Plastids



 What is plastids?

Plastids are distinct  and double __membrane bounded organelles of the plant  cells, with exception of fungi, and blue  green algae and gametes and zoospores of plants.The plastids¹ are  visible under light microscope. Their detailed structure, however, has been shown by electron microscope. The term plastid was introduced by E. Hoachel in 1866.

   Plastids:

Plastids are semi^ autonomous  organelles having DNA and double membrane envelope which store or synthesis various types of organic compounds with exception of  unicellular organism e.g. Euglenda. Plastids are found in plant cells only. Plastids  develop from colourless precursors called proplastids which have the ability to divide and differentiate into various  types of plastids. 



Location and Types:

The plastids are  cytoplasmic organelles found in the plant cells and in certain protists. They are of two main types:
Leucoplasts and chrlmoplasts. . The  leucoplasts are colorless and occur in the cells not exposed to sunlight. The chromoplsats are coloured and occur in the cells exposed to sunlight. The chrlmoplasts having light _ absorbing green pigment, the chlorophyllare known as the  chloroplasts ( G. chloros = green: plastos= formed). The chrlmoplasts containing pigments other then chlorophyll are found in certain algae and in the flowers, fruits and roots of higher plants. All plastids have a common origin, the  propladtids, and share many common features. One type of plastid can change into another. The proplastids can divide and differentiate into different kinds of plastids.

Types of plastids: 
 Based upon the structure, pigments and function, the plastids are classified into chloroplasts chromoplats  and Leucoplasts. 

(i) Chloroplasts:They are the greenish possessing  photosynthetic  pigments,  chlorophylls and carotenoids. They take part in the synthesis  of food from inorganic  raw materials in the presence  of radiation  energy. Chloroplasts of algae other than green ones  are  called chromatophores (e.g., rhodoplasts of red algae, phaeoplasts of brown algae).
 
Chloroplasts are found in the cells of green parts of plant. They contain the green pigment chlorophyll associated with carotenoids. They are concerned with photosynthesis. 

(ii) Chromoplasts They are yellow or reddish during because  of the presence of carotenoid pigments. They are formed either from leucoplasts or chloroplasts. Chlorophyll are absent. Change of colour in tomato and chilli  from green to reddish  during the ripening  is due to transformation of  chloroplasts to chromoplasts. The organe colour of carrot roots is due to chromoplasts .The pigments are often found in crystallised state so that the shape of the plastids  can be like needles, spindles  or irregular.  Chromoplasts provide colour to many  flowers for attaching  pollinating  insects, provide bright red or organe colour to fruits for attaching  animals  for dispersal and they are also the site of synthesis of membrane lipids.


 chromoplasts are coloured plastids because of the preponderance of carotenoids. They are formed in the coloured  parts of plant such as petals of flowers, pericarps of fruit  etc.

(iii) Leucoplasts: They are colourless  plastids. They possess internal lamella  but grana and photosynthetic pigments are absent. Leucoplasts have variable  size and form e.g., rounded, oval, cylindrical,  filamentous, etc. Three types of special Leucoplasts are: 

(a) Amyloplasts: They are the starch containing  leucoplasts  which are several times larger than the original size of Leucoplasts. It contains a simple or compound starch grain covered by a special protein sheath, e.g., Potato tuber, Rice, Wheat. 

(b)  Elaioplasts ( Lipidoplasts, Oleoplasts): They store fat, e.g., Tube Rose.

(c) Aleuroplasts, proteoplasts or Proteinoplasts: They contain protein e.g., aleurone cells of Mazia grain, endosperm cells of Castor.

Number :The number of chloroplasts  per cell of algae is usually  fixed  for a species. The minimum number of one chloroplasts per cell being found in green alga  Ulothrix and several species of Chlamydomonas. An intermodal  cell of Chara ( an alga) possesses  several hundred  chloroplasts. 

Shape: Chloroplasts  have various  shapes. They may be plat like ( e.g, Ulothrix), cup_ shaped (e.g., Chlamydomonas), ribbon_ like ( e.g., Spirogyra), polygonal or stellate ( e.g., Zygnema) and reticulate ( e.g., Oedogonium). Disc_ shaped with oval or circular  outline are generally  found  in the chloroplasts  of higher plants. Rarely,  lens_ shaped, rounded  or club _ shaped chloroplasts may be present.


Size: The size of the chloroplasts  differ in different species. Larger size is generally  seen in case of polyploid  cells as compared to diploid  and haploid cells.Normally it is much smaller than that of the cells. In many algae the chloroplast may occupy almost the whole length  of  the cell, e.g., Spirogyra.

