Absorption spectrum, action spectrum. and Differentiate between the absorption spectrum and the action spectru: Concepts & Explanation

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

Edited By Irshad Anwar | Updated on Jul 02, 2025 07:03 PM IST

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 Of Absorption Spectrum

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

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 other pigments.

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

Other accessory pigments, including chlorophyll b and carotenoids, 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 In Photosynthesis

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

The action spectrum refers to the graph showing the relative effectiveness of different wavelengths of light in driving the photosynthetic process. An action spectrum differs from an absorption spectrum in showing the amount of absorption by pigments at specific wavelengths but measures the actual rate of photosynthesis or physiological responses of plants for different wavelengths. It gives information regarding those wavelengths that best facilitate photosynthesis.

Concept And Definition Of Action Spectrum

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.

In this case, such a spectrum would normally correspond to the absorption spectra of photosynthetic pigments, though often extra peaks or variations occur depending on how different wavelengths may contribute effectively to the photosynthetic process on the whole.

How it is measured

The action spectrum is the measurement of a plant's physiological responses, such as growth rates and oxygen production, about the 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.

Relationship between light absorption and physiological effects.

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.

Influence of different wavelengths on photosynthesis.

The various wavelengths of light have different extents to which they influence photosynthesis. 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.

Differences Between Absorption Spectrum and Action Spectrum

Feature	Absorption Spectrum	Action Spectrum
Focus	Light absorption by pigments	Overall biological response to light
Specificity	Specific to individual pigments (e.g., chlorophyll a, chlorophyll b)	Includes multiple pigments and their combined effects
Experimental Approach	Methods: Use of spectrophotometer to measure light absorption at different wavelengths.	Methods: Measurement of physiological responses (e.g., photosynthesis rate) under different wavelengths.
Examples	Real-life Examples: Different chlorophyll types (e.g., chlorophyll a and b) in various plants.	Real-life Examples: Variation in photosynthesis rates in different plants under various light conditions (e.g., greenhouses vs. natural sunlight).

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Frequently Asked Questions (FAQs)

1. What is the absorption spectrum and how is it measured?

The absorption spectrum describes the degree to which any pigment absorbs different wavelengths. It does so by graphing which wavelengths of light a pigment—such as chlorophyll—absorbs the most. The graph is derived by passing light through a sample in a spectrophotometer, recording the amount of absorption at different wavelengths, and graphing out the results.

2. How does the action spectrum relate to photosynthesis?

It is the action spectrum that describes how, concerning photosynthesis, the photosynthetic rate varies with the different wavelengths of light. That is to say, it would be a measure of the relative effectiveness of light at various wavelengths in driving the photosynthetic process, integrating the contributions of the different pigments which capture light energy. The action spectrum will, therefore, tend to reflect the more effective wavelengths in inducing photosynthesis and the impact that light of different colours has on the general rate of photosynthesis.

3. What are the main differences between the absorption spectrum and the action spectrum?

The major difference is:

Focus: While the absorption spectrum describes how individual pigments absorb light at particular wavelengths, the action spectrum measures their efficiency in driving photosynthesis as a function of wavelength.
Scope: Absorption spectra relate to the nature and type of pigment absorption. In contrast, the action spectrum will represent the summative effects of different pigments and their contribution toward photosynthesis.
Measurement: With the help of a spectrophotometer, the absorption spectrum is obtained as the amount of light that the pigments absorb. Action spectra are obtained by measuring physiological responses like photosynthetic rates at different light wavelengths.

4. Why are absorption spectra important in plant physiology?

Absorption spectra are important in plant physiology because they identify those wavelengths of light that are most easily absorbed by the pigments, for example, chlorophyll. Information of this kind is required if one is to understand the real capturing of light energy by plants for photosynthesis and how to optimize light conditions for growth, not least in the design of artificial lighting systems used in agriculture and for research purposes.

