Photosynthesis starts with energy from the sun. This energy is in the form of light. White light is actually composed of a spectrum of colors, each with a different wavelength and energy. Light exists as photon particles that travel in waves. Waves are characterized by the wavelength, lambda, which is the distance between successive crests. Energy is inversely proportional to the wavelength of light. The shorter the wavelength, the greater the energy of the wave. The wavelengths of visible light range from 400 to 700 nanometers. A nanometer is one billionth of a meter.

The major light-absorbing pigment in the thylakoid membrane of plants is chlorophyll. Chlorophyll is a green pigment with a chemical structure that resembles the organic ring structure of the heme group in hemoglobin. One difference between chlorophyll and heme is that a magnesium ion, rather than an iron ion, occupies the central position of the chlorophyll molecule. Chlorophyll and accessory pigments absorb visible light. Chlorophyll absorbs light from the red and blue regions of the light spectrum, leaving primarily green light to be reflected back to the eye.

Plant leaves contain two kinds of chlorophyll, chlorophyll a and chlorophyll b, which help plants capture light more efficiently. They also contain accessory pigments such as beta-carotene, which allow them to harvest most of the energy available in sunlight. The absorption spectra of these pigments show the different wavelengths of light absorbed by the pigments.

The light-absorbing pigments of leaves are arranged in photosystems around their reaction centers. When a chlorophyll molecule absorbs a photon of light, one of its electrons moves to a higher energy level called an excited state. The energy absorbed by the chlorophyll molecule is transferred to neighboring molecules until it reaches pigment molecules in the reaction center. The pigment molecules trap the excitation energy for use in a chemical reaction. The trapped excitation energy drives the transfer of electrons from water to NADP+.

Three protein complexes in the thylakoid membrane–photosystem II, cytochrome complex b6f, and photosystem I–make up the electron transport chain that accomplishes this task. The flow of electrons through the electron transport chain is coupled to the movement of protons into the interior or lumen of the thylakoids. In addition, when water is split, protons are released into the lumen. ATP is produced when the protons flow back across the membrane.

Now that we’ve gotten an overview of how light energy is converted into the chemical energy of ATP and NADPH, let’s see how ATP and NADPH are used to synthesize sugars from carbon dioxide.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education