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The micrograph on the left shows a sphere about 400 microns across with round green cells about 50 microns across inside. The middle micrograph shows a similar view at higher magnification. The micrograph on the right shows a broken sphere that has released some of the cells, while other cells remain inside.
Volvox aureus is a green alga in the supergroup Archaeplastida. This species exists as a colony, consisting of cells immersed in a gel-like matrix and intertwined with each other via hair-like cytoplasmic extensions. (credit: Dr. Ralf Wagner)

True multicellular organisms, such as the sea lettuce, Ulva , are represented among the chlorophytes. In addition, some chlorophytes exist as large, multinucleate, single cells. Species in the genus Caulerpa exhibit flattened fern-like foliage and can reach lengths of 3 meters ( [link] ). Caulerpa species undergo nuclear division, but their cells do not complete cytokinesis, remaining instead as massive and elaborate single cells.

This underwater photo shows fern-like plants growing on the sea bottom.
Caulerpa taxifolia is a chlorophyte consisting of a single cell containing potentially thousands of nuclei. (credit: NOAA)

Amoebozoa

The amoebozoans characteristically exhibit pseudopodia that extend like tubes or flat lobes, rather than the hair-like pseudopodia of rhizarian amoeba ( [link] ). The Amoebozoa include several groups of unicellular amoeba-like organisms that are free-living or parasites.

The micrograph shows amoebas with lobe-like pseudopodia.
Amoebae with tubular and lobe-shaped pseudopodia are seen under a microscope. These isolates would be morphologically classified as amoebozoans.

Slime molds

A subset of the amoebozoans, the slime molds, has several morphological similarities to fungi that are thought to be the result of convergent evolution. For instance, during times of stress, some slime molds develop into spore-generating fruiting bodies, much like fungi.

The slime molds are categorized on the basis of their life cycles into plasmodial or cellular types. Plasmodial slime molds are composed of large, multinucleate cells and move along surfaces like an amorphous blob of slime during their feeding stage ( [link] ). Food particles are lifted and engulfed into the slime mold as it glides along. Upon maturation, the plasmodium takes on a net-like appearance with the ability to form fruiting bodies, or sporangia, during times of stress. Haploid spores are produced by meiosis within the sporangia, and spores can be disseminated through the air or water to potentially land in more favorable environments. If this occurs, the spores germinate to form ameboid or flagellate haploid cells that can combine with each other and produce a diploid zygotic slime mold to complete the life cycle.

 Illustration shows the plasmodium slime mold life cycle, which begins when 1n spores germinate, giving rise to cells that can convert between amoeboid and flagellated forms. Fertilization of either cell type results in a 2n zygote. The zygote undergoes mitosis without cytokinesis, resulting in a single-celled, multinucleate mass visible to the naked eye. A photo inset shows that the plasmodium is bright yellow and looks like vomit. As the plasmodium matures, holes form in the center of the mass. Stalks with bulb-shaped sporangia at the top grow up from the mass. Spores are released when the sporangia burst open, completing the cycle.
The life cycle of the plasmodial slime mold is shown. The brightly colored plasmodium in the inset photo is a single-celled, multinucleate mass. (credit: modification of work by Dr. Jonatha Gott and the Center for RNA Molecular Biology, Case Western Reserve University)

The cellular slime molds function as independent amoeboid cells when nutrients are abundant ( [link] ). When food is depleted, cellular slime molds pile onto each other into a mass of cells that behaves as a single unit, called a slug. Some cells in the slug contribute to a 2–3-millimeter stalk, drying up and dying in the process. Cells atop the stalk form an asexual fruiting body that contains haploid spores. As with plasmodial slime molds, the spores are disseminated and can germinate if they land in a moist environment. One representative genus of the cellular slime molds is Dictyostelium , which commonly exists in the damp soil of forests.

 The cellular slime mold asexual life cycle begins when 1n spores germinate, giving rise to solitary amoeboid cells. The solitary amoebas undergo mitosis, and may aggregate to form aggregated amoebas. The aggregated amoebas are able to migrate. A stalk with a fruiting body at the top forms in the aggregated amoebas. Cells migrate up the stalk and form spores that disperse, completing the asexual life cycle. The cellular slime mold sexual life cycle begins when solitary amoebas undergo fertilization, resulting in a 2n zygote. The zygote undergoes mitosis and meiosis, resulting in more 1 n solitary amoebas.
Cellular slime molds may exist as solitary or aggregated amoebas. (credit: modification of work by “thatredhead4”/Flickr)

View this site to see the formation of a fruiting body by a cellular slime mold.

Opisthokonta

The opisthokonts include the animal-like choanoflagellates, which are believed to resemble the common ancestor of sponges and, in fact, all animals. Choanoflagellates include unicellular and colonial forms, and number about 244 described species. These organisms exhibit a single, apical flagellum that is surrounded by a contractile collar composed of microvilli. The collar uses a similar mechanism to sponges to filter out bacteria for ingestion by the protist. The morphology of choanoflagellates was recognized early on as resembling the collar cells of sponges, and suggesting a possible relationship to animals.

