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Heterocyst - Structure, Function, and Examples

Heterocyst is a specialized cell in some cyanobacteria that fixes atmospheric nitrogen for the colony. Heterocyst, diagram structure, function, symbiotic relationship, and examples are provided in the article below.
authorImageKrati Saraswat4 Jun, 2025
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Heterocyst

Heterocyst: Multicellular organisms can have more diverse and efficient structures, functions, and behaviors compared to unicellular ones. Heterocyst-forming cyanobacteria serve as an excellent model for studying cellular differentiation and the formation of multicellular patterns in multicellular prokaryotes. Cyanobacteria are gram-negative prokaryotes capable of oxygenic photosynthesis.

These organisms have evolved various specialized cells, including nitrogen-fixing heterocysts, spore-like akinetes, and cells within motile hormogonia filaments. The most thoroughly studied example is the formation of heterocysts in the filamentous cyanobacterium Anabaena (also known as Nostoc) sp. strain PCC 7120. Heterocyst formation provides an example of cell differentiation and developmental biology. The filaments consist of only two types of cells, arranged in a one-dimensional pattern like beads on a string. NEET biology notes on heterocysts are provided in the article below.

Heterocyst Structure

Heterocysts are specialized cells found in some filamentous cyanobacteria, such as Nostoc and Anabaena. These cells play a vital role in nitrogen fixation, the process of converting atmospheric nitrogen (N2) into a form usable for biological processes.

Cell The Unit of Life

Here’s a detailed breakdown of the structure of heterocysts:
Shape and Size: Heterocysts are generally larger and rounder than vegetative cells in the filament. They appear pale yellow due to the absence of photosynthetic pigments.
Cell Wall: A notable characteristic of heterocysts is their thick cell wall. Unlike vegetative cells that have a single layer, heterocysts develop three additional wall layers. The outermost layer is glycolipidic, serving as a barrier to oxygen diffusion. This is crucial because the enzyme needed for nitrogen fixation, nitrogenase, is deactivated by oxygen. The middle layer is homogenous, and the innermost layer has a laminated structure.
Internal Components : Vegetative cells contain gas vesicles for buoyancy and photosynthetic lamellae for capturing sunlight. In contrast, heterocysts lack gas vesicles and have fewer photosynthetic lamellae.

Sexual Reproduction in Flowering Plants

However, they contain cyanophycin granules, which are believed to store carbon and nitrogen sources for nitrogen fixation. Most importantly, heterocysts are rich in the enzymes nitrogenase and hydrogenase, which are essential for converting atmospheric nitrogen into ammonia. Overall, the specialized structure of heterocysts allows them to create a low-oxygen environment suitable for nitrogenase activity, protecting this vital enzyme from oxygen inactivation. In return, the vegetative cells in the filament supply fixed nitrogen to the heterocysts for their survival. This symbiotic relationship enables cyanobacteria to thrive in environments with limited nitrogen availability.

Heterocyst Location

Heterocysts are specialized cells found within the strands of filamentous cyanobacteria. These cells are adapted to convert atmospheric nitrogen (N2) into a usable form, such as ammonia (NH3), through a process called nitrogen fixation. This conversion is crucial as it allows organisms to use nitrogen for building proteins and nucleic acids. Not all cyanobacteria have heterocysts; they are unique to filamentous types like Nostoc, Cylindrospermum, and Anabaena . Heterocysts are typically positioned intermittently along the filament, rather than being uniformly distributed among the vegetative cells. Key points about heterocyst distribution include:
  • Intermittent placement: Heterocysts are scattered at intervals among the vegetative cells within the filament.
  • Formation trigger: They develop in response to nitrogen starvation.
Heterocysts create a microaerobic environment essential for the nitrogenase enzyme to operate effectively, as this enzyme is deactivated by oxygen.
NEET Biology Notes
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Formation of Heterocysts

Heterocysts, also known as heterocytes, are special cells found in certain thread-like cyanobacteria when they lack nitrogen. Some well-known examples include Nostoc, Cylindrospermum, and Anabaena. The following steps details on how heterocysts form:
  1. Cell Enlargement: A normal cell grows larger in size.
  2. Decrease in Granular Inclusions: The number of granular inclusions inside the cell decreases.
  3. Change in Orientation of Photosynthetic Layers: The arrangement of the photosynthetic layers shifts.
  4. Development of Triple-Layered Cell Wall: The cell wall becomes triple-layered.
These specialized cells capture nitrogen from the air (dinitrogen, N₂) using an enzyme called nitrogenase, which provides nitrogen for making essential molecules in the filament. Their unique structure and function involve gene activity changes, additional cell wall layers, and adjustments to create a low-oxygen environment.

