Plastid

Advance Study materials
Plastids

The term plastid was applied by Schimper, 1885, (Greek word Plastikas, i.e., formed or moulded) to those organelles which are primarily concerned with food storage. The term plastid was introduced by E. Haeckel in 1866. Plastids are semi-autonomous organelles having DNA and double membrane envelope which store or synthesizes various types of organic compounds. Plastids develop from colourless precursors called proplastids. Proplastids have the ability to divide and differentiate into various types of plastids. The sum total of all plastids in a cell is called Plastidome. This term was introduced by Dangeard, 1920.The chloroplast with nitrogen fixing genes are called Nitroplast. Traditionally, the plastids are classified into three categories as under:

(a)   Chloroplast : They are the green plastids.

(b)    Chromoplast : They are the coloured plastids, other than greens.

(c)    Leucoplast : They are the colourless plastids. They are of three types :

  1. Amyloplast : They store carbohydrates. An amyloplast is several times larger than the original size of leucoplast. It contains a simple or compound starch grain covered by a special protein sheath, e.g., Potato tuber, Rice, Wheat
  2. Elaioplast(Lipidoplasts, Oleoplasts) : They store fats. e.g., Tube Rose   
  3. Aleuroplast(Proteoplasts or Proteinoplasts) : They store proteins. The protein in the amorphous, crystalloid or crystallo-globoid state (e.g., aleurone cells of Maize grain, endosperm cells of Castor).

     The three plastids are interconvertible as under :

Sometimes, they are classified on the basis of pigmentation, into the following two categories:

(a)    Chromoplast:

         (Gk. chroma— colour, plastos— moulded). The plastids are yellow or reddish in colour because of the presence of carotenoid pigments. Chlorophylls are absent. Chromoplasts are formed either from leucoplasts or chloroplasts. Change of colour from green to reddish during the ripening of Tomato and Chili is due to transformation of chloro­plasts to chromoplasts. The orange 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,

(i) Chromoplasts provide colour to many flowers for attracting pollinat­ing insects, (ii) They provide bright red or orange colour to fruits for attracting animals for dispersal, (iii) They are also the site of synthesis of membrane lipids.

         They are sub-divided on the basis of pigmentation as under :

    1. Chloroplasts: They contain chlorophyll a and b e.g. green algae and other green plants.
    2. Phaeoplast: They contain fucoxanthin e.g., brown algae, diatoms etc.
    3. Rhodoplast: They contain phycoerythrin e.g., red algae.
    4. Blue green Chromoplast: They contain phycocyanin e.g. blue-green algae.

         Lycoprene in red tomato and Capsanthin in red chilies are present in plastids. These belong to the category of carotenoids. These are also present in carrots.

 (b)    Leucoplast:

         (Gk. leucos— white, plastos— moulded).  They are colourless plastids which generally occur near the nucleus in nongreen cells and possess internal lamellae. Grana and photosynthetic pigments are absent. Leucoplasts have variable size and form, e.g., rounded, oval, cylindrical, filamentous, etc.

         It is also stated that bacterial 'chromoplasts' contain bacteriochlorophyll. Since the prokaryotes lack plastids, this classification is untenable. 

Chloroplast

Number

The number of chloroplasts per cell of algae is usually fixed for a species. The minimum number of one chloroplast per cell is found in green alga Ulothrix and several species of Chlamydomonas, two in Zygnema. However, different species of a genus may have different number of chlo­roplasts, e.g., 1 in Spirogyra indica and 16 in S. rectospora. A photosynthetic leaf parenchyma cell has 20—140 chloroplasts. An internodal cell of Chara (an alga) has several hundred chloroplasts’.

Shape

In algae the chloroplasts have various shapes. They may be plate 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,Cladophora). Discoid in Vaucheria.. The chloroplasts of higher plants are generally disc-shaped with oval or circular outline. Rarely, they may be lens-shaped, rounded or club-shaped.

