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Is human zygote unicellular or multicellular

Is human zygote unicellular or multicellular


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Is the human zygote which is formed by fusion of sperm and egg a unicellular structure? Bcz at some places the 2 celled stage which is formed due to cleavage has been labelled as the zygote. So is the zygote 2 celled stage or the unicellular structure which is formed initially.


A zygote is produced when the gametes, a sperm and egg cell, combine with each other. This is unicellular. That one cell then duplicates into two cells, and then four cells. People don't really have a need to name this stage, but anyone would understand it if you just called it two-cell or four-cell within the context of zygotic development. As more cells are created (16+), the solid mass of undifferentiated cells is called a morula. This occurs around 3-5 days after fertilization. This then becomes a sort of hollow sphere of cells. These cells are now called blastomeres and the whole structure is called a blastula. When you hear a cell name with "blast" in it, it typically means it is a precursor to some other type of cell. In this case, those 16 or so cells are the precursor to a future human's entire body.


Is human zygote unicellular or multicellular - Biology

Biology

The cell organization in unicellular

In this subtopic, we will learn more on:
- The living process in amoeba and paramecium
- The organization in multicellular organism in the form of tissue, organ, system and organism

ORGANISM
-Living thing that capable of undergo vital function & living process.

One single cell only
Eg: Paramecium, bacteria, Amoeba

More than one cell
Eg: plant & animal


Unicellular organism – Amoeba sp

>Characteristic
-Enclosed by plasma membrane
-Changes shape as respond towards stimuli (eg: food)
-Ectoplasm: gel-like outer part of cytoplasm
-Endoplasm: inner cytoplasm

>Habitat
-Freshwater, ponds, damp soil
-Free living, some parasitic

>Respiration
-Exchange of O2 and CO2 by simple diffusion through plasma membrane

>Excretion
-Waste product removed from cell by simple diffusion
-Osmoregulation by contractile vacuole (removal of excess water)

>Find more on: Locomotion, Feeding and Reproduction


- The video on Amoeba locomotion -

- The video on Amoeba feeding -

>Multicellular organism is organism that consist of more than one cell
Example: plant, animal

In multicellular organism, life begin from a single cell that known as zygote.
-Example: human

The cell grow, change shape and differentiate to carry out specific function.
-Means that a single cell do not have to carry out all vital process such in a single Amoeba to sustain life.

Cells of multicellular organism differentiate and undergo specialisation to perform task efficiently.
-Example: - Red blood cell for transport
- White blood cell for defence
- Nerve cell for impulse transmission

Paramecium is a freshwater protozoan. Answer the following questions.

(a)How many contractile vacuoles present in the paramecium? (1 marks)
(b) (4 marks)
i- Water is always diffusing into the paramecium. Why is this so?
ii- Name the process by which the water enters the paramecium.
iii- Name the substance that is eliminated when the contractile vacuole contracts.
(c)What would be happen to the paramecium of the contractile vacuole stops contracting? (1 marks)
(d)Describe how the paramecium gets its
i- Oxygen supply
ii- Food (4 marks)


  • Cell structure: eukaryotes, unicellular and multicellular
  • Cell wall: (sometimes) polysaccharide
  • Nutrition: autotrophic, heterotrophic
  • Placed in this category by exclusion / cannot be placed in any other kingdom
    • Slime moulds / fungi characteristics
    • Protozoa / heterotrophic and ingest food
    • Algae / photosynthesis
    • Cell structure: eukaryotes, multicellular and unicellular (yeast)
    • Cell wall: chitin
    • Nutrition: heterotrophic / saprotrophic decomposers or parasitic
    • Genus Penicillium
      • Body of a fungus is composed of thin filaments called hyphae / form a mycelium
      • Secret enzymes / external digestion / absorbs resulting nutrients
      • Erect hyphae that grow upwards from the mycelium carry their reproductive spores
      • Chains of spores on the erect hyphae / coloured mould visible on stored food

      All animal-like protists (protozoa) are unicellular. This includes the Rhizopoda,the ciliates,the flagellates,and the Sporozoa). Many plant-like protists (algae) and fungi-like protists (molds) are also unicellular organisms.

      Although the majority of protists are unicellular, some are multicellular organisms. One notable example is the giant kelp, which is a type of brown algae and can reach lengths of up to 65m (215 feet). Other examples of multicellular protists include seaweeds, such as red algae and green algae. Slime mold cells may also clump together to form multicellular structures.


      Eukaryotic Microorganisms

      The domain Eukarya contains all eukaryotes, including uni- or multicellular eukaryotes such as protists, fungi, plants, and animals. The major defining characteristic of eukaryotes is that their cells contain a nucleus.

      Protists

      Protists are unicellular eukaryotes that are not plants, animals, or fungi. This is a highly diverse group. Algae and protozoa are examples of protists groups.

