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Plastid: Plastids are largest in size among the
organelles present in cytoplasm. They can
be seen clearly under microscope. Plastids are not
present in Fungi, Bacteria and Animal cells.
Plastids are mainly of two types: Leucoplast and Chromo plastid or Chloroplast. Leucoplastids are colorless and Chromo plastids are colored. Chromo plastid again is of
two types Chloroplast and Chloroplast.
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Chloroplast: They possesses green pigments named
chlorophyll in excess as a result they are green. Other pigments are also present to
some extent. Every cell may contain one or
more chloroplast. In higher plants the
shapes of chloroplast is lenceolate. In Algal cells their shapes are of various types, e.g. cup shaped, spiral,
reticulate, star shaped.semicircular etc.
Structure of chloroplast: The following parts make Chloroplast :1. A
bi-layered semipermiable membrane surrounds the whole chloroplast. It is composed of protein and
lipid. This is called lipoprotein.2. A hygroscopic matrix surrounded by a
membrane is there. This matrix is called
Struma.3. In the stroma, 40-80 well-arranged barrel shaped grana (sing: granum)
are present. In one granum there are 5-25 granum
disc. Insidethe granum disc, there are spaces. Probably chlorophyll and
other.photosynthetic materials are present in this chamber.4. Some granum discs
of two adjacent grana are connected by minute tubular bodies, which are called stroma lamelli.5. Many
crystalline bodies are arranged in the membrane of grana disc. These are called quantosome. In the dark phase of
photosynthesis, carbon-di-oxide is assimilated
mainly in the quantosome. 
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Mitochondria: Krebs cycle, fatty acid cycle, electron
transport system etc.take place in mitochondria (sing: mitochondrion). All
energy producing process
occurs in mitochondria for which mitochondria are compared to the powerhouse of the cell. Number of mitochondria may vary
according to the species. Normally each cell
contains average 300-400 mitochondria. Their shape
may be globular, rod, thread, star-shaped or ring-shaped. The outside of
mitochondria is surrounded by a bi-layered
membrane. The membrane is made by
lipoprotein, which is enriched with lipid and protein. The outer membrane is
smooth but the inner one has a series of enfolding
into the inner cavity of mitochondria. These
enfolding are called cristae. Small stalked granular bodies are arranged on the cristae and they are called auxisome.
Various enzymes.
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Endoplasmic reticulum:In a mature cell a network is found in the
cytoplasm. This is called endoplasmic
reticulum. Endoplasmic reticulum is of two types - smooth and rough. ER having ribosome on its body is called rough
endoplasmic reticulum.
Structure: They are bounded by double-layered semipermiable membrane. Normally they are branched but
may occur in parallel. They are continuous with
the nuclear membrane and the cell membrane. Chemically the membrane is made by lipoprotein. Small granular bodies may be
present on it.
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Ribosome: Ribosomes are small spherical
/granular organelle. They may occur freely in the cytoplasm and may be attached
on the outer wall of endoplasmic reticulum.
Structure: These are spherical and bounded by double layered membrane.
Mostly they are made by protein.
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Golgi bodies/Golgi
apparatus: Golgi bodies may be
flat, spherical or elongated. Normally they are present near the
nucleus. It was first observed by a scientist named Golgi in 1898 AD in the nerve
cells of owl and cat. This organelle is afterward named Golgi apparatus after
his name. In plant cells their number is small for which it is not always visible
under microscope.
Structure: Golgi apparatus is tubular, small vesicle, vacuolar, elongated
vessel like or lamillar bodies. They are vacuolar space bounded by
double-layered membrane. necessary for respiration are well arranged in
auxisome. About 70 types of enzymes and 14 types of co-enzymes are present in
it. Matrix is also present inside the free space of mitochondria. The main
function of mitochondria is to produce energy, like- respiration, oxidative
phosphorilation, electron transport
system etc.
