Why Chlorophyll a is the Chief Pigment of Photosynthesis
Biology · Photosynthesis in Higher Plants · NEET
Chlorophyll a is called the chief (main) pigment because it is the only pigment present at the reaction centre of both photosystems (P680 in PS II and P700 in PS I). It is the pigment that actually loses the excited electron and starts the light reaction. All other pigments (chlorophyll b, xanthophylls, carotenoids) are accessory pigments that only trap light and pass the energy to chlorophyll a. Memory hook: only chlorophyll a "does the work", the rest just "pass the ball" to it.
Accessory pigments (chlorophyll b, xanthophylls, carotenoids) absorb light and pass the energy to chlorophyll a at the reaction centre (P680 in PS II, P700 in PS I). Only chlorophyll a releases the excited electron and starts the light reaction, which is why it is the chief pigment.
Your doubts, answered
Why exactly is chlorophyll a the chief pigment and not chlorophyll b?
Because chlorophyll a is the pigment sitting at the reaction centre of both Photosystem I and Photosystem II. Only the reaction-centre chlorophyll a can lose the excited electron and start the electron transport chain. Chlorophyll b cannot do this step. So chlorophyll a does the actual photochemistry, which makes it the chief pigment.
If chlorophyll b, xanthophylls and carotenoids also absorb light, why are they only accessory?
They absorb light at wavelengths that chlorophyll a absorbs poorly, so they widen the range of light that can be used. But they cannot start the light reaction themselves. They only trap light energy and transfer it to chlorophyll a in the reaction centre. Because they help chlorophyll a instead of doing the chemistry, they are called accessory pigments.
Can a plant carry out photosynthesis with only chlorophyll b and no chlorophyll a?
No. This is a direct NCERT question. Without chlorophyll a there is no reaction-centre pigment to release the excited electron, so the light reaction cannot begin. Chlorophyll b would only absorb light and have nowhere to pass the energy. Plants keep chlorophyll b and other accessory pigments only to capture more wavelengths and to protect against excess light, but chlorophyll a is essential.
What is P680 and P700 and how do they connect to chlorophyll a?
P680 and P700 are both special forms of chlorophyll a at the reaction centres. P680 is the reaction-centre chlorophyll a of Photosystem II that absorbs 680 nm red light. P700 is the reaction-centre chlorophyll a of Photosystem I that absorbs 700 nm red light. Both are chlorophyll a molecules, which is why chlorophyll a is present in the reaction centre of both photosystems.
Does chlorophyll a absorb only one colour of light?
No. Chlorophyll a has two main absorption peaks, one in the blue region (around 430 nm) and one in the red region (around 660-680 nm). It absorbs green light poorly, which is why leaves look green (green light is reflected). Most photosynthesis takes place in the blue and red regions of the visible spectrum.
⚠️ The NEET trap ✗ Chlorophyll a is chief because it is present in the largest amount in the leaf. ✓ Chlorophyll a is chief because it is the pigment at the reaction centre (P680 and P700) that actually loses the excited electron and starts the light reaction. Being most abundant is not the defining reason. 🧠 NEET tests function, not amount. Remember: chief = reaction-centre pigment that does the chemistry, not just the most common pigment.
Real NEET questions
ReNEET 2026
Which pigment has an absorption peak at 700 nm in the photosynthetic reaction centre PS I (P700)?
A · Chlorophyll b
B · Chlorophyll a ✓
C · Xanthophylls
D · Carotenoids
Solution: In Photosystem I the reaction-centre chlorophyll a has an absorption maximum at 700 nm and is therefore called P700. The reaction-centre pigment of both photosystems is chlorophyll a (P680 in PS II, P700 in PS I), which is exactly why chlorophyll a is the chief pigment.
NEET 2025
Match List I (Pigment) with List II (Colour) and choose the correct answer. A. Chlorophyll a B. Chlorophyll b C. Xanthophylls D. Carotenoids ; I. Yellow-green II. Yellow III. Blue-green IV. Yellow to Yellow-orange
A · A-I, B-II, C-IV, D-III
B · A-I, B-IV, C-III, D-II
C · A-III, B-IV, C-II, D-I
D · A-III, B-I, C-II, D-IV ✓
Solution: Paper chromatography separates leaf pigments by colour: chlorophyll a is blue-green (III), chlorophyll b is yellow-green (I), xanthophylls are yellow (II), and carotenoids are yellow to yellow-orange (IV). Chlorophyll a (the chief pigment, blue-green) is always listed first because it is the reaction-centre pigment; the other three are accessory pigments.
NEET 2019 Odisha
One scientist cultured Cladophora in a suspension of Azotobacter and illuminated the culture by splitting light through a prism. He observed that bacteria accumulated mainly in the region of:
A · Violet and green light
B · Indigo and green light
C · Orange and yellow light
D · Blue and red light ✓
Solution: This is Engelmann's action-spectrum experiment. Aerobic bacteria collect where the most oxygen is released, that is where photosynthesis is highest. Photosynthesis peaks in the blue and red regions because chlorophyll a (the chief pigment) absorbs strongly there, so bacteria gather in the blue and red light regions.
Solved Photosynthesis in Higher Plants NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Is chlorophyll a the primary pigment or the chief pigment?
Both terms mean the same thing. Chlorophyll a is called the primary, main, or chief pigment because it sits at the reaction centre and starts the light reaction. All other pigments are accessory pigments.
What is the colour and formula difference of chlorophyll a?
Chlorophyll a is blue-green in colour. Its chemical formula is C55H72O5N4Mg. Chlorophyll b is yellow-green and has the formula C55H70O6N4Mg. For NEET, remember chlorophyll a is blue-green and is the chief pigment.
Why do leaves appear green if chlorophyll a is the chief pigment?
Chlorophyll a absorbs blue and red light strongly but reflects green light. Since green light is reflected back to our eyes, leaves look green even though that light is not used well for photosynthesis.
Are P680 and P700 different pigments?
No, both are chlorophyll a molecules. They differ only in the wavelength they absorb because of the proteins around them: P680 absorbs 680 nm (in PS II) and P700 absorbs 700 nm (in PS I).
Why is this concept important for NEET?
NEET repeatedly asks about reaction-centre pigments (P680, P700), pigment colours in chromatography, and accessory versus chief pigments. Knowing that chlorophyll a is the chief pigment because it works at the reaction centre helps you answer these directly.
