BIO170 General Biology Freeman/Mac Leod FMCC
GREEN ALGAE
Objective: After completing this exercise, you should be able to do the following:
Describe the steps in the evolution of land plants from algal ancestors.
Identify and describe structures of Chlamydomonas, Volvox, Ulva and Chara.
Describe the life cycle of Chlamydomonas, Volvox, Ulva and Chara.
Introduction
The green algae are a diverse lineage with a fairly undefined phylogeny. Together green
algae total about 7000 species. Green algae are important primary producers in near shore
ocean environments and in all types of freshwater habitats. You can find examples of
unicellular, motile and non-motile, filamentous and multicellular species in a variety of
environments.
Within this lineage, we can see species with characteristics that indicate a possible
evolutionary path for the land plants. Your focus will be here in this investigation.
In this exercise you will observe the unicellular algae Chlamydomonas, the colonial algae
Volvox, the multicellular algae Ulva and Chara, a close relative of terrestrial plants.
A. CHLAMYDOMONAS
Chlamydomonas is a unicellular
flagellated (with 2 flagella) alga
(Figure 1). It contains a single cupshaped chloroplast. An eyespot helps
it orient toward light. This organism
is capable of both asexual and sexual
reproduction (Figure 2).
Figure 1: Chlamydomonas
Figure 2: Life cycle of Chlamydomonas
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BIO170 General Biology Freeman/Mac Leod FMCC
Procedure:
1. Refer to page 35 in your photo atlas as you work through this exercise.
2. Make a wet mount of Chlamydomonas and observe it using a compound
microscope.
3. Also observe a prepared slide of Chlamydomonas.
4. Draw and label the following terms in your notebook.
a. Nucleus, chloroplast, cell wall
B. VOLVOX
Volvox is a colonial organism composed of a hollow ball of 500-50,000 cells, each of
which is very similar to Chlamydomonas. Each of the cells has 2 flagella that point away
from the colony. The beating of these flagella results in the whole colony rolling through
their environment. Each cell has a cup shaped chloroplast. It is hypothesized that a
Chlamydomonas-like ancestor may have given rise to the progressively more complicated
colonial organism (Figure 3).
Figure 3. Proposed evolutionary pathway of Volvox
Volvox reproduces both asexually and sexually. In asexual reproduction, daughter
colonies are created inside a mother colony via mitosis. When mature, the daughter
colonies are released as independent colonies. Adverse environmental conditions trigger
sexual reproduction. At that time, some of the colony cells take on the specialized
function of producing eggs or sperm. The egg is retained by the mother colony where
fertilization takes place. The resulting zygote undergoes immediate meiosis to form
diploid cells that will grow into a new colony (Figure 4).
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BIO170 General Biology Freeman/Mac Leod FMCC
Figure 4. Life cycle of Volvox
Procedure:
1. Refer to page 35 in your photo atlas as you work through this exercise.
2. Make a wet mount of Volvox and observe it using a compound microscope.
3. Observe a prepared slide of asexual reproduction in Volvox.
4. Draw and label the following terms in your notebook.
a. Volvox cell, daughter colony
C. ULVA
Ulva is a multicellular organism composed of
only 2 cell layers. You could imagine that, if
you squashed Volvox, you would end up with
these two cell layers. Like Chlamydomonas
and Volvox, each cell contains a cup-shaped
chloroplast. Ulva differs from
Chlamydomonas and Volvox in that it is
sessile. There are specialized cells that form a
holdfast to anchor the organism to the
substrate. The remaining cells form a
relatively undifferentiated sheet called the
blade (Figure 5)
blade
Figure 5. Ulva
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BIO170 General Biology Freeman/Mac Leod FMCC
Though the individual cells of Ulva are not flagellated, during sexual reproduction, the
gametes are and they resemble Chlamydomonas. In Ulva, we can see all phases of the
alternation of generations life cycle. This organism has a multicellular sporophyte and
gametophyte phase. Because these individuals look very similar, you can only identify
them by the reproductive cells they produce. Sporophytes (diploid) produce spores
(haploid) via meiosis and gametophytes (haploid) produce gametes (haploid) via mitosis
(Figure 6).
Figure 6. Life Cycle of Ulva
Procedure:
1. Refer to page 39 in your photo atlas as you work through this exercise.
2. Make a wet mount of Ulva and observe it with the naked eye and using a zoom
stereomicroscope.
3. Draw and label the following terms in your notebook.
a. blade, holdfast, cells
D. CHARA
Chara is a member of the Charophyceae (stonewort) lineage. This group is seen to be the
sister lineage to all the land plants. In Chara we can see further specialization of cells:
rhizoids, stems, branches and reproductive structures: the oogonium which contains
eggs and antheridium that contains sperm (Figure 7).
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BIO170 General Biology Freeman/Mac Leod FMCC
Figure 7. Structure of the Charophyceae
The macroscopic organism that you will observe is the haploid gametophyte. During
sexual reproduction, the oogonium produces an egg that is retained. The antheridia
produce sperm that are released and swim to an oogonium to fertilize the egg. The
resulting zygote undergoes meiosis to form the spores that disperse and eventually
grow into another Chara plant. Chara does not display the alternation of generations
that we saw in Ulva.
Procedure:
1. Refer to page 39 in your photo atlas as you work through this exercise.
2. Observe a prepared slide of Chara using a compound microscope.
3. Draw and label the following terms in your notebook.
a. Stem, branch, oogonium, antheridium
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