Algae: Chlorophyceae (Green Algae) Notes | Study Biology Class 11 - NEET

What are Algae?

Algae exist in environments ranging from oceans, rivers, and lakes to ponds, brackish waters (both fresh & marine) and even snow. Algae are usually green, but they can be found in a variety of different colours.

Definition: “Alga is a term that describes a large and incredibly diverse group of eukaryotic, photosynthetic life forms. These organisms do not share a common ancestor and hence, are not related to each other (polyphyletic).”

Algae

Algae can be Unicellular and Multicellular

• Unicellular algae are made up of only one cell that carries out all of the functions needed by them.
  Example: Chlorella, Chlamydomonas
  Chlorella (left), Chlamydomonas (Right)
• Multicellular algae are made up of more than one cell, with differentiated groups of cells performing specific roles,but they lack the true stems, leaves, or roots of the more complex, higher plants.
  Example: Brown algae and Kelp.
  Brown AlgaeMost algae require a moist or watery environment; hence, they are ubiquitous near or inside water bodies. Anatomically, they are similar to another major group of photosynthetic organisms – the land plants. However, that is where the differences end as algae lack many structural components typically present in plants, such as true stems, shoots, and leaves. Furthermore, they also do not have vascular tissues to circulate essential nutrients and water throughout their body.

Characteristics of Algae

• 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.

Nature of Algae

• Algae are found in both fresh and marine water.
• Algae are found in many forms like unicellular 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, galactans, 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.
  Thallus structure of 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 in Algae

1. Vegetative Reproduction
   By fragmentation, where each fragment develops into a thallus.
   Example: Spirogyra
   Fragmetation in Spirogyra
2. Asexual Reproduction
   By the production of flagellated zoospores which on germination give rise to new plants.
   Example: Chlamydomonas
   1- Mitosis, 2- Meiosis, 3- Nuclear membrane breaks down
3. 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 archegonium or 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: The sexual reproduction in which the fusion of gametes of similar size takes place is called isogamous. 
  Example: Chlamydomonas debaryanum, Ulothrix, Ectocarpus 
  
• Anisogamous: Fusion of two gametes dissimilar in size, as in some species of Chlamydomonas is termed as anisogamous. 
  Example: Chlamydomonas braunii
• Oogamous: The fusion of large immobile female gametes with small motile male gametes is known as oogamy.
  Example: Chlamydomonas coccifera, Sargassum, Volvox, Fucus 

Note

• Chlamydomonas exhibits complete evolution of sexual reproduction.
  i.e. Isogamous- Simplest, Oogamous- Advanced.
• 
  Structure of a vegetative 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.

Types of Algae

1. Chlorophyceae: Green Algae
   
   Green Algae
   • It is a large, informal grouping of algae having the primary photosynthetic pigments chlorophyll a and b, along with auxiliary pigments such as xanthophylls and beta carotene.
   • Higher organisms use green algae to conduct photosynthesis for them. Other species of green algae have a symbiotic relationship with other organisms.
   • Members are unicellular, multicellular, colonial and flagellates. Prominent examples of green algae include Spirogyra, Ulothrix, Volvox, etc.
2. Phaeophyceae: Brown Algae
   
   Brown Algae
   • Brown algae are a group of algae belonging to class Phaeophyceae. They are named due to their colour, which varies from brown to olive green.
   • They are mostly found in marine environments.
   • There are around 1500 species of brown algae, which vary greatly in their size and shape.
   • They are multicellular and the colour depends on the ratio of chlorophyll and the pigment, fucoxanthin.
   • They contain seaweeds, some of the notable examples are Ectocarpus, Fucus, giant kelps, Sargassum, etc.
3. Xanthophyceae: Yellow-Green Algae
   
   Yellow-Green Algae
   • Yellow-green algae or the Xanthophyceae (xanthophytes) are an important group of heterokont algae (two unequal length flagella).
   • Most live in freshwater, but some are found in marine and soil habitats. 
   • They vary from single-celled flagellates to simple colonial and filamentous forms.
4. Rhodophyceae: Red Algae
   
   Red Algae
   • It is a distinctive species found in marine as well as freshwater ecosystems. 
   • The pigments phycocyanin and phycoerythrin are responsible for the characteristic red colouration of the algae.
   • Other pigments that provide green colouration (such as chlorophyll a) are present. However, they lack chlorophyll b or beta-carotene.

Chlorophyceae (Green Algae)

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

Different forms of Green algae (Structure):

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. Types of Unicellular Algae: 
   • Chlamydomonas: Motile unicellular algae. This alga moves with the help of flagella.
     Unicellular Organism
   • Chlorella: Non-motile unicellular algae. 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
2. Coenocytic
   Some green algae are coenocytic, i.e. multi-nucleated. 
   Example: Caulerpa

   Note: According to the 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. A colony with a fixed number of cells is called coenobium.
   Example: Volvox- Motile colony Hydrodictyon- Non-motile colony (called a water net).
   Volvox
4. Multicellular Filamentous
   Most algae are multi-cellular filamentous.
   Example: Ulothrix- Known as pond wool, Spirogyra- Known as pond silk.
   SpirogyraNote: Some green algae are heterotrichous i.e. prostrate branches and erect branches. Erect system of filaments in the plant body develops from prostate system of filaments. Branches in prostrate branches are just above the ground, but branches in erect branches are held upright. Green algae such as Fritschiella, Draparnaldia, and others have a heterotrichous habit.  (Fritschiella tuberosa has an approach to the early land plants).
   Fritschiella
5. Multicellular Thalloid or Parenchymatous
   Some algae are multi-cellular in length & width.
   Example: Ulva- also called sea lettuce.  It is an edible green alga.
   
   
6. Photosynthetic Pigments
   • Chlorophyll- Chl 'a' and Chl 'b'
   • Carotene-  β-carotene
   • Xanthophyll- Luteaxanthin and Violaxanthin- 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.

Question related to Algae has been asked in NEET 2017 & 2020

Q.1. An example of colonial alga is: ( NEET 2017)
(a) Spirogyra
(b) Chlorella
(c) Volvox
(d) Ulothrix

Ans: (c) Volvox

Volvox is a unicellular green alga found in freshwater bodies. It is photosynthetic in nature. It forms colonies of around 50,000 cells which are spherical in shape.

Q.2. Which of the following pairs include unicellular algae? (NEET 2020)

(a) Gelidium & Gracilaria
(b) Anabaena & Volvox
(c) Chlorella & Spirulina
(d) Laminaria & Sargassum
Ans: (c)

• Chlorella and Spirulina are unicellular green algae, which are commercially very important.
• Anabaena is Cyanobacteria, Laminaria, Sargassum, Gelidium, Gracilaria are multicellular algae.
• Volvox is unicellular colonial algae.

So, the correct answer is 'Chlorella and Spirulina'

Economic Importance of Algae

• Food: Chlorella is used as food, because after Spirulina, Chlorella has the largest amount of protein.
• Antibiotics: Chlorellin antibiotic is obtained from Chlorella.
• Space: In space, Chlorella is used as a source of food and O₂.
• Commercial importance: Agar is widely used in the preparation of jelly and ice cream.Agar is derived from Gracilaria and Gelidium. Carrageenan is used in chocolates, paints, and toothpaste as an emulsifier. Its source is red algae.
• Biological indicator: Algae such as Euglena and Chlorella are used to check water pollution.
• Pisciculture: Algae is very important in fish farming since it helps in the production process.

To practice questions on the topic of "Green Algae", you just read, attempt the tests given below:

• Test: Algae - 1
• Test: Algae - 2