individual cells (as seen in Volvox)
· Repeated division of nuclei with no cytoplasmic division (as in Bryopsis)
· Formation of true multicellular forms as in Ulva.
Most chlorophytes have complex life histories involving sexual and asexual reproductive stages.
· Some are conjugating algae which produce amoeboid gametes.
· A majority produce biflagellated gametes.
The life cycle of chlamydomonas is a good example of the life history of a unicellular chlorophyte. Nate, a mature Chlamydomonas is a single haploid cell.
· During asexual reproduction, the flagella are resorbed and the cell divides twice by mitosis to form four cells (more in some species)
-the duaghter cells develop and emerge as swimming zoospores. Zoospore development includes formaition of flagella and cell walls.
-zoospores grow into mature cells, thus completing asexual reproduction
· Sexual reproduction is simulated by environmental stress from such things as a shortage of nutrients
-during sexual reproduction, many gametes are produced by mitotic division within the wall of the parent cell. The gametes escape the parent cell wall.
-Gmaetes of opposition mating strains (+ and -) pair off and cling together by the tips of their flagella.
· the gametes are morphologically indistinguishable and their fusion is known as isogamy
- the slow fusion of the gametes forms a diploid zygote which secretes a resistant coat that protects it from harsh environmental conditions.
- When dormancy of the zygote is broken, four haploid individuals (two of each mating type) are produced by meiosis
- These haploid cells emerge from the coat and develop into mature cells, thus completing the sexual life cycle
Many features of chlamydomonas sex are believed to have evolved early in the chlorophyte lineage. Using this basic life cycle, many refinements that evolved among the chlorophytes have been identified.
· Some green algae produce gametes that differ from vegetative cells and, in some species, the male gamete differs in size or morphology from the female gamete (anisogamy)
· Many species exhibit oogamy, a type of anisogamy in which a flagellated sperm fertilizes a nonmotile egg
· Some multicellular species also exhibit alternation of generations.
-Ulva produces isomorphic thalli for its diploid sphorophyte and haploid gametophytes
VIII. systematists continue to refine their hypothesis eukaryotic phylogeny
new evidence from fossils, cell ultrastructure and molecular comparisons have eukaryotic stimulated much debate about relationships among protists
· many systematists feel the kingdom Protista of the five kingdom system is obselete since it includes all of the groups covered in this chapter
· current debate is in part concerned with subdivision of the eukaryotes into phyla:
-how many phyla should there be, what are their boundaries, and what to name the new phyla.
· Another compnent of the debate concerns higher taxonomic levels and many researchers feel the five-kingdom system does not reflect the current understanding of phylogeny and should be abandoned
A hypothetical phylogeny of eukaryotes based on molecular systematics and other evidence is shown in figure 26.26
· The tree relates some of the phyla discussed in this chapter to the origin of eukaryotes by serial endosymbiosis among prokaryotic ancestors and the later diversification of other prokaryotes
· Note that chloroplasts probably evolved from cyanobacterial endosymbionts relatively early but that several lineages of heterotrophic eukaryotes lost their chloroplasts during their subsequent evolution.
The proposed eight kingdom system differs from the five kingdom system in several ways
· Prokaryotes are split into separate kingdom for archaebacteria and eubacteria.
· The early branching of archaezoans from the eukaryotic tree is also recognized by inclusion of the kingdom Archeazoa.
· The kingdom Protista is retained for most of the species covered in this chapter, however, the brown algae and some related phyla are separated into the kingdom Chromista.
· Organisms in the kingdom Chromista are distinguished from those in other kingdoms by:
-the presence of unusual chloroplasts that have two additional membranes outside of the chloroplast envelope.
-a small amount of cytoplasm
-the presence of a vestigial nucleus
-Recent evidence indicates these chloroplasts are descended from eukaryotic (endosymbionts (probably red algae)
- the Oomycota (water molds) are also placed on this branch even though
they lost their chloroplasts
· the green algae (which have an obviously close relationship with plants) and red algae are moved to the plant kingdom
a consensus has not been reached among biologists about wether to replace the five kingdom system with an alternative classification system.
