Question

Describe the ultra-structure of a chloroplast.

Answer 1

Chloroplast has two unit membranes separated by a space called periplastidial space. The heterogeneous nature of chloroplast is due to the presence of disc-like structures called the grana and the colourless matrix called stroma. The grana are the site for the light reactions and the stroma for dark reactions. The grana are interconnected through stroma lamellae or frets, running parallel to each other. The granum is made of a stack of closed compartment called thylakoids. The membranes of the thylakoids contain a layer of particles called quantasomes. Each quantasome has 230 molecules of chlorophyll.

The chlorophyll molecule consists of porphyrin head made of four pyrrole rings with Mg⁺ in the centre and a long alcohol (phytol) tail joined to the fourth pyrrole ring. In each quantasome, the chlorophyll molecule is arranged along with other pigments between lipid and protein layer. The hydrophobic tail lies towards the lipid layer and hydrophilic porphyrin head extends towards the lipid layer and hydrophilic porphyrin head extends towards the protein layer. The pigment molecules associated with PSI complex are located towards the outer half of the thylakoid membrane and those with PS II towards the inner half.

Fig: Electron microscopic structure of a chloroplast