The following points highlight the two main parts of cyanobacterial cell. The parts are: 1. Outer Cellular Covering 2. Cytoplasm.
Part # 1. Outer Cellular Covering of Cyanobacterial Cell:
It can be differentiated into following parts:
A. Slime layer or Mucilaginous sheath:
Presence of mucilaginous sheath is the characteristic feature of cyanobacteria. It consists of fibrils reticulately arranged within the matrix to give a homogeneous appearance (Fig. 4 A). Fibrils are made up of peptic acids and mucopolysaccharides. It retains the absorbed water and protects the cell against dessication.
B. Cell Wall:
It is present between the slime layer and plasma membrane. It is a rigid and complex structure and resembles the cell wall of bacteria. It is made of four layers. Carr and Whitton (1973) named all these four layers as L I, L II, L III and L IV (Fig. 4 A).
L I is a transparent space and occurs between the L II and plasmembrane. L II and L III are mucopolymer, made up of alanine, glucosamine, peptidoglycan, muramic acid, glutamic acid and α-diaminopimelic acid. The L IV is undulating, wavy and made of liposaccharides and proteins.
C. Plasma Membrane:
It is present below the cell wall. It is made up of protein-lipid-protein layers. The cytoplasmic membrane and its invaginations are the sites of biochemical functions, normally associated with the membrane bounded structures like mitochondria, endoplasmic reticulum and Golgi bodies of the eukaryotic cells.
Part # 2. Cytoplasm of Cyanobacterial Cell:
It is differentiated into two regions (Fig. 4B):
It is the outer or peripheral pigmented region. This region consists of flattened vesicle like structures called thylakoids or photosynthetic lamellae. These lamellae contain chlorophyll V, carotenoids and three phycobilins—C-phycocyanin, allophycocyanin and C-phycoerythrin.
Photosynthetic lamellae are arranged in parallel rows close to the periphery of the cell or they are distributed irregularly throughout the cell. In between the lamellae, occur certain granules of 400 A° diameter. These granules contain phycobilin pigment and are called cyanosomes y or phycobilisomes.
It is the inner or central colourless region. It is often called nucleoid or incipient nucleus. It consists of DNA fibrils. DNA is not surrounded with protein materials (histones). Like bacteria, small circular DNA segments occur in addition to nucleoid. These are known as plasmids or transposons. 70S ribosomes are also present in this region (Fig. 4 C).