CHLOROPLASTS (Cell biology)

● PLASTIDS 

 1. Plastids are organelles enclosed by a double membrane found in all plants. 

2. The largest cytoplasmic organelles in the cell are plastids. 

3. Plastids are of different types varying in shape, size, çolour and function.

• Proplastids are found in embryonie tissue, composed only of an inner membrane, outer membrane, a small amount of stromal space with DNA. 

• Etioplasts are colourless plastids found in such tissue as leaves deprived of light. 

• Leucoplasts are colourless plastids. Oval, spherical, rodlike or filamentous leucoplasts occur in large numbers in cells of fruits, seeds, tubers and rhizemes. Leucoplasts store nutrients. They are classified into three types: 

(1) Amyloplast of potato stores starch 

(li) Elaloplast stores droplets of oil or fats. 

(il) Proteinoplast stores protein grains (Aleuroplast). 

• Chromoplasts are coloured plastids with yellow, orange and red carotenoids and other pigments. Chromoplasts are responsible for colours in flowers, ripening fruits, autumn leaves and some root like carrot. Chromoplast arises from proplastid or chloroplast. 

● Chloroplast 

4. The term chloroplast was coined by A.J.W. Schimper in 1883. 

5. The shape, size and distribution of chloroplasts may vary in different cells within a species. 

6. In higher plants there are 20 to 40 chloroplasts per cell. 

7. The chloroplasts are of the size range of few micron. 

8. The average diameter of chloroplast in higher planis is 4-6 µm.

9. If we separate the cell organelles of living cell, chloroplasts should be alive. 

10. Besides leucoplast, starch grains can develop in chloroplast. 

11. Chloroplasts are the photosynthetic organelles of green plants and contain the pigment chlorophyll. 

12. Chloroplasts are sites of photosynthesis by which a plant is able to capture light energy using H20 and CO2. 

13. Chloroplast is the organelle which acts as factory for the synthesis of sugars in autotrophic eukaryotes. 

● Structure 

14. Each chloroplast is covered with a double membrane containing a proteinaceous matrix called stroma. 

15. Stroma is the ground substance of chloroplast and contains a small circular double-helical DNA, ribosomes and several enzymes. 

16. Ribosomes of chloroplasts are 70S type containing 23S and 16S RNA. 

17. Many sheet-like lamellae called thylakoids occur in stroma. 

18. The term thylakoid was given by Menke (1961). About 20-50 thylakoids are placed one above the other like a stack of coins to form a granum. 

19. About 40-100 grana usually occur in a chloroplast. 

20. Many membraneous tubules called stroma lamellae inter- connect thylakoids of different grana.

21. Quantasomes are photosynthetic units present on the surface of grana. 

22. Quantasomes were discovered by Park and Biggins in 1954. 

23. The term 'quantasome' was introduced by Park and Pon in 1961. 

24. Each quantasome contains 230 chlorophyll molecules. 

25. Chlorophyll molecule has a complex porphyrin ring with a long phytyl chain. 

26. The metal magnesium is located at the centre of chlorophyll molecule. 

27. The blood pigment haem is almost identical to chlorophyll but it contains iron instead of magnesium. 

28. Chlorophyll has four pyrrole rings, so called a tetrapyrrole. 

● Types of Chlorophyll 

1. Chlorophyll 'a'  C55H72O5N4Mg

2. Chlorophyll 'b'  C55H70O6N4Mg

3. Chlorophyll 'c'  C35H32O5N4Mg

4. Chlorophyll 'd'  C54H70O6N4Mg

5. Chlorophyll 'e'  Formula not fully known 

29. Chlorophyll 'a' and 'b' differ in the nature of groups attached to position X of porphyrin ring. 

30. Chlorophyll 'a' has a methyl group (-CH3) while chlorophyll 'b' has an aldehyde group (-CHO). 

31. Most plants contains two or three times more chlorophyll 'a' than chlorophyll 'b'

32. Anthocyanin does not occur in chloroplast. 

33. Absence of chlorophyll in plants is known as albinism.