Chemical  Composition:
Protein_ 50_ 60%, 
Lipids _ 25_ 30%

Chlorophyll _ 5_ 10%,
Carotenoids ( carotenes and xanthophylls) _ 1_ 2%
DNA _ upto 0.5% 

RNA  _ 2_ 3%

Vitamins K and E, quinones , Mg, Fe, Co, Mn P, etc.  _ in traces.


Chromoplasts are of different types:


(a) Chromatophores:These are found in photosynthetic bacteria and blue_ green algae. Fucoxanthin in main pigment. 

(b) Phaeoplasts: These are present in brown, algae, diatoms and dinoflagellates. 

(c) Leucoplasts are colourless plastids and are not associated with any pigment. They are found in storage organs where light is not available eg. underground stem and roots.They are also found in parenchymatous cells, embryonic cells, sex cells etc. They are meant for the storage of food materials.  Depending on storage material, they are classified into  amyloplasts ( starch), elaioplasts ( lipid and found in seeds) and aleuroplast ( proteins, are found in seeds).

These three structure are homogeneous structures and are capable of getting converted into one another. The chloroplasts gets converted into leucoplast in the absence of light.  Similarly,  Leucoplasts become chloroplasts when exposed to sunlight. In ripening fruits the chloroplasts become converted into Chromoplasts. 




What are the Chloroplastsq?


The term " Chloroplasts " was given by Schimper ( 1883). In higher plants these are usually discoid or spherical. Their number varies from one ( Chlamydomonas, ulothrix,), one _ sixteen ( Spirogyra), twenty_ forty  per cell in higher plants to upon five hundred per cell.


Their  size varies from 4__ 6  ųm .In green algae, the chloroplasts is cup__ shaped ( Chlamydomonas), girdleshaped ( ulothrix), spiral and ribbon shaped ( Spirogyra), reticulate ( Oedogonium) and stellate ( Zygenma). The chloroplasts are the most common type of plastids, and are of prime biological importance as they provide food ( energy) to practically all organisms through photosynthesis. 

Location:The chloroplasts were first seen by Leeuwenhoek. They are characteristic of green protists and plant cells. They are often seventy distribution in the cell. Bacteria, blue_ green algae, many proteins, fungi and animals lack chloroplasts. 



Morphology:

The chloroplasts vary in form, size and number, but have a uniform structure. 

(a) Form: The chloroplasts of higher plants are usually spherical, ovoid, disposal, or lens_ shaped. In lower plants ( algae), they are more varied in form, and may be cup_ shaped ( Chlamydomonas),spiral ( Spirogyra), girdle_ shaped ( Ulothrix), net _ like ( Oedogonium) or star_ like ( Zygnema).

(b) Size: The chloroplasts generally range from 4 to 10 ųm in long axis. The chloroplasts of shade plants are larger than those of sun plants. Polyploid cells also have larger chloroplasts than other cells.


(c) Number:Algal cell usually has a single very large chloroplasts. However, the intermodal cell of the alga Chara may have several hundred chloroplasts. Higher plants often have 20__ 40 chloroplasts per cell, but sometimes about 500 or even more.

Ultrastructure:

Chloroplasts are double membrane bounded organelles. Each is 40_ 60 A° thick,lipoproteinous and trilaminar in nature.Between two membranes, there is fluid_ filled intermembranous periplastidial space (25_75A°). The interior of chloroplast is filled with homogeneous proteinaceous transparent substance known as matrix or stroma. The stroma contains starch grains, eosinophilic granules, 70S ribosomes DNA.The multienzyme complex of the dark reaction of photosynthesis ( Calvin cycle) is located in the stroma. Several green disclick structures called thylacoids are embedded in the stroma, 10 to 100 thylakoids are piled one over the other forming a granum of the grana contains the photosynthetic pigments. The thylakoids of the grana  give rise to several colourless projections which connect the adjacent grana. These are called fret channels which are usually devoid of pigments. In  algae the chloroplasts are known  as  lamellate chloroplasts and the grana are typically absent.

In each granum, the thylakoids or lamellae are arranged parallel to one another to form a stack. The membranes of granum Thylakoids  are unit membranes and formed of layers of protein and phospholipids. The chlorophyll and carotenoid molecules are arranged in between protein and phospholipid layer of thylakoids, thylakoids are structural and functional elements of chloroplast.

