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Algae | Class 11 Biology

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  • Last Updated : 02 Sep, 2022
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Algae, bryophytes, pteridophytes, gymnosperms, and angiosperms are all members of the plant kingdom. The first land plants appeared around 470 million years ago, and a spectacular amount of plant biodiversity has evolved over the aeons.

The plant kingdom is thought to play one of the most important roles in human life. Human survival would be impossible without the existence of plants. It is an open secret that plants are the primary source of the various vital components required for the survival of life on Earth.

The most important component is oxygen, without which most species would become extinct. This oxygen is released by plants during the photosynthesis process, in which green plants use the energy obtained from sunlight to help convert carbon dioxide into oxygen. This oxygen that humans have access to is critical for breathing and survival.




  • Algae can be found in a wide range of habitats, including oceans, rivers, and lakes, as well as ponds, brackish water, and even snow. Algae are typically green but can be found in a variety of colours. Snow algae, for example, contain carotenoid pigments in addition to chlorophyll, giving the surrounding snow a distinct red colour.
  • Algae are chlorophyll-containing, simple, thalloid, autotrophic, aquatic plants.
  • They are found in moist stones, soils, and wood, as well as in the presence of fungi and animals.
  • Algae are common near or inside bodies of water because they require a moist or watery environment. They share structural similarities with terrestrial plants, a significant group of photosynthetic organisms.
  • The similarities stop there, however, because algae lack many structural components found in plants, such as true stems, shoots, and leaves.
  • They also lack vascular tissues, which allow vital nutrients and water to circulate throughout their bodies.
  • Example: Lichen is the combination of algae and fungi, Algae grow on the sloth bear’s body.


Some of the general characteristics of algae are listed below.

  • Algae can photosynthesize like plants and have specialised structures and cell organelles found only in animals, such as centrioles and flagella.
  • Algae are photosynthetic organisms that can be unicellular or multicellular in nature.
  • Algae lack a well-defined body, so structures such as roots, stems, or leaves are absent, and Algaes can be found anywhere there is enough moisture.
  • Algal reproduction takes both asexual and sexual forms. Spore formation is used in asexual reproduction.
    Algae are self-contained organisms, though some can form symbiotic relationships with other organisms.


  • Algae reproduce through vegetative, asexual, and sexual means.
  • Asexual reproduction occurs through the formation of flagellated zoospores, which germinate and give rise to new plants. Palmella Stage-During this stage of asexual reproduction, the spores colonise and resemble the algae Palmella. For example, Ulothrix and Chlamydomonas.
  • Vegetative reproduction is accomplished through fragmentation, with each fragment developing into a thallus.
  • Sexual reproduction occurs through the fusion of two gametes; if the gametes, with or without flagella, are similar in size, they are isogamous; if they are dissimilar in size, they are anisogamous; and if the female gamete is large, non-motile, and the male gamete is smaller, motile, the gametes are referred to as
  • Isogamous gametes with flagella are found in Chlamydomonas and without flagella in Spirogyra, anisogamous gametes are found in some Chlamydomonas species, and oogamous gametes are found in Volvox and Fucus.

Classification of algae 




  • Phaeophyceae, or brown algae, are mostly found in marine environments. Laminarin or mannitol are used to store food.
  • They contain chlorophylls A and C, as well as carotenoids and xanthophylls.
  • The plant body is usually held to the substratum by a holdfast and consists of a stalk, the stipe, and a leaf-like photosynthetic organ called the frond.
  • They range in colour from olive green to brown, depending on the amount of the xanthophyll pigment fucoxanthin present.
  • Sexual reproduction can be isogamous, anisogamous, or oogamous, with pyriform gametes bearing two laterally attached flagella.
  • Vegetative reproduction occurs through fragmentation, while asexual reproduction occurs through pear-shaped biflagellate zoospores with two unequally attached flagella.
  • The cellulosic wall of vegetative cells is usually covered on the outside by a gelatinous coating of align.
  • Ectocarpus, Dictyota, Laminaria, Sargassum, and Fucus are the most common forms.


