Light travels as tiny packets of energy called photons. When this energy encounters any material, three primary interactions can occur: reflection, transmission, or absorption. Light absorption is the physical process where the energy of a photon is captured by the material, transferring energy to the atoms and molecules within a living organism. This transfer is the necessary first step that allows organisms to harness or defend against the power of the electromagnetic spectrum.
The Molecular Process of Light Absorption
The absorption of light is governed by the principles of quantum mechanics at the atomic level. Electrons within a molecule exist only at specific, discrete energy levels. For a molecule to absorb a photon, the energy of that incoming photon must precisely match the difference between an electron’s current energy level and a higher, unoccupied energy level. If this energy is an exact match, the photon is consumed, and the electron is instantly boosted to the higher, “excited” energy state.
This requirement for a precise energy match explains why molecules are selective about the color, or wavelength, of light they absorb. Wavelengths that are not absorbed are reflected or transmitted, which is why objects have color. An electron in this excited state is unstable and must quickly release the absorbed energy to return to its lower, resting state. This energy can be dissipated as heat, re-emitted as a lower-energy photon known as fluorescence, or used to drive a chemical reaction in biological systems.
Light Absorption in Photosynthesis
The most widespread biological use of light absorption is photosynthesis, the process by which plants, algae, and some bacteria convert light energy into chemical energy. This process begins with specialized pigment molecules, primarily chlorophylls, embedded in the membranes of chloroplasts. Chlorophyll a and b absorb light most effectively in the blue-violet and red regions of the visible spectrum.
The absorbed energy is then transferred through a network of accessory pigments, such as carotenoids, acting as a light-harvesting complex. This energy funneling ensures that the excitation energy reaches a specialized pair of chlorophyll molecules in the reaction center. Upon receiving this energy, an electron is ejected from the reaction center chlorophyll and enters an electron transport chain.
The flow of this energized electron ultimately leads to the synthesis of the energy-carrying molecules adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide phosphate (NADPH). These chemical carriers then power the subsequent metabolic reactions that fix carbon dioxide into glucose. Carotenoids also serve a photoprotective role by absorbing excess light energy, dissipating it as heat to prevent damage to the photosynthetic machinery.
Light Absorption for Vision and Protection
Vision
In the animal kingdom, light absorption is the initiating event for the sense of sight. Photoreceptor cells in the retina contain the light-sensitive pigment rhodopsin, which is composed of a protein called opsin bound to a small molecule called 11-cis-retinal. The 11-cis-retinal molecule has a bent configuration in its resting state.
When a photon strikes the retina, it is absorbed by the retinal molecule, causing an extremely rapid structural change. The molecule straightens out, isomerizing from the 11-cis form to the all-trans form. This instantaneous molecular change activates the rhodopsin protein, triggering a cascade of biochemical signals within the cell. This signaling pathway ultimately results in an electrical impulse transmitted to the brain, which is interpreted as vision.
Protection
Organisms also use light absorption for self-defense, most notably through the pigment melanin in human skin. Melanin acts as a broadband light filter, absorbing high-energy ultraviolet (UV) radiation across a wide spectrum to protect underlying tissues. The primary form, eumelanin, intercepts UV photons before they can reach and damage the cellular DNA.
The high energy of the absorbed UV light is converted into harmless heat, a process that occurs on an ultrafast timescale. This conversion prevents the energy from creating destructive chemical reactions. Melanin also functions as an antioxidant, scavenging the reactive oxygen species, or free radicals, generated by UV exposure. This two-pronged action of physical absorption and chemical neutralization is an effective defense against sun damage.