IX. Multicelluarity originated independently many times
Early eukaryotes were more complex than prokaryotes and this increase in complexity allowed for greater morphological variations to evolve.
·
Extant protists are more complex in structure and show
a greater diversity of morphology than the simpler prokaryotes
·
The ancestral stock which gave rise to new waves of
adaptative radiations were the protists with multicellular bodies.
Multicellular evolved several times among the early eukaryotes and gave rise to the multicellular algae, plants, fungi, and animals.
Most researchers believe that the earliest multicellular forms arose from unicellular ancestors as colonies or loose aggregates of interconnected cells.
· Mulitcellular algae, plants, fungi, and animals probably evolved from several lineages of protists that formed by amalgamations of individual cells
· Evolution of multicellularity from colonial aggregates involved cellular specialization and division of labor
- The earliest specialization may have been locomotors capabilities provided by flagella
- As cells became more interdependent, some lost their flagella and performed other functions.
-
·
Further division of labor may have separated sex cells
from somatic cells
-this type of specialization and cooperation is seen today in colonial species such as Volvox (a green algae)
-Gametes specialized for reproduction are dependent on somatic cells while developing
· May additional steps were involved in the evolution of specialized cells capable of performing all the nonreproductive function in a mutlicellular organism.
- extensive division of labor exists among the different tissues that comprise the thalli of seaweeds.
· multicellular forms more complex than filamentous algae appeared approximately 700 million years ago.
· A variety of animal fossils has been found in late Precambrian strata and many new forms evolved in the Cambrian period (about 570 million years ago)
· Seaweeds and other complex algae were also abundant during the Cambrian period.
· Primative plants are believed to have evolved from certain green algae living in shallow waters about 400 million years ago.
chapter 26 objectives.
1.
List
characteristics that distinguish protists from organisms in the other four
kingdoms
2.
explain
why some biologists prefer to use the term “undulipodia” for eukaryotic
flagella and cilia.
3.
Briefly
summarize and compare the two major models of eukaryotic origins, the
autogenous hypothesis and the endosymbiotic hypothesis.
4.
provide
three major lines of evidence for the endosymbiotic hypothesis.
5.
explain
why some critics are skeptical about the eubacterial origins for chloroplasts
and mitochondria.
6.
explain
why modern biologists recommend expanding the original boundaries of the
Kingdom Protista.
7.
Explain
what is meant by the statement that the kingdom Protista us a polyphyletic group.
8.
List
the six major protozoan phyla and distinguish among them based upon locomotor structures, reproduction
(sexual and asexual) and habitat.
9.
Describe
amoeboid movement.
10.explain thy the phylum
Apicomplexa is so named.
11.Outline the life cycle of
Plasmodium.
12. Indicate the organism
that causes African sleeping sickness and explain how it is spread and why it
is difficult to control.
13. Describe the function of
contractile vacuoles in freshwater ciliates
14. distinguish between
macronuclei and micronuclei
15. Using diagrams, describe
conjugation in Paramecium caudatum.
16.Explain how accessory
pigments can be used to classify algae and determine phylogenetic relationships
among divisions.
17. Distinguish among the
following algal divisions based upon pigments, sell wall components, storage
products, reproduction, number and position of flagella and habitat:
a.
Dinoflagellata d. Phaeophyta
b.Bacillariophyta e. Phodophyta
c. Chrysophyta
f. Chlorophyta
18.
Describe three possible evolutionary trends that led to multicellularity in
the chlorophyta.
19. Outline the life cycles of Chlamydomonas, Ulva and
Laminaria and indicate whether the stages are haploid or diploid.
20. Distinguish between isogamy and oogamy; sporophyte and
gametophyte; and isomorphic and heteromorphic generations
21. Compare the life cycles of plasmodial and cellular slime
molds and describe the major differences between them.
22. Provide evidence that the oomycetes are not closely
related to true fungi.
23. Give examples of oomycetes and describe their economic
importance.
24. Explain the most widley accepted hypothesis for the
evolution of multicellularity