Plastids
They are double walled DNA containing largest organelle in plant cells,
discovered by Haeckel (1865). These are developed from colourless proplastids found in meristems.
Three types of plastids are (i) Leucoplasts: Largest, colourless, found in unexposed parts and store starch (amyloplast), fat (claioplast) or Protein (aleuronplast). (ii) Chromoplasts: Second largest plastids, have carontenoids to provide attractive colour to fruits, seeds, flowers. (iii) Chloroplasts: Green plastids discovered by Sachs (1862) but named Chloroplast by Schimper (1885) store starch temporarily; shape variable, maximum variation in shape is found in green algae.
Shape is planoconvex or discoid; each chloroplast has two membranes. Its matrix
(stroma) has prokaryotic naked circular DNA (0.5%), RNA, vitamin E and K,
plastoglobules (osmiophillic globules), starch particles; 70 S ribosomes, minerals (Fe,Mg, Cu, Mn, Zn, Co) and enzymes of dark reaction of photosynthesis.
50% of matrix is filled with Rubisco enzyme. In matrix are found double membrane bounded tubular sacs called thylakoids (structural unit of chloroplast) which are stacked to form grana; number of grana per chloroplast is 40-100 and each granum has 2-100 stackedthylakoids. Inner membrane of thylakoid bears quantasomes (functional unit of chloroplast, discovered by Park and Biggins (1962), size 180 × 150 × 100 Å, called photosynthetic units (PSU) where primary act of photosynthesis (i.e. release of e–) occurs.
A quantasome has 230 chlorophyll molecules (160 chl a + 70 chl b) and
about 50 carotenoid molecules. One of the molecules of Chl a acts as reaction (trap)
centre. It is P700 in PS I and P680 in PS II. Two grana are joined by Frets channel
(stroma lamella). Chloroplasts and mitochondria are energy transducing. DNA
containing, semiautonomous, double walled organelles and called cell within cell
because they have their own protein machinery and show cytoplasmic inheritance.
No life is possible on this earth without chloroplast.
Chl a is C55 H72 O5
N4
Mg – blue green and has – CH3
group.
Chl b is C55 H70 O6
N4
Mg – greenish colour and has – CHO group.
Pyrenoid is a proteinaceous body around which starch is stored in green algae.
Chloroplasts are extremely fragile osmotically and burst in H2O and hence,
chloroplasts are isolated from green leaves using sugar solution.
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