9.2: Light as Energy
The energy required to carry out photosynthesis is light— typically electromagnetic radiation from the sun. The range of all possible wavelengths is known as the electromagnetic spectrum.
A photon is a discrete electromagnetic particle or bundle of energy. Photons are characterized by their frequency, wavelength, and amplitude, similar to the properties of a wave. Waves with higher frequencies transmit more energy and have shorter wavelengths than longer wavelengths that transmit less energy and have lower frequencies.
Photosynthetic Absorption Spectrum
For most organisms, photosynthesis relies on a small segment of the electromagnetic spectrum—namely, the visible portion that ranges from 380 to 750 nanometers, from blue to green to red. However, a few organisms can carry out photosynthesis using infrared light.
In plants, different pigment molecules absorb specific wavelengths of light, giving each molecule a distinct absorption spectrum. For example, chlorophyll a— the most abundant pigment molecule in leaves— only absorbs red and blue light. Chlorophyll a reflects the green portion of the spectrum, letting plant leaves appear green to the human eye. Plants also use additional pigments to absorb light. For example, they have phycocyanin that absorbs orange and red light, carotenes that absorb ultraviolet, violet, blue, blue-green, and orange-red light, and xanthophylls that absorb blue and ultraviolet light.