Bacteriochlorophyll differs from chlorophyll $a$ in having:

Identify $X$ and $Y$ from the figure.
$X$ $-$ $Y$

Who revealed the chemical composition of chlorophyll,carotene,and xanthophyll?

Pigment system-$I$ receives radiant energy and releases an electron from which reaction center?

Photosystem-$I$ contains

In the figure given below,the black line (upper) indicates the action spectrum for photosynthesis and the lighter line (lower) indicates the absorption spectrum of chlorophyll-$a$. Answer the following:
$(a)$ What does the action spectrum indicate? How can we plot an action spectrum? Explain with an example.
$(b)$ How can we derive an absorption spectrum for any substance?
$(c)$ If chlorophyll-$a$ is responsible for the light reaction of photosynthesis,why do the action spectrum and absorption spectrum not overlap?