Each chlorophyll molecule has a porphyrin ring towards the protein layer and phytol tail towards the phospholipid layer. The carotenoid pigments are arranged alternating with the chlorophyll molecules. Along the inner side of thylakoid membrane, there are a number of small rounded paracrystalline bodies, called quantasomes ( a quantasomes is the photosynthetic unit which can trap a mole of quantum of light and can bring about photosynthetic act). Each quantasome is about 180 A° × 150 A° × 130 A° in size and contains about 230 chlorophyll molecules ( Park and Biggins) and other photosynthetic pigments ( Chl.a, Chl. b, Carotenes and Xanthophylls).



Structure of plastids:


A chloroplasts is a vesicle bounded by an envelop of two unit membranes and filled with a fluid matrix like a mitochondrion .







(i) Membranes: The two membranes are separated by a narrow, fluid_ filled intermembrane space Each membrane is about 90_ 100 A thick. Both the membranes resemble the plasma membrane in molecular structure, being a lipoprotein sandwich.

(a) Outer Membrane: The outer membrane is smooth and freely permeable to small molecules. It has porin proteins which form channels for the passage of molecules. 

(b) Inner Membrane: The inner membrane is semipermeable and rich in proteins including carrier molecules like the inner mitochondrial membrane. It has permeases that control the passage of molecules. 

   Like the inner mitochondrial membrane, it is also greatly unfolded, but the infolds become free in the mature chloroplasts to lie as lamellae in the matrix. The inner membrane is attached to the Thylakoids at certain places. 

(ii) Matrix: It is colourless,granular, colloidal ground substance called stroma.It contains proteins,  lipids,  small ( 70S) ribosomes, small circular double helical  DNA molecule, RNA  molecules ( rRNA, tRNA and mRNA), enzymes, lipid droplets called plastoglobuli,and certain metal ions( Fe,Mn, Mg). The  enzymes catalyse the synthesis of photosynthetic pigments, photos of water, photophosphorylation, dark assimilation of carbon dioxide, and synthesis of starch,  lipids, etc.

(iii) Lamellae: The lamellae, after separation from the inner membrane, usually take the form of closed, flattened ovoid sacs, the Thylakoids, which lie closely packed in piles, the grana. A thylakoid encloses a very narrow intrathylakoid space bounded by a unit membrane.The Thylakoids of adjacent grana are interconnected by branching, membranous tubules termed frets, or stromal lamellae.The latter are also called intergranal thylacoids.  A chloroplast may have 40 to 100 grams in its matrix . A granum is composed to 20 __ 50 thylacoids in the chloroplasts of higher plants. Algae have a few thylacoids that do not form grana. They are said to be agranal.The thylakoid membrane contains photosynthetic pigments, namely, chlorophyll a, chlorophyll b, carotenoids ( carotene, xanthophy and plastoquinone .The pigments occur in groups termed  photosytems( former name quantosomes) The thylacoid membranes also contain  coupling factors  that bring about  ATP synthesis. 

         In green algae, the chloroplast contains one or more special protein bodies, the pyrenoids.They accumulate starch.

       Origin:Like mitochondria, the chloroplasts are self_ duplicating organelles. They grow by expansion and then fission. They also arise by division of their precursors, the proplastids.

        Autonomy: Like the mitochondria, the chloroplasts are partly independent or semiautonomous organelles. They can manufacture some of their proteins with the help of their DNA,  RNAs,  enzymes and ribosomes and get others from the cytoplasm formed under the control of nuclear  DNA.Moreover, the chloroplasts can divide to multiply their number.


         Evolution:is held by certain workers that some small photosynthetic prokaryotic cells, namely, blue _ green algae, got into a certain large heterotrophic  eukaryotic cell, viz., a protist. The association provide beneficial for both the partners. The large heterotroph could use  ATP  and organic materials produced by the small photographs through  photosynthesis. The small photographs got from their large heterotroph host cells protection and also food at times when photosynthesis was not possible, i.e.,in dark environment. The host phototrophs changed and in due course of time became modified into chloroplasts, the photosynthetic organelles, of the host.

    Thus, like the mitochondria, the chloroplasts also have a symbiotic or prokaryotic origin.

Functions of the Chloroplasts:

The chloroplasts have many functions __ 
(i) Photosynthesis: The chloroplasts trap the radiant energy of sunlight and transform it into the chemical energy of glucose formed water and carbon dioxide. The process is called photosynthesis. 



(ii)Oxygen Supply: Chloroplasts provide oxygen, the byproduct of photosynthesis, to all aerobic organisms for respiration. 

(iii) Starch Storage: Chloroplasts temporarily store starch grains during the day_ time in the pyrenoid, the starch_ forming organelle. At night,the starch is transferred to regions of growth and storage. 

(iv) Utilise CO2: Chloroplasts fix CO2,thereby keeping its concentration in the normal. 

(v) Change into Chloroplasts: Chloroplasts may change into chlomoplasts in flowers and fruits to attract animals. 

