A) What is pheophytin?
B) What is the significance of `C_(4)` cycle?

Explanation

A. Pheophytin is a modified chlrophyll a molecule in which two H atoms have replaced the central `Mg^(2+)`. It accepts the electrons from `P_(680)` and passes them to a special molecule of quinone.
B) `C_(4)` cycle is of great adaptive advantage to the tropical plants which can continue even under much low `CO_(2)` concentration when `C_(3)` plants stop photosynthesis. The `C_(4)` cycle is more efficient than the `C_(3)` cycle due to absence of photorespiration in `C_(4)` plants.