In this article we will discuss about the ultrastructure of eukaryotic algal cell with the help of suitable diagrams.
Cell Wall:
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.
Plasma Lemma:
It is present just below the cell wall and consists of two opaque layers which remain separated by less opaque zone (Fig. 1).
Protoplast:
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:
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).
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:
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:
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.