5. How can action spectra be used in agriculture?

Optimisation of light conditions for plant growth can be realized in agriculture by using action spectra. Thus, knowing the wavelengths of light that most stimulate photosynthesis, a grower will tune light systems in greenhouses or controlled environments to optimize plant growth and productivity. Given the action spectra, one can choose the light conditions suitable for various plant species to have good crop yields using a minimum of resources.

6. What is an absorption spectrum?

An absorption spectrum is a graph that shows how much light of different wavelengths is absorbed by a substance, such as chlorophyll. It reveals which colors of light are most effectively absorbed by the pigment, providing insight into its chemical structure and function in photosynthesis.

7. What causes the peaks in an absorption spectrum?

Peaks in an absorption spectrum occur at wavelengths where the pigment absorbs light most strongly. These peaks correspond to specific electron transitions within the molecule's structure. For chlorophyll, the main peaks are typically in the blue and red regions of the visible spectrum.

8. How do scientists measure an absorption spectrum?

Scientists measure an absorption spectrum using a spectrophotometer. This device passes light of different wavelengths through a sample and measures how much light is transmitted or absorbed at each wavelength. The resulting data is used to create the absorption spectrum graph.

9. What is the "green gap" in the absorption spectrum of chlorophyll, and why is it significant?

The "green gap" refers to the region in the absorption spectrum where chlorophyll absorbs less light, corresponding to green wavelengths. This gap is significant because it explains why plants appear green and highlights the potential for improving photosynthetic efficiency by enhancing absorption in this range.

10. Why do plants appear green if chlorophyll absorbs red and blue light?

Plants appear green because chlorophyll absorbs red and blue light most strongly, while reflecting or transmitting green light. The reflected green light is what we perceive, giving plants their characteristic color despite chlorophyll's peak absorption in other parts of the spectrum.

11. What information can be derived from an action spectrum of photosynthesis?

An action spectrum of photosynthesis reveals which wavelengths of light are most effective in driving the process. This information helps identify the primary pigments involved in photosynthesis and provides insights into the light-dependent reactions' efficiency at different wavelengths.

12. How does the action spectrum of photosynthesis vary between different species of plants?

The action spectrum of photosynthesis can vary between plant species due to differences in pigment composition and ratios. For example, some algae have different primary photosynthetic pigments, leading to action spectra that peak at different wavelengths compared to land plants.

13. How does the Emerson effect relate to the action spectrum of photosynthesis?

The Emerson effect demonstrates that the combination of red and far-red light results in a higher rate of photosynthesis than the sum of their individual effects. This phenomenon is reflected in the action spectrum and helped reveal the existence of two photosystems working in tandem in photosynthesis.

14. How does understanding absorption and action spectra contribute to optimizing artificial lighting for plant growth?

Understanding these spectra allows for the design of artificial lighting that matches the most effective wavelengths for photosynthesis. By providing light that aligns with the peaks in the action spectrum, growers can maximize photosynthetic efficiency and plant growth while minimizing energy waste.

15. Why is it important to consider both the absorption spectrum and the action spectrum when studying photosynthesis?

Considering both spectra provides a more complete picture of the photosynthetic process. The absorption spectrum shows what light is captured, while the action spectrum reveals how effectively that captured light drives photosynthesis. Together, they help explain the efficiency and limitations of light utilization in plants.

16. How does an action spectrum differ from an absorption spectrum?

An action spectrum shows the effectiveness of different wavelengths of light in driving a biological process, like photosynthesis. While an absorption spectrum shows which light is absorbed, an action spectrum demonstrates which absorbed light actually contributes to the process. They may not always match exactly due to factors like energy transfer efficiency.

17. Why might the absorption spectrum of chlorophyll not perfectly match its action spectrum for photosynthesis?

The absorption spectrum may not perfectly match the action spectrum because not all absorbed light energy is used efficiently in photosynthesis. Factors such as energy transfer between pigments, heat dissipation, and the presence of accessory pigments can cause differences between what light is absorbed and what light is most effective for photosynthesis.