The Mesomycetozoa form a small group of parasites, primarily of fish, and at least one form that can parasitize humans. Their life cycles are poorly understood. These organisms are of special interest, because they appear to be so closely related to animals. In the past, they were grouped with fungi and other protists based on their morphology.

Section summary

The process of classifying protists into meaningful groups is ongoing, but genetic data in the past 20 years have clarified many relationships that were previously unclear or mistaken. The majority view at present is to order all eukaryotes into six supergroups: Excavata, Chromalveolata, Rhizaria, Archaeplastida, Amoebozoa, and Opisthokonta. The goal of this classification scheme is to create clusters of species that all are derived from a common ancestor. At present, the monophyly of some of the supergroups are better supported by genetic data than others. Although tremendous variation exists within the supergroups, commonalities at the morphological, physiological, and ecological levels can be identified.

Art connections

[link] Which of the following statements about Paramecium sexual reproduction is false?

  1. The macronuclei are derived from micronuclei.
  2. Both mitosis and meiosis occur during sexual reproduction.
  3. The conjugate pair swaps macronuclei.
  4. Each parent produces four daughter cells.

[link] C

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[link] Which of the following statements about the Laminaria life cycle is false?

  1. 1 n zoospores form in the sporangia.
  2. The sporophyte is the 2n plant.
  3. The gametophyte is diploid.
  4. Both the gametophyte and sporophyte stages are multicellular.

[link] C

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Questions & Answers

What is cytoplasm
Nitta Reply
Parasitic adaptation of tapeworm
Angela Reply
Describe osmoregulator or osmoconformers and how these tools allow animals to adapt to different environment.
Alick Reply
describe the secondary function of a leaf to a plant.
Twizera Reply
hi I'm asking a question about HIV infection, can HIV infection transmitted from Mother to unbron child? please help me I So confused.
Khushboy Reply
no it can't
Ahmad
yes it can but their is an injection that can be used to prevent it.
Gift
Ahmad how no?
Khushboy
Hi im new
innocent
no, unless when giving birth
Bright
YES
Kaole
it is can be transmitted but there is an injection that the mother is injected to prevent the disease
Nia
OK
Kaole
I agree with nia
peter
what is the difference between primary and secondary active transport in detail? I didn't understand the steps in the textbook specifically
Fathima Reply
you are a doctor?
Mohammed
what is the meaning of connective tissue?
Mohammed Reply
what are the characteristics of living things
Owolo Reply
hi
Mohammed
what's the meaning of connective tissue?
Mohammed
Reproduction, adaptation, interaction, movement, growth, respiration, made of cells, responsive to environment (homeostasis), metabolic action (consumption of food converted into energy)
garret
Movement, reproduction, nutrition, irritability, growth, excretion, respiration, death, adaptation, competition
peter
state two most important factors that favour exponential growth of population of a gazelle in a pack
Eliza Reply
growth
Ben
what are the two types of electron microscope
Sharlom Reply
light microscope and early microscope
Sama
Enzymes are biological catalyst which alter any reaction and protein in nature
Nkoue Reply
Thanks
Gaudi
Your welcome sir
Nkoue
guyz you enjoying
Royd
What is translation and transcription
Nkoue
Transcription is making RNA from DNA. Translation is going from RNA to proteins.
Eric
full meaning of RNA and DNA
Charity
Nkoue what homeostits means?
Mohammed
Mohammed khalfan, In biology, homeostasis is the state of steady internal, physical, and chemical conditions maintained by living systems.[1] This dynamic state of equilibrium is the condition of optimal functioning for the organism and includes many variables, such as body temperature and fluid bal
Khushboy
what is the definition of enzymes
Royd Reply
enzymes are biological catalyst that speed up chemical reaction.
Alale
What are enzymes?
Gaudi
Enzymes are made of proteins and lower the energy of activation. In other words, they bring things together which helps to lower the amount of energy for a reaction to go forward.
Eric
they are catalyses that speeds up chemical reaction.... e.g they break down the food we consume.
Azeez
These are catalyst that speeds up the chemical reaction.
Micheal
What is connective tissue?
Mohammed
what homeostits means ?
Mohammed
what is a spirogyra
Talabi Reply
Spirogyra is a filamentous chlorophyte green algae of the order Zygnematales. It is named for the helical or spiral arrangement of the chloroplasts. That is characteristic of the genus. It is commonly found in freshwater habitats. And there are more than 400 species of Spirogyra in the world.
Eric
what is the mean of biology
Bello Reply
what is cell
Bello
A cell is the smallest living unit.
Eric
Hi I'm new in this group can someone please help with the list features shared by plants and charopytes that are not shared with most other eukaryotes
Wendy
iz a basic units of a living thing?
simongc
what is eutrophication
Chinaza Reply
hi.. I'm asking a question about HIV infection.... Can HIV infection transmitted from Mother to unbron child?
Khushboy
Eutrophication is an enrichment of water by nutrient salts that causes structural changes to the ecosystem such as: increased production of algae and aquatic plants, depletion of fish species, general deterioration of water quality and other effects that reduce and preclude use.
Khushboy

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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