What is the Difference Between Absorption and Action Spectrum in Photosynthesis?

When studying plant biology and preparing for competitive exams like the Emirates Standardized Test (EmSAT) in the UAE, understanding how plants interact with light is crucial. Two of the most frequently confused concepts in plant physiology are the absorption spectrum and the action spectrum.

Although they look similar at a glance, they measure entirely different aspects of plant science. In this complete guide, we will break down the differences between the two, their similarities, and why they matter for your biology students or exam candidates.

Difference Between Absorption and Action Spectrum in Photosynthesis Graph

1. What is an Absorption Spectrum?

An absorption spectrum is a physical or biochemical measurement. It is a graph that plots the percentage of light energy absorbed by different photosynthetic pigments (such as Chlorophyll a, Chlorophyll b, and carotenoids) at different wavelengths of the light spectrum.

What it measures: The light absorption capacity of individual pigments.
How it is measured: Using an instrument called a spectrophotometer.
Key Peaks: Chlorophylls absorb light most strongly in the blue-violet range (~430–450 nm) and the red range (~640–660 nm).
The Trough: It drops significantly in the green-yellow region (~550 nm), which is why plants reflect or transmit green light, giving them their characteristic green color to the human eye.

Absorption spectra of chlorophyll a and b

2. What is an Action Spectrum?

An action spectrum, on the other hand, is a biological measurement. It is a graph showing the overall rate of photosynthesis driven by different wavelengths of light. Instead of measuring what individual pigments absorb, it measures the physiological effectiveness of those wavelengths in creating energy.

What it measures: The rate of the photosynthetic process (such as the volume of oxygen produced or carbon dioxide consumed).
How it is measured: By measuring oxygen evolution or $CO_2$ uptake.
Key Peaks: Like the absorption spectrum, the rate of photosynthesis is highest in the blue and red regions.
The Trough: Shows the lowest rate of photosynthesis in the green region.

[Insert Diagram: Action Spectrum Graph Here]

Alt text: Action spectrum of photosynthesis

3. Comparison of Absorption vs. Action Spectrum

Use this quick-reference comparison table for your revision notes and study guides.

Feature	Absorption Spectrum	Action Spectrum
Definition	A graph showing the wavelengths of light absorbed by individual pigments (e.g., chlorophyll).	A graph showing the overall rate of photosynthesis at different wavelengths of light.
Primary Focus	Measures the capacity of pigments to capture photons of light.	Measures the effectiveness of different light wavelengths in driving the chemical reaction.
Method of Measurement	Measured using a spectrophotometer in vitro.	Measured by oxygen release or carbon dioxide consumption in vivo.
Typical Shape	Shows multiple narrow peaks corresponding to specific pigment absorption.	Follows the cumulative absorption spectrum of all pigments but is broader.
Significance	Explains the physical properties of the light-absorbing molecules.	Explains the biological efficiency of light in producing energy.

4. Why are they slightly different?

If you overlay an absorption spectrum with an action spectrum, they are very similar, but they do not match perfectly. Why is this the case?

Accessory Pigments: Not all light absorbed by accessory pigments (like carotenoids) is transferred to Chlorophyll a with 100% efficiency.
In Vitro vs. In Vivo: The absorption spectrum is often measured on extracted pigments (in vitro), while the action spectrum is measured in a living leaf (in vivo).
Green Light Penetration: Some green light penetrates deeper into the leaf tissue and is absorbed by the lower layers of chloroplasts, which might not show up if you only measure the absorption of a single layer.

Advanced LevFrequently Asked Questions (FAQs) for EmSAT / Advanced Biology

Q1: Why is the action spectrum peak in the red region relatively larger or broader than the absorption spectrum peak?

Answer: In the action spectrum, the peak in the red region is often higher because photosystems (specifically P680 and P700 reaction centers) utilize red light directly with maximum quantum efficiency. While accessory pigments absorb light across various wavelengths, the direct energy transfer to these reaction centers makes red light extremely efficient for driving the chemical reactions of photosynthesis.

Q2: Why does the action spectrum show significant photosynthesis occurring in the green region, despite low pigment absorption?

Answer: In a living leaf (in vivo), green light is not completely wasted. The light penetrates deeper into the leaf's spongy mesophyll tissue, where underlying chloroplasts scatter, absorb, and utilize the light. This makes the action spectrum in the green region (~500–550 nm) significantly higher than the absorption spectrum of isolated pigments (in vitro).

Q3: What is the main difference in the tools used to measure these two spectra?

Answer: The absorption spectrum is measured using a spectrophotometer, which quantifies the percentage of light absorbed by dissolved or isolated pigments in a solvent. The action spectrum is measured by evaluating the physiological rate of the biological process, such as $\text{O}_2$ production or $\text{CO}_2$ consumption, across different wavelengths of light.

el MCQs

Question 1:

Which of the following best explains why the action spectrum and absorption spectrum do not align perfectly in living leaves?

A) Absorption spectra measure only oxygen evolution.
B) Accessory pigments transfer energy to chlorophyll $a$ with varying efficiency, and green light penetrates deeper into the spongy mesophyll tissue.
C) Chlorophyll $b$ absorbs only in the red region of the spectrum.
D) Spectrophotometers cannot measure wavelengths above 700 nm.

