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Chloroplast is the photosynthetic apparatus in plants. The green leaves perform the photosynthesis which is the source of food for all living organisms. Chloroplast is usually discoid in shape and is surrounded by two unit membrane. The proteinaceous fluid that is filled inside the chloroplast is called stroma. Membrane bound sac like structure present in the stroma are called as the thylakoid. Each thylakoid seen in the granum is called as the grana thylakoid.

The light reaction takes place in the granum and dark reaction takes place in the stroma. Chloroplast contains chlorophyll, carotenoid, xanthophyll, cytochrome, DNA, RNA etc. The photosynthetic pigments are present in the grana of the chloroplast. The smallest unit of pigment molecule which is capable of carrying out light reaction is known as photosynthetic unit or quantosome. The photosynthetic pigments are chlorophyll a, chlorophyll b, carotenoids, xanthophyll and phycobilins etc. The photosynthetic pigment constitutes the pigment system known as photosystem.

There are two types of photosystems they are PS I and PS II.

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