Final answer:
Phytochromes are related to red and far-red colors of light. They play a crucial role in plant growth and development by acting as a biological light switch, responding to changes in the light environment and regulating physiological activities and gene expression.
Step-by-step explanation:
Phytochromes are a family of chromoproteins that are related to red and far-red colors of light. These chromoproteins have a linear tetrapyrrole chromophore which is similar to chlorophyll. Phytochromes exist in two photo-interconvertible forms, Pr and Pfr. The Pr form absorbs red light and is immediately converted to the Pfr form. Conversely, the Pfr form absorbs far-red light and can quickly revert back to Pr. The physiologically active form of the phytochrome is Pfr, which is activated by red light and can induce various responses, including the activation of genes. Exposure to far-red light converts Pfr back to Pr, inhibiting the phytochrome's activity. This system allows plants to respond to the light environment and thus optimizes their growth as a biological light switch.