11th Class Biology Cell - The Unit Of Life Plastids

Plastids are semiautonomous organelles having DNA, RNA, Ribosomes and double membrane envelope. These are largest cell organelles in plant cell.

History

(1) Haeckel (1865) discovered plastid, but the term was first time used by Schimper (1883).

(2) A well organised system of grana and stroma in plastid of normal barley plant was reported by de Von Wettstein.

(3) Park and Biggins (1964) gave the concept of quantasomes.

(4) The term chlorophyll was given by Pelletier and Caventou, and structural details were given by Willstatter and Stall.

(5) The term thylakoid was given by Menke (1962).

(6) Fine structure was given by Mayer.

(7) Ris and Plaut (1962) reported DNA in chloroplast and was called plastidome.

Types of plastids : According to Schimper, Plastids are of 3 types: Leucoplasts, Chromoplasts and Chloroplasts.

Leucoplasts : They are colourless plastids which generally occur near the nucleus in nongreen cells and possess internal lamellae. Grana and photosynthetic pigments are absent. They mainly store food materials and occur in the cells not exposed to sunlight e.g., seeds, underground stems, roots, tubers, rhizomes etc. These are of three types.

(1) Amyloplast : Synthesize and store starch grains. e.g., potato tubers, wheat and rice grains.

(2) Elaioplast (Lipidoplast, Oleoplast) : They store lipids and oils e.g., castor endosperm, tube rose, etc.

(3) Aleuroplast (Proteinoplast) : Store proteins e.g., aleurone cells of maize grains.

Chromoplasts : Coloured plastids other than green are kown as chromoplasts. These are present in petals and fruits. These also carry on photosynthesis. These may arise from the chloroplasts due to replacement of chlorophyll by other pigments.

Green tomatoes and chillies turn red on ripening because of replacement of chlorophyll molecule in chloroplasts by the red pigment lycopene in tomato and capsanthin in chillies. Thus, chloroplasts are changed into chromatoplast.

All colours (except green) are produced by flavins, flavenoids and cyanin. Cyanin pigment is of two types one is anthocyanin (blue) and another is erythrocyanin (red). Anthocyanin are water soluble pigments and found in cell sap of vacoule.

Chloroplast : Discovered by Sachs and named by Schimper. They are greenish plastids which possess photosynthetic pigments.

Number : It is variable. Number of chloroplast is 1 in Spirogyra indica, 2 in Zygnema, 16 in S.rectospora, up to 100 in mesophyll cells. The minimum number of one chloroplast per cell is found in Ulothrix and species of Chlamydomonas.

Shape : They have various shapes

Size : It ranges from \[3-10\,\mu \,m\] (average \[5\,\mu \,m)\] in diameter. The discoid chloroplast of higher plants are \[4-10\,\mu \,m\] in length and \[2-4\,\mu \,m\] in breadth. Chloroplast of Spirogyra may reach a length of 1 mm.  Sciophytes (Shade plant) have larger chloroplast.

Chemical composition :  Proteins 50 – 60%; Lipids 25 – 30%; Chlorophyll – 5- 10 %; Carotenoids (carotenes and xanthophylls) 1 –2%; DNA – 0.5%, RNA 2 – 3%; Vitamins K and E; Quinines, Mg, Fe, Co, Mn, P, etc. in traces.

Ultrastructure : It is double membrane structure. Both membranes are smooth. The inner membrane is less permeable than outer but rich in proteins especially carrier proteins. Each membrane is \[90100\text{ }{AA}\] thick. The inter-membrane space is called the periplastidial space. Inner to membranes, matrix is present, which is divided into two parts.

(1) Grana : Inner plastidial membrane of the chloroplast is invaginated to form a series of parallel membranous sheets, called lamellae, which form a number of oval – shaped closed sacs, called thylakoids. Thylakoids are structural and functional elements of chloroplasts.

Along the inner side of thylakoid membrane, there are number of small rounded para-crystalline bodies, called quantasomes (a quantasome is the photosynthetic unit).

Each quantasome contains about 230 chlorophyll molecules (160 chl. 'a' and 70 chl. 'b') and 50 carotenoid molecules.

In eukaryotic plant cells, a number of thylakoids are superimposed like a pile of coins to form a granum. The number of thylakoids in a granum ranges from 10-100 (average number is 20-50). Adjacent grana are interconnected by branched tubules, called stromal lamellae or Fret-channel or Fret membrane's.

(2) Stroma : It is transparent, proteinaceous and watery substance. Dark reaction of photosynthesis occurs in this portion. Stroma is almost filled with “Rubisco” (about 15% of total enzyme, protein) enzyme \[C{{O}_{2}}\]is accepted by this enzyme. CO₂ assimilation results in carbohydrate formation. It has 20 – 60 copies of naked circular double stranded DNA.

Pigments of chloroplast

Chlorophyll a : \[{{C}_{55}}\,{{H}_{72}}\,{{O}_{5}}{{N}_{4}}Mg\] (with methyl group)

Chlorophyll b : \[({{C}_{55}}{{H}_{74}}{{O}_{6}}{{N}_{4}}Mg)\](with aldehyde group)

Chlorophyll c : \[{{C}_{35}}{{H}_{32}}{{O}_{5}}\,{{N}_{4}}Mg\]

Chlorophyll d : \[\alpha ,\beta ,\gamma \]

Bacteriochlorophyll \[({{C}_{55}}{{H}_{74}}{{O}_{6}}{{N}_{4}}Mg)\] or chlorobium chlorophyll present in photosynthetic bacteria. These pigment are red in acidic and blue in alkaline medium.

Carotenoids : These are hydrocarbons, soluble in organic solvents. These are of two types :

(1) Carotenes : \[{{C}_{40}}{{H}_{56}}{{O}_{2}},\] derivatives of vitamin A. Carrot coloured \[\alpha ,\beta ,\gamma \] carotene, lycopene, etc. \[\beta -\]carotene most common.

(2) Xanthophyll : \[{{C}_{40}}{{H}_{56}}{{O}_{2}},\]yellowish in colour, fucoxanthin, violaxanthin. Molar ratio of carotene and xanthophyll in young leaves is 2 : 1.

Origin of chloroplast : Plastids, like the mitochondria, are self duplicating organelles. These develop from colourless precursors, called proplastids. They are believed to be evolved from endosymbiont origination.

Functions

(1) It is the site of photosynthesis, (light and dark reaction).

(2) Photolysis of water, reduction of \[NADP\] to \[NADP{{H}_{2}}\] take place in granum.

(3) Photophosphorylation through cytochrome \[{{b}_{6}}\,f,\] plastocyanine and plastoquinone etc.

(4) They store starch or factory of synthesis of sugars.

(5) Chloroplast store fat in the form of plastoglobuli.

(6) They maintain the percentage of \[C{{O}_{2}}\]and \[{{O}_{2}}\] in atmosphere.