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Botany · Photosynthesis in Higher Plants

Action Spectrum & Absorption Spectrum

Two graphs — one measured in a cuvette, the other measured in a living leaf — together explain which pigments drive photosynthesis and why two cooperating photosystems are necessary. NCERT Section 11.4 anchors this subtopic; NEET asks it directly through the Emerson Enhancement Effect and Engelmann's experiment. Expect at least one question per three-year window, often disguised in a "which discovery proved two photosystems" format.

NCERT Grounding

NCERT Class 11 Biology, Chapter 11 (Photosynthesis in Higher Plants), Section 11.4 presents three graphs side by side: the absorption spectrum of chlorophyll a and b plus carotenoids (Figure 11.3a), the action spectrum of photosynthesis (Figure 11.3b), and a superimposed comparison of the two (Figure 11.3c). The text states: "These graphs, together, show that most of the photosynthesis takes place in the blue and red regions of the spectrum; some photosynthesis does take place at other wavelengths of the visible spectrum." The NIOS Biology Chapter 11 reinforces the definitions directly: "An action spectrum is a graph showing the effectiveness of different wavelengths of light in stimulating the process of photosynthesis … An absorption spectrum is a graph representing the relative absorbance of different wavelengths of light by a pigment."

"Chlorophyll a is the chief pigment associated with photosynthesis — but accessory pigments enable a wider range of wavelengths to be utilised."

NCERT Class 11 Biology, Section 11.4

Absorption Spectrum

The absorption spectrum of a pigment is obtained by passing white light through a solution of that pigment and measuring, at each wavelength, the fraction of incident light that is absorbed rather than transmitted. The instrument used is a spectrophotometer: it splits light into its component wavelengths (or scans across them with a monochromator) and records absorbance as a function of wavelength. The resulting curve is a property of the pigment molecule itself — its electronic structure determines which photon energies are captured.

For the photosynthetic pigments of higher plants, the key absorption maxima are tabulated below.

Pigment Blue / Violet Peak Red / Orange Peak Colour Reflected Location
Chlorophyll a ~430 nm ~680 nm Blue-green Thylakoid; reaction centre P680 / P700
Chlorophyll b ~450 nm ~640 nm Yellow-green Thylakoid; antennae complex
Carotenoids (carotene + xanthophylls) 400–500 nm (broad) None significant Yellow-orange Thylakoid; antennae complex

A crucial observation: all three pigment groups absorb poorly in the green region (500–560 nm). Green light is largely reflected, which is why leaves appear green to the eye. This fact directly predicts that the action spectrum will also show a trough in the green region.

Figure 1 — Absorption Spectra of Photosynthetic Pigments Absorption Spectra of Photosynthetic Pigments 400 450 500 550 600 650 700 750 Wavelength (nm) Absorbance 0 0.5 1.0 430 nm 680 nm 450 nm 640 nm 400–500 nm Chlorophyll a Chlorophyll b Carotenoids

Figure 1. Schematic absorption spectra of the three major photosynthetic pigment groups. Chlorophyll a (teal, solid) shows peaks at approximately 430 nm and 680 nm. Chlorophyll b (green, dashed) shows peaks at approximately 450 nm and 640 nm. Carotenoids (amber) absorb broadly across 400–500 nm. All three groups show minimal absorption in the green region (500–560 nm).

Action Spectrum

The action spectrum of photosynthesis is obtained experimentally by measuring the rate of a photosynthetic output — typically O2 evolution or CO2 fixation — at each wavelength of incident monochromatic light, while holding light intensity constant. The resulting graph plots photosynthetic rate (y-axis) against wavelength (x-axis).

The action spectrum of photosynthesis shows:

Maximum — Violet-Blue (~430–450 nm)

Highest photosynthetic rates here because both chlorophylls and carotenoids absorb strongly; all pigments contribute simultaneously.

