Heterocyst (Simplified)

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Heterocysts are specialized, thick-walled cells that can be found in the trichomes of blue-green algae. They earn their name because they are distinct from regular vegetative cells. These cells appear pale-yellow in color. There are two types of heterocysts:

  • Terminal heterocyst: Located at the terminal end of the trichome.
  • Intercalary heterocyst: Positioned between the vegetative cells of the trichome or laterally.

Heterocysts develop from vegetative cells that enlarge and transform into proheterocysts before eventually becoming heterocysts. The envelope of a heterocyst is exceptionally thick, consisting of two layers—the outer sheath and the inner cell wall. The sheath is further divided into three layers: an outer fibrous layer, a middle homogeneous layer, and an inner laminated layer. This sheath envelops the heterocyst, leaving only the junction between the heterocyst and the adjacent vegetative cell exposed. The heterocyst's cell wall consists of four layers: L1, L2, L3, and L4.

Inside the cell wall lies the plasma membrane, which encloses the protoplasm. The protoplasm contains thylakoids, ribosomes, photosystem-1, acryllipid, glycolipid, and the nucleoid (genetic material). At each end of the heterocyst, there is a constriction with a pore.

In certain species, the protoplast of a heterocyst divides twice, resulting in the production of four asexual spores. These spores develop into filaments. Terminal heterocysts possess a polar pore, while intercalary heterocysts have two polar pores.

Heterocysts have the following components:

  1. Cell wall made up of polysaccharides.
  2. Plasma membrane that surrounds the protoplasm.
  3. Protoplasm containing thylakoids, ribosomes, photosystem-1, acryllipid, glycolipid, and the nucleoid.
  4. Polar pore, closed by polar nodule, composed of solid mucilaginous substances.

Functions of Heterocysts include:

  1. Production of asexual spores.
  2. Involvement in hormogone formation.
  3. Acting as a storehouse of reserve food and essential salts.
  4. Serving as the site of nitrogen fixation.
  5. Stimulating cell division and growth.

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