Heterocyst Function

Essentially, heterocysts act as small factories within cyanobacteria, converting nitrogen gas from the atmosphere (N2), which most organisms cannot use, into ammonia (NH3). Ammonia is a form of nitrogen that can be utilized to make amino acids and other important organic molecules. The main functions of heterocysts are as follows:
1. Creating an Oxygen-Free Environment: Nitrogenase, the enzyme responsible for nitrogen fixation, is highly sensitive to oxygen. Heterocysts create a microaerobic environment (very low oxygen levels) using two main methods:
  • Thick Glycolipid Walls: Heterocysts have thick cell walls made of glycolipids, which act as a barrier, preventing oxygen from entering.
  • Absence of Photosystem II: Unlike other cells in the filament, heterocysts lack photosystem II, a part of photosynthesis that produces oxygen.
2. Protecting Nitrogenase: By maintaining an oxygen-free environment, heterocysts protect nitrogenase from being deactivated.
3. Regulating Nitrogen Fixation: Heterocysts also play a role in controlling nitrogen fixation based on environmental nitrogen availability. When fixed nitrogen sources like ammonium or nitrate are scarce, heterocyst formation is encouraged. Conversely, abundant fixed nitrogen sources suppress the formation of heterocysts. Heterocysts are essential for cyanobacteria to thrive in environments where nitrogen is limited. By providing a suitable environment for nitrogenase and regulating nitrogen fixation, heterocysts enable cyanobacteria to capture atmospheric nitrogen and convert it into a usable form. This process supports their growth and ensures their survival.
NEET Biology Notes
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Slime moulds hypotonic solution Post fertilisation

Heterocyst Examples

Heterocysts are specialized cells in cyanobacteria, also known as blue-green algae, that are responsible for fixing nitrogen. They develop when the cyanobacteria face a shortage of nitrogen. Some examples of cyanobacteria that produce heterocysts include:
  1. Nostoc punctiforme: This filamentous cyanobacterium forms heterocysts to fix nitrogen under low nitrogen conditions.
  2. Cylindrospermum stagnale: Another example is Cylindrospermum stagnale, which also develops heterocysts to fix nitrogen.
  3. Anabaena sphaerica: Anabaena species, such as Anabaena sphaerica, form heterocysts when they lack nitrogen.
These specialized cells are interesting because they create a low-oxygen environment to protect the nitrogenase enzyme, which is sensitive to oxygen. By converting atmospheric nitrogen (dinitrogen) into nitrates and nitrites, heterocysts play a crucial role in maintaining the nitrogen balance in their ecosystem.
NEET Biology Notes
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Nostoc Spirogyra Ribosomes Liverworts

Symbiotic Relationships of Heterocysts

Heterocysts are specialized cells in some filamentous cyanobacteria, particularly those in the order Nostocales. These cells play a crucial role in nitrogen fixation, a process essential for converting atmospheric nitrogen into a form that plants and other organisms can use. The symbiotic relationship involving heterocysts typically occurs in associations where cyanobacteria provide fixed nitrogen to other organisms, such as plants or bacteria, in exchange for essential nutrients or protection. For example, in symbiotic associations like lichens, certain cyanobacteria form heterocysts to fix nitrogen, benefiting the fungal partner with essential nitrogen compounds. The fungus provides the cyanobacteria with a protected environment and sometimes essential nutrients. This mutualistic relationship allows both organisms to thrive in limited nitrogen availability. Physics Wallah offers top-notch, affordable online coaching for Class 12 NEET UG Exam preparation in multiple languages. Our experienced faculty provide live and recorded classes, Q&A sessions, and comprehensive study materials to help students excel. Enroll now for effective NEET preparation!
NEET Exam Important Links
NEET Syllabus NEET Biology Diagrams
NEET Biology MCQ NEET Biology Chapter wise Weightage
NEET Biology Notes NEET Previous Year Question papers

Heterocyst FAQs

Q 1. What is a heterocyst and its function?

Ans. A heterocyst is a specialized cell in cyanobacteria responsible for nitrogen fixation. It forms when the cyanobacteria face a shortage of fixed nitrogen (NH4 or NO3). Heterocysts play a crucial role in converting atmospheric nitrogen into a form usable by the organism under aerobic conditions.

Q 2. Where are heterocysts found?

Ans. Heterocysts are transparent, thick-walled cells located intermittently along filaments of blue-green algae. They are specialized nitrogen-fixing cells produced by certain types of filamentous cyanobacteria.

Q 3. What is the color of a heterocyst?

Ans. Heterocysts are transparent, thick-walled cells found at intermittent positions within filaments of blue-green algae.

Q 4. Which algae have heterocysts?

Ans. Heterocysts are present in various species of filamentous blue-green algae. They are distinguished by their slightly larger size and thicker cell walls than vegetative cells.

Q 5. What is the shape of a heterocyst?

Ans. Heterocysts are generally round and larger than vegetative cells. They have less granular cytoplasm, reduced pigmentation, and thickened cell walls. Two refractile polar bodies are present in intercalary heterocysts, while terminal heterocysts have one.

Q 6. Which enzyme is present in heterocyst?

Ans. Heterocysts are specialized cells transformed from vegetative cells and serve as sites for nitrogen fixation. They contain an enzyme called nitrogenase, crucial for converting atmospheric nitrogen into a biologically useful form. Additionally, these cells perform oxygenic photosynthesis.
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