High activity zone

Second Peak — Red (~660–680 nm)

Chlorophyll a and b both absorb here; the reaction centre P680 of PSII is directly excited, driving the full Z-scheme.

P680 territory

Minimum — Green (~500–560 nm)

Green light is reflected by chlorophyll; very little is absorbed, so photosynthetic rate is near its minimum — giving the action spectrum its characteristic trough.

Exam favourite

Drop — Far Red (>680 nm)

Rate falls sharply beyond 680 nm — the red drop discovered by Emerson. P700 of PSI absorbs here but cannot function efficiently without PSII input.

Red Drop territory

Engelmann's Experiment

The first experimental action spectrum of photosynthesis was produced by T.W. Engelmann (1843–1909). His experimental design was elegant and required no specialised instruments beyond a prism.

Engelmann's Experimental Protocol (1882)

4 steps
  1. Step 1

    Prism Dispersion

    White light is split by a glass prism into its spectral components — a miniature rainbow of VIBGYOR — projected across a glass slide.

    Physics of dispersion
  2. Step 2

    Algal Filament Placement

    A filament of the green alga Cladophora (NCERT text) or Spirogyra is laid across the slide, so different parts of the filament lie in different wavelengths of light.

    Living detector
  3. Step 3

    Bacterial Suspension

    The slide is bathed in a suspension of aerobic bacteria (motile, O2-seeking). Bacteria migrate toward regions of highest O2 concentration — regions of highest photosynthetic rate.

    O2 biosensor
  4. Step 4

    Observation

    Bacteria cluster densely at the blue-violet end and the red end of the spectrum. Very few bacteria are found in the green region. This maps directly to the action spectrum.

    First action spectrum

The NCERT text records: "He observed that the bacteria accumulated mainly in the region of blue and red light of the split spectrum. A first action spectrum of photosynthesis was thus described. It resembles roughly the absorption spectra of chlorophyll a and b."

Why the Action Spectrum Exceeds the Absorption Spectrum of Chlorophyll a Alone

When the action spectrum is overlaid on the absorption spectrum of chlorophyll a alone (NCERT Figure 11.3c), the match is imperfect: the action spectrum shows significant photosynthetic activity at wavelengths — particularly in the blue-violet region around 450–480 nm — where chlorophyll a absorbs only weakly. The explanation lies in accessory pigments and resonance energy transfer.

Absorption Spectrum of Chl a Alone vs. Action Spectrum of Whole Plant

Absorption Spectrum (Chl a only)

Narrow

Coverage of visible spectrum

  • Strong absorption at 430 nm (blue) and 680 nm (red)
  • Minimal absorption at 450–480 nm and 500–640 nm
  • Reflects what one pigment's electrons can absorb
  • Measured on an isolated pigment solution in a spectrophotometer
vs

Action Spectrum (Whole Leaf)

Broader

Coverage of visible spectrum

  • High activity at 430 nm AND at 450–480 nm (carotenoid + Chl b zones)
  • Secondary peak at 640–680 nm (Chl b + Chl a combined)
  • Reflects combined harvest of all antennae pigments
  • Measured as O2 evolution or CO2 fixation in a living system

The mechanism is Förster resonance energy transfer (FRET): when an accessory pigment (chlorophyll b, carotene, xanthophyll) absorbs a photon, its excited electron does not drive photochemistry directly. Instead, it transfers excitation energy — without a physical electron transfer — to an adjacent chlorophyll a molecule in the antennae complex. This energy migrates, pigment by pigment, until it reaches the reaction centre chlorophyll a (P680 or P700), where it is used to drive electron ejection. The result: every photon absorbed by any antennae pigment is eventually funnelled to the same reaction centres, and the action spectrum is the sum of all antennae contributions.

Accessory pigments serve a dual role stated in NCERT: they "not only enable a wider range of wavelengths of incoming light to be utilised for photosynthesis but also protect chlorophyll a from photo-oxidation."