      Algae (singular: alga) are plant-like protists that can be either unicellular or multicellular (Figure (PageIndex<4>)). Their cells are surrounded by cell walls made of cellulose, a type of carbohydrate. Algae are photosynthetic organisms that extract energy from the sun and release oxygen and carbohydrates into their environment. Because other organisms can use their waste products for energy, algae are important parts of many ecosystems. Many consumer products contain ingredients derived from algae, such as carrageenan or alginic acid, which are found in some brands of ice cream, salad dressing, beverages, lipstick, and toothpaste. A derivative of algae also plays a prominent role in the microbiology laboratory. Agar, a gel derived from algae, can be mixed with various nutrients and used to grow microorganisms in a Petri dish. Algae are also being developed as a possible source for biofuels.

      Figure (PageIndex<4>): Assorted diatoms, a kind of algae, live in annual sea ice in McMurdo Sound, Antarctica. Diatoms range in size from 2 &mum to 200 &mum and are visualized here using light microscopy. (credit: National Oceanic and Atmospheric Administration)

      Protozoa (singular: protozoan) are protists that make up the backbone of many food webs by providing nutrients for other organisms. Protozoa are very diverse, though often more animal-like. Some protozoa move with help from hair-like structures called cilia or whip-like structures called flagella. Others extend part of their cell membrane and cytoplasm to propel themselves forward. These cytoplasmic extensions are called pseudopods (&ldquofalse feet&rdquo). A few protozoa are photosynthetic many others feed on organic material. Some are free-living, whereas others are parasitic, only able to survive by extracting nutrients from a host organism. Most protozoa are harmless, but some are pathogens that can cause disease in animals or humans (Figure (PageIndex<5>)).

      Figure (PageIndex<5>): Giardia lamblia, an intestinal protozoan parasite that infects humans and other mammals, causing severe diarrhea. (credit: modification of work by Centers for Disease Control and Prevention)

      Fungi

      Fungi (singular: fungus) are also eukaryotes. Some multicellular fungi, such as mushrooms, resemble plants, but they are actually quite different. Fungi are not photosynthetic, and their cell walls are usually made out of chitin rather than cellulose. The microscopic fungi broadly get split into two groups: yeasts and molds.

      Yeasts are unicellular fungi included within the study of microbiology. There are more than 1000 known species. Yeasts are found in many different environments, from the deep sea to the human navel. Some yeasts have beneficial uses, such as causing bread to rise and beverages to ferment but yeasts can also cause food to spoil. Some even cause diseases, such as vaginal yeast infections and oral thrush (Figure (PageIndex<6>)).

      Figure (PageIndex<6>): Candida albicans is a unicellular fungus, or yeast. It is the causative agent of vaginal yeast infections as well as oral thrush, a yeast infection of the mouth that commonly afflicts infants. C. albicans has a morphology similar to that of coccus bacteria however, yeast is a eukaryotic organism (note the nuclei) and is much larger. (credit: modification of work by Centers for Disease Control and Prevention)

      Molds are colonial (many attached single cells) that make long filaments which form visible colonies (Figure (PageIndex<7>)). Molds are found in many different environments, from soil to rotting food to dank bathroom corners. Molds play a critical role in the decomposition of dead plants and animals. Some molds can cause allergies, and others produce disease-causing metabolites called mycotoxins. Molds have been used to make pharmaceuticals, including penicillin, which is one of the most commonly prescribed antibiotics, and cyclosporine, used to prevent organ rejection following a transplant.

      Figure (PageIndex<7>): Large colonies of microscopic fungi can often be observed with the naked eye, as seen on the surface of these moldy oranges.

      1. Name two types of protists and two types of fungi.
      2. Name some of the defining characteristics of each type.

      Helminths

      Multicellular parasitic worms called helminths are not technically microorganisms, as most are large enough to see without a microscope. However, these worms fall within the field of microbiology because diseases caused by helminths involve microscopic eggs and larvae. These features place them in the animal kingdom like us. One example of a helminth is the guinea worm, or Dracunculus medinensis, which causes dizziness, vomiting, diarrhea, and painful ulcers on the legs and feet when the worm works its way out of the skin (Figure (PageIndex<8>)). Infection typically occurs after a person drinks water containing water fleas infected by guinea-worm larvae. In the mid-1980's, there were an estimated 3.5 million cases of guinea-worm disease, but the disease has been largely eradicated. In 2014, there were only 126 cases reported, thanks to the coordinated efforts of the World Health Organization (WHO) and other groups committed to improvements in drinking water sanitation. 1,2

      Figure (PageIndex<8>): The beef tapeworm, Taenia saginata, infects both cattle and humans. T. saginata eggs are microscopic (around 50 µm), but adult worms like the one shown here can reach 4&ndash10 m, taking up residence in the digestive system. (b) An adult guinea worm, Dracunculus medinensis, is removed through a lesion in the patient&rsquos skin by winding it around a matchstick. (credit b: modification of work by Centers for Disease Control and Prevention)


      Ciliates

      The genus Vorticella belongs in this group.

