Ultrastructure of Plastid:
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 90-100 Angstrum thick.The inner-membrane space is called the periplastidial space. Inner to membrane,matrix is present,which is divided into two parts.
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).
Park and Biggins(1964) discovered quantasome.Each quantasome contains about 230 chlorophyll molecules(160 chl.'a' and 70 chl.'b')and 50 carotenoid molecules.
In eukaryotes 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.
It is transparent,proteinaceous and watery substances.Dark reaction of photosynthesis occurs in this portion.Stroma is almost filled with "Rubisco"(about 15% of total enzyme, protein.) enzyme CO2 is accepted by this enzymes.Co2 assimilation results in carbohydrate formation.It has 20-60 copies of naked circular double stranded DNA.
Pigments of chloroplast:Chlorophyll a
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.
It is the site of photosynthesis,(light and dark reaction).
Photolysis of water,reduction of NADP to NADPH2take place in granum.
Photophosphorylation through cytochrome b6 f,plastocyanine and plastoquinone etc.
They store starch or factory of synthesis of sugars.
Chloroplast store fat in the form of plastoglobuli. They maintain the percentage of CO2 and O2 in atomsphere.