Download Summer Bio153S Lab 2: Protists and Primitive Plants week of July

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Summer Bio153S Lab 2: Protists and Primitive Plants
week of July 12
There will be a pre-lab quiz based on the microscopy document. The
pre-lab quiz will be 10 questions and will be worth 1% of your final
mark. There is a short assignment to hand in at the end of the lab
(worth 3% of your final mark). Please bring your textbook to this lab.
Part 1: Protists
You will examine a number of protists in live cultures and on prepared slides.
Some of the protists described in the background material may not be available
in live cultures; in these cases, slides will be provided.
The taxonomy of the group informally known as protists is contentious and under
constant revision, so we’ll focus on the functional differences among the species
– their locomotion, their structure, and their modes of reproduction.
To make wet mounts of live cultures:
1.
Place a drop of the culture on a microscope slide.
2.
Carefully lower the coverslip over the drop of culture, avoiding air
bubbles.
3.
Examine first under low power.
4.
Sketch what you see – these sketches will be your study notes for the lab
practical exam!
Diatoms
Diatoms are related to the green algae, and
are a major component of phytoplankton.
They contribute greatly to global
photosynthesis. They lack flagella, and have
silica-based cell walls (frustules) that fit
together like the halves of a petri dish.
Individual diatoms are diploid.
Reproduction is mostly asexual, with
daughter cells each receiving one-half of the
parent cell wall. Because the halves of the
cell wall are not the same size, one daughter cell tends to be smaller than the
parent. After several generations of asexual reproduction, average size of
individuals in the population declines, due to this unequal division. Reduction of
body size in the population can trigger sexual reproduction.
Trypanosomes
Trypanosomes are unicellular, parasitic protists with a
single anterior flagellum. They have considerable impact
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on human health: Trypanosoma brucei, carried by the tsetse fly, causes sleeping
sickness in Equatorial Africa and T. cruzi, carried by the bedbug Rhodnius,
causes Chagas disease in South America. Trypanosomes have a glycoprotein
coat that exhibits antigenic variation: the cell surface antigens change every
few days to avoid detection by the host’s immune system. This makes it
extremely difficult to devise an effective vaccine against these diseases.
Trypanosomes are genetically unusual – approximately 75% of their genes have
no similarity with any other known eukaryote. Similar to prokaryotes, their
genes lack introns; however, their telomere sequences are very similar to those
in humans.
Plasmodium
Plasmodium is the organism that causes malaria. Like the
trypanosomes, it is a unicellular parasitic protist with a
complex life cycle that is split between an insect vector
(about 60 species of Anopheles mosquito) and a
vertebrate host. It affects humans and some of the great
apes. The life cycles of the 4 species (P. falciparum, P.
ovale, P. vivax and P. malariae) are complex and vary
among the species, but in general, sporozoites are injected into the human
bloodstream while a mosquito feeds; these travel to the liver and lodge in the
hepatocytes. After multiplying in the liver for several days, they break out and
infect red blood cells. The blood cells rupture, releasing merozoites. The
rupture of the cells produces the spiking fevers characteristic of the disease.
After the acute phase of the disease is over, relapses can occur even several
years after the initial infection, because the parasite may remain dormant in the
liver. Malaria kills more than 1 million people a year.
Amoeba proteus
This organism derives its name from the Greek
God Proteus, who could assume many forms. It
is a relatively large protist, and moves using
extensions of the cytoplasm called
pseudopodia. The edge of the pseudopodium
is relatively stiff compared to the liquid interior:
transitions from gel to sol to gel account for the
extension of the pseudopodium. This
cytoplasmic streaming is readily visible under the light microscope. Food is
engulfed and contained within food vacuoles. Contractile vacuoles expel
excess water.
The ciliates: Didinium, Paramecium and Stentor
The ciliates are unicellular protists of enormous complexity. They are covered
with cilia that propel them through water and aid in feeding. Some cilia are
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fused into cirri, which can produce a more forceful beat. Others are modified
into trichocysts, which can be discharged to catch prey or repel predators. The
examples we will examine today are all freshwater species.
Ciliates are mostly diploid. Most ciliates have 2 types of nuclei – the
macronucleus and the micronucleus. The macronucleus has many copies of
the genome, and is transcriptionally active. The micronucleus has one copy of
the diploid genome, and is important in conjugation. Ciliates reproduce asexually
by fission, but can also participate in an exchange of genetic material with
another ciliate of an opposite mating type. This is not true sexual reproduction,
because no new individual arises from conjugation. During conjugation, two
cells join together, dissolving their cell membranes at the site of junction and
forming a cytoplasmic bridge. The macronucleus disintegrates and the
micronucleus undergoes meiosis. One haploid micronucleus from each cell
migrates across the cytoplasmic bridge and fuses with the micronucleus in the
other cell. This recombined micronucleus gives rise to a new macronucleus. This
periodic genetic rejuvenation is necessary for survival; cell lines that are
prevented from undergoing conjugation eventually die out.
