GROWTH AND DIVISION OF CHLOROPLASTS OR PROPLASTID

• The microscopic evidence suggests that chloroplasts usually arise by growth and division of existing chloroplasts. 
• In algae, which contain only one or two large chloroplasts per cell, the fate of individual chloroplasts is easy to watch. Light microscopists discovered that each chloroplast in cells of the green alga spirogyra divides in half prior to cell division. chloroplasts isolated from higher plant tissues have also been reported to divide when placed in an artificial growth medium. 
• Not all plant cells contain chloroplasts. 
• The various cell types found in higher plants all arise from a rapidly dividing, undifferentiated tissue called meristem. 
• Meristem cells lack chloroplasts, but they have small organelles called proplastids that contain chloroplast DNA. 
• Proplastids measure 1 μm or less in diameter and consist of an undifferentiated stroma surrounded by a double-membrane envelope. 

• Depending on where they occur in the plant and how much light they receive, proplastids develop into different kinds of plastids designed to serve different functions. 
• Some proplastids differentiate into amyloplasts, which are starch-filled particles that predominate in starchy vegetables such as potatoes. 
• Other proplastids acquire red, orange or yellow pigments, forming chromoplasts that give flowers and fruits thyeir distinctive colours. Although proplastids can develop into a variety of organelles, their main fate in photosynthetic tissues is to evolve into chloroplasts. 
• The conversion of proplastids into chloroplasts requires light, which triggers an enlargement of the proplastid and the formation of membrane tubules that project from the inner membrane into the stroma. 
• The tubules then spread into flat sheets that line up in parallel to form thylakoids. 
• At the same time, chlorophyll and other components of the photosynthetic electron transfer chain are produced and incorporated into the thylakoids. 
• If developing plant seedlings are allowed to grow in the dark, a different sequence of events takes place. 
• The proplastids still enlarge and membrane sheets invaginate from the inner membrane, but these sheets condense into a set of interconnected membrane tubules known as a prolamellar body. 
• The membranes of a typical prolamellar body arc arranged as highly ordered hexagonal arrays of membrane tubules. 
• At this stage the plastid is called an etioplast. l.ksides developing from proplastids in dark-grown seedlings, etioplasts also arise when green plants containing mature chloroplasts are transferred from light to darkness. 
• Under such conditions, the thylakoid membranes break up into tubules that fuse together to produce a typical prolamellar body converting the chloroplast into an etioplast. 
• They act as chloroplast precursors that can develop into functional chloroplasts when ex- posed to light. 
• Within a few hours after an etioplast is illuminated, the prolamellar body becomes transformed into an irregular mass of tubules; membrane sheets then grow out from the tubules and develop into thylakoid membranes. 
• This conversion of an etioplast into a chloroplast is accompanied by the formation of components of the photosynthetic electron transfer chain, such as chlorophyll. Etioplasts contain high concentrations of protochlorophyllide, which is a metabolic precursor of chlorophyll. 
• When etioplasts are illuminated, the absorption of light by protochlorophyllide triggers its rapid conversion to chlorophyll. 

MCQ’s

1. What is the primary way chloroplasts arise?
   • A) Spontaneous generation
   • B) Growth and division of existing chloroplasts ✅
   • C) Formation from mitochondria
   • D) Absorption of pigments from the environment
2. In which organism is it easiest to observe chloroplast division?
   • A) Higher plants
   • B) Fungi
   • C) Green alga Spirogyra ✅
   • D) Cyanobacteria
3. What triggers chloroplast division in Spirogyra?
   • A) Cell division ✅
   • B) Water availability
   • C) High CO₂ concentration
   • D) Temperature changes
4. What is the function of chloroplasts?
   • A) Protein synthesis
   • B) Photosynthesis ✅
   • C) Respiration
   • D) DNA replication
5. What type of plant tissue gives rise to different cell types?
   • A) Epidermis
   • B) Xylem
   • C) Meristem ✅
   • D) Phloem
6. Which organelle is found in meristematic cells?
   • A) Mitochondria
   • B) Proplastids ✅
   • C) Ribosomes
   • D) Lysosomes
7. What is the typical size of a proplastid?
   • A) 10 μm
   • B) 5 μm
   • C) 1 μm or less ✅
   • D) 100 μm
8. What structure encloses a proplastid?
   • A) Single membrane
   • B) Double-membrane envelope ✅
   • C) No membrane
   • D) Protein shell
9. What is the undifferentiated internal matrix of a proplastid called?
   • A) Thylakoid
   • B) Stroma ✅
   • C) Lumen
   • D) Prolamellar body
10. What factor determines the type of plastid a proplastid becomes?

