Algae: Characteristics and Examples NEET Notes | EduRev

Plant Kingdom

All the multicellular eukaryotic plants are placed in Kingdom Plantae. They are autotrophic, i.e. they manufacture their food by photosynthesis.

Kingdom-Plantae

Following plant, groups are included in Kingdom-Plantae:

• Algae
• Bryophyta
• Pteridophyta
• Gymnosperm
• Angiosperm 

• Algae are chlorophyll-bearing, simple, thalloid, autotrophic, and largely aquatic plants.
• The term "Algae" was given by Linnaeus.
• Phycology- Study of algae
• Father of Phycology- Fritsch → Book → "Structure & Reproduction of algae" 
• Father of Indian phycology- Mandayam Osuri Parthasarathy Iyengar.

Algae

➢ Nature

• Algae are found in both fresh and marine water.
• Algae are found in many forms like filamentous, colonial.
• Algae are surrounded by a mucilaginous sheath, and below the sheath, the cell wall is present which is made up of cellulose and pectin but mainly made up of cellulose, Galatians, mannans, and mineral-like calcium carbonate.
• On the basis of structure, algae are thalloid, i.e. plant body is not differentiated into root, stem, and leaves. The tissue system is also absent in algae.
• On the basis of nutrition, algae are photoautotrophic. They have chloroplast in which photosynthetic pigments are present. Classification of algae is mainly based on pigments. Chl-a and b carotene are universal pigments of algae.

Reproduction

➢ Vegetative Reproduction

• By fragmentation, where each fragment develops into a thallus.

➢ Asexual Reproduction

• By the production of flagellated zoospores which on germination give rise to new plants.

➢ Sexual Reproduction

• The male sex organ is called antheridium, and the female is called oogonium. The sex organs of algae are unicellular & jacketless. But exceptionally sex organs of green algae Chara (Chara- green algae- known as stonewort) are multi-cellular and jacketed.
• The male sex organ of Chara is known as globule, and the female is known as nucule.
• The plant body of algae is haploid, so sexual reproduction takes place through zygotic meiosis. So their life cycle is haplontic. But exceptionally brown algae are diploid so that sexual reproduction takes place through gametic meiosis in them. So their life cycle is Diplo haplontic.
• Algae reproduce by zygotic meiosis, i.e. first division in the zygote is meiosis so, the embryo is not formed.

Sexual reproduction is of three types:

• Isogamous: Chlamydomonas Debaryanum, Ulothrix, Ectocarpus 
• Anisogamous: Chlamydomonas Braunii 
• Oogamous: Chlamydomonas Coccifera, Sargassum, Volvox, Fucus 

Note

• Chlamydomonas exhibits complete evolution of sexual reproduction.
  Isogamous- Simplest, Oogamous- Advanced.
• Chlamydomonas
• Ulothrix exhibits the origin of sexual reproduction. The classification of algae is mainly based on the photosynthetic pigments. In addition to this, cell wall composition and stored food are also the basis of classification.

Algae Is Divided into Following Divisions

• Chlorophyta: Green Algae
• Phaeophyta: Brown Algae

Brown Algae

• Xanthophyta: Yellow-Green Algae

Yellow Algae

• Rhodophyta: Red Algae
  Red Algae

Chlorophyta (Green Algae)

• Green algae are the most advanced algae. It is believed that green algae are the ancestors of the higher plants.
• Habitat: Green algae are cosmopolitan in nature.

Different forms of Green algae (Structure). Green algae are found in many forms:
➢ Unicellular 

• A unicellular organism, also known as a single-celled organism, is an organism that consists of only one cell, unlike a multicellular organism that consists of more than one cell.

Unicellular Organism

• Chlamydomonas: Motile unicellular algae. This alga moves with the help of flagella.
• Chlorella: Non-motile unicellular alga. Calvin discovered the "Calvin Cycle" by experimenting with Chlorella.

Chlorella Unicellular Alga

• Acetabularia: Umbrella plant - It is the largest unicellular plant. The diameter of its cell is 10 cm. Hammerling experimented on Acetabularia.
  Acetabularia Umbrella Plant

➢ Coenocytic: Some green algae are coenocytic, i.e. multi-nucleated. 

• Example: Caulerpa

Note: According to five kingdom system the algae described above should be placed in Protista but exceptionally due to their life cycle, it is similar to green algae. They are placed in Plantae. But now modern scientist place above algae in protista.

Coenocytic

➢ Colonial

• Some green algae are found in colonies. They form a colony of cells. The number of cells in a colony is fixed. A colony with a fixed number of cells is called coenobium.
  Example: Volvox- Motile colony Hydrodictyon- Non-motile colony (called a water net).

➢ Multicellular Filamentous

• Mostly algae are multi-cellular filamentous.
  Example: Ulothrix- Known as pond wool, Spirogyra- Known as pond silk.

Note: 
Some green algae are heterotrichous i.e. two types of branches, prostrate and erect- Fritschiella, Stigeoclonium, Coleochaete (Fritschiella tuberosa has approach to the early land plants).

➢ Multicellular Thalloid or Parenchymatous

• Some algae are multi-cellular in length & width.
  Example: Ulva- also called sea lettuce.

➢ Photosynthetic Pigments

• Chlorophyll- Chl 'a' and Chl 'b'
• Carotene- b carotene
• Xanthophyll- Luteaxanthin and Violoxanthin- Yellow-colored

Note: The green colour of green algae is due to chlorophyll.

• Stored food: In green algae, the food is stored in the form of starch. 

Note: On the basis of pigments (Chl 'a', Chl 'b', Carotenoids), stored food (starch) & cell wall (made up of cellulose or pectin), green algae are considered similar to higher plants.

Economic Importance

• Food: Chlorella is used as food, because after Spirulina, Chlorella has the largest amount of protein.
• Antibiotics: Chlorellin antibiotic is obtained from Chlorella.
• Space research: In space, Chlorella is used as a source of food and O₂.
• Parasitic algae: Cephaleuros algae remains parasitically in the leaves of the tea plants and causes the disease 'red rust'.