Stentor
Stentor may be free-swimming or sessile, adhering to
the substrate due to a mucoid substance produced at
the posterior of the cell. It is trumpet shaped, with a
row of cilia at the opening which create currents that
drive food into the funnel and down to the cytostome
at the base. Stentor will spit out particles it finds
distasteful. It can contact rapidly due to the presence
of myonemes – contractile fibres in its cytoskeleton. It
is an omnivore, feeding on rotifers, algae and other
ciliates. It frequently harbours symbiotic green
algae.
Paramecium
Paramecium has an oral groove lined
with cilia. It rotates on its axis while it
swims, sweeping food into its gullet. When
it bumps into an object, calcium channels
in the cell membrane open, and the cell
rapidly depolarizes (loses electrical
charge). This reverses to direction of the
cilia, and Paramecium swims backwards.
Although it feeds mostly on bacteria, it
also has trichocysts which it uses to repel
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predators such as Didinium.
Didinium
Didinium is a voracious predator, feeding
almost exclusively on Paramecium. It
discharges a trichocyst from its anterior
end to catch its prey, and engulfs it. It
can prey on cells larger than itself.
Phylum: Phaeophyta: the brown algae
Fucus
The brown algae are a monophyletic group, but they vary a great deal in size
and structure. The kelps can be 30 meters long and weigh 300 pounds. The
brown algae contain plastids with a goldenbrown pigment, which gives them their name.
The body is called a thallus, and it can be
differentiated into many parts, such as a
holdfast and air bladders. In Fucus, the
thallus is diploid.
Chlamydomonas, Volvox and Ulva -- Phylum: Chlorophyta
These 3 organisms are green algae. Green algae are now considered part of the
Kingdom Plantae, and thus are not included in the protists. However, they have
been included here to show their life-cycles, which are evolutionary precursors of
the higher plants.
1. Chlamydomonas
Chlamydomonas is a motile, unicellular green alga
with 2 flagella. Molecular analysis has shown that it is
polyphyletic group – all convergent in having 2
flagella, a chloroplast with a photosensitive red
eyespot (the stigma) and a glycoprotein cell wall.
Chlamydomonas is haploid, and can reproduce
sexually or asexually. During asexual reproduction,
the parent nucleus divides mitotically several times;
cell walls form around the daughter cells within the
parent cell; and the daughter cells secrete an enzyme
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to break down the parent cell wall. Sexual reproduction occurs when the haploid
cells are starved of nitrogen. Chlamydomonas has 2 mating types called “+”
and “-”. These 2 types may fuse to form a zygote. Because the 2 types of
gametes appear the same, this type of fertilization is called isogamous.
Fertilization is a 2-step process: first, the cells fuse (plasmogamy); then the
nuclei fuse (karyogamy). The diploid zygote undergoes a period of dormancy,
after which it undergoes meiosis to produce 4 haploid daughter cells (2 “+” and
2 “-”).
2. Volvox
Volvox is a colonial green alga commonly found in
freshwater ponds. Each sphere is made up of 50060,000 cells, similar in structure to such unicellular
algae as Chlamydomonas, each with 2 flagella. The
flagella of the colony beat in unison to propel the
sphere through the water. The colony is held together
by a hollow ball of mucilage secreted by the cells, and
by a net of cytoplasmic strands connecting the
individuals. The colony cells are haploid and most have
photosynthetic function, but some are specialized for sexual reproduction.
Although the colony is spherical, it has an anterior and posterior end: cells at the
anterior have larger eyespots, used to orient the sphere towards sunlight. Some
of the posterior cells are specialized to mitotically produce gametes. Sexual
reproduction in Volvox is oogamous – that is, a large non-motile egg cell is
fertilized by a smaller, motile sperm. Sperm packets from a male colony
penetrate a female colony by means of special enzymes, and then fertilize the
eggs within. The resulting zygote can lie dormant for some time; then it
undergoes meiosis to produce 4 haploid daughter cells which divide mitotically to
form new colonies. Volvox can also reproduce asexually. Specialized
reproductive cells undergo repeated mitosis to form daughter colonies (the
small bright-green spheres within the large colony. In the daughter colonies, the
flagella are oriented inward; they turn inside out to become motile. Daughter
colonies are hatched when the outer cells of the colony die.
