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Algae: Alternation of generation, Classification, Chlorophyceae, Practice Problems and FAQs

Algae are autotrophic organisms which contain a variety of pigments. They are one of the major producers in the marine environment. 

Did you know which algae is used as a super food in the International Space Station? 

Yes, it is Chlorella, the unicellular green algae which is rich in proteins. It is also used for the production of oxygen there. Let’s dive deep into the details of green algae. 

algal bioreactor

Table of Contents

Alternation of Generations in Algae

Like all members of the plant kingdom, the life cycle of algae shows alternation of generation. The haploid gametophyte generation that develops from haploid spores gives rise to gametes by mitosis. Then the zygote is formed by the fusion of haploid gametes. The diploid sporophytic generation develops from the diploid zygote. The sporophyte produces haploid spores by meiotic division.

alternation of generations in algae

Depending on the prevalence of the haploid and diploid generations, the life cycle of algae can be of three types as follows: 

  • Haplontic - The haploid gametophyte phase is more dominant here. Most of the red, brown and green algae exhibit a haplontic life cycle.
  • Diplontic - The diploid sporophyte phase is more dominant here. Examples include Fucus.
  • Haplo-diplontic - The haploid and diploid phases are equally prevalent here. Examples include Polysiphonia, Ectocarpus, kelps, etc.

Classification of Algae

Algae is divided into three classes based on the nature of pigments, storage material and complexity of structure. These classes are - Chlorophyceae, Phaeophyceae and Rhodophyceae.


These algae are green in colour due to the predominance of green pigments. They are of different types based on the complexity of the thallus as follows:

Morphological form












Common Characteristics of Green Algae

They have a cell wall with the outer layer made up of pectose and the inner layer made up of cellulose. Their reserve food material is starch but some algae also store food in the form of oil droplets. Photosynthetic pigments include chlorophyll a and chlorophyll b which are present within chloroplasts. Pyrenoids are non-membrane bound organelles in the chloroplasts which store proteins and starch. Chloroplasts can be of various shapes such as: 

green algae

Reproduction in Green Algae

They reproduce by vegetative, asexual and sexual methods. 

  • Vegetative reproduction - It is done by fragmentation. For example, fragmentation in Spirogyra.

    fragmentation in Spirogyra

  • Asexual reproduction - It is done by the production of motile zoospores. For example zoospore formation in Chlamydomonas.

    zoospore formation in chlamydomonas

  • Sexual reproduction - It ocurs by isogamy (fusion of equal sized gametes), anisogamy (fusion of unequal sized gametes) or oogamy (fusion of a large immotile female gamete with a smaller motile male gamete).

Life Cycle in Green Algae

Green algae shows all three types of life cycle:

  • Haplontic : The haploid gametophyte is the dominant generation. Sporophytic generation is represented only by the diploid zygote which undergoes meiosis to form haploid spores. Examples include Chlamydomonas, Ulothrix, Spirogyra etc.
  • Diplontic - The diploid phase is the dominant generation. Gametes produced by meiosis represent the haploid phase. Examples include Caulerpa
  • Haplodiplontic - In this both the haploid and diploid phases are well developed. The haploid gametophyte produces haploid gametes. Fusion of gametes results in the formation of the zygote (2n). Zygote develops into a sporophyte which produces haploid spores through meiosis. Spores then develop into the gametophyte. Examples include Cladophora.

Economic Importance of Green Algae

  • They can be used as food. Examples include Chlorella, Ulva (sea lettuce) etc.
  • Chlorellin is an antibiotic extracted from Chlorella.
  • Some green algae helps in sewage oxidation. Examples include Chlamydomonas

Practice Problems of Algae

Question 1. In Ulothrix, meiosis occurs in which part of the body?

(A) hold fast
(B) zygote
(C) zoospore
(D) Filament cells

Solution : Ulothrix belongs to the class Chlorophyceae. It is a filamentous algae found attached to some solid objects. They reproduce normally by vegetative reproduction and spore formation. During sexual reproduction mainly isogametes are formed. Fusion of gametes results in the formation of zygotes. Zygote later develops into a thick walled zygospore. Zygospore undergoes meiotic division and forms meiospores which germinate to form new filaments. Hence in Ulothrix, meiotic division takes place through the zygote.

Thus, the correct option is B.

Question 2. An algal species reproduces with the help of fusion between a small motile gamete and large immotile gamete. What type of reproduction do you think occurs in this algae?

(A) Oogamy
(B) Isogamy 
(C) Anisogamy 
(D) Reproduction by asexual flagellated zoospores. 

Solution : In algae, reproduction by fusion of gametes is known as sexual reproduction. The type of reproduction in which a smaller motile male gamete fuses with a larger immotile female gamete is known as oogamy.

Thus, the correct option is A.

Question 3. Read the following statements (A - D) and find out the incorrect statement.

(A) Algae usually reproduce vegetatively by fragmentation, 
(B) Algae reproduce asexually by the formation of different types of spores.
(C) Algae are divided into four classes. 
(D) Algae are chlorophyll bearing simple, thalloid, autotrophic organisms. 

Solution : Algae are simple chlorophyll-bearing, eukaryotic, autotrophic organisms in which the plant body is a thalloid. Algae reproduce vegetatively by fragmentation and asexually with the help of spores. Algae is divided into 3 classes as follows:

  • Chlorophyceae or green algae
  • Phaeophyceae or brown algae
  • Rhodophyceae or red algae

Thus, the correct option is C.

Question 4. Identify the odd one from the options given below.

(A) Chlamydomonas
(B) Chlorella
(C) Ulothrix
(D) Porphyra

Solution : Chlamydomonas, Chlorella and Ulothrix belong to the class Chlorophyceae or green algae whereas Porphyra belong to the class Rhodophyceae or red algae. 

Thus, the correct option is D.

FAQs of Algae

Question 1. What is the basis of classification in Algae?

Solution: Algae are classified based on the nature of pigments, storage materials and complexity of structure. 

Question 2. Give one example for unicellular, colonial and filamentous green algae.


  • Unicellular green algae - Chlamydomonas
  • Colonial green algae - Volvox
  • Filamentous green algae - Spirogyra

Question 3.Write down the life cycle exhibited by the following green algae:

Chlamydomonas, Cladophora, Spirogyra, Ulothrix


  • Chlamydomonas - Haplontic
  • Cladophora - Haplodiplontic
  • Spirogyra - Haplontic
  • Ulothrix - Haplontic

Question 4. Define pyrenoids.

Solution: Storage bodies called pyrenoids are present in the green algae. They store starch and proteins. 

Other Related Topics

The Living World Biological Classification Plant Kingdom
Animal Kingdom Morphology of Flowering Plants Anatomy of Flowering Plants
Structural Organization in Animals Cells: The Unit of Life Biomolecules
Cell Cycle and Division Transport in Plants Mineral Nutrition
Photosynthesis in Higher Plants Respiration in Plants Plant Growth and Development
Digestion and Absorption Breathing and Exchange of Gases Body Fluids and Circulation
Excretory Products and their Elimination Locomotion and Movement Neural Control and Coordination
Chemical Coordination and Integration

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