Q.What are heterocysts?
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Cyanobacteria
Imagine a living thing that can do what plants do — capture sunlight and make food — but is built more like a bacterium. That is exactly what cyanobacteria are. They are often called blue-green algae, but that name is misleading. They are not algae at all; they are bacteria. The "blue-green" comes from the colour they give to water when they grow in large numbers, and from the pigment that helps them photosynthesise.
You have probably seen them without knowing it. That greenish scum that forms on the surface of a stagnant pond or a slow-moving river in summer? That is a cyanobacterial bloom. They thrive in water rich in nutrients — especially phosphorus and nitrogen from fertilisers or sewage — and when conditions are right, they multiply explosively.
The Precise Statement
Cyanobacteria are a group of Gram-negative bacteria that obtain energy through oxygenic photosynthesis — the same type of photosynthesis that plants use, producing oxygen as a byproduct. They contain chlorophyll a (the same green pigment found in plants) and accessory pigments called phycobilins (which give them their blue or red hues). Some species can also fix atmospheric nitrogen — convert inert nitrogen gas (N₂) into ammonia (NH₃) — inside specialised cells called heterocysts.
6 CO₂ + 6 H₂O --(light, in the presence of cyanobacteria)--> C₆H₁₂O₆ + 6 O₂
This is the same overall reaction as plant photosynthesis. Cyanobacteria were the first organisms on Earth to perform oxygenic photosynthesis, and over billions of years, they transformed the planet's atmosphere from one rich in carbon dioxide to one rich in oxygen.
Why They Matter in Your Syllabus
You will encounter cyanobacteria in three main contexts:
1. As photosynthetic autotrophs — They are producers in aquatic ecosystems, forming the base of many food webs. Unlike true algae, they are prokaryotes (no nucleus, no membrane-bound organelles).
2. As nitrogen fixers — Some cyanobacteria (e.g., Anabaena, Nostoc) have heterocysts, thick-walled cells where the enzyme nitrogenase converts N₂ into ammonia. This is crucial because nitrogenase is destroyed by oxygen, and heterocysts provide an oxygen-free environment. These cyanobacteria often live in symbiotic relationships — for example, Anabaena lives inside the water fern Azolla, which is used as a green manure in rice paddies.
3. As indicators of pollution — Cyanobacterial blooms in lakes and ponds signal eutrophication (excess nutrients from human activity). Some species produce toxins (cyanotoxins) that can poison livestock and humans. …
Some filamentous cyanobacteria possess specialised cells that provide the oxygen-free micro-environment required by the nitrogen-fixing enzyme nitrogenase, which is otherwise inactivated by oxygen. …
Heterocysts are specialised nitrogen-fixing cells seen in cyanobacteria such as Nostoc and Anabaena.
Certain filamentous cyanobacteria (blue-green algae, Kingdom Monera), such as Nostoc and Anabaena, possess specialised large cells with thick walls called heterocysts, interspersed along their filaments. These cells provide the anaerobic micro-environment required by the enzyme nitrogenase, which is inactivated by oxygen, allowing the cyanobacterium to fix atmospheric nitrogen (N2) into a usable form even …
- CBSE 2022Set TERM11 markMCQQ.The cyanobacteria is also referred as(a) Slime mould(b) Blue green algae(c) Protista(d) Golden algae
›Reveal solutionSolution
Cyanobacteria = blue-green algae, a photosynthetic group of Monera.
Cyanobacteria are photosynthetic prokaryotes (kingdom Monera) that contain chlorophyll a along with the pigment phycocyanin, which gives many species a bluish-green colour -- hence the common name 'blue-green algae'. They may be unicellular, colonial, or filamentous (e.g., Nostoc, Anabaena, Oscillatoria), are found in fresh water, marine, and terrestrial environments, and some fix atmospheric nitrogen using specialised cells called heterocysts. They are …
- CBSE 2022Set ANNUAL1 markMCQQ.The functions of heterocysts include(a) Recycling nutrients(b) Nitrogen fixation(c) Help in photosynthesis(d) All the above
›Reveal solutionSolution
Heterocysts are specialised cyanobacterial cells whose main function is biological nitrogen fixation.
Cyanobacteria (blue-green algae) such as Nostoc and Anabaena are photosynthetic prokaryotes. Some filamentous forms differentiate a few cells along the filament into heterocysts — larger, thick-walled cells that lack Photosystem II.