Green Algae


  • It is a large, loosely knit grouping of algae that contains the primary photosynthetic pigments chlorophyll a and b, as well as auxiliary pigments like xanthophylls and beta carotene.
  • Green algae are used by higher organisms to perform photosynthesis. Other green algae species have symbiotic relationships with other organisms.
  • Vegetative reproduction is usually accomplished through fragmentation or by different types of spores, asexual reproduction is accomplished through flagellated zoospores produced in zoosporangia.
  • sexual reproduction is accomplished through the formation of sex cells, which can be isogamous, anisogamous, or oogamous.
  • Chlorophyll a and b are photosynthetic pigments.
  • Some foods are stored as starch, while others are stored as oil droplets.
  • Pyrenoids, or storage bodies, are present.
  • The inner layer of the cell wall is composed of cellulose, while the outer layer is composed of pectose.
  • Unicellular, multicellular, colonial, and flagellates are all members. Spirogyra, Ulothrix, Volvox, and other prominent examples of green algae


  • Rhodophyceae is also known as red algae. They are mostly marine and only occasionally freshwater. For instance, Betrachospermum.
  • They can be found in well-lit areas as well as deep ocean depths, and the thallus is multicellular.
  • Chlorophyll a, d, and phycoerythrin are pigments found in red algae.
  • Food is stored as Floridian starch, which is structurally similar to amylopectin and glycogen.
  • The presence of the red pigment r-phycoerythrin causes the colour of algae to be red.
  • Oogamy is a sexual reproduction process that results in complex post-fertilization developments.
  • Asexual reproduction by nonmotile spores and vegetative reproduction by fragmentation
  • Polysiphonia, Porphyra, Gracilaria, Gelidium, Betrachospermum, and other red algae are commonly found.

Economic Importance 

  • Algae fix half of the total carbon dioxide fixation on Earth through the process of photosynthesis.
  • It is very important in the purification of air and water.
  • Some algae can be eaten. Chlorella, Laminaria, Porphyra, Sargassum, Ulva, and Spirulina are some examples.
  • The algae form the foundation of all aquatic animals’ food cycles.
  • Algin and carrageenan, which are water-holding substances or hydrocolloids, are obtained from red algae.
  • In tissue culture experiments, it is used to prepare a culture medium.
  • It has medicinal properties because antibiotics are made from it.
  • For example, Chlorella and Polysiphonia.
  • Polysiphonia and Laminaria are also used as mineral sources.
  • Chlorella and Acetabularia are important in biological research.
  • It also serves as a food supplement for astronauts. For example, Chlorella and Spirulina.
  • Gelidium and Gracilaria produce agar. It is used to cultivate microorganisms.
  • Some algae are used as animal feed. Examples, Laminaria, Sargassum, and Fucus.

Not an Algae  

Blue-green algae 

  • Blue-green algae were once among the most well-known types of algae. Blue-green algae, on the other hand, are prokaryotes and are not currently classified as algae (because all algae are classified as eukaryotic organisms).
  • These organisms, also known as cyanobacteria, live in moist or aquatic environments, similar to other algae.
  • Dams, rivers, reservoirs, creeks, lakes, and oceans are examples of these. This type of bacteria obtains energy through the process of photosynthesis.
  • Some blue-green algae species are important to the environment because they fix nitrogen in the soil. As a result, these are also known as nitrogen-fixing bacteria.
  • Other types of blue-green algae, on the other hand, can be toxic to humans. They can be neurotoxic (causes paralysis in the respiratory or nervous systems) or hepatotoxic (causes the liver to fail).
  • Furthermore, some can serve as environmental health indicators, indicating the extent of pollution.
  • For example, Nostoc, Anabaena, and others.

