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9.2: Light as Energy

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Light as Energy

9.2: Light as Energy


The energy required to carry out photosynthesis is light—typically radiation from the sun. The electromagnetic particles are called photons and carry a defined amount of energy, dependent on the wavelength. All wavelengths fall within a range called the electromagnetic spectrum. Plants carry out photosynthesis using wavelengths that range from 400 to 700 nm. They primarily use a pigment called chlorophyll a that absorbs red and blue light and reflects green light which is why leaves appear green.


A photon is a discrete electromagnetic particle or bundle of energy. It is always in motion and travels at the speed of light. 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. The range of all possible wavelengths is known as the electromagnetic spectrum.

The Photosynthetic Absorption Spectrum

For most organisms, photosynthesis relies on a small segment of the electromagnetic spectrum—namely, the visible portion that ranges from 400 to 700 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.

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