Class 11 Biology: A Comprehensive Guide to Algae (With NCERT Notes & Mind Maps)

  

Algae



Introduction to Algae

  • Characteristics:
    • Chlorophyll-bearing, simple, thalloid, and autotrophic organisms.
    • Largely aquatic, found in freshwater and marine environments.
    • Can also inhabit moist stones, soils, wood, and live in symbiotic relationships (e.g., lichens with fungi, on sloth bears).

Forms and Sizes of Algae

  • Variable Forms:
    • Colonial forms like Volvox.
    • Filamentous forms like Ulothrix and Spirogyra.
    • Massive marine forms like kelps.

Reproduction in Algae

  1. Vegetative Reproduction:

    • By fragmentation.
    • Each fragment develops into a new thallus.
  2. Asexual Reproduction:

    • Through spores, primarily zoospores (flagellated and motile).
  3. Sexual Reproduction:

    • Isogamous: Fusion of similar-sized gametes (e.g., Ulothrix).
    • Anisogamous: Fusion of dissimilar-sized gametes (e.g., species of Eudorina).
    • Oogamous: Fusion of a large non-motile female gamete and a smaller motile male gamete (e.g., Volvox, Fucus).

Importance of Algae

  • Ecological Role:

    • Major contributors to carbon dioxide fixation through photosynthesis.
    • Increase dissolved oxygen levels in their environment.
    • Primary producers in aquatic food cycles.
  • Economic Importance:

    • Edible species like Porphyra, Laminaria, and Sargassum.
    • Production of hydrocolloids such as algin (from brown algae) and carrageen (from red algae).
    • Agar from Gelidium and Gracilaria used in microbial culture, ice-cream, and jelly production.
    • Chlorella as a protein-rich food supplement, even for space travelers.

Classes of Algae

  1. Chlorophyceae (Green Algae):

    • Characteristics:

      • Unicellular, colonial, or filamentous.
      • Green due to chlorophyll a and b.
      • Chloroplasts can be discoid, plate-like, reticulate, cup-shaped, spiral, or ribbon-shaped.
      • Storage bodies (pyrenoids) contain protein and starch.
      • Cell wall composed of cellulose and pectose.
    • Reproduction:

      • Vegetative by fragmentation.
      • Asexual by flagellated zoospores.
      • Sexual can be isogamous, anisogamous, or oogamous.
    • Examples: Chlamydomonas, Volvox, Ulothrix, Spirogyra, Chara.

  2. Phaeophyceae (Brown Algae):

    • Characteristics:

      • Mostly marine.
      • Size ranges from simple filamentous forms to large kelps.
      • Contain chlorophyll a, c, carotenoids, and fucoxanthin (xanthophyll pigment).
      • Store food as laminarin or mannitol.
      • Cell wall has cellulose and a gelatinous algin coating.
      • Structure includes holdfast, stipe, and frond.
    • Reproduction:

      • Vegetative by fragmentation.
      • Asexual by biflagellate zoospores.
      • Sexual can be isogamous, anisogamous, or oogamous with pyriform gametes.
    • Examples: Ectocarpus, Dictyota, Laminaria, Sargassum, Fucus.

  3. Rhodophyceae (Red Algae):

    • Characteristics:

      • Predominantly marine, especially in warmer regions.
      • Red due to r-phycoerythrin pigment.
      • Can live in well-lit regions or deep ocean areas.
      • Multicellular with complex body structures.
      • Store food as floridean starch (similar to amylopectin and glycogen).
    • Reproduction:

      • Vegetative by fragmentation.
      • Asexual by non-motile spores.
      • Sexual reproduction by non-motile gametes.
    • Examples: Porphyra, Polysiphonia, Gelidium, Gracilaria.

Visual Summary (Mind Map)

Algae

  • Characteristics

    • Chlorophyll-bearing
    • Thalloid
    • Autotrophic
    • Aquatic
  • Forms and Sizes

    • Colonial: Volvox
    • Filamentous: Ulothrix, Spirogyra
    • Massive: Kelps
  • Reproduction

    • Vegetative: Fragmentation
    • Asexual: Zoospores
    • Sexual:
      • Isogamous: Ulothrix
      • Anisogamous: Eudorina
      • Oogamous: Volvox, Fucus
  • Importance

    • Ecological: CO2 fixation, Oxygen production, Primary producers
    • Economic: Food, Hydrocolloids (algin, carrageen), Agar, Chlorella
  • Classes

    • Chlorophyceae (Green Algae)
      • Characteristics: Chlorophyll a, b, pyrenoids, cellulose cell wall
      • Examples: Chlamydomonas, Volvox, Ulothrix, Spirogyra, Chara
    • Phaeophyceae (Brown Algae)
      • Characteristics: Chlorophyll a, c, fucoxanthin, laminarin/mannitol storage
      • Examples: Ectocarpus, Dictyota, Laminaria, Sargassum, Fucus
    • Rhodophyceae (Red Algae)
      • Characteristics: r-phycoerythrin, floridean starch
      • Examples: Porphyra, Polysiphonia, Gelidium, Gracilaria

 Also Read:

  Bryophytes - NOTES


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