The document summarizes the cell structure of prokaryotic and eukaryotic algal cells. Prokaryotic cells like cyanobacteria have an outer cellular covering including a mucilaginous sheath and cell wall, as well as a cytoplasm differentiated into a pigmented chromoplasm and central colorless centroplasm containing DNA. Eukaryotic algal cells have a cell wall, plasma lemma, and protoplast containing organelles like a chloroplast, mitochondria, and flagella. The chloroplast contains thylakoids and pyrenoids and the cell has structures like an eyespot.

1. PROKARYOTIC AND
EUKARYOTIC ALGAL
CELL STRUCTURE

2. PROKARYOTIC ALGAL
CELL STRUCTURE

3. The two main parts of cyanobacterial
cell.
• Outer Cellular Covering
• Cytoplasm.

4. 1. Outer Cellular Covering
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.
• Fibrils are made up of peptic
acids and
mucopolysaccharides.
• It retains the absorbed water
and protects the cell against
dessication.

5. 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.
• 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.

6. 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.

7. 2. Cytoplasm of Cyanobacterial Cell:
• It is differentiated into two regions
– Chromoplasm
– Centroplasm
a. 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.

8. • 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.

9. (2) Centroplasm:
• 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

11. 4. The Cytoplasm:
• The cytoplasm of cyanobacterial cell, like that
of bacteria, is incredibly boring.
• It lacks eukaryotic organelles such as
chloroplasts, mitochondria, endoplasmic
reticulum, Golgi bodies.
• But, it possesses photosynthetic apparatus,
ribosomes, and a large number of subcellular
inclusions such as glycogen or α-granules,
polyphosphate bodies, polyhedral bodies,
cyanophycin granules, and the genetic
material.

12. (i) Photosynthetic apparatus:
• In place of the chloroplasts of photosynthetic
eukaryotes, cyanobacteria have flattened
vesicular structures called thylakoids or
lamellae, which resemble the individual
thylakoids of the true chloroplasts of
photosynthetic eukaryotes.

13. • The lamellae or thylakoids are both
physiologically or structurally complex and
possess photosynthetic pigments.
• The principal pigment of all cyanobacteria is
chlorophyll a.
• In addition, there are β-carotene and other
accessory pigments, namely, phycobiliproteins.
• The phycobiliproteins are phycocyanin (PC),
allophycocyanin (AP), allophycocyanin-B (APB),
and phycoerythrin.
• By possessing phycocyanin and phycoerythrin
accessory pigments, the cyanobacteria resemble
with red algae.

14. • However, the necessary pigments of these
organisms are generally organized into
organelles called phycobilisomes and trap
light energy of lower wavelengths, which
cannot be absorbed by chlorophyll a, and pass
it on to the chlorophyll a.
• This is the reason why cyanobacteria, like
green algae, can exploit deeper waters where
the quality and quantity of illumination is
inappropriate for the photosynthetic plants.

16. (ii) Ribosomes:
• These are the sites of protein synthesis.
Cyanobacterian ribosomes occur freely in the
cytoplasm and are identical to those of
bacteria in being 70S ribosomes.

17. (iii) Glycogen or α-granules:
• Glycogen or α-granules are the sites for
storage of excess photosynthetic products.
• The latter is used as energy source in darkness
or when CO2 supply is limiting.

18. (iv) Polyphosphate bodies:
• These are the spherical
structures formed as a
result of the aggregation
of high molecular weight
linear polyphosphates.
• These subcellular
inclusions are also called
metachromatin granules
or volutin granules and
serve as phosphate stores
and are consumed during
periods of phosphate
starvation.
• These structures develop
mostly in those
cyanobacteria that grow
in a phosphate-rich
environment.

19. (v) Polyhedral bodies:
• All cyanobacteria store their ribulose 1, 5-
bisphosphate carboxylase (RUBP carboxylase)
enzyme in structures referred to as polyhedral
bodies.

20. (vi) Cyanophycin granules:
• Cyanobacteria growing in nitrogen-rich
environment produce structures, called
cyanophycin granules, which are made up of
arginine and aspartic acid.

21. (vii) Genetic material:
• The genetic material of cyanobacteria is made up
of naked DNA fibrils found dispersed in the
central region of the cytoplasm.
• Like other prokaryotes, they lack membrane-
bound organized nucleus.
• The exact number of genomes per cell is not yet
known
• it has recently been reported that Agmenellum
contains 2, 3 or more copies of its genetic
material.
• The molecular weight of the cyanobacterial
genome is considered to range from 2.7 to 7.5 x
109 daltons.

22. (viii) Plasmids:
• All the naturally occurring plasmids in
cyanobacteria are phenotypically cryptic.
• They are covalently closed circular DNAs and
their genetic compositions and complete
function is not yet known.
• However, plasmid-mediated transfer of
genetic material has been reported in certain
cyanobacteria.

23. EUKARYOTIC ALGAL
CELL STRUCTURE

25. 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
• 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.

26. 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.
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

27. 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 oval 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).

28. • 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.

29. 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.
blepaharoplast

30. • 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

31. 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

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