Absorption and Action Spectrum: Did you know every living thing is bioluminescent? We cannot observe emitted lights as our eyes can only perceive visible light. If only we use a handy infrared camera,  can we observe the emitted lights? Although we humans can see lights of the visible range (400- 700nm), plants may require wavelengths above the visual spectrum, i.e., far-red/infrared (>700 nm). Plants absorb light of different wavelengths with the help of various kinds of pigments. Each pigment has a specific range of absorption, which can be studied In-vitro by plotting the absorption spectrum graph, and the action spectrum can study the rate of photosynthesis. Read further to learn more about absorption and action spectrum.

Light

Light has dual nature. It acts as both waves as well as particles. Wave nature explains Its electromagnetic behaviour and properties like frequency, wavelength etc. The visible spectrum ranges between 400nm to 700nm, which constitute seven visible colours of VIBGYOR. Each colour of light possesses its own wavelength and speed. Different wavelengths and speeds cause the splitting of white light into seven different colours when it is passed through a prism. Part of the spectrum that is used for photosynthesis is called photosynthetically active radiation (PAR).

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Fig: Visible Spectrum

Photons can explain the particle nature of light. Photons can be defined as packets of energy from electromagnetic radiation. When a photon strikes a pigment molecule, it produces a high energy state. The high energy state of the reaction centre ultimately triggers a chain of reactions called light reactions. Only a photon of a specific wavelength can bring the change in the state of energy; hence, each pigment has its absorption spectrum.

Fig: Absorption of Light Through Different Pigments

Pigments

Pigments can be defined as light-absorbing, coloured molecules. Furthermore, different pigments can be separated by paper chromatography. There are mainly two types of pigment: Chlorophyll, Carotenoid.

Chlorophyll

There are five major types of chlorophyll: a, b, c, d, and bacteriochlorophyll.  In higher plants, chlorophyll a and chlorophyll b are the main photosynthetic pigments. Chlorophyll a appears bright or blue-green in the chromatogram, whereas; chlorophyll b appears yellow-green. The chlorophyll molecule consists of 2 parts:

1. Porphyrin ring: It makes the head part of chlorophyll which has a central magnesium atom surrounded by a nitrogen-containing porphyrin ring.
2. Phytol chain: It makes the tail of chlorophyll. It is hydrophobic in nature and is made up of carbon and hydrogen.

The structure of chlorophyll is similar to the haemoglobin molecule found in the RBC of mammals. Chlorophyll a is the principal pigment, whereas other pigments are accessory pigment or antenna pigment.

Carotenoid

Carotenoids are ubiquitous and essential pigments in photosynthesis. Carotenoids are categorized into two major types: Carotene and Xanthophyll. Xanthophylls appear yellow in the chromatogram, and carotenoids appear yellow to yellow-orange. These are a group of accessory pigments. Carotenoids impart bright colours in fruits, for example:

1. The red colour of the tomato is due to lycopene
2. The yellow colour of corn seeds is due to zeaxanthin
3. The orange colour of an orange peel is due to β-carotene.

Absorption Spectrum

The absorption spectrum is a graph that can be defined as a range of different wavelengths absorbed by a pigment. The cellular and molecular make-up of different species are the major factors that decide the range of absorption spectra, and therefore it differs in plants and different kinds of algae. An instrument can quantify the absorption of radiation by a substance called a spectrophotometer. Spectrophotometers measure light intensity as a function of wavelength. A solution of a pigment is placed in a spectrometer to produce the graph based on the wavelength absorbed by the pigment. The light absorbed by the pigment is plotted against wavelength to produce an absorption spectrum. Common observations of the absorption spectrum of chlorophyll a, chlorophyll b and carotenoid are listed below.

1. Both chlorophyll a and b show maximum absorption in the blue-violet and orange-red regions of the visible spectrum. Green and light are absorbed very little.
2. The green plants use blue and red light as the energy source for photosynthesis.
3. The absorption spectrum of chlorophyll a shows peaks at about 680 and 700 nm.
4. Carotenoids absorb radiant energy between 449 and 490 nm.
5. Carotenes show absorption peaks at 449 and 478 nm.
6. The xanthophylls show peaks at 440 and 490 nm.

Fig: Absorption Spectrum of Chlorophyll a, b and Carotenoid

Action Spectrum

An action spectrum is defined as a graph that depicts the overall rate of photosynthesis against the wavelength of light. It implies that the action spectrum is simply a graphical representation of biological effectiveness as a function of the wavelength of incident light. The rate of photosynthesis can be studied by measuring the amount of oxygen evolved or the amount of carbon dioxide absorbed. It shows that maximum photosynthesis occurs in the blue-violet zone or red zone.

Fig: Action Spectrum

Absorption Spectrum Vs Action Spectrum

Absorption Spectrum	Action spectrum
It is a graphical representation of the amount of light of different wavelengths absorbed by a pigment.	It is a graphical representation of biological effectiveness as a function of the wavelength of incident light.
It represents the amount of light absorbed by a pigment.	It represents the overall rate of photosynthesis.
It can be studied directly In-vitro.	It can be studied indirectly, i.e., the amount of oxygen evolved, etc.
It can be studied using the spectrometer.	It can be studied by measuring the amount of oxygen evolved,  or amount of carbon dioxide absorbed, amount of NADP+ reduced.

Fig: Absorption Spectrum Vs. Action Spectrum

Summary

Light has dual nature. It acts as both waves as well as particles. Additionally, the visible spectrum ranges between 400nm to 700nm. Part of the spectrum that is used for photosynthesis is called photosynthetically active radiation (PAR). Pigments can be defined as light-absorbing, coloured molecules. There are mainly two types of pigment: Chlorophyll and carotenoid. There are five major types of chlorophyll: a, b, c, d, and bacteriochlorophyll. In higher plants, chlorophyll a and chlorophyll b are the main photosynthetic pigments. Carotenoids are ubiquitous and essential pigments in photosynthesis.

Carotenoids are categorized into two major types: carotene and xanthophyll. Light particle or photon of a specific wavelength of light produces a high energy state when it strikes a pigment molecule. A high energy state triggers a chain of reactions called light reactions. The absorption spectrum is a graph that can be defined as the range of different wavelengths absorbed by a pigment. The cellular and molecular make-up of the plant are the major factors that decide the range of absorption spectra, and therefore it differs depending upon the species. An action spectrum is defined as a graph that depicts the overall rate of photosynthesis against the wavelength of light. An action spectrum is simply a graphical representation of biological effectiveness as a function of the wavelength of incident light.

Frequently Asked Questions (FAQs) from the Absorption and Action spectrum

Q.1. What does the action spectrum indicate?
Ans: Action spectra indicate the relative effectiveness of various wavelengths supporting photochemical reactions, such as for phytochrome.

Q.2. What does the absorption spectrum of Chlorophyll-a indicate?
Ans: The absorption spectrum of chlorophyll shows peaks at about 680 and 700 nm.

Q.3. What is the process of obtaining an absorption spectrum?
Ans: Absorption spectrum can be obtained In-vitro by making a solution of a pigment and subjecting it to spectroscopy.

Q.4. In which light rate of photosynthesis is maximum?
Ans: Maximum photosynthesis occurs in red light.

Q.5. In which light rate of photosynthesis is minimum?
Ans: Minimum photosynthesis occurs in green light.

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