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Ultrastructure of Eukaryotic Algal Cell:
Chlamydomonas, a member of green algae (chlorophyceae) is found almost in all places. It is simple, motile, unicellular, fresh water alga. Its ultrastructure can be divided into following parts (Fig. 1, 2):
Cell Wall of Eukaryotic Algal Cell:
The cell is bounded by a thin, cellulose cell wall. Cellulose layer is finely striated with parallel cellulose fibrils (Fig. 1). In many species there is a pectose layer external to it which dissolves in water and forms a mucilaginous pectin layer. According to Roberts et. al. (1972), Hills (1973) the cell wall in C. Reinhardt consists of seven layers.
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Plasma Lemma of Eukaryotic Algal Cell:
It is present just below the cell wall and consists of two opaque layers which remain separated by less opaque zone (Fig. 1).
Protoplast of Eukaryotic Algal Cell:
It is bounded by plasma lemma. It is differentiated into cytoplasm, nucleus, chloroplast with one or more pyrenoids, mitochondria, Golgi bodies, two contractile vacuoles, a red eye spot and two flagella.
Chloroplast of Eukaryotic Algal Cell:
In majority of the species of Chlamydomonas, cytoplasm contains of a single, massive cup shaped chloroplast which almost fills the oral or pear shaped body of the cell. It is surrounded by a double-layered unit membrane. It bears number of photosynthetic lamellae (disc or thylakoids).
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The lamellae are lippo-proteinaceous in nature and remain dispersed in a homogeneous granular matrix (stroma). About 3-7 thylakoids bodies fuse to form grana like bodies. Matrix also contains ribosomes, plastoglobuli, microtubules and many crystals like bodies.
Flagella of Eukaryotic Algal Cell:
The anterior part of thallus bears two flagella. Both the flagella are whiplash or acronematic type, equal in size. Each flagellum originates from a basal granule or blepharoplast and comes out through a fine canal in cell wall. It shows a typical 9+ 2 arrangement. Fibrils remain surrounded by a peripheral fibril. According to Ringo (1907), 2 central ones are singlet fibrils and 9 peripheral ones are doublet fibrils (Fig. 2).
Stigma or Eyespot of Eukaryotic Algal Cell:
The anterior side of the chloroplast contains a tiny spot of orange or reddish colour called stigma or eyespot. It is photoreceptive organ concerned with the direction of the movement of flagella. The eye spot is made of curved pigmented plate. The plate contains 2-3 parallel rows of droplets or granules containing carotenoids (Fig. 3).
The other structures such as mitochondria, Golgi bodies, endoplasmic reticulum and nucleus are also bounded by double-layered unit membrane.
Ultrastructure of Cyanobacterial Cell:
The cell exhibits a typical prokaryotic structure. It can be differentiated into two parts:
1. Outer cellular covering
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2. Cytoplasm
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.
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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:
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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.
2. Cytoplasm of Cyanobacterial Cell:
It is differentiated into two regions (Fig. 4B):
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(1) Chromoplasm
(2) Centroplasm
(1) Chromoplasm:
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.
(2) Centroplasm:
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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).
Cytoplasmic Inclusions:
The membrane bound organelles such as the plastids, endoplasmic reticulum, vacuoles, mitochondria and the dictyosomes are absent. However, the chromoplasm contains a large number of inclusions.
These are ribosomes, a granules, (3 granules, structural granules, polyhedral bodies, gas vacuoles and vacuoles like inclusions (Fig. 4 C). α granules are also called mitochondrion granules and are said to be the region of storage. β granules are thought to be equivalent to cyanophycin (cyanophycean) granules.
Structural granules are considered as modified β or cyanophycean granules. Polyhedral bodies are also found in the central region. They are associated with the genetic material but their function is unknown.
In some cyanobacteria e.g., Oscillatoria, gas filled vacuoles (pseudo vacuoles) are present in the peripheral part of the cell. A gas vacuole is made up of a large number of units called vesicles. Gas vacuoles provide a buoyancy regulating mechanism.