(vi) Lipid Storage: Fat droplets are stored as plastoglobuli in the matrix.


 (vii) Synthesis of Organic Acid:  Chloroplasts may produce amino acids and fatty  acids. 

(viii) Food Supply: Chloroplasts provide food and chemical energy to practically all organisms. 

 (ix) Greenery:  Chloroplasts impart the pleasing greenery to the earth.


Functions:

1): These are mainly concerned with the synthesis of carbohydrate using light energy, i.e. the site of photosynthesis. 

2): In some plants like Mirabilis jalapa, the chloroplasts DNA is founded to be involved in cytoplasmic inheritance and it also codes for structural proteins of thylakoid membranes. 

3): The leucoplasts serve to store reserve materials.

4): They maintain O2/CO2 balance in biosphere. 

5): These prevent  global warming. 

Resemblance between Chloroplasts and Mitochondria: 

Both chloroplasts and mitochondria: 

1: Are enveloped by 2 unit membranes: outer smooth and permeable, and inner semipermeable and infolded;

2: Carry  enzymes in the infolds;

3: Are energy transducing organelles;

4: Have circular proteinless DNA, all the  three RNAs and 70s ribosomes;

5: Are semiautonomous organelles;

6: Multiply by fission;

7: Are assumed to have symbiotic origin from prokaryotes; 

8: Form ATP by a similar mechanism; 

9: Produce amino  acids and  fatty acids; 

10: Occur in eukaryotes only,being absent in prokaryotes. 


Similarities between Mitochondria,   chloroplasts and Prokaryotes: All the three have similar  ribosomes ( in size 70s, and Composition of subunits), similar DNA (circular with little protein) and similar mode of multiplication ( division).

Leucoplasts:

The leucoplasts are colorless plastids ( G. leukos= white; Plastos = formed). They may be oval, spherical, rod_ like or filamentous. They occur in the cells not exposed to sunlight in seeds,fruits, tubers and rhizomes. They have internal lamellae but lack grana and photosynthetic pigments. They store reserve food materials. They are of 3 types __ 

(i) Amyloplasts: The amyloplasts synthesize and store starch grains enclosed by a protein sheath. They occur in the potato tubers,rice grains and wheat grains. 



(ii) Proteinoplasts ( Proteoplasts,Aleuronoplasts): The proteinoplasts store proteins.They are found in the cells of maize grains.

(iii) Elaioplasts( Lipidoplasts): The elaioplasts store  lipids ( oils). They are seen in the cells of the endosperm of castor seeds.

Chromoplasts:

The Chromoplasts are coloured plastids ( G. chroma= colour; plastos= formed).They usually occur in cells exposed to light such as flowers and fruits. They may occur in roots too, e.g., carrots,beets.They contain fat_ soluble carotenoids, a group of pigments including carotene and xanthophylls which provide yellow, orange and red colors and carry on photosynthesis. They lack chlorophyll. They may arise from the chloroplasts by development of a pigment as in carrots.Green tomatoes and chilies turn  red on ripening by replacing chlorophyll with the red pigment lycopin, early non pigmented carrot roots later become colored by developing the pigment carotene.

Functions:

The Chromoplasts have four main functions:
  
(i) They impart bright colours to the flowers to attract insects for pollination. 

(ii) They give attractive colours to the fruits for alluring certain animals for dispersal.


(iii) They also carry on photosynthesis. 

(iv) They synthesize membrane lipids.

QNo:Differences Between Mitochondria and Chloroplasts 

 Mitochondria

(1): Cristae remain attached to inner membrane. 

(2) Cristae produce ATP by break_ down of glucose 

(3) : Lack pigments. 

(4): Occur in practically all eukaryotic cells.

(5): Consume organic compounds in their activity, leading to decrease in weight.

(6): Produce CO2 and H2O by break down of organic compounds. 

(7) Use oxygen. 

(8): Function all the time.

(9): Present in gametes.

(10): Do not trap light energy. 

(11): Are smaller in size, 1 to 4 ųm.

Chloroplasts 

(1) : Thylakoids separate from inner membrane. 

(2): Thylakoids produce ATP by active of light.

(3): Have pigments. 

(4): Occur only in green eukaryotic cell exposed to sunlight.

(5): Produce organic compounds, leading to increase in weight.

(6): Use CO2 and H2O as raw materials to synthesize organic compounds. 

(7): Release oxygen. 

( 8): Function only in sunlight.

(9): Gametes have proplastids. 

(10): Trap light energy in the chemical bonds of organic compounds. 

( 11): Are larger in size, 4_ 10 ųm.



































































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