Algae Biofuel 

  • Algae can now be used as a fuel source thanks to recent advances in science and technology.
  • Global demand for petroleum products, combined with deteriorating environmental health, has prompted the use of environmentally friendly alternatives such as algal biofuel.
  • As a result, algae fuel is becoming a more viable alternative to traditional fossil fuels. It is used in the manufacture of everything from “green” diesel to “green” jet fuel. It is comparable to other biofuels derived from corn and sugar cane.

Difference between algae and fungi 

  • Both algae and fungi lack vascular tissues. Eukaryotic cells are found in both.
  • Algae and fungi both reproduce asexually through fragmentation.
  • Reproductive organs are not protected.
  • The mode of nutrition of algae and fungi differs. Algae are autotrophic organisms. They have chlorophyll and are capable of photosynthesis.
  • Fungi are heterotrophic, which means they rely on others for nutrients. They feed on decomposing organic matter.
  • Some algae and fungi, such as lichens, have a symbiotic relationship. In lichens, the algal partner feeds the fungi, and the fungi, in turn, feed the algae.

Frequently Asked Questions

Question 1: Define Algae? 


Algae are a large and extremely diverse group of eukaryotic, photosynthetic lifeforms. These organisms do not share a common ancestor and are thus unrelated (polyphyletic).”

Question 2: Explain how algae are beneficial to humans. 


  • Photosynthesis fixes carbon dioxide and increases dissolved oxygen levels. Primary producers of compounds high in energy.
  • Certain marine brown and red algae produce large amounts of commercially useful hydrocolloids.
    Align, for example, is produced by brown algae, whereas carrageen is produced by red algae.
  • Many Porphyra, Laminaria, and Sargassum species are eaten.

Question 3: Explain the reproduction in algae? 


  • Asexual Reproduction: Flagellated zoospores in the Chlorophyceae, biflagellated zoospores in the Phaeophyceae, and non-motile spores in the Rhodophyceae.
  • Vegetative reproduction occurs through fragmentation.
  • Sexual reproduction in Chlorophyceae and Phaeophyceae is isogamous, anisogamous, or oogamous.
  • Rhodophyceae are oogamous and have non-motile gametes.

Question 4: What are the general characteristics of red algae ( Rhodophyceae )? 


  • They are made up of multicellular filaments and blades. Apical growth, Complex oogamy, and a diffuse growth pattern (triphasic).
  • Food is stored in the form of starch and galactan sulphate polymers. Their cell walls are made up of cellulose and a variety of carbohydrates.
  • This type of red algae is typically found in tropical marine environments.
  • The mode of nutrition can be saprophytic, parasitic, or epiphytic. Grows on solid surfaces on its own or in association with other algae.
  • The presence of a pit in the cell wall allows cytoplasmic connections to be maintained.

Question 5: What is the Phaeophyceae (Brown Algae) family?


  • Brown algae contain chlorophyll a, chlorophyll c, xanthophylls, and carotenoids. Food is stored in the form of complex carbohydrates such as laminarin or mannitol.
  • Brown algae can be found in marine environments. They range in complexity from simple branched filamentous forms to highly branched forms.
  • Isogamous, anisogamous, or oogamous sexual reproduction is possible. Ectocarpus, Dictyota, Laminaria, Sargassum, and Fucus are some examples.

Question 6: What is the family of green algae (Chlorophyceae)?


  • Algae, and green Chlorophyll a and b are the primary pigments. The cell wall is composed of an inner layer of cellulose and an outer layer of pectose.
  • Pyrenoids consist of starch and proteins.
  • Chloroplast contains pigments and pyrenoids.
  • Chlamydomonas, Volvox, Spirogyra, Ulothrix, and Chara are a few examples.

Question 7: What are the benefits of algae?


Algae have been shown to fix 50% of carbon dioxide. Because they are photosynthetic, they increase the oxygen content of their surroundings. They are primary producers and provide food for many organisms. Many commercial products, such as align, are derived from algae.

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