The plastid (Greek: πλαστός; plastós: formed, molded – plural plastids) is a major organelle found in the cells of plants and algae. Plastids are the site of manufacture and storage of important chemical compounds used by the cell. They often contain pigments used in photosynthesis, and the types of pigments present can change or determine the cell's colour. They possess a double-stranded DNA molecule, which is circular, like that of prokaryotes.

2. INTRODUCTION
The plastid (Greek: πλαστός; plastós: formed, molded –
plural plastids) is a major organelle found in the cells of plants and
algae. Plastids are the site of manufacture and storage of important
chemical compounds used by the cell. They often contain
pigments used in photosynthesis, and the types of pigments present
can change or determine the cell's colour. They possess a double-stranded
DNA molecule, which is circular, like that of prokaryotes.

3. PLASTIDS IN
PLANTS
Those plastids which contain pigments can carry out photosynthesis.
Plastids can also store products like starch and can synthesise fatty
acids and terpenes, which can be used for producing energy and as
raw material for the synthesis of other molecules. For example, the
components of the plant cuticle and its epicuticular wax, are
synthesized by the epidermal cells from palmitic acid, which is
synthesized in the chloroplasts of the mesophyll tissue. All plastids
are derived from proplastids which are present in the meristematic
regions of the plant. Proplastids and young chloroplasts commonly
divide by binary fission, but more mature chloroplasts also have this
capacity.

4. In plants, plastids may differentiate into several forms, depending
upon which function they play in the cell. Undifferentiated plastids
(proplastids) may develop into any of the following variants:
Chloroplasts green plastids: for photosynthesis; see also etioplasts,
the predecessors of chloroplasts
Chromoplasts coloured plastids: for pigment synthesis and storage
Gerontoplasts: control the dismantling of the photosynthetic
apparatus during senescence
Leucoplasts colourless plastids:
for monoterpene synthesis; leucoplasts sometimes differentiate into
more specialized plastids:
Amyloplasts: for starch storage and detecting gravity
Elaioplasts: for storing fat
Proteinoplasts: for storing and modifying protein
Tannosomes: for synthesizing and producing tannins and
polyphenols

5. Depending on their morphology and function, plastids have the ability
to differentiate, or redifferentiate, between these and other forms.
Each plastid creates multiple copies of a circular 75–250 kilobase plastome.
The number of genome copies per plastid is variable, ranging from more
than 1000 in rapidly dividing cells, which, in general, contain few plastids, to
100 or fewer in mature cells, where plastid divisions have given rise to a
large number of plastids. The plastome contains about 100genes encoding
ribosomal and transfer ribonucleic acids (rRNAs and tRNAs) as well as
proteins involved in photosynthesis and plastid
gene transcription and translation. However, these proteins only represent a
small fraction of the total protein set-up necessary to build and maintain the
structure and function of a particular type of plastid. Plantnuclear genes
encode the vast majority of plastid proteins, and the expression of plastid
genes and nuclear genes is tightly co-regulated to coordinate proper
development of plastids in relation to cell differentiation.
Plastid DNA exists as large protein-DNA complexes associated with the inner
envelope membrane and called 'plastid nucleoids'. Each nucleoid particle
may contain more than 10 copies of the plastid DNA. The proplastid contains
a single nucleoid located in the centre of the plastid. The developing plastid
has many nucleoids, localized at the periphery of the plastid, bound to the
inner envelope membrane. During the development of proplastids to
chloroplasts, and when plastids convert from one type to another, nucleoids
change in morphology, size and location within the organelle. The
remodelling of nucleoids is believed to occur by modifications to the

6. Many plastids, particularly those responsible for photosynthesis,
possess numerous internal membrane layers.
In plant cells, long thin protuberances called stromules sometimes
form and extend from the main plastid body into the cytosol and
interconnect several plastids. Proteins, and presumably smaller
molecules, can move within stromules. Most cultured cells that are
relatively large compared to other plant cells have very long and
abundant stromules that extend to the cell periphery.

