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The NCERT photosynthesis chapter tells you there are two photosystems, PS I and PS II, and that they are named in the order of their discovery. It does not tell you how anyone found out there were two. Much of that story starts with Robert Emerson's experiments on green algae. This post explains his three main findings, works through the numbers, and links his work to the Z-scheme you draw in exams.
Who was Robert Emerson?
A history by the photosynthesis researchers Govindjee and Rajni Govindjee at the University of Illinois gives the outline. Emerson was born in New York City in 1903, earned a master's degree in zoology at Harvard in 1925 and a PhD in botany in Berlin in 1927 under Otto Warburg, and died in 1959. He did much of his later work at the University of Illinois at Urbana. The same history lists his main discoveries: the photosynthetic unit, the minimum number of light quanta needed per oxygen molecule, the red drop and the enhancement effect.
There is an Indian thread in this story. Govindjee and Rajni Varma (later Rajni Govindjee) were graduate students at the University of Allahabad in 1955, as a photograph caption on Govindjee's university page records, and later worked in the Illinois photosynthesis group. Their history notes that Rajni Govindjee showed in 1961 that the enhancement effect also appears in the Hill reaction.
Finding 1: the photosynthetic unit
With his student William Arnold, Emerson shone very short, bright flashes of light on suspensions of algal cells. Their 1932 paper in the Journal of General Physiology, The photochemical reaction in photosynthesis, found that about 2,480 chlorophyll molecules were present for each molecule of carbon dioxide reduced per flash, and that one unit took about 0.02 second at 25°C to complete its cycle.
So most chlorophyll molecules do not do the chemistry themselves. They gather light and pass the energy on. NCERT describes the same idea as a light-harvesting complex, or antenna, around a single reaction-centre chlorophyll a.
A worked example: using the 1932 numbers
Use sample numbers for the first part.
Suppose a sample of algae holds 1,000,000 chlorophyll molecules. If about 2,480 serve each unit, the number of units is 1,000,000 ÷ 2,480 ≈ 403.
If one unit needs about 0.02 second to finish a cycle, the most flashes it can use per second is 1 ÷ 0.02 = 50. Flashes closer together than 0.02 second would waste light.
Now the enhancement idea, again with sample numbers. Suppose far-red light alone gives a photosynthesis rate of 4 units, shorter red light alone gives 10, and the two together give 18.
Sum of separate rates: 4 + 10 = 14.
Together: 18, which is more than 14.
Enhancement ratio: 18 ÷ 14 ≈ 1.29.
A ratio above 1 is the signature of the effect.
Finding 2: the red drop
In 1943 Emerson and Charleton Lewis published The dependence of the quantum yield of Chlorella photosynthesis on wave length of light. They measured the quantum yield, the amount of oxygen produced per quantum of light absorbed, at different wavelengths. The yield fell sharply at the far-red end of the spectrum. This became known as the red drop.
Finding 3: the enhancement effect
In 1957 Emerson, Ruth Chalmers and Carl Cederstrand published Some factors influencing the long-wave limit of photosynthesis in PNAS. A 2019 review in Annals of Botany sums up the result: the rate of photosynthesis in two lights given together was higher than the sum of the rates in each light given separately. The review traces the idea of two light reactions and two photosystems to the red drop and the enhancement effect, and says the discovery led to the Z-scheme proposed by Robert Hill and Fay Bendall in 1960.
The NCERT Class 11 photosynthesis chapter (2026-27 reprint) gives the modern result: PS I has a reaction centre absorbing at 700 nm (P700) and PS II at 680 nm (P680). The Annals of Botany review calls the light absorbed mainly by PS I "light 1" and notes that far-red light is of this kind, so on its own it cannot keep both photosystems running at full speed.
Common mistakes in exam answers
Assuming PS I acts first because it is numbered first. NCERT says the names follow the order of discovery, not the order of action.
Mixing up the two effects. The red drop is low efficiency in far-red light alone. Enhancement is the boost when a shorter wavelength is added.
Thinking every chlorophyll molecule is a reaction centre. Most are antenna pigments.
For the full chapter, see our guide to photosynthesis for NEET, and for the earlier question of where oxygen comes from, read about Cornelis van Niel.
What can a student take from Emerson?
He trusted careful measurement over expectation. A drop in efficiency where chlorophyll still absorbs light was a puzzle, not an error to ignore, and following it up changed the field. When a result in your practice does not fit, write it down and ask what it would take to explain it. The Z-scheme is easier to understand when you can see energy levels change; the Fermi Simulation Lab has interactive simulations for ideas that are easier to see than read.
Next step: sketch the Z-scheme from memory, label P680 and P700, and mark which one far-red light mainly drives.
