Absorption spectrum, action spectrum. and Differentiate between the absorption spectrum and the action spectru

What Is Absorption Spectrum?

Absorption spectrum refers to a graph of the length ranges of light absorbed by a pigment or group of pigments. It accounts for specific wavelengths at which a pigment absorbs light and which it reflects. In the case of chlorophyll, its absorption spectrum details peaks in the blue-violet and red regions corresponding to the effective wavelengths of absorption.

Concept And Definition

The absorption spectrum of chlorophyll usually shows strong peaks of absorption in the blue-violet region, at approximately 430-450 nanometers, and in the red region, at about 660-680 nanometers. This can also define peaks of wavelengths where chlorophyll is best absorbed.

Lower views of absorbance were generally used in the green wavelength range of 500-550 nanometers, which is reflected to turn out a green colour for the plants. It helps outline the exact wavelengths that light could be captured for photosynthesis by chlorophyll.

The amount of light absorbed by the pigment in different wavelength ranges is recorded on an absorption spectrum using a spectrophotometer. Place a sample of chlorophyll in the path of light and it shows the intensity of light, which gets transmitted through the sample at different wavelengths. This graph-plotted information forms the absorption spectrum.

Mechanism of Absorption

The mechanism of absorption includes:

Interaction of Light with Pigments

As a result of this interaction, photons are going to be absorbed by the chlorophyll molecules, thereby raising the energy of electrons to a higher level. This energy will drive the light-dependent reactions of photosynthesis. The wavelengths that get absorbed correspond exactly to the energy levels required to quench the electron. Thereby driving this process of conversion of light energy into chemical energy.

Role of Chlorophyll and Accessory Pigments

Chlorophyll, along with other accessory biological pigments, like carotenoids and phycobilins, absorbs light energy and subsequently translocates it to photosynthetic reaction centres. Chlorophyll is responsible for primarily absorption in the blue-violet and red portions of the spectrum.

These accessory pigments also capture light within other ranges of wavelengths, and hence the zone of light absorption by the photosynthetic apparatus is considerably extended. These pigments cooperate in optimizing light absorption and safeguarding the photosynthetic apparatus.

Importance Of Absorption Spectrum

The absorption spectrum is important in providing information on how different pigments contribute to photosynthesis. From the spectrum, scientists could obtain information on which wavelengths of light were absorbed most efficiently and relate them with photosynthesis efficiency. The knowledge helps in optimising conditions for the growth of plants and enhancing artificial photosynthetic systems.

Action Spectrum – Definition and Relationship

The action spectrum refers to the graph showing the relative effectiveness of different wavelengths of light in driving the photosynthetic process. An action spectrum gives information regarding those wavelengths that best facilitate photosynthesis.

The action spectrum generally refers to the various changes in photosynthesis rates under different wavelengths of light. It is characterised by peaks where the photosynthetic rate is highest for the wavelengths that most effectively drive photosynthesis.

Evaluation Method

The action spectrum is the measurement of the responses of plants physiology, such as growth rates and oxygen production, light of different wavelengths. In this case, it normally takes place in a growth chamber or photosynthetic apparatus with light filters that limit the amount of illumination at particular wavelengths.

Normally, one would monitor the photosynthesis rate by the evolution of oxygen or carbon dioxide uptake to know which wavelength was best at promoting photosynthesis.

Mechanism Of Action

The relation to the amount of light absorbed and the physiological effects was based on how it is absorbed by chlorophyll and other pigments, resulting in excited electrons that trigger the light-dependent reactions of photosynthesis.

Differences Between Absorption Spectrum and Action Spectrum

The key difference between the absorption and action spectrum is given in the table below:

Relationship Between Absorption and Action Spectrum

Different wavelengths of light may have different effects on efficiency. For example, those wavelengths corresponding to peaks of the action spectrum are more efficient in photosynthesis since they align better with the absorption characteristics of the pigments involved.

Wavelengths that best correspond to peaks in the absorption spectrum of chlorophyll and other pigments will tend to increase photosynthesis maximally. For example, the normal peaks of maximum effectiveness are blue and red light, and green light has a lesser effect since it is reflected rather than absorbed.

Importance In Photosynthesis

Knowing the action spectrum is of central importance for optimising photosynthesis concerning use in agriculture and ecology. It allows the selection of appropriate light conditions for controlled environments to ensure maximal photosynthetic efficiency—for example, in greenhouses or under artificial lighting systems. Light conditions could be tuned toward the optimization of plant growth and productivity by taking into account the action spectrum given by the scientist/grower.

Absorption Spectrum NEET MCQs (With Answers & Explanations)

Important topics for NEET are:

• Absorption and Action Spectrum

• Key differences between absorption and action spectrum

Practice Questions for NEET

Q1. Which of the following is absorbed maximally by chlorophyll a?

1. Red wavelength

2. Blue wavelength

3. Both a and b

4. None of these

Correct answer: 1) Red wavelength

Explanation:

Chlorophyll a absorbs more red wavelengths (~665–680 nm) than chlorophyll b (~640 nm) and is the primary pigment in photosynthesis, directly participating in light-dependent reactions. In contrast, chlorophyll b acts as an accessory pigment, absorbing more blue light (~455 nm) and transferring the energy to chlorophyll a, thereby broadening the spectrum of light available for photosynthesis. This difference in absorption allows plants to maximize light capture and efficiency in various lighting conditions.

Hence, the correct answer is option 1) Red wavelength.

Q2. The absorption spectrum can be measured by:

1. Spectrophotometer

2. Auxanometer

3. Potometer

4. All of these

Correct answer: 1) Spectrophotometer

Explanation:

• A spectrophotometer is a piece of equipment used in laboratories to detect how much light at various wavelengths is absorbed by a sample. It can provide an absorption spectrum, which displays the amount of light at different wavelengths that the sample absorbs.

• In disciplines like chemistry, biology, and physics this spectrum helps in determining the precise wavelengths that a material absorbs, which helps examine the characteristics of molecules, including plant pigments like chlorophyll.

Hence, the correct answer is option 1) Spectrophotometer.

Q3. The action spectrum of photosynthesis follows the pattern of the absorption spectrum of chlorophyll but is not identical because of

1. Absorption by accessory pigments

2. Absorption variation of chlorophyll

3. Both a and b

4. None of these

Correct answer: 1) Absorption by accessory pigments

Explanation:

The wavelengths of light that chlorophyll absorbs most effectively are displayed in its absorption spectrum; these wavelengths are mostly in the blue and red portions of the spectrum.

The efficiency of various light wavelengths in promoting photosynthesis is demonstrated by the action spectrum of photosynthesis. Although the pattern resembles that of chlorophyll's absorption spectrum, it differs because accessory pigments, such as carotenoids, also absorb light and give chlorophyll energy.

Hence, the correct answer is option 1)Absorption by accessory pigments.

Also Read:

• MCQs on Absorption Spectrum

• C4 Pathway

• Carbon Fixation

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