Biology · Plant Kingdom · NEET
The name is old and misleading. Long ago, scientists saw a green, slimy, water-living organism that made food by photosynthesis, so they called it an alga. Later, the microscope showed its cell has NO true nucleus and NO membrane-bound organelles. That makes it a prokaryote (a bacterium), not a true alga. So the name stayed the same, but the classification changed. NCERT clearly writes: "the cyanobacteria that are also referred to as blue green algae are not 'algae' any more."
Cell type. Cyanobacteria are PROKARYOTIC (no true nucleus, DNA lies free in the cytoplasm). True algae are EUKARYOTIC (they have a real nucleus and organelles like a proper chloroplast). This single difference is enough to place them in two different kingdoms. For NEET, remember: prokaryote = Monera, eukaryote with cell wall + chlorophyll = Plantae (algae).
Kingdom Monera. In Whittaker's Five Kingdom system, all prokaryotes go into Monera. Cyanobacteria are photosynthetic prokaryotes, so they sit with bacteria in Monera. True algae go into Kingdom Plantae. NCERT reason: "Fungi, and members of the Monera and Protista having cell walls have now been excluded from Plantae."
Doing photosynthesis does not make something a plant. Cyanobacteria have chlorophyll a and are photosynthetic autotrophs, just like green plants. But their chlorophyll is NOT inside a membrane-bound chloroplast, it lies in folded membranes in the cytoplasm. Function (making food) is similar; STRUCTURE (prokaryotic cell) is completely different. Classification follows cell structure, so they stay in Monera.
They are cyanobacteria = bacteria (Monera), NOT true algae. Nostoc and Anabaena also fix atmospheric nitrogen in special cells called heterocysts. This is a favourite NEET trap: a question calls Spirulina or Anabaena a "unicellular alga," but strictly they are cyanobacteria in Monera. Always read the option carefully.
Their detailed features (heterocysts, blooms, nitrogen fixation, gelatinous sheath) are in the Biological Classification chapter under Kingdom Monera. In the Plant Kingdom chapter, NCERT only mentions them once, to tell you they are NOT counted as algae. So for NEET, link this fact to both chapters.
Which of the following pairs is of unicellular algae?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Yes. "Blue-green algae" is just the old common name for cyanobacteria. NCERT uses both names but stresses they are cyanobacteria, i.e. bacteria in Kingdom Monera, not true algae.
Prokaryotic. They have no true (membrane-bound) nucleus and no membrane-bound organelles. This is the core reason they are separated from true algae, which are eukaryotic.
The change came with modern classification (based on Whittaker's Five Kingdom system, 1969). Prokaryotes with cell walls, including cyanobacteria, were placed in Monera and excluded from Plantae.
Some do. Nostoc and Anabaena fix atmospheric nitrogen in special thick-walled cells called heterocysts. This is an extra reason they behave like bacteria, not plants.
Strictly no. Spirulina is a cyanobacterium (Monera). Some exam keys loosely call it a unicellular alga, so read NEET options carefully and know the exception.
