Question

Which pigment has absorption peak at 700 nm in the photosynthetic reaction centre PS I (P700) ?

• A. Carotenoids
• B. Chlorophyll b
• C. Chlorophyll a
• D. Xanthophylls

Hint:

Remember the golden rule of photosynthesis: Chlorophyll a is the "King" pigment. It is the *only* pigment that can form the operational reaction centres (P680 in PS II and P700 in PS I). All other pigments (Chlorophyll b, Carotenoids, Xanthophylls) are merely "soldiers" or accessory pigments forming the antenna complex to harvest light and protect the King!

Answer 1

The Correct Option is C

Step 1: Understand the reaction centre.
Each photosystem has a reaction centre surrounded by accessory light-harvesting pigments.
Step 2: Identify the core pigment.
The reaction centre molecule in both PS I and PS II is always a special chlorophyll a.
Step 3: Decode P700.
In PS I this pigment is named P700, where P means pigment and 700 is its absorption peak at 700 nm in the red region. So the pigment is chlorophyll a.
Step 4: Reject carotenoids and xanthophylls.
Carotenoids and xanthophylls are accessory pigments absorbing in the blue region and never form the reaction centre.
Step 5: Reject chlorophyll b.
Chlorophyll b is an accessory pigment that passes energy on to chlorophyll a, so it is not the P700 core.
Step 6: Conclude.
The pigment with the 700 nm peak at the PS I reaction centre is chlorophyll a.
\[ \boxed{\text{Chlorophyll a}} \]