Photosynthesis is the biological process that sustains nearly all life on Earth, allowing plants, algae, and some bacteria to convert light energy into chemical energy. This conversion is nuanced; the quality of the light—its spectral composition—is as important as the quantity. The specific wavelengths of solar radiation determine how efficiently a plant can synthesize the sugars necessary for growth and metabolism.
Deconstructing Sunlight
Sunlight is a complex stream of electromagnetic radiation composed of various wavelengths. This radiation spans a wide spectrum, from invisible, high-energy gamma rays to long-wavelength radio waves. Only a small portion of this total output, known as the visible light spectrum, is used by plants for energy production.
Visible light ranges roughly from 400 to 700 nanometers (nm) in wavelength, which humans perceive as the colors of the rainbow. When white light passes through a prism, it separates into distinct colors because each corresponds to a different wavelength. Shorter wavelengths, such as violet and blue light, carry higher energy, while longer wavelengths, like red light, carry lower energy. The light waves that fall within this 400-700 nm range are collectively termed Photosynthetically Active Radiation (PAR).
The Catchers: Plant Pigments
Plants possess specialized organic molecules called pigments to intercept solar radiation and begin the energy conversion process. The most well-known is chlorophyll, housed within the chloroplasts inside plant cells. Chlorophyll $a$ is universally present in all photosynthetic organisms and serves as the primary pigment that converts light energy into chemical energy.
Chlorophyll $b$ functions alongside chlorophyll $a$ as an antenna pigment, absorbing light at different wavelengths and transferring that energy to the reaction center. Accessory pigments, such as carotenoids, further support this system by absorbing violet and blue-green light that chlorophylls may miss. Carotenoids also serve a protective function, helping to dissipate excessive light energy as heat to shield the photosynthetic machinery from damage.
The Action Spectrum: Which Colors Matter Most
The most effective wavelengths for photosynthesis are described by the action spectrum, which maps the rate of sugar production against the specific colors of light supplied. This spectrum reveals that plants do not use all colors equally, showing two distinct peaks of high activity. The first peak occurs in the blue-violet region (approximately 430–470 nm), and the second peak is in the red-orange region (around 640–670 nm).
These peaks correspond precisely to the wavelengths most strongly absorbed by chlorophylls $a$ and $b$. Blue light is absorbed by both chlorophyll types and carotenoids, while red light is largely absorbed by chlorophylls. In the center of the visible spectrum, the rate of photosynthesis drops significantly in the green and yellow light range (around 500–600 nm). This green light is largely reflected by the chlorophyll, which is why plants appear green to the human eye. Although green light is absorbed less efficiently, it can penetrate deeper into the leaf tissue and canopy, contributing to carbon fixation in lower layers.
Beyond Photosynthesis: Light’s Other Roles
Light serves the plant as more than just an energy source; it is also an informational signal that guides growth and development. This regulatory role is governed by specialized photoreceptors separate from photosynthetic pigments. One function is phototropism, the directional growth or bending of a plant toward or away from a light source.
This movement is controlled by blue light receptors called phototropins, which sense the direction of the light and trigger an uneven distribution of growth hormones to bend the stem. Another light-mediated process is photoperiodism, where plants use the duration of the day and night cycle to determine when to perform certain actions. Photoperiodism regulates events like flowering, dormancy, and seed germination, and is primarily sensed by phytochrome receptors sensitive to red and far-red light.