Structure of Chloroplast

 Each chloroplast is surrounded by a double membrane system. The external surface of the outer membrane

is smooth whereas the inner membrane is thrown into lamellated structure showing two distinct regions,

called Stroma and Grana Lamellae.

Stroma

Stroma is the main site for the dark reaction of photosynthesis. It forms the matrix (ground substance) of

the chloroplast. In the matrix, the lamellae are loosely arranged. The lamellae found in the stroma are called as

Stroma Lamellae. Besides these structures, ribosomes and osmophilic granules are also found in the stroma.

Besides photosynthesis, lipid, protein, and nucleic acid metabolisms are also found to take place in the

stroma region of the chloroplast.

Grana

Granum (= singular) is the most but complicated portion of the chloroplast. Light reaction of photosynthesis

takes place only in the grana region of chloroplasts. About 40-60 grana are present in each chloroplast.

In the granum region, the lamellae are compactly arranged just like a stack of coins arranged one above the

other back to back.

Grana lamellae from sac-like structures called Thylakoids. In higher plants, these thylakoids are closely

packed together. Park and Begins (1964,1967) observed some projections known as Thylakoid Bodies. On these

thylakoid bodies, some specialized smaller and rounded particles are found which are named Quantasomes.

These quantasomes are known to contain chlorophyll molecules (200-300 chlorophylls per quantasome) arranged

in a monolayer (single layer). Later on, these Quantasomes are referred to as Photosynthetic Units.

Photosynthetic Pigments

There are three types of photosynthetic pigments, viz.,

1. Chlorophylls

2. Carotenoids

3. Phycobillins

Properties of photosynthetic pigments

* Chlorophylls and carotenoids are insoluble in water and can be extracted only with organic solvents like

acetone, petroleum ether, etc.

* Carotenoids include carotenes and xanthophylls (or carotenols)

* Different pigments absorb light of different wavelengths and show characteristic absorption peaks both

in vivo and in vitro (refer to Table below)

* They show fluorescence

* Chlorophylls and carotenoids are insoluble in water and can be extracted only with organic solvents.

* Phycobillins are soluble in water.




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