BOTANY Higher Secondary Second Year - Textbooks Online

cytochrome, DNA, RNA, manganese, etc. Chloroplasts are generally
considerably larger than mitochondria.
The space enclosed by the envelope is filled with matrix called stroma.
In the stroma, many grana are embedded. In each granum, several disc
shaped lamellae are found. These disc shaped structures are called
thylakoids. They resemble a stack of coins. This structure is known
granum. Generally a chloroplast contains 40 to 60 grana. The
photosynthetic pigments are found in grana. The stroma contains circular
DNA, RNA and enzymes for starch synthesis.
5.2.3. Photochemical and biosynthetic phases
The pigments involved in photosynthesis are called photosynthetic
pigments. They are chlorophyll ‘a’, chlorophyll ‘b’, carotenoids,
xanthophyll and phycobilins. Magnesium is an essential component for
the formation of chlorophyll. Chlorophyll ‘a’ is a universal pigment present
in the plants in which water is one of the raw materials for photosynthesis.
Chlorophylls are highly efficient in absorbing solar energy and they are
directly linked to photosynthetic electron transport. Photosynthetic pigments
other than chlorophyll ‘a’ are generally called accessory pigments eg.
chlorophyll ‘b’, carotenoids and xanthophyll, whereas chlorophyll ‘a’ is
regarded as primary pigment.
Photosynthetic pigments occur in the granum. They constitute the
pigment system called photosystem. About 250 to 400 pigment molecules
are present in a photosystem. Two types of photosystems are found in the
granum. Photosystem I (PS I) has less accessory pigments and more
chlorophyll ‘a’, while photosystem II (PS II) has more accessory pigments
and less chlorophyll ‘a’. The primary function of photosystems is to trap
light energy and converts it to chemical energy. The energy absorbed by
accessory pigments is transferred to the chlorophyll ‘a’. The granal lamella
where the photosynthetic pigments are aggregated to perform photosynthetic
activities is called active centre.
Mechanism of photosynthesis
The overall reaction of photosynthesis can be written as follows.
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