18. What role do accessory pigments play in expanding the action spectrum of photosynthesis?

Accessory pigments, such as carotenoids and phycobilins, absorb light at wavelengths not efficiently absorbed by chlorophyll. They transfer this energy to chlorophyll, effectively expanding the range of wavelengths that can drive photosynthesis and broadening the action spectrum.

19. How does the absorption spectrum of chlorophyll relate to the light-dependent reactions of photosynthesis?

The absorption spectrum of chlorophyll directly influences the light-dependent reactions of photosynthesis. Wavelengths that are strongly absorbed (primarily red and blue) provide the energy needed to excite electrons in chlorophyll molecules, initiating the electron transport chain and driving the production of ATP and NADPH.

20. How does the absorption spectrum of a leaf differ from that of isolated chlorophyll?

The absorption spectrum of a leaf is broader than that of isolated chlorophyll due to the presence of other pigments and the complex structure of the leaf. Factors like light scattering within the leaf and the presence of accessory pigments contribute to a more comprehensive absorption across the visible spectrum.

21. What is the relationship between the absorption spectrum and the energy levels of electrons in chlorophyll molecules?

The absorption spectrum directly reflects the energy levels of electrons in chlorophyll molecules. Peaks in the spectrum correspond to wavelengths that provide the exact energy needed to excite electrons from their ground state to higher energy levels, which is crucial for initiating the photosynthetic process.

22. What techniques are used to measure the action spectrum of photosynthesis?

The action spectrum is typically measured by exposing plants or isolated chloroplasts to different wavelengths of light and measuring the resulting photosynthetic activity. This can be done by measuring oxygen production, carbon dioxide uptake, or fluorescence emission at each wavelength.

23. How does the absorption spectrum of chlorophyll change when it's part of a photosystem complex?

When chlorophyll is part of a photosystem complex, its absorption spectrum can shift slightly due to interactions with proteins and other pigments. This shift, known as the "red shift," allows the photosystem to capture light at slightly longer wavelengths than isolated chlorophyll.

24. What is the significance of the red drop phenomenon in the action spectrum of photosynthesis?

The red drop refers to the sudden decrease in photosynthetic efficiency at wavelengths longer than about 680 nm. This phenomenon helped scientists discover that photosynthesis requires the cooperation of two photosystems, as longer wavelengths can only excite one photosystem effectively.

25. How do carotenoids contribute to the overall absorption spectrum of a leaf?

Carotenoids absorb light in the blue and green regions of the spectrum, complementing chlorophyll's absorption. This broadens the overall absorption spectrum of the leaf, allowing for more efficient light capture across a wider range of wavelengths and contributing to photoprotection.

26. What is the quantum yield of photosynthesis, and how does it relate to the action spectrum?

The quantum yield of photosynthesis is the number of molecules of product formed (or substrate consumed) per photon absorbed. The action spectrum essentially shows how the quantum yield varies with wavelength, indicating which colors of light are most efficiently used in the photosynthetic process.

27. How does the absorption spectrum of chlorophyll change in different solvents, and why is this relevant to understanding photosynthesis in vivo?

The absorption spectrum of chlorophyll can shift depending on the solvent due to changes in the molecule's electronic environment. This is relevant because it demonstrates how the protein environment within photosystems can influence chlorophyll's light-absorbing properties in living plants.

28. What role does chlorophyll fluorescence play in studying photosynthesis, and how does it relate to the absorption spectrum?

Chlorophyll fluorescence occurs when excited chlorophyll molecules return to their ground state by emitting light. This phenomenon is used to study photosynthetic efficiency non-invasively. The fluorescence spectrum is related to, but distinct from, the absorption spectrum, providing complementary information about the photosynthetic apparatus.

29. How do shade-adapted plants modify their pigment composition to optimize their action spectrum?

Shade-adapted plants often increase their ratio of chlorophyll b to chlorophyll a and may have more accessory pigments. This adaptation broadens their action spectrum, allowing them to more efficiently capture the limited and spectrally altered light available in shaded environments.