In Sciophytes (Shade loving plants), chloroplasts are larger while in Heliophytes (Sun loving plants) chloroplasts smaller.

Size

Like shape, the size of the chloroplasts is different in different species. The discoid chloro­plasts of higher plants are 4—10 um in length and 2—4 um in breadth. The size is generally larger in case of polyploid cells as compared to diploid and haploid cells. Normally it is much smaller than that of the cell. However, in many algae the chloroplast may occupy almost the whole length of the cell, e.g., Spirogyra. The chloroplast of Spirogyra may reach a length of 1 mm.

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, (Fe, Mg, Cu, Mn, Zn, Co) , etc. other  in traces.

However, spinach leaf cells contain about 56% protein, 32% lipids and 8% chlorophyll. The lipids here include xanthophylls, carotenoids, triglycerides, steroids, waxes and phospholipids.

 Ultrastructure :

The chloroplasts are bound by double unit membranes where each membrane is 90—100 A0 thick. The two membranes are separated by 100-300 A0 wide periplastideal space. The outer membrane may be attached to endoplasmic reticulum. At places the inner membrane is con­nected to thylakoids. As in mitochondria, the outer membrane is more permeable than the inner membrane. The inner membrane has more of proteins including carrier proteins. 

Matrix:

The ground substance of a chloroplast is known as matrix or stroma. It is semi-fluid colloidal complex that contains DNA, RNA, ribosomes, plastoglobuli and enzymes. Chloroplast or ct DNA is naked, circular or occa­sionally linear. A chloroplast may have several copies of it Many copies present in each chloroplast, e.g., potato 22 copies, wheat 900 copies. In pea (270 copies) Chloroplast DNA = 12% total cell DNA.DNA makes the chloroplast genetically autonomous be­cause it can both replicate and transcribe to form RNA. Chloroplast ribosomes are 70 S. They resemble the ribosomes of prokaryotes. With the help of ribosomes the chloroplast is able to synthesize most of the enzymes required by it. The important enzymes present in chloroplast are those that take part in synthesis of photosynthetic pigments, photolysis of water, photophosphorylation, dark assimilation of CO2, synthesis and degradation of starch, synthesis of lipids, etc Plastoglobuli are lipid droplets of 10 — 500 nm diameter. They may contain some enzymes, vitamin K and chloroplast matrix of higher plants may store starch temporarily, as starch grains. It is known as assimilation starch. In green algae (e.g., Spirogyra, Ulothrix), the chloroplasts possess special starch storing structures called pyrenoids.    

Genes Encoded by Chloroplast DNA.

Function

Number of genes

Genes for the genetic apparatus

 

rRNAs (23S, 16S, 5S, 4.5S)

4

tRNAs

30

Ribosomal proteins

21

RNA polymerase subunits

4

Genes for photosynthesis

 

Photosystem I

5

Photosystem II

12

Cytochrome bf complex

4

ATP synthase

6

Ribulose bisphosphate carboxylase

1

(Rest part of study materials after registration).

Details heading of rest part study materials:-

2. Thylakoids
3. Quantasome
4. Chlorophyll pigment and their distribution in plant kingdom
5. Proplastid
6. Formation of Etioplast
7. Granal & Agranal with distribution in plant kingdom
8. Autonomy
9. Similarities and dissimilarities between Mitochondria and Chloroplasts
10. Protein import into the chloroplast stroma
11. Import of proteins into the thylakoid lumen
12. The antenna complex and photochemical reaction center in a photosystem
13. Electron flow during photosynthesis in the thylakoid membrane
14. Changes in redox potential during photosynthesis.
15. Three types of photosynthetic reaction centers compared .
16. A comparison of the flow of H+ and the orientation of the ATP synthase in mitochondria and chloroplasts .
17. A phylogenetic tree of the proposed evolution of mitochondria and chloroplasts and their bacterial ancestors
18. Function

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