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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

who is the father of biology
Sean Reply
what are prokaryotic cells
Kusubira Reply
having cell that lack membrane - bound nuclei.
Lubera
Please each person should suggest ur answers
Franklina Reply
differentiate between autelogical and synecology
Christiana Reply
differentiate between autelogical and synecology
Christiana Reply
answer please
Edmond
Autecology is the study of individual organism or individual species. It is also known as population ecology. Synecology is the study of group of organisms of different species which are associated together as a unit in form of a community.
Daniel
Autecology is the study of the environment in relation to only one species in contrast while synecology is the study of the environment affecting groups of species coexisting in an area.
Franklina
Describe the factors affecting homotasis
Grace Reply
Fluids and electrolytes Energy and nutrition Immune response mediators
Franklina
prove starch in a mango leaf
Ruth Reply
prove starch in a leaf
Ruth
draw and name the plant cell
Nalongo
wat is classification
Nalongo
levels of classification
Nalongo
classification of what
Ezenwa
Nidhipal
Nidhi
Iodine solution is used to test leaves for the presence of starch.
Franklina
Identify the caste of termites or Honey bees
Mensah Reply
Soldier Winged reproductive Worker King and queen
Franklina
what is biology
Jackson Reply
biology is a study of living organism
Brilliant
what is the myelin sheath?
Myelin sheath is an insulting layer that forms around nerves, including those in the brain and spinal cord.
Franklina
description on how an enzyme such as pepsin breaks down a substrate
linister Reply
The specific reaction catalyzed by pepsin is the acid hydrolysis of the peptide bond. This reaction will break down protein into smaller units to enable the digestive process
Franklina
what is biology
Kanzy Reply
study of life
Tufail
the study of life..meaning studying all aspects of life in humans,animals and everything inside the global
Nalukui
study of life
Memory
is the branch of science which deal with the study of living and non living things
David
Is the study of living organisms or living things
Franklina
draw the taxonomic hierarchy of classification
KC Reply
Kingdom Class Order Family Genus Species
Franklina
explain the following bush burning, deforestation, over grazing
Thomas Reply
what is biodiversity
Tracy
No idea
nana
Please what are alleles
nana
Deforestation: this is the cutting down of trees without planting new ones
Brilliant
over grazing is the continuously planting of crops in one particular land.
Friday
what is afforestation
Friday
Bush burning is the removal of the natural vegetation cover, that protects the soil surface through the use of fire. Deforestation is the actor process of establishing a forest especially on land not previously forested.
Franklina
Over grazing is the practice of grazing toi many livestock for too long a period on a land unable to recover its vegetation.
Franklina
what is physiology
Joseph Reply
the study of physical things
Nalukui
dealing with those things which you can see by your neck eyes
Odwa
Further explanation please
nana
the study of things tt interact with time,energy
David
Is the scientific study of chemistry and physics of the structure of the body
Franklina

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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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