Red Drop and the Emerson Enhancement Effect

The Red Drop (Emerson, 1943)

Robert Emerson, measuring quantum yield (moles O2 per photon absorbed) at various wavelengths, found that photosynthetic efficiency remained relatively constant across 600–680 nm but then fell sharply beyond 680 nm — even though chlorophyll a still absorbs light up to about 700 nm. This precipitous decline in efficiency in the far-red region became known as the red drop.

680 nm

Red Drop Onset

Beyond this wavelength, quantum yield of photosynthesis drops steeply despite continued chlorophyll a absorption. Corresponds to the absorption maximum of PSII reaction centre P680.

+ 700 nm

PSI P700 Peak

Reaction centre of Photosystem I. Absorbs far-red light efficiently but cannot sustain photosynthesis alone — it requires electrons from PSII (water splitting) to function.

The Emerson Enhancement Effect (Emerson, 1957)

Emerson then conducted a decisive experiment: he simultaneously illuminated plants with far-red light (~700 nm) and shorter red light (~650 nm). The rate of photosynthesis produced by the two beams together was greater than the sum of the rates produced by each beam alone. This synergistic increase is the Emerson Enhancement Effect.

Figure 2 — Emerson Enhancement Effect Emerson Enhancement Effect — Bar Comparison Rate of Photosynthesis 25 50 75 Far-red (~700 nm) ~16% Red alone (~650 nm) ~35% Expected sum ~51% Both together ~78% ↑ Enhancement

Figure 2. Schematic representation of the Emerson Enhancement Effect. Far-red light (~700 nm) alone produces very low photosynthetic rates. Red light (~650 nm) alone produces moderate rates. When both beams illuminate simultaneously, the rate exceeds the arithmetical sum of the two individual rates — the enhancement effect — demonstrating cooperative action of PSI and PSII.

The logical consequence of these two observations — red drop and enhancement — was the hypothesis that photosynthesis requires two separate photochemical systems that must cooperate:

  • Photosystem II (P680): absorbs light at 680 nm and below; drives water splitting and electron entry into the Z-scheme.
  • Photosystem I (P700): absorbs far-red light at ~700 nm; reduces NADP+ to NADPH; depends on electrons fed from PSII.

When only far-red light (700 nm) is provided, PSI is excited but PSII is not adequately activated. Without PSII water-splitting, there are insufficient electrons to sustain the cyclic and non-cyclic electron flows necessary for high O2 evolution — hence the red drop. When both wavelengths are supplied, both photosystems operate concurrently at maximum efficiency, producing more O2 than either alone. This constitutes definitive evidence for the two-photosystem model, later formalised as the Z-scheme.

Worked Examples

Worked example 1

A student measures the rate of O2 evolution from a leaf illuminated with monochromatic light at 430 nm, 550 nm, and 680 nm respectively (same intensity in each case). Rank the rates from highest to lowest and explain.

Answer: 430 nm > 680 nm > 550 nm.
At 430 nm (blue-violet): chlorophyll a, chlorophyll b, and carotenoids all absorb strongly; combined antennae harvest is maximal — highest rate.
At 680 nm (red): chlorophyll a absorbs strongly (this is its red peak at P680); chlorophyll b also absorbs nearby at 640 nm; high photosynthesis — second highest.
At 550 nm (green): all pigments reflect most of this wavelength; virtually no absorption by any photosynthetic pigment — lowest rate (trough of action spectrum).

Worked example 2

Explain why the action spectrum of photosynthesis is broader than the absorption spectrum of chlorophyll a.

Answer: The action spectrum reflects the combined light-harvesting capacity of all pigments in the antenna complexes of PSI and PSII: chlorophyll a, chlorophyll b, carotene, and xanthophylls. Accessory pigments absorb wavelengths — particularly the 450–480 nm blue region — where chlorophyll a alone absorbs weakly. Through resonance energy transfer (FRET), excitation energy captured by these accessory molecules is channelled to the reaction centre chlorophyll a (P680 or P700). The photochemical reaction is therefore driven by photons originally absorbed by accessory pigments, making the effective photosynthetic range wider than what chlorophyll a alone can capture. Hence, the action spectrum is broader than the absorption spectrum of isolated chlorophyll a.

