Organization of Algal Thalli

Algal thalli exhibit a diverse range of organizational structures, varying from unicellular microscopic forms to large macroalgae like giant kelp, exceeding one hundred feet in length.

Chlaymdomonas

1. Unicellular Thalli

   • Simple motile unicell: Chlamydomonas
   • Non-motile unicell: Chlorella

2. Colonial Organization

   • Cells are grouped into colonies.
     • Motile colonies: Volvox
     • Non-motile colonies: hydrodictyon
     • Definite-shaped colony: Coenobium (e.g., Volvox)

3. Filamentous Thalli

   • Unbranched filaments: Ulothrix
   • Simple branched filaments: Cladophora
   • Complex filaments: Ectocarpus, Polysiphonia, Sargassum, and Laminaria

4. Parallelism Among Different Divisions

   • No single thallus type is restricted to a specific division.
   • Striking parallelism is observed among different algal divisions.

Types of Cell Structures in Algae

A. Prokaryotic Cell Organization (Class Cyanophyceae/Mixophyceae)

• Presence of a primitive or incipient nucleus
• Lack of nuclear membranes and histones
• DNA is organized into fibrils.
• Chlorophyll in photosynthetic lamellae or thylakoids
• Absence of chloroplast, mitochondria, Golgi body, and endoplasmic reticulum

1. Characteristic Features of Prokaryotic Algae
   • Simple cells lacking the nuclear membrane, mitochondria, and plastids
   • Reproduction without mitosis
   • Cell wall composition: mucopeptide (specific to Cyanophyceae)

B. Eukaryotic Cell Organization

• Presence of a well-organized nucleus
• Membrane-bounded organelles: plastids, mitochondria, and Golgi bodies

1. Membrane-bound Organelles in Eukaryotes
   • Nucleus
   • Plastids
   • Mitochondria
   • Golgi Bodies

Examples of Eukaryotic Algae

• Ectocarpus
• Polysiphonia
• Sargassum
• Laminaria

Flagella in Algae

Flagella, singularly known as flagellum, are thread-like structures crucial for cell movement and are present in almost all classes of algae, excluding Rhodophyceae and Cyanophyceae.

1. Flagellar Structure

   • Each flagellum consists of:
     • 2 central fibrils
     • 9 peripheral double fibrils (9+2 arrangements)

2. Types of flagellaA.Whiplash or Acronematic Flagella:Flagella with a smooth surface

   B. Tinsel Flagella or Pleuromematic Flagella: flagella with a surface covered with fine hair-like appendages called mastigonemes.

   Mastigonemes arrangement types:

   (i) Pantonematic: Two opposite rows of mastigonemes;

   (ii) Pantocronematic: Terminal fibril;

   (iii) Stichonematic: Mastigonemes develop only on one side of the flagellum.

3. Isoknot and Heteroknot

• If the flagella of a cell are similar, it is known as isoknot.
• If the flagella are dissimilar, it is called heterooknot.

1. Characteristic Features of Flagella

   • Size, number, and arrangement are specific to the class of algae or genera.

2. Flagella in Different Algal Classes

   • Chlorophyceae:
     • Motile stages possess two or four anteriorly inserted whiplash flagella of equal length.
   • Phaeophyceae and Xanthophyceae:
     • Have one whiplash and one tinsel flagellum of unequal length.

Axonemes, Basal Bodies, and Their Structure

1. Axoneme:

   • The axoneme is a structural component found in the core of eukaryotic flagella and cilia.
   • It is responsible for the beating and movement of flagella or cilia.

   Structure of Axoneme:

   
   
   

   • Microtubule Arrangement:

     • The axoneme is primarily composed of microtubules, which are tubular structures made of protein.
     • Microtubules are arranged in a characteristic pattern, usually in a "9+2" arrangement.
     • This arrangement consists of a central pair of microtubules surrounded by nine doublets.

   • Dynein Arms:

     • Dynein arms are molecular motor proteins attached to the microtubules.
     • They generate force during the sliding movement between adjacent microtubules, causing the bending and flexing of the axoneme.

2. Basal Bodies:

   • Basal bodies are cylindrical structures located at the base of cilia and flagella.
   • They serve as the anchoring point and organizing center for the formation of axonemes.

   Structure of Basal Bodies:

   • Microtubule Arrangement:

     • Similar to axonemes, basal bodies also have a microtubule arrangement, often in a "9+0" pattern.
     • The central pair found in axonemes is absent in basal bodies, giving them a "9+0" structure.

   • Transition Zone:

     • The region where the microtubules transition from the basal body to the axoneme is known as the transition zone.

   • Role in Flagellar/Ciliary Function:

     • Basal bodies play a crucial role in the initiation and organization of axonemal microtubules.
     • They anchor and organize the microtubules, providing structural support for the growth and movement of flagella or cilia.

3. Composition:

• Both axonemes and basal bodies are composed of protein structures, with microtubules being a fundamental component.
• Other associated proteins, such as dynein arms, contribute to the mechanical movement of the axoneme.

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