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Chloroplasts are the green plastids which act as photosynthetic apparatus. In higher plants, they are mostly oval, spherical or discoid.

The major parts of chloroplast are:

1. Envelop:

  • Each chloroplast is bounded by two smooth selectively permeable cytoplasmic membranes with an inter-membrane space which is filled with fluid.
  • These  membranes are composed of lipo-protein sub-units.

2. Stroma:

  • It is the colourless hydrophilic ground substance (matrix) which fills the internal space of chloroplast.
  • It contains various enzymes required for the synthesis of glucose during dark phase of photosynthesis.
  • Matrix contains 70S ribosomes, 2-6 copies circular DNA, water and enzymes.
  • The most abundant enzyme is Rubisco (Ribulose Diphosphate Carboxylase)

3. Grana:

  • It is embedded in the ground substance-Stroma
  • It consists of many granums connected with each other by intergranal lamellae
  • Each granum is made up of stack of thylakoids.
  • Thylakoids are membrane-bound flattened, disc-shaped vesicles.
  • Each thylakoid consists of alternate layers of lipids and proteins embedded with quantasomes.
  • The chlorophyll pigments are organized into numerous photosynthetic units called quantasomes.
  • Each quantasome contains about 230 to 300 units of chlorophyll molecules.
  • They are capable of trapping light energy and converting it into ATP during the light reaction of photosynthesis.

Functions:

  1. Chloroplasts are the site of photosynthetic reactions
  2. Evolution of oxygen
  3. Synthesis of ATP during phosphorylation
  4. Synthesis of glucose
  5. Photosensitivity in algae
  6. Formation of chromoplast
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