Because the nitrogen-fixing enzyme nitrogenase is destroyed by oxygen, the cell needs an oxygen-free micro-environment to fix atmospheric N2 into ammonia. The heterocyst provides exactly this: by shutting down oxygen-evolving (PSII) photosynthesis and developing a thick wall that limits gas diffusion, it creates the anaerobic conditions nitrogenase needs. Ordinary vegetative cells of the filament continue photosynthesis and supply the heterocyst with carbon compounds, while the heterocyst supplies fixed ni …
- CBSE 2022Set ANNUAL1 markMCQQ.First autotrophic organism was?(a) Fungi(b) Spirogyra(c) Blue green algae(d) Microplasma
›Reveal solutionSolution
Cyanobacteria (blue-green algae) were the first autotrophs → (c).
- Blue-green algae (cyanobacteria) are among the most ancient organisms and were the first oxygenic photosynthetic autotrophs, enriching the early atmosphere with oxygen. …
- CBSE 2021Set ANNUAL1 markQ.What are heterocysts?
›Reveal solutionSolution
Heterocysts are specialised nitrogen-fixing cells seen in cyanobacteria such as Nostoc and Anabaena.
Certain filamentous cyanobacteria (blue-green algae, Kingdom Monera), such as Nostoc and Anabaena, possess specialised large cells with thick walls called heterocysts, interspersed along their filaments. These cells provide the anaerobic micro-environment required by the enzyme nitrogenase, which is inactivated by oxygen, allowing the cyanobacterium to fix atmospheric nitrogen (N2) into a usable form even …
- CBSE 2021Set botany1 markMCQQ.An element playing important role in nitrogen fixation is:(a) Copper(b) Manganese(c) Zinc(d) Molybdenum
›Reveal solutionSolution
Nitrogen fixation is carried out by the enzyme nitrogenase, whose active site contains iron and molybdenum, so the element playing an important role in nitrogen fixation is (D) Molybdenum.
Concept. Biological nitrogen fixation is the conversion of atmospheric nitrogen (N2) into ammonia (NH3) by microorganisms such as Rhizobium, Azotobacter and cyanobacteria. This reaction is catalysed by the enzyme nitrogenase, which is a molybdenum-iron (Mo-Fe) protein. The metal ions molybdenum and iron at its active centre are essential for the enzyme to reduce N2. Molybdenum is therefore an important micronutrient (trace element) for nitrogen fixation.
Evaluating the options:
- (A) Copper - a micronutrient, but its main role is in enzymes of redox reactions/electron transport, not nitrogen fixation. …
- CBSE 2020Set ANNUAL1 markMCQQ.Match the correct pair for: Heterocyst(a) Hibernation(b) C4 plants(c) Nitrogen fixation(d) Immunity(e) Bacteria
›Reveal solutionSolution
Heterocyst -> Nitrogen fixation.
Heterocysts are specialized, thick-walled cells found in filamentous, nitrogen-fixing cyanobacteria such as Nostoc and Anabaena. They create a micro-anaerobic environment (since the nitrogen-fixing enzyme nitrogenase is inactivated by oxygen) allowing t …
- CBSE 2018Set ANN1 markQ.Fill in the blank. In Anabaena specialized cells called ______ help in nitrogen fixation.
›Reveal solutionSolution
In Anabaena, specialised thick-walled cells called heterocysts carry out nitrogen fixation.
Anabaena is a filamentous blue-green alga (cyanobacterium). Nitrogen fixation is carried out by the enzyme nitrogenase, which is destroyed by oxygen. To keep this enzyme in an oxygen-free environment, some vegetative cells of the filament differentiate into large, thick-walled cells called heterocysts. …
- CBSE 2017Set ANNUAL1 markQ.Name the cell for N2 fixation in Blue green algae.
›Reveal solutionSolution
The heterocyst is a large, thick-walled specialised cell in filamentous cyanobacteria where atmospheric nitrogen is fixed.
Cyanobacteria such as Nostoc and Anabaena are photosynthetic (release O2), yet the enzyme that fixes atmospheric N2 into ammonia — nitrogenase — is irreversibly inactivated by oxygen. To resolve this conflict, certain cells along the filament differentiate into heterocysts:
- They develop a thick, multi-layered outer wall that greatly restricts gas (O2) diffusion into the cell, creating a micro-anaerobic environment.
- They lack photosystem II, so they do not evolve oxygen themselves. …
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