• A) pH levels
• B) Oxygen concentration
• C) Light exposure ✅
• D) Soil nutrients

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11. What is an amyloplast?

• A) A chloroplast in developing cells
• B) A plastid that stores starch ✅
• C) A pigment-containing plastid
• D) A type of mitochondrion

12. In which plants are amyloplasts most abundant?

• A) Leafy vegetables
• B) Starchy vegetables like potatoes ✅
• C) Fruits
• D) Grasses

13. What do chromoplasts store?

• A) Chlorophyll
• B) Lipids
• C) Red, orange, and yellow pigments ✅
• D) Starch

14. What is the primary role of chloroplasts?

• A) Respiration
• B) Photosynthesis ✅
• C) Water transport
• D) Protein synthesis

15. What triggers the transformation of proplastids into chloroplasts?

• A) Darkness
• B) Water absorption
• C) Light exposure ✅
• D) DNA replication

16. Which membrane structure forms first during chloroplast development?

• A) Thylakoid membranes
• B) Tubules from the inner membrane ✅
• C) Prolamellar body
• D) Envelope membrane

17. What pigment is synthesized during chloroplast development?

• A) Carotenoids
• B) Anthocyanins
• C) Chlorophyll ✅
• D) Xanthophylls

18. What happens if plant seedlings grow in darkness?

• A) They develop amyloplasts
• B) They develop etioplasts ✅
• C) They form more mitochondria
• D) They produce more xylem

19. What structure is found in etioplasts?

• A) Prolamellar body ✅
• B) Thylakoids
• C) Chromoplast pigments
• D) Starch granules

20. What is the shape of the prolamellar body?

• A) Spherical
• B) Hexagonal ✅
• C) Cuboidal
• D) Irregular

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21. What happens when a chloroplast is kept in darkness?

• A) It turns into an amyloplast
• B) It disintegrates
• C) It converts into an etioplast ✅
• D) It stops photosynthesis but retains structure

22. What happens to the thylakoid membranes in the dark?

• A) They expand
• B) They break into tubules ✅
• C) They turn red
• D) They absorb more chlorophyll

23. What is the main pigment precursor in etioplasts?

• A) Anthocyanin
• B) Protochlorophyllide ✅
• C) Xanthophyll
• D) Carotene

24. How do etioplasts respond to light?

• A) They turn into chromoplasts
• B) They develop a prolamellar body
• C) They convert into chloroplasts ✅
• D) They store starch

25. What happens to the prolamellar body upon illumination?

• A) It enlarges
• B) It transforms into tubules ✅
• C) It stores pigments
• D) It produces ATP

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31. What is produced as etioplasts turn into chloroplasts?

• A) Thylakoids ✅
• B) Amyloplasts
• C) Chromoplasts
• D) Xylem

32. What component forms in the thylakoid membranes?

• A) Mitochondria
• B) Photosynthetic electron transfer chain ✅
• C) Ribosomes
• D) Starch granules

33. What molecule absorbs light to initiate chloroplast conversion?

• A) Carotenoid
• B) Protochlorophyllide ✅
• C) Xanthophyll
• D) Hemoglobin

34. How fast does an etioplast convert into a chloroplast upon illumination?

• A) A few minutes
• B) A few hours ✅
• C) A few days
• D) Several weeks

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41. What is the primary function of thylakoids?

• A) Water storage
• B) Light-dependent photosynthesis ✅
• C) DNA replication
• D) Protein transport

42. Where is chloroplast DNA located?

• A) In the stroma ✅
• B) In the cell nucleus
• C) In the cytoplasm
• D) In the thylakoid membrane

43. What is the evolutionary origin of chloroplasts?

• A) They evolved from mitochondria
• B) They evolved from cyanobacteria ✅
• C) They formed from the nucleus
• D) They appeared independently

44. What structures provide chloroplasts with their own division mechanism?

• A) Spindle fibers
• B) Binary fission machinery ✅
• C) Ribosomes
• D) Cytoskeleton