Light Harvesting Complex: Antenna and Reaction Centre
Biology · Photosynthesis in Higher Plants · NEET
In each photosystem, hundreds of pigment molecules bound to proteins form the Light Harvesting Complex (LHC). All these pigments except one special chlorophyll a make up the antenna, which absorbs different wavelengths of light and funnels the energy to a single chlorophyll a molecule called the reaction centre. Memory hook: the antenna is like many hands passing a bucket of water to one central tap, the reaction centre, where the real work of pushing out an electron happens.
Antenna pigments of the light harvesting complex absorb many wavelengths and funnel the energy to a single reaction-centre chlorophyll a, which then releases an electron. PS I uses P700, PS II uses P680, named in order of discovery.
Your doubts, answered
What is the Light Harvesting Complex (LHC)?
The LHC is a group of hundreds of pigment molecules bound to proteins, present inside each photosystem (PS I and PS II). It contains chlorophyll a, chlorophyll b, xanthophylls and carotenoids. Its job is to absorb light of many different wavelengths so photosynthesis becomes more efficient. NCERT states the pigments are organised into two discrete light harvesting complexes within PS I and PS II.
What is the antenna and how is it different from the reaction centre?
The antenna is the light-harvesting part of the photosystem, made of ALL the pigments EXCEPT one molecule of chlorophyll a. The antenna only absorbs light and passes (funnels) that energy inward. The single chlorophyll a molecule left out forms the reaction centre, where the trapped energy finally excites an electron that leaves the molecule. So: antenna = collects energy, reaction centre = releases the electron.
How many chlorophyll a molecules form the reaction centre?
Only ONE. Each photosystem has just a single chlorophyll a molecule at its reaction centre. All the other pigment molecules (including other chlorophyll a molecules) belong to the antenna. This single-molecule point is a favourite NEET trap.
What is the difference between P700 and P680?
Both are reaction-centre chlorophyll a molecules, but they sit in different photosystems. In PS I the reaction centre chlorophyll a absorbs maximally at 700 nm, so it is called P700. In PS II it absorbs maximally at 680 nm, so it is called P680. The 'P' stands for pigment and the number is the absorption peak in nanometres.
Why does the antenna have so many different pigments?
Different pigments absorb different wavelengths of light. Chlorophyll a absorbs mainly blue and red, while accessory pigments like chlorophyll b, xanthophylls and carotenoids capture wavelengths that chlorophyll a misses. By having many pigments, the antenna captures a wider range of light and makes photosynthesis more efficient, then funnels all that energy to the reaction centre.
Are PS I and PS II named in the order they work?
No. This is a common confusion. PS I and PS II are named in the sequence of their DISCOVERY, not the sequence in which they function during the light reaction. During non-cyclic electron transport, PS II actually acts first, then PS I.
⚠️ The NEET trap ✗ The reaction centre is made of many chlorophyll a molecules, and PS I works before PS II because it is number one. ✓ The reaction centre is a SINGLE chlorophyll a molecule per photosystem; the rest of the pigments form the antenna. PS I and PS II are named by order of discovery, not order of function, and PS II actually acts first in the Z-scheme. 🧠 NEET loves the words 'single' and 'discovery'. Remember: ONE chlorophyll a = reaction centre, and the numbers I and II are discovery labels, not a work timetable.
Real NEET questions
2023
The reaction centre in PS II has an absorption maxima at
A · 680 nm ✓
B · 700 nm
C · 660 nm
D · 780 nm
Solution: Each photosystem has one reaction-centre chlorophyll a. In PS I it absorbs maximally at 700 nm (P700); in PS II it absorbs maximally at 680 nm and is called P680. So the PS II reaction centre absorbs at 680 nm.
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 PS I the reaction-centre pigment is a single chlorophyll a molecule with an absorption peak at 700 nm, hence named P700. The reaction centre is always chlorophyll a in both photosystems (P680 in PS II).
2016
Emerson's enhancement effect and Red drop have been instrumental in the discovery of:
A · Photophosphorylation and non-cyclic electron transport
B · Two photosystems operating simultaneously ✓
C · Photophosphorylation and cyclic electron transport
D · Oxidative phosphorylation
Solution: Emerson's enhancement effect and the Red drop showed that photosynthesis needs two photosystems (with two light harvesting complexes, PS I P700 and PS II P680) working together. Efficiency rises when both operate at once, revealing the two-photosystem design.
Solved Photosynthesis in Higher Plants NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
What is a light harvesting complex in simple words?
It is a set of hundreds of pigment molecules attached to proteins inside a photosystem. It catches sunlight of many colours and sends the energy to one central chlorophyll a, the reaction centre.
Is the antenna the same as the reaction centre?
No. The antenna is all the pigments except one chlorophyll a and only collects and passes energy. The reaction centre is that single chlorophyll a where an electron is finally excited and released.
How many reaction centre chlorophyll a molecules are there per photosystem?
Just one. Every other pigment in the photosystem is part of the antenna.
Where is the light harvesting complex located?
Inside the thylakoid membranes of the chloroplast, as part of Photosystem I and Photosystem II.
What do P700 and P680 stand for?
P means pigment. P700 is the reaction-centre chlorophyll a of PS I that absorbs best at 700 nm. P680 is the reaction-centre chlorophyll a of PS II that absorbs best at 680 nm.