Worked example 3

What would happen to the rate of photosynthesis if a plant were illuminated exclusively with light at 710 nm? What would happen if 650 nm light were added simultaneously?

710 nm alone: Very low rate — this is in the far-red region beyond the red drop threshold (680 nm). PSI (P700) would be excited, but PSII (P680) would be poorly activated. Without adequate PSII activity, water splitting is insufficient, electron supply to PSI is limited, and quantum yield is drastically reduced. Cyclic photophosphorylation (PSI only) may still occur, producing some ATP but no NADPH and no O2.
Adding 650 nm simultaneously: Rate would increase dramatically — exceeding the arithmetic sum of the two individual rates. This is the Emerson Enhancement Effect: 650 nm activates PSII (P680) effectively, supplying electrons and driving water splitting; 710 nm continues to excite PSI (P700); both systems cooperate in the full Z-scheme, maximising NADPH and ATP production and O2 evolution.

Common Confusion & NEET Traps

Red Drop vs. Emerson Enhancement Effect — Side by Side

Red Drop

1943

Emerson and Lewis

  • Observation: quantum yield falls sharply beyond 680 nm
  • Implication: far-red light absorbed by P700 (PSI) alone cannot sustain efficient photosynthesis
  • Suggests that a second, shorter-wavelength-dependent system (PSII) is required
  • Measured with monochromatic far-red illumination only
+

Emerson Enhancement Effect

1957

Emerson et al.

  • Observation: rate with far-red + red > sum of rates individually
  • Implication: the two wavelength-classes drive two separate photosystems that act synergistically
  • Constitutes direct proof of PSI + PSII cooperating in series
  • Measured with simultaneous bichromatic illumination

NEET PYQ Snapshot — Action Spectrum & Absorption Spectrum

Direct and concept-based questions from previous NEET papers on this subtopic.

NEET 2016 — Q.62

Emerson's enhancement effect and Red drop have been instrumental in the discovery of:

  1. Two photosystems operating simultaneously
  2. Photophosphorylation
  3. Calvin cycle
  4. Photorespiration
Answer: (1)

Why: The red drop showed that quantum yield collapses beyond 680 nm (suggesting PSI alone is insufficient); the enhancement effect showed that adding shorter-wavelength light (activating PSII) raises the rate above the sum — together, these two findings proved that two photosystems (PSI and PSII) must operate simultaneously in series. Photophosphorylation (2) is related to ATP synthesis, Calvin cycle (3) is the dark reaction, and photorespiration (4) involves RuBisCO oxygenase activity — none of these are proven by the red drop or enhancement effect.

Concept

Which of the following correctly explains why the action spectrum of photosynthesis is broader than the absorption spectrum of chlorophyll a?

  1. Chlorophyll a has more absorption peaks at all wavelengths
  2. Accessory pigments absorb additional wavelengths and transfer energy to chlorophyll a
  3. Green light is converted to red light inside the leaf before absorption
  4. The action spectrum is measured at higher light intensity
Answer: (2)

Why: Accessory pigments (chlorophyll b, carotenoids) absorb wavelengths where chlorophyll a absorbs weakly (especially 450–480 nm). They transfer this energy to the chlorophyll a reaction centre via resonance transfer, effectively broadening the usable spectrum. Options (1), (3), and (4) are factually incorrect.

Concept

In Engelmann's experiment, aerobic bacteria clustered around the region of Spirogyra/Cladophora exposed to blue and red light because:

  1. These regions had the highest temperature
  2. O2 evolution was maximum in blue and red light
  3. CO2 concentration was lowest in these regions
  4. The bacteria absorbed blue and red light directly
Answer: (2)

Why: The aerobic bacteria were O2-seeking. Photosynthesis is most efficient at blue and red wavelengths (action spectrum peaks), so O2 evolution is greatest there. Bacteria migrated to these high-O2 zones. The experiment mapped the action spectrum using bacteria as a biological O2 detector.