The colonial structure of Volvox shows an early type of multicellularity, with the
beginnings of cell differentiation. However, it appears to be an evolutionary
dead end – this is not the ancestor of the multicellular plants
The type of life-cycle exhibited by Volvox and Chlamydomonas is called a
haploid life-cycle, because the diploid zygote does not divide mitotically to
produce a diploid multicellular structure, but undergoes meiosis to produce
haploid daughter cells. These divide mitotically to produce the multicellular
organism. Thus, this type of life-cycle has a gametophyte, but no sporophyte –
the only diploid part of the life-cycle is the diploid zygote.
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3. Ulva
Sea lettuce is a multicellular green alga that
grows on rocks in shallow marine
environments. It consists of a flat thallus
that is 2 cells thick, but can be a meter long.
The thallus is anchored to the substrate by
a holdfast produced by extensions of cells at
the base of the thallus. Ulva is
anisogamous – that is, both gametes are
motile, but one (the female) is larger than
the other. Ulva has both a sporophyte and gametophyte generation, which
are isomorphic: the sporophyte and the gametophyte look identical. However,
the diploid sporophyte has sporangia that produce spores by meiosis; these
spores give rise to the haploid gametophyte which produces gametes.
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Examine a slide of Ulva and note the structure of the thallus. Sketch for
your study notes.
Your TA will select a protist or alga that you will draw from observing the live
culture.
Part II: Primitive terrestrial plants
1. The bryophytes (mosses, liverworts and hornworts)
The most elementary of the extant land plants are 3 groups (liverworts,
hornworts and mosses) collectively called bryophytes. Phylogenetically they
are separate groups, but share some general, primitive features:
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the gametophyte (gamete – producing structure) is more conspicuous and
persistent than the sporophyte (spore-producing structure)
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anchored to the ground by short threads of cells called rhizoids (these are
not roots, because they are not specialized to absorb water or nutrients)
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produce eggs and motile sperm: the eggs are held in the flask-shaped
archegonium in the female gametophyte, and the sperm swims up to
the egg through a fluid-filled protective tube of cells
a) Liverworts (there are about 6000 species) are the most primitive of land
plants, and form the sister group of all other terrestrial plants. Their structure is
simple; often consisting only of flat sheets of parenchymatous tissue a few cells
thick (a thallus). These are prostrate on the ground, attached by unicellular
outgrowths called rhizoids. Other species superficially resemble mosses. All
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liverworts lack stomata. Marchantia reproduces both sexually and asexually. It
produces two types of spore that give rise to male and female gametophytes
(gamete-producing generation). Marchantia also reproduces vegetatively
(asexually) by means of gemmae (clumps of cells held in structures called
gemmae cups on the upper surface of the gametophyte).
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Examine a specimen of the liverwort, Marchantia. Draw the gross structure,
including the thallus, rhizoids, and gemmae cups. Sexual structures are not
produced at this time of year, so look at preserved specimens and/or
diagrams to see the gross morphology of the antheridia and archegonia.
Marchantia is one of the most complex thallose liverworts. It has an upper layer of
tissue consisting of diamond shaped compartments containing specialized
photosynthetic tissue, each compartment having an opening to the outside. When
the plants are under water stress, the openings close up. Turn the plant over and
you will see a series of scales running lengthwise along the middle of the thallus.
These form an external water conducting system.
b) Mosses show some limited structural specialization compared to the liverworts.
They always have a stem and leaves and in most genera there is some cellular
specialization. The centre of the stem has elongated cells capable of conducting
water (hydroids), and these extend into the leaves to form one or more veins. A
few mosses such as Polytrichum are more complex than this as they have
specialized conducting cells known as leptoids which move carbohydrates and
other complex substances through the plant. Polytrichum also has complex leaves
that are several cells thick with a series of flaps of photosynthetic tissue on the
upper surface. This leaf is so designed that when dry it rolls up, covering this
photosynthetic tissue. Sphagnum occurs in vast quantities throughout the cool
temperate regions of the world. It is the major component in horticultural "peat
moss". The unique structure of Sphagnum has enabled it to play such an
important part in the formation of bogs, fens and similar environments. Like other
mosses Sphagnum has a stem and leaves. The stem produces clusters of 3
branches which as they age turn downwards to cover the stem. Internally, the
stem has a central region of hydroids surrounded by a thick-walled group of cells
and an outer region of large thin-walled water storage cells. The stem and
branches act as a wick drawing up water from the ground to the tip of the plant.
The surface tension of water is sufficiently high that water can be raised as much
as 200 m by this method. In addition to the stem, the leaves also closely overlap
and consist of a diamond shaped pattern of thin living cells enclosing large dead,
spirally thickened, water-storage cells. The water storage cells have openings to
the exterior.