      Paramecium

      Figure 3. Paramecium caudatum X 100

      The pellicle (outer covering) of paramecium is covered with hundreds of cilia. They have numerous organelles including a gullet (oral groove) and an anal pore. Ciliates have a large macronucleus and a smaller micronucleus.

      The micronucleus is involved in sexual and asexual reproduction. Other nuclear activities are handled by the macronucleus. The macronucleus is polyploid (approximately 860 N in Paramecium aurelia) and the micronucleus is diploid.

      Figure 4. Paramecium X 200

      During reproduction, the macronucleus disintegrates. Later, a micronucleus will develop into a macronucleus. Most reproduction is asexual (mitosis). Sexual reproduction is by conjugation.

      The micronucleus will divide by meiosis 3 of the 4 resulting nuclei will disintegrate as will the macronucleus. The remaining haploid nucleus will divide by mitosis producing an individual with two haploid nuclei. Two conjugating individuals will each exchange one of the nuclei. The two haploid nuclei will then fuse producing a diploid nucleus.



      Welcome to the Living World

      Aristotle’s classification: Plants to trees, shrubs & herbs. Animals to those with red blood & without red blood.

      Linnaeus: 2-Kingdom classification (Plantae & Animalia).

      • Prokaryotes & eukaryotes under Plants.
      • Unicellular and multicellular organisms in same group.
      • No differentiation between fungi and plants.

      Polysaccharide + amino acid

      Multicellular, loose tissue

      Most abundant microorganisms.

      • Halophiles: Live in salty areas.
      • Thermoacidophiles: In hot springs.
      • Methanogens: In marshy areas and guts of ruminant animals. Produce methane (biogas).

      Rigid cell wall and a flagellum.

      • Have chlorophyll a.
      • Cyanobacteria (blue-green algae) colonies have gelatinous sheath. Some fix nitrogen in heterocysts. E.g. Nostoc & Anabaena.

      b. Chemosynthetic autotrophs: Oxidize inorganic substances and release energy.

      c. Heterotrophic bacteria: Most abundant. Decomposers.

      Some have flagella or cilia.

      Reproduction: Asexual & sexual (cell fusion → zygote).

      • Diatoms & golden algae (desmids).
      • Diatoms have siliceous cell walls. Their cell wall deposit is called diatomaceous earth.
      • Mostly marine and photosynthetic.
      • Cell wall: stiff cellulose plates.
      • Most have 2 flagella.
      • Red dinoflagellates (E.g. Gonyaulax)- sea appears red (red tides).
      • Have a protein rich layer (pellicle) & 2 flagella.
      • Photosynthetic in sunlight. Heterotrophs in darkness.
      • E.g. Euglena.
      • Saprophytic protists.
      • Suitable condition → form an aggregation (plasmodium).
      • Unfavourable conditions → plasmodium differentiates → fruiting bodies bearing spores.
      • Amoeboid protozoans: Move & capture prey by pseudopodia (false feet). E.g. Amoeba, Entamoeba (parasite).
      • Flagellated protozoans: Have flagella. Parasites cause diseases like sleeping sickness. E.g. Trypanosoma.
      • Ciliated protozoans: Move by cilia. E.g. Paramoecium.
      • Sporozoans: Have infectious spore-like stage. E.g. Plasmodium (malarial parasite).

      Except yeasts, fungi are filamentous.

      Hyphae: Thread-like structures of the body.

      Mycelium: Network of hyphae.

      • Coenocytic hyphae: Continuous tubes with multinucleated cytoplasm.
      • Septate hyphae: Have septae or cross walls.
      • Vegetative propagation: Fragmentation, fission & budding.
      • Asexual: Spores (conidia, sporangiospores & zoospores).
      • Sexual: By oospores, ascospores & basidiospores. They are produced in fruiting bodies.
      1. Plasmogamy: Fusion of protoplasm between two motile or non-motile gametes.
      2. Karyogamy: Fusion of two nuclei.
      3. Meiosis in zygote to give haploid spores.

      In some fungi, 2 haploid cells fuse → diploid cells (2n).

      In ascomycetes & basidiomycetes, a dikaryotic stage or dikaryophase (2 nuclei) occurs. Such condition is called a dikaryon. Later, parental nuclei fuse → diploid.


      What are stem cells, and what do they do?

      Cells in the body have specific purposes, but stem cells are cells that do not yet have a specific role and can become almost any cell that is required.

      Stem cells are undifferentiated cells that can turn into specific cells, as the body needs them.