7. CHLOROPLAST
Chloroplasts /ˈklɔrəplæsts/ are organelles, specialized subunits,
in plant and algal cells. Their main role is to conduct photosynthesis, where
the photosynthetic pigment chlorophyll captures the energy from sunlight,
and stores it in the energy storage molecules ATP and NADPH while
freeing oxygen from water. They then use the ATP and NADPH to make
organic molecules from carbon dioxide in a process known as the Calvin
cycle. Chloroplasts carry out a number of other functions, including fatty
acid synthesis, much amino acid synthesis, and the immune response in
plants.
A chloroplast is one of three types of plastid, characterized by its high
concentration of chlorophyll. (The other two types, the leucoplast and
the chromoplast, contain little chlorophyll and do not carry out
photosynthesis.) Chloroplasts are highly dynamic—they circulate and are
moved around within plant cells, and occasionally pinch in two to reproduce.
Their behaviour is strongly influenced by environmental factors like light
colour and intensity. Chloroplasts, like mitochondria, contain their own DNA,
which is thought to be inherited from their ancestor—a
photosyntheticcyanobacterium that was engulfed by an early eukaryotic cell.
Chloroplasts cannot be made by the plant cell, and must be inherited by

8. With one exception (a member of the genus Paulinella), all
chloroplasts can probably be traced back to a single endosymbiotic
event (the cyanobacteria being engulfed by the eukaryote). Despite
this, chloroplasts can be found in an extremely wide set of
organisms, some not even directly related to each other—a
consequence of many secondary and even tertiary endosymbiotic
events.
The word chloroplast is derived from the Greek
words chloros (χλωρός), which means green, andplastes , which
means "the one who forms".[1]
Functions
(1) Absorption of light energy and conversion of it into biological
energy.
(2) Production of NAPDH2 and evolution of oxygen through the
process of photosys of water.
(3) Production of ATP by photophosphorylation. NADPH2 and ATP are
the assimilatory powers of photosynthesis. Transfer of CO2 obtained

9. (4) Breaking of 6-carbon atom compound into two molecules of
phosphoglyceric acid by the utilization of assimilatory powers.
(5) Conversion of PGA into different sugars and store as stratch. The
chloroplast is very important as it is the cooking place for all the
green plants. All heterotrophs also depend on plasts for this food.

10. CHROMOPLAST
Chromoplasts are plastids, heterogeneous organelles responsible
for pigment synthesis and storage in specific
photosynthetic eukaryotes. It is thought that like all other plastids
including chloroplasts and leucoplasts they are descended from
symbiotic prokaryotes.
Functions
1.Chromoplast imparts colour and found in some cells of
more complex plants.
The three types of plastids are chloroplasts, chromoplasts and
leucoplasts.
2.Chromoplasts are coloured plastids other than green and found in
coloured parts of plants such as petals of the flower, pericarp of the
fruits etc. Due to the presence of carotenoid pigments they are

11. 3.The primary functions of chromoplasts are synthesis and
accumulation of carotenoid pigments.
4.Though similar to chloroplasts in size, chromoplasts differ
significantly in shape, often appear angular.
5.Found in fruits, flowers, roots, and stressed and aging leaves;
available in large quantities in ripe tomatoes, carrot tap roots,
daffodil flower petals, sweet potato tubers or red peppers.

12. LEUCOPLAST
Leucoplasts are a category of plastid and as such are organelles found
in plant cells. They are non-pigmented, in contrast to other plastids such as
the chloroplast.
Lacking pigments, leucoplasts are not green. They are colourless, so they are
predictably located in roots and non-photosynthetic tissues of plants. They
may become specialized for bulk storage of starch, lipid or protein and are
then known as amyloplasts,elaioplasts, or proteinoplasts [also called a
leuroplast] respectively. However, in many cell types, leucoplasts do not have
a major storage function and are present to provide a wide range of essential
biosynthetic functions, including the synthesis of fatty acids, many amino
acids, and tetrapyrrole compounds such as haem. In general, leucoplasts are
much smaller than chloroplasts and have a variable morphology, often
described as amoeboid. Extensive networks of stromules interconnecting
leucoplasts have been observed in epidermal cells of roots, hypocotyls,
and petals, and in callus and suspension culture cells of tobacco. In some
cell types at certain stages of development, leucoplasts are clustered around
the nucleus with stromules extending to the cell periphery, as observed for
proplastids in the root meristem.

13. Etioplasts, which are pre-granal, immature chloroplasts but can also
be chloroplasts that have been deprived of light, lack active pigment
and can be considered leucoplasts. After several minutes exposure to
light, etioplasts begin to transform into functioning chloroplasts and
cease being leucoplasts.
function
1. Leucoplasts are another type of plastid - the colourless plastids
and common to cells of higher plants.
2 .These colourless plastids are involved in the storage of various
materials (carbohydrates, fats, oils and proteins), especially starch.
3. Plastids storing carbohydrates are called amyloplasts, plastids
storing fats and oils are called elaioplasts and plastids storing protein
are called proteinoplasts.
4. If leucoplasts are exposed to light they can develop into
chloroplasts and vice versa.

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