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.

Photons

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.

The energy for photosynthesis is light, typically electromagnetic radiation from the sun. Light behaves as particles—discrete packets of energy called photons and as waves that oscillate along a path.

The wavelength is measured as the distance between corresponding points of two consecutive waves, such as crests. The electromagnetic spectrum includes gamma rays with wavelengths in picometers to radio waves with wavelengths as long as kilometers.

A wavelength is inversely proportional to the energy it transmits. That is, long wavelengths have less energy than short ones.

Although the sun emits a broad spectrum of electromagnetic radiation, only a part of this spectrum, ranging from around 380 to 750 nanometers, is visible to the human eye and can support life on earth.

In plants and other photosynthetic organisms, chloroplasts contain different pigments  that absorb specific wavelengths of visible light. For instance, the chlorophyll a pigment only absorbs red and blue light, causing green to be reflected. This is why most leaves appear green in color. 

Photosynthesis transforms the light energy into the chemical energy stored in glucose and other organic molecules. These molecules serve as energy sources for non-autotrophic organisms in the food pyramid.