Correct Answer: B Explanation: Accessory pigments (like carotenoids) transfer their absorbed light energy to chlorophyll $a$ with some energy loss, and in vivo green light penetrates deeper into the leaf layers to be utilized by chloroplasts, leading to a higher action spectrum in that region.

Question 2:

When experimentally measuring the action spectrum of a plant, which physiological parameter is most commonly recorded?

A) The amount of chlorophyll $a$ extracted in a solvent.
B) The rate of carbon dioxide consumption or oxygen evolution.
C) The reflection of light by the leaf cuticle.
D) The rate of starch synthesis in the light-independent reactions.

Correct Answer: B Explanation: The action spectrum measures the physiological rate of photosynthesis, which is conventionally tracked via oxygen gas produced during the light-dependent reactions or carbon dioxide utilized during the Calvin cycle.

Question 3:

If you extract chlorophyll $a$ and measure its absorption spectrum, the highest absorption peaks are typically observed in which of the following wavelength ranges?

A) Green and yellow (500–550 nm)
B) Violet-blue and red (430–450 nm and 640–660 nm)
C) Orange and yellow (600–630 nm)
D) UV and far-red (300–400 nm and 700–720 nm)

Correct Answer: B Explanation: Chlorophyll $a$ absorbs light maximally in the violet-blue (~430–450 nm) and red (~640–660 nm) portions of the spectrum while reflecting green light.

Question 4:

Which of the following graphs measures the physiological effectiveness of different wavelengths of light in driving photosynthesis?

A) Absorption spectrum
B) Action spectrum
C) Emission spectrum
D) Transmission spectrum

Correct Answer: B Explanation: This graph plots the rate of photosynthesis against different wavelengths of light.

Question 5:

To measure the absorption spectrum of chlorophyll $a$ , which of the following instruments is most commonly used?

A) Gas chromatograph
B) Spectrophotometer
C) Calorimeter
D) Mass spectrometer

Correct Answer: B Explanation: This instrument quantifies the amount of light absorbed by dissolved or isolated pigments at specific wavelengths.

Question 6:

Chlorophylls absorb light most strongly in which regions of the visible spectrum?

A) Green and yellow
B) Blue and red
C) Ultraviolet and infrared
D) Orange and blue

Correct Answer: B Explanation: Chlorophyll molecules absorb light maximally at wavelengths around 430-450 nm and 640-660 nm.

Question 7:

The trough observed in the absorption spectrum of chlorophyll in the green-yellow region (approx. 550 nm) indicates which of the following?

A) Maximum absorption of light energy
B) Maximum rate of oxygen evolution
C) Reflection and transmission of green light
D) High concentration of carotenoids

Correct Answer: C Explanation: Because the pigments do not absorb much of this region, the light is reflected and transmitted.

Question 8:

Why does the action spectrum in a living leaf exhibit higher photosynthetic activity in the green region compared to the absorption spectrum of extracted pigments?

A) Because accessory pigments absorb only in the green region.
B) Because green light penetrates deeper into the spongy mesophyll to be absorbed by lower chloroplasts.
C) Because chlorophyll $a$ is synthesized rapidly in green light.
D) Because spectrophotometers are inaccurate at 550 nm.

Correct Answer: B Explanation: The internal architecture of a leaf allows green light to be effectively utilized by lower tissue layers.

Question 9:

How do accessory pigments contribute to the difference between the absorption and action spectra?

A) They destroy chlorophyll $a$ in the light-harvesting complex.
B) They absorb light and transfer energy to chlorophyll $a$ with varying efficiency.
C) They reflect all available light to prevent overheating.
D) They prevent the light-dependent reactions from running too quickly.

Correct Answer: B Explanation: Energy loss during transfer between accessory pigments and the reaction center causes a slight difference between the two spectra.

Question 10:

The action spectrum for photosynthesis shows the highest rate of $\text{CO}_2$ uptake corresponding to which wavelength range?

A) 500-530 nm
B) 430-450 nm and 640-660 nm
C) 300-400 nm
D) 680-720 nm

Correct Answer: B Explanation: These wavelength ranges correspond to the peak absorption of chlorophylls and maximum photosynthetic activity.

Question 11:

What physiological measurement is typically used to create an action spectrum in vivo?

A) Absorbance per unit volume
B) Rate of oxygen evolution
C) Concentration of pigment in a solvent
D) Volume of water transpired

Correct Answer: B Explanation: The rate of photosynthesis is conventionally tracked via the oxygen gas produced during the light-dependent reactions.

Question 12:

Which of the following wavelengths is considered to have the lowest efficiency in driving the light reactions of photosynthesis?

A) 430 nm
B) 660 nm
C) 550 nm
D) 450 nm

Correct Answer: C Explanation: This green wavelength is poorly absorbed by primary pigments and is the least efficient.

Question 13:

What is the main conclusion to be drawn by comparing the absorption and action spectra of green plants?

A) Only chlorophyll $b$ is responsible for photosynthesis.
B) Photosynthesis is driven entirely by green light.
C) Light absorption by pigments is directly linked to photosynthetic efficiency but not perfectly identical.
D) Plants cannot use blue light.

Correct Answer: C Explanation: Although they share the same peaks and troughs, factors like accessory pigments and in vivo vs. in vitro conditions create slight variations.