Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy. This conversion begins with the capture of photons by specialized pigments within the chloroplasts. While chlorophyll A is the primary molecule executing the final energy conversion, it is supported by other light-absorbing compounds. These accessory pigments are integral to the overall efficiency and survival of the organism.
What Defines Accessory Pigments
Accessory pigments are any light-absorbing compound in a photosynthetic organism other than chlorophyll A, the molecule found at the reaction center where light energy is first transformed into chemical energy. These molecules are located within the light-harvesting complexes embedded in the thylakoid membranes. Their structures differ slightly from chlorophyll A, allowing them to absorb distinct wavelengths of light.
The two main classes of these pigments are the carotenoids and the phycobilins. Carotenoids, including carotenes and xanthophylls, are responsible for the yellow, orange, and red colors seen in many organisms and are present in nearly all photosynthetic life. Phycobilins, such as phycoerythrin and phycocyanin, are water-soluble pigments found in cyanobacteria and red algae. Accessory pigments broaden the range of light an organism can use and subsequently pass that energy to chlorophyll A.
Capturing a Broader Spectrum of Light
The primary function of accessory pigments is to expand the usable range of the visible light spectrum for photosynthesis. Chlorophyll A absorbs light most effectively in the blue-violet and red ends of the spectrum. This leaves a significant gap in the middle, particularly in the green and yellow wavelengths, which are largely reflected and cause leaves to appear green.
Accessory pigments efficiently fill this gap by absorbing the wavelengths that chlorophyll A misses. Carotenoids strongly absorb light in the blue-green range, while phycobilins in aquatic organisms absorb the green and orange light that penetrates deeper water. This extended absorption profile ensures the organism captures a greater percentage of the available solar energy.
Once an accessory pigment absorbs a photon, the energy elevates an electron to an excited state. This excitation energy is rapidly transferred through a series of pigment molecules until it reaches the chlorophyll A molecule at the reaction center. This process, known as resonance energy transfer, passes energy but not the electron itself. This energy funnel maximizes the light energy delivered to the reaction center, increasing the overall efficiency of the light-dependent reactions.
Essential Role in Photoprotection
Beyond harvesting light, accessory pigments, particularly carotenoids, serve a protective function against damage from excessive sunlight. When light intensity is high, the photosynthetic system can become overloaded, meaning captured energy exceeds the system’s capacity to process it. This over-excitation can lead to the formation of highly reactive oxygen species (ROS), a process known as photo-oxidation.
ROS are unstable molecules that can cause damage to the proteins and lipids of the thylakoid membrane, including the chlorophyll A molecules. Carotenoids act as a safety valve to prevent this cellular damage. They absorb the excess energy directly from over-excited chlorophyll molecules or from the developing ROS.
The absorbed excess energy is then safely dissipated, most commonly as harmless heat, often facilitated by the xanthophyll cycle. By diverting this surplus energy away from the reaction centers, accessory pigments prevent the destruction of the photosynthetic apparatus. This photoprotection ensures the plant’s survival and functionality under intense solar radiation.