      Scientists and doctors are interested in stem cells as they help to explain how some functions of the body work, and how they sometimes go wrong.

      Stem cells also show promise for treating some diseases that currently have no cure.

      Stem cells originate from two main sources: adult body tissues and embryos. Scientists are also working on ways to develop stem cells from other cells, using genetic “reprogramming” techniques.

      Adult stem cells

      Share on Pinterest Stem cells can turn into any type of cell before they become differentiated.

      A person’s body contains stem cells throughout their life. The body can use these stem cells whenever it needs them.

      Also called tissue-specific or somatic stem cells, adult stem cells exist throughout the body from the time an embryo develops.

      The cells are in a non-specific state, but they are more specialized than embryonic stem cells. They remain in this state until the body needs them for a specific purpose, say, as skin or muscle cells.

      Day-to-day living means the body is constantly renewing its tissues. In some parts of the body, such as the gut and bone marrow, stem cells regularly divide to produce new body tissues for maintenance and repair.

      Stem cells are present inside different types of tissue. Scientists have found stem cells in tissues, including:

      • the brain
      • bone marrow
      • blood and blood vessels
      • skeletal muscles
      • skin
      • the liver

      However, stem cells can be difficult to find. They can stay non-dividing and non-specific for years until the body summons them to repair or grow new tissue.

      Adult stem cells can divide or self-renew indefinitely. This means they can generate various cell types from the originating organ or even regenerate the original organ, entirely.

      This division and regeneration are how a skin wound heals, or how an organ such as the liver, for example, can repair itself after damage.

      In the past, scientists believed adult stem cells could only differentiate based on their tissue of origin. However, some evidence now suggests that they can differentiate to become other cell types, as well.

      Embryonic stem cells

      From the very earliest stage of pregnancy, after the sperm fertilizes the egg, an embryo forms.

      Around 3–5 days after a sperm fertilizes an egg, the embryo takes the form of a blastocyst or ball of cells.

      The blastocyst contains stem cells and will later implant in the womb. Embryonic stem cells come from a blastocyst that is 4–5 days old.

      When scientists take stem cells from embryos, these are usually extra embryos that result from in vitro fertilization (IVF).

      In IVF clinics, the doctors fertilize several eggs in a test tube, to ensure that at least one survives. They will then implant a limited number of eggs to start a pregnancy.

      When a sperm fertilizes an egg, these cells combine to form a single cell called a zygote.

      This single-celled zygote then starts to divide, forming 2, 4, 8, 16 cells, and so on. Now it is an embryo.

      Soon, and before the embryo implants in the uterus, this mass of around 150–200 cells is the blastocyst. The blastocyst consists of two parts:

      • an outer cell mass that becomes part of the placenta
      • an inner cell mass that will develop into the human body

      The inner cell mass is where embryonic stem cells are found. Scientists call these totipotent cells. The term totipotent refer to the fact that they have total potential to develop into any cell in the body.

      With the right stimulation, the cells can become blood cells, skin cells, and all the other cell types that a body needs.

      In early pregnancy, the blastocyst stage continues for about 5 days before the embryo implants in the uterus, or womb. At this stage, stem cells begin to differentiate.

      Embryonic stem cells can differentiate into more cell types than adult stem cells.

      Mesenchymal stem cells (MSCs)

      MSCs come from the connective tissue or stroma that surrounds the body’s organs and other tissues.

      Scientists have used MSCs to create new body tissues, such as bone, cartilage, and fat cells. They may one day play a role in solving a wide range of health problems.

      Induced pluripotent stem cells (iPS)

      Scientists create these in a lab, using skin cells and other tissue-specific cells. These cells behave in a similar way to embryonic stem cells, so they could be useful for developing a range of therapies.

      However, more research and development is necessary.

      To grow stem cells, scientists first extract samples from adult tissue or an embryo. They then place these cells in a controlled culture where they will divide and reproduce but not specialize further.

      Stem cells that are dividing and reproducing in a controlled culture are called a stem-cell line.

      Researchers manage and share stem-cell lines for different purposes. They can stimulate the stem cells to specialize in a particular way. This process is known as directed differentiation.

      Until now, it has been easier to grow large numbers of embryonic stem cells than adult stem cells. However, scientists are making progress with both cell types.

      Researchers categorize stem cells, according to their potential to differentiate into other types of cells.

      Embryonic stem cells are the most potent, as their job is to become every type of cell in the body.

      The full classification includes:

      Totipotent: These stem cells can differentiate into all possible cell types. The first few cells that appear as the zygote starts to divide are totipotent.

      Pluripotent: These cells can turn into almost any cell. Cells from the early embryo are pluripotent.

      Multipotent: These cells can differentiate into a closely related family of cells. Adult hematopoietic stem cells, for example, can become red and white blood cells or platelets.