30. What is the significance of the blue peak in the action spectrum of photosynthesis?

The blue peak in the action spectrum is significant because it corresponds to high-energy photons that can drive photosynthesis efficiently. It also aligns with one of the major absorption peaks of chlorophyll, demonstrating the direct relationship between light absorption and photosynthetic activity.

31. How does the concept of photosynthetically active radiation (PAR) relate to the action spectrum of photosynthesis?

Photosynthetically active radiation (PAR) refers to the range of light wavelengths (typically 400-700 nm) that can drive photosynthesis. The action spectrum helps define this range by showing which wavelengths within PAR are most effective for photosynthesis, guiding the design of plant lighting and our understanding of plant productivity.

32. Why doesn't the action spectrum of photosynthesis extend significantly into the ultraviolet or infrared regions?

The action spectrum doesn't extend significantly into UV or IR regions because chlorophyll and other photosynthetic pigments have not evolved to efficiently absorb or utilize these wavelengths. UV light can be damaging to cellular components, while IR light typically doesn't provide enough energy per photon to drive the photosynthetic reactions.

33. How does the absorption spectrum of a single chlorophyll molecule differ from that of a chloroplast?

A single chlorophyll molecule has sharp, well-defined absorption peaks, while a chloroplast shows broader, less distinct peaks. This difference is due to the diverse environments of chlorophyll molecules within the chloroplast, interactions with proteins and other pigments, and the presence of multiple types of chlorophyll and accessory pigments.

34. What is the importance of the "red edge" in remote sensing of vegetation, and how does it relate to the absorption spectrum of chlorophyll?

The "red edge" refers to the sharp change in reflectance of vegetation between red and near-infrared wavelengths. It's directly related to chlorophyll's strong absorption in the red region and low absorption in the near-infrared. This feature is crucial for remote sensing of vegetation health and productivity using satellite or aerial imagery.

35. How do changes in the absorption spectrum of leaves during senescence reflect changes in pigment composition?

During senescence, the absorption spectrum of leaves changes as chlorophyll breaks down and other pigments, like carotenoids, become more visible. This results in a decrease in the characteristic chlorophyll absorption peaks and an increase in absorption and reflection of yellows and reds, causing the familiar autumn colors.

36. What is the role of phycobilins in expanding the action spectrum of certain algae and cyanobacteria?

Phycobilins are accessory pigments found in red algae and cyanobacteria that absorb light in the green and yellow regions of the spectrum, where chlorophyll absorption is low. By transferring this energy to chlorophyll, phycobilins significantly expand the action spectrum, allowing these organisms to thrive in aquatic environments where available light is spectrally limited.

37. How does the absorption spectrum of chlorophyll contribute to our understanding of its molecular structure?

The absorption spectrum of chlorophyll provides insights into its molecular structure, particularly its conjugated system of alternating single and double bonds. The specific wavelengths absorbed correspond to the energy required for electron transitions within this system, helping scientists deduce the molecule's electronic configuration and structural features.

38. What is the significance of the Soret band in the absorption spectrum of chlorophyll?

The Soret band is a strong absorption peak in the blue region of chlorophyll's spectrum. It represents a high-energy electron transition and is significant because it accounts for much of chlorophyll's ability to absorb blue light, which is crucial for photosynthesis, especially in aquatic environments where blue light penetrates water more deeply.

39. How does the action spectrum of photosynthesis inform our understanding of the evolution of photosynthetic organisms?

The action spectrum of photosynthesis provides clues about the evolutionary history of photosynthetic organisms. The fact that most plants show similar action spectra suggests a common ancestral photosynthetic system. Variations in action spectra among different groups (e.g., purple bacteria, green sulfur bacteria) indicate diverse evolutionary adaptations to different light environments.

40. What is the relationship between the absorption spectrum of chlorophyll and the absorption spectra of other biological pigments like hemoglobin or rhodopsin?

