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Questions and Answers
What is the characteristic of the Gonidia cells compared to the vegetative cells?
What is the purpose of the Gonidia cells in Volvox?
What is the term for the process of a somatic cell losing its motility organs and enlarging to form a gonidium?
What is the term for the hollow sphere structure formed during autocoenobium formation?
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What is the type of sexual reproduction exhibited by Volvox?
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What occurs after the formation of zygotes in Volvox?
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What is the fate of three of the resulting nuclei after meiosis and mitosis in Volvox?
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What is the final outcome of the remaining nucleus after meiosis and mitosis in Volvox?
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What is the structure of the chloroplasts in green algae?
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What is the function of pyrenoids in chloroplasts?
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What is the characteristic of the cell wall of Chlamydomonas?
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What is the shape of the chloroplast in Chlamydomonas?
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How many flagella does a motile form of Chlamydomonas have?
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What is the characteristic of the cells in a Pandorina colony?
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What is the number of cells in a Pandorina colony?
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What is the characteristic of Volvox cells?
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What surrounds the Volvox colony?
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How do the cells in a Volvox colony communicate?
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Study Notes
Volvox
- The greater part of the coenobium is vegetative (somatic)
- Only a few cells located on the posterior side of the coenobium can give rise to a new colony or reproductive organs
- These cells are termed Gonidia and are characterized by their large size (ten times more than vegetative cells or even more)
Reproduction in Volvox
- Asexual reproduction: Gonidium is responsible for the formation of autocoenobium
- Autocoenobia formation is the mode of asexual reproduction in Volvox
- Sexual reproduction: is Oogamous, taking place by the formation of antheridia and oogonia in the posterior half of the colony
Characteristics of Volvox
- Volvox is the most advanced colony in this group because it exhibits a considerable degree of complexity
- Number of cells ranges from 1000 to 50 000 cells
- All cells of the colony are arranged in a single layer within the periphery of the colonial matrix
- The colony is surrounded by a marked mucilaginous lamella
- The colony moves and rotates slowly by the flagella and behaves like a single individual
Division: Chlorophyta
- Chlorophyta is considered the largest algal division
- It comprises a great variety of species which vary in form, size, habitat, reproduction, and life cycle
- Pigments of these algae are much similar to those of higher plants
- They are composed of chlorophyll a, and b, and several carotenes and xanthophylls
- Pigments are carried in well-organized chloroplasts that resemble those of higher plants
Characteristics of Chlorophyta
- Most chloroplasts have one or more pyrenoids
- Motile forms contain light-sensitive structures, the stigma
- Motile reproductive units of non-motile green algae contain stigma as well
- Chloroplasts have many shapes, including cup-shaped, parietal, spiral, reticulate, and ribbon-like
- Cells of green algae are surrounded by a firm wall composed mainly of polysaccharides (usually cellulose) and pectins
Classification of Chlorophyta
- This division comprises only one class; Chlorophyceae, which is divided into 15 orders
- The basis of classification includes:
- Form of the thallus (unicellular, coenobic, filamentous, foliose, or siphonous)
- Motility of both vegetative and reproductive cells
- Number of nuclei in the cell (uninucleate or multinucleate)
Order I: Volvocales
- This order includes two families: Chlamydomonaceae and Volvocaceae
- Family 1: Chlamydomonaceae, which includes unicellular forms
- Family 2: Volvocaceae, which includes colonial forms with common gelatinous matrices
Chlamydomonas
- Chlamydomonas is one of the largest algal genera (more than three hundred species)
- It occurs in both freshwater and saline water
- The cell wall is chemically composed of glycoprotein
- The cell contains a single chloroplast, which is mostly cup-shaped, but other shapes are known
- It contains one or more pyrenoids and a stigma, which is embedded in it
- The cell is provided with a pair of contractile vacuoles
- The nucleus is embedded in the cavity of the chloroplast and is connected with the peripheral cytoplasm with cytoplasmic strands
Asexual Reproduction in Chlamydomonas
- It takes place by zoospores, which is common and achieved when the cell becomes immotile, rests, and undergoes one or more mitotic divisions
- The mother cell wall ruptures, and the cells acquire two flagella and swim again, repeating an asexual life cycle
- Under certain conditions, these cells fail to release from mother cells and form a mucilaginous material, known as the palmella stage
Sexual Reproduction in Chlamydomonas
- It takes place by Isogamy, anisogamy, and Oogamy
- Isogamy: it takes place between two gametes, morphologically similar but physiologically different
- Anisogamy: it takes place between two gametes which are morphologically and physiologically different
- Oogamy: it occurs when a cell becomes immotile and rests, acting as an ogonium, and a motile gamete adheres to it, and fusion takes place to yield a zygote
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Description
Learn about the reproductive biology of Volvox, a type of green algae, including its unique characteristics and reproductive processes.
