Algae: Characteristics and Examples NEET Notes | EduRev

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

Fig: Kingdom- Plantae

FOLLOWING PLANT GROUPS ARE INCLUDED IN KINGDOM-PLANTAE:
1. Algae
2. Bryophyta
3. Pteridophyta
4. Gymnosperm
5. Angiosperm

ALGAE:
(a) Algae are chlorophyll-bearing, simple, thalloid, autotrophic and largely aquatic plants.
(b) Term "Algae" was given by Linnaeus
(c) Phycology- Study of algae
(d) Father of Phycology- Fritsch → Book → "Structure & Reproduction of algae" 
(e) Father of Indian phycology- Mandayam Osuri Parthasarathy Iyengar 

Fig: Algae

Nature:
1. Algae are found in both fresh and marine water.
2. Algae are found in many forms like filamentous, colonial.
3. Algae are surrounded by mucilaginous sheath and below the sheath, cell wall is present which is made up of cellulose and pectin but mainly made up of cellulose, galactans, mannans and mineral like calcium carbonate.
4. On the basis of structure, algae are thalloid i.e. plant body is not differentiated into root, stem and leaves. Tissue system is also absent in algae.
5. 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
1. Vegetative 
2. Asexual 
3. Sexual Vegetative Reproduction
1. Vegetative reproduction is by fragmentation, where each fragment develops into a thallus.
2. Asexual reproduction is by the production of flagellated zoospores which on germination give rise to new plants.
3. Sexual Reproduction
(a) Male sex organ is called antheridium and 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 stone wort) are multi-cellular and jacketed.
The male sex organ of Chara is known as globule and female is known as nucule.
(b) 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.
(c) Algae reproduce by zygotic meiosis i.e. first division in zygote is meiosis so embryo is not formed.
Sexual reproduction is of three types:
(a) Isogamous: Chlamydomonas debaryanum, Ulothrix, Ectocarpus 
(b) Anisogamous: Chlamydomonas braunii 
(c) Oogamous: Chlamydomonas coccifera, Sargassum, Volvox, Fucus 
Note:
1. Chlamydomanas exhibits complete evolution of sexual reproduction.
Isogamous- Simplest, Oogamous- Advanced

2. 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
1. Chlorophyta: Green Algae
2. Phaeophyta: Brown Algae

Fig: Brown Algae

3. Xanthophyta: Yellow- Green Algae

Fig: Yellow Algae

4. Rhodophyta: Red Algae

Fig: Red Algae

(1) Chlorophyta

Fig: Chlorophyta

Green Algae:
(a) Green algae are the most advanced algae. It is believed that green algae are the ancestors of the higher plants.
(b) Habitat: Green algae are cosmopolitan in nature.

Different forms of Green algae (Structure): Green algae are found in many forms.
1. 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. 

Fig: Unicellular Organism

(a) Chlamydomonas: Motile unicellular algae. This algae moves with the help of flagella.

(b) Chlorella: Non-motile unicellular alga.

• Calvin discovered "Calvin Cycle" by experimenting on Chlorella.

Fig: Chlorella Unicellular Alga

(c) Acetabularia: Umbrella plant - It is the largest unicellular plant. The diameter of its cell is 10 cm.

• Hammerling experimented on Acetabularia.
  

Fig: Acetabularia umbrella Plant

2. 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.

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

4. 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).

5. Multicellular thalloid or Parenchymatous: Some algae are multi-cellular in length & width. 
Example: Ulva- Also called as sea lettuce.
Photosynthetic pigments:
Chlorophyll- Chl 'a' and Chl 'b'
Carotene- b carotene
Xanthophyll- Luteaxanthin and Violoxanthin- Yellow coloured

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