While the specific absorption peaks differ, the principle is similar for all biological pigments: the absorption spectrum reflects the molecule's ability to capture specific wavelengths of light based on its structure. Like chlorophyll, hemoglobin and rhodopsin have characteristic spectra that relate to their functions in oxygen transport and vision, respectively.

41. How does the pH of the cellular environment affect the absorption spectrum of chlorophyll?

The pH of the cellular environment can slightly shift the absorption spectrum of chlorophyll. In more acidic conditions, the spectrum may show a small shift towards shorter wavelengths (blue shift), while in more alkaline conditions, it may shift towards longer wavelengths (red shift). This is due to changes in the electronic structure of the chlorophyll molecule in different pH environments.

42. What is the significance of the fact that the action spectrum of photosynthesis is broader than the absorption spectrum of chlorophyll alone?

The broader action spectrum indicates that photosynthesis utilizes light more efficiently than would be possible with chlorophyll alone. This broadening is due to the presence of accessory pigments and the organization of pigments within photosystems, demonstrating the sophisticated light-harvesting adaptations of photosynthetic organisms.

43. How does the concept of complementary chromatic adaptation in cyanobacteria relate to their absorption and action spectra?

Complementary chromatic adaptation is the ability of some cyanobacteria to alter their pigment composition in response to changes in light quality. This adaptation allows them to optimize their absorption spectrum to match the available light, effectively shifting their action spectrum to maintain efficient photosynthesis in varying light environments.

44. What role does the absorption spectrum play in the phenomenon of photoinhibition?

The absorption spectrum is crucial in understanding photoinhibition, a decrease in photosynthetic capacity due to excess light. Wavelengths that are strongly absorbed, as shown in the absorption spectrum, are more likely to cause photoinhibition when present in high intensities, leading to damage of the photosynthetic apparatus.

45. How do artificial photosynthetic systems attempt to mimic the absorption and action spectra of natural photosynthesis?

Artificial photosynthetic systems often try to replicate the broad absorption of natural systems by using multiple light-absorbing components or synthetic molecules with wide absorption ranges. They aim to create an action spectrum that efficiently utilizes a broad range of the solar spectrum, sometimes even extending beyond the range used by natural photosynthesis.

46. What is the importance of understanding the absorption spectrum in the development of solar cells inspired by photosynthesis?

Understanding the absorption spectrum of photosynthetic pigments guides the development of bio-inspired solar cells. It informs the design of light-harvesting materials that can absorb across a wide range of wavelengths, mimicking the efficiency of natural photosynthesis and potentially improving the performance of solar energy conversion devices.

47. How does the absorption spectrum of chlorophyll change when the molecule is oxidized or reduced?

When chlorophyll is oxidized or reduced, its absorption spectrum can change significantly. Oxidation typically leads to a decrease in absorption intensity and possible shifts in peak positions, while reduction can cause new absorption bands to appear. These changes reflect alterations in the molecule's electronic structure and are important in understanding its role in the electron transport chain.

48. What is the relationship between the absorption spectrum and the color of light reflected by photosynthetic organisms?

The color of light reflected by photosynthetic organisms is complementary to the light absorbed, as shown in the absorption spectrum. Wavelengths that are not strongly absorbed are reflected or transmitted, giving the organism its characteristic color. For example, plants appear green because chlorophyll absorbs strongly in the red and blue regions but reflects green light.

49. How does the absorption spectrum of chlorophyll contribute to our understanding of energy transfer in photosynthesis?

The absorption spectrum of chlorophyll reveals which wavelengths of light can excite its electrons. This information is crucial for understanding how energy is captured and transferred within the photosynthetic apparatus. The spectrum helps explain why certain wavelengths are more effective in driving photosynthesis and how energy can be passed between different pigment molecules.

50. What is the significance of the difference between the absorption spectra of chlorophyll a and chlorophyll b?

The slight differences in the absorption spectra of chlorophyll a and b allow for more efficient light capture across the visible spectrum. Chlorophyll b absorbs light at slightly different waveleng