The Red Drop is the sudden fall in photosynthesis efficiency when light beyond 680 nm (far-red) is used alone. The Emerson Enhancement Effect is the big rise in photosynthesis when far-red light (700 nm) and shorter red light (680 nm) are given together, more than the sum of the two alone. Together these two findings proved that photosynthesis needs TWO photosystems working at the same time: PS I (P700) and PS II (P680). Memory hook: "Drop happens with ONE light, Enhancement happens with TWO lights = TWO photosystems."
Far-red light alone (beyond 680 nm) causes the red drop as efficiency falls. Combining 680 nm and 700 nm light gives the enhancement effect, proving PS II (P680) and PS I (P700) must work together.
Your doubts, answered
What exactly is the 'Red Drop'?
When you give plants only far-red light (wavelength longer than 680 nm), the efficiency of photosynthesis suddenly drops, even though this light is still absorbed by chlorophyll. This sharp fall is called the red drop. It happens because far-red light alone can excite only PS I (P700) and cannot drive PS II (P680) well, so the two systems no longer work together.
What is the Emerson Enhancement Effect?
When far-red light (about 700 nm) and shorter-wavelength red light (about 680 nm) are given TOGETHER, the rate of photosynthesis becomes much higher than the sum of the two lights given separately. This extra boost is the enhancement effect. It shows both photosystems must be switched on at the same time to get full efficiency.
What did these two observations prove for NEET?
They proved that photosynthesis is run by TWO distinct photosystems acting together, not one. NCERT states the pigments are organised into two light-harvesting complexes: PS I with reaction centre P700 (peak 700 nm) and PS II with reaction centre P680 (peak 680 nm). This is the single most-tested point from this topic.
Why is far-red light alone (beyond 680 nm) less efficient?
Far-red light can be absorbed by PS I (P700) but is too weak in energy to drive PS II (P680) properly. With PS II idle, the electron chain between the two systems breaks, so oxygen release and NADPH formation fall. That is why efficiency drops sharply beyond 680 nm.
Red drop vs enhancement effect - are they opposite things?
They are two parts of the same story. Red drop shows the PROBLEM (one long-wavelength light alone is inefficient). Enhancement effect shows the SOLUTION (adding a second, shorter-wavelength light restores and boosts efficiency). Both point to the same conclusion: two photosystems are needed.
⚠️ The NEET trap ✗ Emerson enhancement effect discovered photophosphorylation / cyclic electron transport. ✓ It helped discover that TWO photosystems (PS I and PS II) operate simultaneously. 🧠 NEET 2016 tested this exact option. The answer is 'two photosystems operating simultaneously', NOT photophosphorylation. Enhancement = TWO lights = TWO photosystems.
Real NEET questions
NEET 2016 Phase 1
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: The red drop (fall in efficiency beyond 680 nm) and the enhancement effect (boost when 680 nm and 700 nm light are combined) together revealed that photosynthesis needs two photochemical systems acting together. NCERT states pigments are organised into two light-harvesting complexes: PS I (P700) and PS II (P680), working in series. Hence the correct answer is 'two photosystems operating simultaneously' (B), not any phosphorylation mode.
Solved Photosynthesis in Higher Plants NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Robert Emerson. His work on red drop and the enhancement effect led to the understanding that two photosystems (PS I and PS II) work together in the light reaction of photosynthesis.
At what wavelengths do PS I and PS II work?
NCERT says PS I reaction centre chlorophyll a peaks at 700 nm (called P700) and PS II reaction centre chlorophyll a peaks at 680 nm (called P680).
Does red drop mean far-red light is not absorbed?
No. Far-red light IS absorbed by chlorophyll, but it drives only PS I. Because PS II stays inactive, overall efficiency drops. Absorption still happens; the efficient two-system teamwork does not.
Is the Emerson enhancement effect directly in NCERT Class 11?
NCERT presents the outcome of this work: two photosystems (PS I P700 and PS II P680) organised into two light-harvesting complexes that act together. NEET questions test this conclusion, so learn the P700/P680 link firmly.
