Chromophore - Development of plant
The chromophore is a tetrapyrrole molecule like chlorophyll, but unlike chlorophyll it is an open tetrapyrrole and contains no metal ion. It is covalently attached to the protein.
Figure: The structure of chromophore and its attachment to protein. Phytochrome chromophore is an open tetrapyrrole. It is attached to the protein at a specific site and undergoes proton shift on conversion from Pr to Pfr.
It has been shown that it is the Pfr form of phytochrome that is biologically active. Changes in the structure occur during conversion of Pr to Pfr which probably make the Pfr form active.