      Oligopotent: These can differentiate into a few different cell types. Adult lymphoid or myeloid stem cells can do this.

      Unipotent: These can only produce cells of one kind, which is their own type. However, they are still stem cells because they can renew themselves. Examples include adult muscle stem cells.

      Embryonic stem cells are considered pluripotent instead of totipotent because they cannot become part of the extra-embryonic membranes or the placenta.

      Stem cells themselves do not serve any single purpose but are important for several reasons.

      First, with the right stimulation, many stem cells can take on the role of any type of cell, and they can regenerate damaged tissue, under the right conditions.

      This potential could save lives or repair wounds and tissue damage in people after an illness or injury. Scientists see many possible uses for stem cells.

      Tissue regeneration

      Tissue regeneration is probably the most important use of stem cells.

      Until now, a person who needed a new kidney, for example, had to wait for a donor and then undergo a transplant.

      There is a shortage of donor organs but, by instructing stem cells to differentiate in a certain way, scientists could use them to grow a specific tissue type or organ.

      As an example, doctors have already used stem cells from just beneath the skin’s surface to make new skin tissue. They can then repair a severe burn or another injury by grafting this tissue onto the damaged skin, and new skin will grow back.

      Cardiovascular disease treatment

      In 2013, a team of researchers from Massachusetts General Hospital reported in PNAS Early Edition that they had created blood vessels in laboratory mice, using human stem cells.

      Within 2 weeks of implanting the stem cells, networks of blood-perfused vessels had formed. The quality of these new blood vessels was as good as the nearby natural ones.

      The authors hoped that this type of technique could eventually help to treat people with cardiovascular and vascular diseases.

      Brain disease treatment

      Doctors may one day be able to use replacement cells and tissues to treat brain diseases, such as Parkinson’s and Alzheimer’s.

      In Parkinson’s, for example, damage to brain cells leads to uncontrolled muscle movements. Scientists could use stem cells to replenish the damaged brain tissue. This could bring back the specialized brain cells that stop the uncontrolled muscle movements.

      Researchers have already tried differentiating embryonic stem cells into these types of cells, so treatments are promising.

      Cell deficiency therapy

      Scientists hope one day to be able to develop healthy heart cells in a laboratory that they can transplant into people with heart disease.

      These new cells could repair heart damage by repopulating the heart with healthy tissue.

      Similarly, people with type I diabetes could receive pancreatic cells to replace the insulin-producing cells that their own immune systems have lost or destroyed.

      The only current therapy is a pancreatic transplant, and very few pancreases are available for transplant.

      Blood disease treatments

      Doctors now routinely use adult hematopoietic stem cells to treat diseases, such as leukemia, sickle cell anemia, and other immunodeficiency problems.

      Hematopoietic stem cells occur in blood and bone marrow and can produce all blood cell types, including red blood cells that carry oxygen and white blood cells that fight disease.

      People can donate stem cells to help a loved one, or possibly for their own use in the future.

      Donations can come from the following sources:

      Bone marrow: These cells are taken under a general anesthetic, usually from the hip or pelvic bone. Technicians then isolate the stem cells from the bone marrow for storage or donation.

      Peripheral stem cells: A person receives several injections that cause their bone marrow to release stem cells into the blood. Next, blood is removed from the body, a machine separates out the stem cells, and doctors return the blood to the body.

      Umbilical cord blood: Stem cells can be harvested from the umbilical cord after delivery, with no harm to the baby. Some people donate the cord blood, and others store it.

      This harvesting of stem cells can be expensive, but the advantages for future needs include:


      122 Classifications of Fungi

      By the end of this section, you will be able to do the following:

      • Identify fungi and place them into the five major phyla according to current classification
      • Describe each phylum in terms of major representative species and patterns of reproduction

      The kingdom Fungi contains five major phyla that were established according to their mode of sexual reproduction or using molecular data. Polyphyletic, unrelated fungi that reproduce without a sexual cycle, were once placed for convenience in a sixth group, the Deuteromycota, called a “form phylum,” because superficially they appeared to be similar. However, most mycologists have discontinued this practice. Rapid advances in molecular biology and the sequencing of 18S rRNA (ribosomal RNA) continue to show new and different relationships among the various categories of fungi.

      The five true phyla of fungi are the Chytridiomycota (Chytrids), the Zygomycota (conjugated fungi), the Ascomycota (sac fungi), the Basidiomycota (club fungi) and the recently described Phylum Glomeromycota ((Figure)).


      Chytridiomycota: The Chytrids

      The only class in the Phylum Chytridiomycota is the Chytridiomycetes . The chytrids are the simplest and most primitive Eumycota, or true fungi. The evolutionary record shows that the first recognizable chytrids appeared during the late pre-Cambrian period, more than 500 million years ago. Like all fungi, chytrids have chitin in their cell walls, but one group of chytrids has both cellulose and chitin in the cell wall. Most chytrids are unicellular however, a few form multicellular organisms and hyphae, which have no septa between cells (coenocytic). The Chytrids are the only fungi that have retained flagella. They produce both gametes and diploid zoospores that swim with the help of a single flagellum. An unusual feature of the chytrids is that both male and female gametes are flagellated.