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Examine a specimen of Sphagnum and sketch the arrangement of the
branches on the stem. Strip off a leaf of Sphagnum and examine it under
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high power. Draw, showing the two types of cells: the living cells forming a
diamond - patterned network, and the dead water storage cells with special
thickenings.
An additional important feature of Sphagnum is that it is very slow to decay owing
to the acidic nature of the decayed plant and to the presence of mild antibiotics.
The water-transporting and storage functions of the stem and leaves can still be
performed by dead tissue. This capability results in huge deposits of Sphagnum
peat being formed which may be several thousand years old at the base. There is
so much water stored in Sphagnum that it can be literally "wrung out" to extract
water even though the plant does not feel wet to the touch. Dry Sphagnum can
absorb more than 20 times its own weight of water.
Sexual reproduction in mosses and liverworts
Land plants show a few important differences from any of the groups of algae.
Sexual reproduction is entirely oogamous, but the egg is contained in a
multicellular structure called an archegonium. The archegonium has a very
precise and consistent structure, although it is reduced in higher plants.
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Examine the slide of Marchantia archegonium and note the venter, egg,
ventral canal cell, neck and neck canal. Apart from the archegonium,
the other main feature which distinguishes them from green algae is the
production of a complex multicellular spore producing structure known as a
capsule. The most specialized capsules are found in mosses (e.g.
Polytrichum). The spores are non-motile and wind dispersed.
There is considerable debate about the origin of the capsule and its evolutionary
significance, as it is homologous with the sporophyte generation of other land
plants. This has led to two conflicting hypotheses:
1. The sporophyte originated in association with the gametophyte as a sporedispersal structure that later became independent of the gametophyte. If this
hypothesis is true, then mosses and liverworts could be ancestral to other
terrestrial plants.
2. The sporophyte originated as an independent plant that later became
permanently dependant upon the gametophyte in mosses and liverworts.
If this second hypothesis is true, then mosses and liverworts cannot be ancestral to
other terrestrial plants. At present, the second hypothesis is more generally
accepted.
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2. Primitive vascular plants: Psilotum, Equisetum, Lycophyta
The earliest known vascular plants appeared in the late Silurian period about 420
million years ago. These were simple plants with a dichotomously branched
horizontal stem system bearing erect dichotomously branched stems. This is a
commonly occurring body organization in several algal groups. The important
difference is that these plants had a small star-shaped region of xylem tissue in
the centre. The living genus Psilotum is remarkably similar to these early simple
land plants although there is much debate over whether there is a direct
relationship.
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Examine a specimen of Psilotum and note that it consists only of a
horizontal underground stem (called a rhizome) without roots and an erect
over-ground stem. It bears small projections on the over-ground stem
resembling small leaves. However there is no vascular tissue in these
projections so they are named prophylls to distinguish them from the
leaves of other vascular plants. Examine a cross-section of the stem of
Psilotum. Find the xylem tissue in the centre (stained red) and locate the tip
of the stars. The small blue cells between these tips are the phloem tissue.
Early vascular plants such as Psilotum, Equisetum and some Lycophyta and almost
all ferns are homosporous. This means that in these plants meiosis results in
only one kind of haploid spore that, on germination, produces a bisexual
gametophyte or haploid plant that produces both male and female gametes.
Lycophyta (e.g., Selaginella, Lycopodium)
Lycophyta is a division of vascular plants that originated early in the Devonian
period, about 390 m. y. ago. Plants very similar to modern Lycopodium occur in
the late Devonian period about 350 m. y. ago. This division of plants shows two
important advances; they have leaves with a strand of vascular tissue and they
produce rather small relatively simple roots. Although the leaves are usually quite
small (less than 10 mm), some extant (i.e. still living) Isoetes have leaves 50 cm
long and some fossil members had leaves over 1 m in length. However the leaves
were nearly always long and narrow and even those that were somewhat lobed did
not have branching vascular tissue. Simple leaves of this type are known as
microphylls.
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Examine a specimen of Lycopodium and/or Selaginella.
Sphenophyta (Equisetum).
Sphenophyta is another division of plants originating in the Devonian period.
Basically similar to the Lycophyta in organization, but whereas in the Lycophyta
the leaves are spirally arranged and branching is dichotomous, in the
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Sphenophyta the stem is organized into nodes and internodes. In the
Sphenophyta the leaves are arranged in rings (whorls) at each node and
branches arise at the same point. The branches arise alternately with the leaves,
an arrangement unique to this division. Observe a stem of the horsetail,
Equisetum and note the arrangement of the tiny leaves and the origin of the
branches at the nodes.
Assignment: Submit your drawings of the live protist or alga, Marchantia
(gross morphology) and Sphagnum (gross morphology and microscopic
structure of the leaf) at the end of your lab session. These drawings are
worth 3% of your final mark.