Ultra-structure of Chloroplast

Ultra-structure of Chloroplast

(site of photosynthesis)

Introduction-

Schimper(1883) coined the term chloroplast for green plants. The process of photosynthesis takes place in green plastids called chloroplasts (Cholos-green & Plastos-moulded). Chloroplast are present in mesophyll cells of leaf.

1. Shape- chloroplasts are spherical, oval or disc shaped in higher plants. The shape varies in algae. Exam- Spirogyra-spiral shaped, Zygnema-star shaped, Ulothrix-girdle.
2. Size- chloroplast measure about 4-10 μm in length, 1-5 μm broad and 2-4 μm in thickness.
3. Number- in some algal cells there is only single chloroplast. In higher plants the number is large. There are 20-40 chloroplast in each cells.
4. Distribution- chloroplast are evenly distributed throughout the cytoplasm.
5. Chemical composition- chloroplast contain 50-60% protein, 23-25% phospholipids, 5-10% chlorophyll, 2-3% RNA, 1-2% other pigments, .5% DNA, vitamins and metal ions in traces.

                    The ultra-structure of chloroplast

parts-

                 A. Peristromium 

                           B. Stroma 

                                       C. Grana 

                                                   D. Stroma lamella 

                                                                  E. Quantasome

 A. Peristromium-

1. A chloroplast is covered by a double layered unit membrane called peristromium.
2. The two membranes are separated by about 100-200 Å thick space called intermembranal space or       periplastideal space. It is filled with fluid.
3. Each membrane is 40-60 Å thick and made up from lipid and protein.
4. Unit membrane encloses stroma and grana. Outer membrane is freely permeable to many solutes and     metabolites while inner membrane is selectively permeable for transportation of certain substances between stroma and cytoplasm.

B. Stroma-

1. The colourless, transparent, homogenous and protaneceous fluid mass present inside the inner unit           membrane is called stroma.
2. It contains the enzymes necessary for Calvin cycle, synthesis of photosynthetic pigments, lipid                 metabolism, replication of DNA, transcription, translation etc.
3. Stroma contains few lipophilic granules osmopillic granules, DNA, RNA for ribosomes, starch etc.
4. The chloroplast DNA is naked and circular is known as plastidome. It provide site for synthesis of photosynthetic enzymes.
5. The most important enzyme in stroma RUBISCO (RUBP Carboxylase oxygenase).

     Function-

1.  Stroma plays an important role in the reduction of CO2 to carbohydrates through dark reaction.
2. Stroma contains DNA, RNA and Ribosomes, thus it controls all the activities of chloroplast.

C. Grana-

1.  These are the dark green bodies present in the stroma which helps in light reaction of photosynthesis.
2. Each granum looks like stack of coins and contains about 20-250 superimposed structure called thylakoids (thylakos=sac, oid=like).
3. Each thylakoid is a membranous sac. The membrane of thylakoid is caked as thylakoid membrane or Grana lamella.
4. The internal space of each thylakoid is called as fret channel or loculus.
5. The end of the disc shaped thylakoid is called as margin, while the membranes between the two thylakoids are called as partition.
6. Inner surface of thylakoids membrane contains many photosynthetic unites called Quantasomes.
7. According to Park and Biggins (1964), each quantasomes contains about 230-300 molecules of Chlorophyll-a, Chlorophyll-b, Carotene, and Xanthophyll. Thus it act as site for light reaction.

      Function-

1.  Grana convert the light energy into chemical energy in the form of ATP & NADPH2.
2. In grana, photolysis of water & liberation of O2 is takes place.

D. Stroma lamellae/ Fret lamellae-

1. The grana are connected with each other by colourless, membranous, long structures called as Intergrana lamellae or stroma lamellae or Fret lamellae.
2. The space present in the stroma lamellae is called Fret channel.
3. They contains the CO2 fixing enzymes.

     Function –

1.  They absorb atmospheric CO2 and transfer it to the stroma for further process.

E. Quantasome-

1.  In thylakoid, many small, rounded, flattened bodies are present called Quantasomes or Oblate particles.
2. They measures about 180X110X120 Å.
3. Each quantasomes contains about 230-300 photosynthetic pigment molecules.
4. The photosynthetic pigments are chlorophyll-a, chlorophyll-b, carotene and xanthophyll. Chlorophyll-a is present in the centre and acts as reaction center.
5. The photosynthetic pigments absorb light energy and convert it into chemical energy.
6. Quantasome also contain the electron carrier like ferridoxin and cytochromes. It is also called the photoreaction center of chloroplast.
7. It consist of both pigment systems, PS-I & PS-II which are involved in the light reaction of photosynthesis.

      Function-

• Quantasome act as a site for light reaction.
• The photosynthetic pigments absorb light energy and convert it into chemical energy.

 FUNCTION OF CHLOROPLAST-

•    It is the site of photosynthesis.
• In grana of chloroplast, light reaction of photosynthesis takes place.
• The dark reaction of photosynthesis occurs in stroma, as enzymes necessary for it are present in stroma.
• As entire process of photosynthesis occurs in chloroplast, it is also called as photosynthetic apparatus.