      The ecological habitat and cell structure of chytrids have much in common with protists. Chytrids usually live in aquatic environments, although some species live on land. Some species thrive as parasites on plants, insects, or amphibians ((Figure)), while others are saprobes. The chytrid species Allomyces is well characterized as an experimental organism. Its reproductive cycle includes both asexual and sexual phases. Allomyces produces diploid or haploid flagellated zoospores in a sporangium.


      Zygomycota: The Conjugated Fungi

      The zygomycetes are a relatively small group of fungi belonging to the Phylum Zygomycota . They include the familiar bread mold, Rhizopus stolonifer, which rapidly propagates on the surfaces of breads, fruits, and vegetables. Most species are saprobes, living off decaying organic material a few are parasites, particularly of insects. Zygomycetes play a considerable commercial role. For example, the metabolic products of some species of Rhizopus are intermediates in the synthesis of semi-synthetic steroid hormones.

      Zygomycetes have a thallus of coenocytic hyphae in which the nuclei are haploid when the organism is in the vegetative stage. The fungi usually reproduce asexually by producing sporangiospores ((Figure)). The black tips of bread mold are the swollen sporangia packed with black spores ((Figure)). When spores land on a suitable substrate, they germinate and produce a new mycelium. Sexual reproduction starts when environmental conditions become unfavorable. Two opposing mating strains (type + and type –) must be in close proximity for gametangia from the hyphae to be produced and fuse, leading to karyogamy. Each zygospore can contain several diploid nuclei. The developing diploid zygospores have thick coats that protect them from desiccation and other hazards. They may remain dormant until environmental conditions are favorable. When the zygospore germinates, it undergoes meiosis and produces haploid spores, which will, in turn, grow into a new organism. This form of sexual reproduction in fungi is called conjugation (although it differs markedly from conjugation in bacteria and protists), giving rise to the name “conjugated fungi”.



      Ascomycota: The Sac Fungi

      The majority of known fungi belong to the Phylum Ascomycota , which is characterized by the formation of an ascus (plural, asci), a sac-like structure that contains haploid ascospores. Filamentous ascomycetes produce hyphae divided by perforated septa, allowing streaming of cytoplasm from one cell to another. Conidia and asci, which are used respectively for asexual and sexual reproduction, are usually separated from the vegetative hyphae by blocked (non-perforated) septa. Many ascomycetes are of commercial importance. Some play a beneficial role for humanity, such as the yeasts used in baking, brewing, and wine fermentation, and directly as food delicacies such as truffles and morels. Aspergillus oryzae is used in the fermentation of rice to produce sake. Other ascomycetes parasitize plants and animals, including humans. For example, fungal pneumonia poses a significant threat to AIDS patients who have a compromised immune system. Ascomycetes not only infest and destroy crops directly they also produce poisonous secondary metabolites that make crops unfit for consumption.

      Asexual reproduction is frequent and involves the production of conidiophores that release haploid conidiospores ((Figure)). Sexual reproduction starts with the development of special hyphae from either one of two types of mating strains ((Figure)). The “male” strain produces an antheridium and the “female” strain develops an ascogonium. At fertilization, the antheridium and the ascogonium combine in plasmogamy, without nuclear fusion. Special dikaryotic ascogenous (ascus-producing) hyphae arise from this dikaryon , in which each cell has pairs of nuclei: one from the “male” strain and one from the “female” strain. In each ascus, two haploid nuclei fuse in karyogamy. Thousands of asci fill a fruiting body called the ascocarp . The diploid nucleus in each ascus gives rise to haploid nuclei by meiosis, and spore walls form around each nucleus. The spores in each ascus contain the meiotic products of a single diploid nucleus. The ascospores are then released, germinate, and form hyphae that are disseminated in the environment and start new mycelia ((Figure)).


      Which of the following statements is true?

      1. A dikaryotic ascus that forms in the ascocarp undergoes karyogamy, meiosis, and mitosis to form eight ascospores.
      2. A diploid ascus that forms in the ascocarp undergoes karyogamy, meiosis, and mitosis to form eight ascospores.
      3. A haploid zygote that forms in the ascocarp undergoes karyogamy, meiosis, and mitosis to form eight ascospores.
      4. A dikaryotic ascus that forms in the ascocarp undergoes plasmogamy, meiosis, and mitosis to form eight ascospores.


