Light energy drives numerous processes in living organisms, from sensing the environment to generating food. To understand which specific colors of light are responsible for a particular biological outcome, scientists use a tool called the action spectrum. This graph visually represents the effectiveness of different light wavelengths in causing a specific biological response. It provides a direct measure of the work being accomplished within a living system when exposed to various parts of the light spectrum.
Defining the Action Spectrum
The action spectrum is a graphical representation that plots the rate of a biological process against the wavelength of light used to stimulate it. The rate of the biological process is the measured “action,” while the specific light wavelength is the independent variable. The resulting curve shows which colors of light are most efficient at driving the response.
Photosynthesis in plants offers the most common example of an action spectrum. Here, the action measured is the rate of oxygen production or carbon dioxide consumption. When plants are exposed to light across the visible spectrum, the action spectrum reveals a high rate of photosynthesis in the blue-violet region (around 400-500 nanometers) and the red region (around 600-700 nanometers). The curve shows a relatively low rate in the green-yellow region. This explains why many plants appear green, since that light is reflected rather than used. The peaks correspond directly to the wavelengths where reaction-driving pigments are most effective at converting light energy into chemical energy.
The Difference from an Absorption Spectrum
The action spectrum is often discussed alongside the absorption spectrum. An absorption spectrum plots the percentage of light absorbed by a specific substance, such as a pigment like chlorophyll, across different wavelengths. This measurement shows the light energy captured, or the input, by the pigment molecules. The peaks indicate the wavelengths the pigment takes in most readily.
The action spectrum, however, measures the biological output, or the actual work being done by the light energy. While the curves for both spectra often show a strong similarity, they are not identical. This difference exists because not all absorbed light energy leads to a chemical reaction; some absorbed energy is lost as heat or re-emitted as fluorescence. Therefore, the absorption spectrum shows what is available to the organism, but the action spectrum shows what is used to perform a task. The close match between the two graphs demonstrates that a particular pigment, like chlorophyll, is the molecule responsible for the light-driven biological action.
Measuring Biological Effectiveness
The concept of the action spectrum was first demonstrated by Theodor Wilhelm Engelmann in 1883. Engelmann used a prism to split white light into its constituent colors, illuminating a strand of filamentous green algae (Spirogyra). He then introduced aerobic bacteria, which are attracted to oxygen.
The algae performed photosynthesis, producing oxygen as a byproduct. Engelmann observed that the bacteria congregated most densely in the areas illuminated by the blue-violet and red light portions of the spectrum. The concentration of bacteria served as a direct, visible measure of the rate of oxygen production at each specific wavelength. This experiment established that the rate of a biological process depends on the specific wavelength of light supplied.
Applications and Significance
The action spectrum serves as a tool for identifying the specific photopigments responsible for any light-driven biological process. By overlaying the action spectrum onto the absorption spectra of various pigments, scientists can pinpoint which molecules are doing the work. For example, the high correlation between the action spectrum of photosynthesis and the absorption spectra of chlorophylls A and B confirms their role as the primary light-harvesting molecules.
The data gathered from action spectra have relevance in practical applications, particularly in horticulture and agriculture. Understanding which wavelengths are most effective for photosynthesis allows growers to optimize artificial light sources for indoor farming and greenhouses. Commercial grow lights are often designed to emit a spectrum heavily weighted toward the blue and red regions to maximize plant growth and yield. Action spectra are also used to study processes like phototropism (plant bending toward light) and the effect of ultraviolet light on DNA damage or vitamin D synthesis in humans.