FAQs — Action Spectrum & Absorption Spectrum

Frequently asked questions from NEET aspirants on this subtopic.

What is the difference between absorption spectrum and action spectrum?

The absorption spectrum graphs the fraction of light absorbed by a pigment at each wavelength, measured by spectrophotometry. The action spectrum graphs the rate of photosynthesis at each wavelength. The action spectrum is broader than the absorption spectrum of chlorophyll a alone because accessory pigments (chlorophyll b, carotenoids) absorb additional wavelengths and transfer that energy to chlorophyll a.

What did Engelmann's experiment prove?

T.W. Engelmann (1882) split white light through a prism onto Spirogyra (or Cladophora) placed in a suspension of aerobic bacteria. Bacteria clustered densely in the blue-violet and red regions of the spectrum — exactly where O2 evolution was greatest — thereby producing the first action spectrum of photosynthesis and demonstrating that blue and red light drive photosynthesis most effectively.

What is the red drop in photosynthesis?

The red drop (Emerson, 1943) is the sudden, steep fall in the quantum yield of photosynthesis at wavelengths beyond 680 nm, even though chlorophyll a still absorbs light up to about 700 nm. This indicated that light absorbed by the far-red-absorbing form of chlorophyll (P700, PSI) alone cannot drive photosynthesis efficiently without the shorter-wavelength-driven system (P680, PSII).

What is the Emerson Enhancement Effect and what did it prove?

Emerson (1957) found that illuminating a plant simultaneously with far-red light (~700 nm) and shorter red light (~650 nm) produced a rate of photosynthesis greater than the sum of the rates obtained with either wavelength alone. This enhancement effect proved that two photochemical systems (PSI and PSII) operate cooperatively: PSI absorbs far-red and PSII absorbs red/blue, and both must function together for maximum efficiency.

Why does the action spectrum exceed the absorption spectrum of chlorophyll a?

Accessory pigments — chlorophyll b, xanthophylls, and carotenoids — absorb wavelengths (especially 400–500 nm blue-violet) not strongly captured by chlorophyll a, and then transfer that excitation energy to chlorophyll a via resonance energy transfer. The action spectrum therefore reflects the combined harvesting capacity of all pigments, making it broader than the absorption spectrum of chlorophyll a alone.

At which wavelengths does chlorophyll a show maximum absorption?

Chlorophyll a shows two absorption peaks: one in the blue-violet region at approximately 430 nm and the other in the red region at approximately 680 nm. It reflects green light (around 500–560 nm), which is why leaves appear green. The reaction centre form P700 (in PSI) has a far-red absorption peak near 700 nm.

What are the absorption peaks of chlorophyll b and carotenoids?

Chlorophyll b absorbs maximally at approximately 450 nm (blue) and 640 nm (red-orange). Carotenoids (carotene and xanthophylls) absorb predominantly in the blue-violet range of 400–500 nm. Both groups transfer absorbed energy to chlorophyll a, extending the usable portion of the solar spectrum for photosynthesis.

Related Topics
Photosynthesis in Higher Plants Back to the full chapter notes and all subtopics. Photosynthetic Pigments Chromatography, pigment roles, and NEET-level detail on Chl a, Chl b, and carotenoids. Light Reaction & Z-Scheme How PSI and PSII cooperate in the full electron transport chain. Splitting of Water Photolysis at PSII, O2 evolution, and the Hill reaction. Cyclic vs Non-Cyclic Photophosphorylation When only PSI operates (cyclic) vs. both photosystems (non-cyclic). Early Experiments in Photosynthesis Priestley, Ingenhousz, Sachs, and the historical groundwork. Factors Affecting Photosynthesis Light intensity, CO2, temperature, water — Blackman's law of limiting factors.