      Basidiomycota: The Club Fungi

      The fungi in the Phylum Basidiomycota are easily recognizable under a light microscope by their club-shaped fruiting bodies called basidia (singular, basidium ), which are the swollen terminal cells of hyphae. The basidia, which are the reproductive organs of these fungi, are often contained within the familiar mushroom, commonly seen in fields after rain, on the supermarket shelves, and growing on your lawn ((Figure)). These mushroom-producing basidiomycetes are sometimes referred to as “gill fungi” because of the presence of gill-like structures on the underside of the cap. The gills are actually compacted hyphae on which the basidia are borne. This group also includes shelf fungi, which cling to the bark of trees like small shelves. In addition, the basidiomycota include smuts and rusts, which are important plant pathogens. Most edible fungi belong to the Phylum Basidiomycota however, some basidiomycota are inedible and produce deadly toxins. For example, Cryptococcus neoformans causes severe respiratory illness. The infamous death cap mushroom (Amanita phalloides) is related to the fly agaric seen at the beginning of the previous section.


      The lifecycle of basidiomycetes includes alternation of generations ((Figure)). Most fungi are haploid through most of their life cycles, but the basidiomycetes produce both haploid and dikaryotic mycelia, with the dikaryotic phase being dominant. (Note: The dikaryotic phase is technically not diploid, since the nuclei remain unfused until shortly before spore production.) In the basidiomycetes, sexual spores are more common than asexual spores. The sexual spores form in the club-shaped basidium and are called basidiospores. In the basidium, nuclei of two different mating strains fuse (karyogamy), giving rise to a diploid zygote that then undergoes meiosis. The haploid nuclei migrate into four different chambers appended to the basidium, and then become basidiospores.

      Each basidiospore germinates and generates monokaryotic haploid hyphae. The mycelium that results is called a primary mycelium. Mycelia of different mating strains can combine and produce a secondary mycelium that contains haploid nuclei of two different mating strains. This is the dominant dikaryotic stage of the basidiomycete life cycle. Thus, each cell in this mycelium has two haploid nuclei, which will not fuse until formation of the basidium. Eventually, the secondary mycelium generates a basidiocarp , a fruiting body that protrudes from the ground—this is what we think of as a mushroom. The basidiocarp bears the developing basidia on the gills under its cap.


      Which of the following statements is true?

      1. A basidium is the fruiting body of a mushroom-producing fungus, and it forms four basidiocarps.
      2. The result of the plasmogamy step is four basidiospores.
      3. Karyogamy results directly in the formation of mycelia.
      4. A basidiocarp is the fruiting body of a mushroom-producing fungus.

      Asexual Ascomycota and Basidiomycota

      Imperfect fungi —those that do not display a sexual phase—were formerly classified in the form phylum Deuteromycota , an invalid taxon no longer used in the present, ever-developing classification of organisms. While Deuteromycota was once a classification taxon, recent molecular analysis has shown that some of the members classified in this group belong to the Ascomycota ((Figure)) or the Basidiomycota. Because some members of this group have not yet been appropriately classified, they are less well described in comparison to members of other fungal taxa. Most imperfect fungi live on land, with a few aquatic exceptions. They form visible mycelia with a fuzzy appearance and are commonly known as mold .


      The fungi in this group have a large impact on everyday human life. The food industry relies on them for ripening some cheeses. The blue veins in Roquefort cheese and the white crust on Camembert are the result of fungal growth. The antibiotic penicillin was originally discovered on an overgrown Petri plate, on which a colony of Penicillium fungi had killed the bacterial growth surrounding it. Other fungi in this group cause serious diseases, either directly as parasites (which infect both plants and humans), or as producers of potent toxic compounds, as seen in the aflatoxins released by fungi of the genus Aspergillus.

      Glomeromycota

      The Glomeromycota is a newly established phylum that comprises about 230 species, all of which are involved in close associations with the roots of trees. Fossil records indicate that trees and their root symbionts share a long evolutionary history. It appears that nearly all members of this family form arbuscular mycorrhizae : the hyphae interact with the root cells forming a mutually beneficial association in which the plants supply the carbon source and energy in the form of carbohydrates to the fungus, and the fungus supplies essential minerals from the soil to the plant. The exception is Geosiphon pyriformis, which hosts the cyanobacterium Nostoc as an endosymbiont.

      The glomeromycetes do not reproduce sexually and do not survive without the presence of plant roots. Although they have coenocytic hyphae like the zygomycetes, they do not form zygospores. DNA analysis shows that all glomeromycetes probably descended from a common ancestor, making them a monophyletic lineage.

      Section Summary

      Chytridiomycota (chytrids) are considered the most ancestral group of fungi. They are mostly aquatic, and their gametes are the only fungal cells known to have flagella. They reproduce both sexually and asexually the asexual spores are called zoospores. Zygomycota (conjugated fungi) produce non-septate hyphae with many nuclei. Their hyphae fuse during sexual reproduction to produce a zygospore in a zygosporangium. Ascomycota (sac fungi) form spores in sacs called asci during sexual reproduction. Asexual reproduction is their most common form of reproduction. In the Basidiomycota (club fungi), the sexual phase predominates, producing showy fruiting bodies that contain club-shaped basidia, within which spores form. Most familiar mushrooms belong to this division. Fungi that have no known sexual cycle were originally classified in the “form phylum” Deuteromycota, but many have been classified by comparative molecular analysis with the Ascomycota and Basidiomycota. Glomeromycota form tight associations (called mycorrhizae) with the roots of plants.

      Visual Connection Questions

      (Figure) Which of the following statements is true?

      1. A dikaryotic ascus that forms in the ascocarp undergoes karyogamy, meiosis, and mitosis to form eight ascospores.
      2. A diploid ascus that forms in the ascocarp undergoes karyogamy, meiosis, and mitosis to form eight ascospores.
      3. A haploid zygote that forms in the ascocarp undergoes karyogamy, meiosis, and mitosis to form eight ascospores.
      4. A dikaryotic ascus that forms in the ascocarp undergoes plasmogamy, meiosis, and mitosis to form eight ascospores.

      (Figure) Which of the following statements is true?

      1. A basidium is the fruiting body of a mushroom-producing fungus, and it forms four basidiocarps.
      2. The result of the plasmogamy step is four basidiospores.
      3. Karyogamy results directly in the formation of mycelia.
      4. A basidiocarp is the fruiting body of a mushroom-producing fungus.

      Review Questions

      The most primitive phylum of fungi is the ________.

      Members of which phylum produce a club-shaped structure that contains spores?

      Members of which phylum establish a successful symbiotic relationship with the roots of trees?

      The fungi that do not reproduce sexually used to be classified as ________.

      A scientist discovers a new species of fungus that introduces genetic diversity during reproduction by creating a diploid zygote. This new species cannot belong to which modern phylum of fungi?

      Critical Thinking Questions

      What is the advantage for a basidiomycete to produce a showy and fleshy fruiting body?

      By ingesting spores and disseminating them in the environment as waste, animals act as agents of dispersal. The benefit to the fungus outweighs the cost of producing fleshy fruiting bodies.

      For each of the four groups of perfect fungi (Chytridiomycota, Zygomycota,
      Ascomycota, and Basidiomycota), compare the body structure and features, and provide an example.

      Chytridiomycota (Chytrids) may have a unicellular or multicellular body structure some are aquatic with motile spores with flagella an example is the Allomyces. Zygomycota (conjugated fungi) have a multicellular body structure features include zygospores and presence in soil examples are bread and fruit molds. Ascomycota (sac fungi) may have unicellular or multicellular body structure a feature is sexual spores in sacs (asci) examples include the yeasts used in bread, wine, and beer production. Basidiomycota (club fungi) have multicellular bodies features includes sexual spores in the basidiocarp (mushroom) and that they are mostly decomposers mushroom-producing fungi are an example.

      Glossary


      Importance

      Protists are responsible for a variety of human diseases including malaria, sleeping sickness, amoebic dysentery and trichomoniasis. Malaria in humans is a devastating disease. It is caused by five species of the parasite Plasmodium, which are transmitted to humans by female Anopheles mosquitoes, according to the Centers for Disease Control and Prevention (CDC). The species Plasmodium falciparum infects red blood cells, multiplies rapidly and destroys them. Infection can also cause red blood cells to stick to the walls of small blood vessels. This creates a potentially fatal complication called cerebral malaria (according to the CDC). The World Health Organization (WHO) states that Plasmodium falciparum is the most prevalent and lethal to humans. According to their recent malaria fact sheet, in 2015 there were an estimated 438,000 deaths due to malaria in the world, the majority of which (90 percent) occurred in Africa. Certain strides have been made in reducing the rates of incidence (occurrence of new cases) and mortality rates in part by supplying insecticide treated mosquito nets, spraying for mosquitoes and improving diagnostics. Between 2000 and 2015 the rate of incidence fell by 37 percent globally and mortality rates fell by 60 percent globally. The WHO has a goal of eliminating malaria in at least 35 countries by 2030.

      Protists also play an important role in the environment. According to a 2009 review article published on the Encyclopedia of Life Sciences (eLS) website, nearly 50 percent of photosynthesis on Earth is carried out by algae. Protists act as decomposers and help in recycling nutrients through ecosystems, according to a 2002 review article published in the journal ACTA Protozoologica. In addition, protists in various aquatic environments, including the open water, waterworks and sewage disposal systems feed upon, and control bacterial populations (ACTA Protozoologica, 2002). "If you took all the protists out of the world, the ecosystem would collapse really quickly," Simpson said.


      Watch the video: Why Are You Multicellular? (September 2022).


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