How Plants Use Light

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Sunlight makes life on Earth possible. Energy from the sun keeps the planet warm enough to sustain life and powers the production of food by plants and other photosynthetic organisms. A plant converts light energy into chemical energy that it can use to build and maintain itself. Animals, including humans, depend on the sugars, proteins, fats, and nucleic acids that plants build using light energy.

Diagram of a chloroplast. The watery stroma (yellow) is separated from the fluid inside the thylakoid’s hollow interior or lumen (green) by thylakoid membranes. Some of the thylakoids are stacked (granal), others are exposed to the stroma. The detailed view of the thylakoid membranes shows proteins that participate in proton pumping and electron transport (blue, red, green, purple). Adapted from Buchanan et al., 2000, Biochemistry and Molecular Biology of Plants, American Society of Plant Biologists (Rockville, MD).

Green plant cells contain chloroplasts, membrane-bound bodies that house the light-harvesting machinery (see Figure). Inside each chloroplast is a network of fluid-filled membrane sacs, called thylakoids, which are surrounded by a watery solution, the stroma. The thylakoid membranes contain the complex assortment of proteins, pigments, and other biomolecules that make photosynthesis possible. In simple terms, the thylakoid membranes act as a system of pumps and dams. Light drives the pumping of protons (H+), tiny*, positively charged particles, from the stroma into the lumen (hollow interior) of the thylakoid. Light-driven proton pumping creates an acidic environment (that is, a high proton concentration [low pH]) within the thylakoid lumen that is necessary for the conversion of violaxanthin to antheraxanthin and zeaxanthin. Like water held behind a dam, or the charge held in a battery, the protons concentrated in the thylakoids have potential energy. The controlled passage of protons through molecular generators in the thylakoid membranes converts this potential energy into chemical energy, in the same way that channeling water through a dam’s generator converts the water’s potential energy into electricity. The chemical energy is subsequently used to synthesize sugar from carbon dioxide gas.

Proton pumping is one half of how light makes photosynthesis possible. The other half involves the transport of electrons, negatively charged particles that are about 1/2,000 the size of a proton. Part of the thylakoid machinery responsible for proton pumping also breaks water into electrons, protons, and the oxygen gas that plants release during photosynthesis. Chlorophyll, the pigment that gives plants their green color, is the key to this process. When chlorophyll molecules absorb light, their electrons gain energy and the molecules are said to be “excited.” Light-excited chlorophyll molecules are highly reactive and can damage the thylakoid membranes. However, excited chlorophyll is also essential to photosynthesis, because some excited chlorophyll molecules are reactive enough to donate an electron to a neighboring acceptor molecule. The donated electron is passed along a chain of molecules, ultimately joining carbon dioxide to make sugar. The donor chlorophyll replaces its missing electron with an electron taken from water, and the process repeats.

Prof. Yamamoto describes the xanthophyll cycle as part of a signaling pathway in the thylakoid membrane that allows the plant to sense excess light and quickly adjust its light absorbing capacity. As described above, light creates an acidic thylakoid environment, facilitating conversion of violaxanthin to antheraxanthin and zeaxanthin. During non-photosynthetic quenching of excess light energy (NPQ), protons, antheraxanthin, and zeaxanthin bind to a target protein in the thylakoid membrane, altering the membrane’s conformation and light absorbance properties. Prof. Yamamoto proposes that antheraxanthin and zeaxanthin may interact with additional target molecules in the thylakoid membrane that have not yet been identified. He credits his colleagues, including Profs. A. Hager (University of Tübingen, Germany), Barbara Demmig-Adams (University of Colorado at Boulder) and Krishna Niyogi (University of California at Berkeley), for their important contributions to this model.

*Protons are VERY small. If each human being alive today were to place one hundred trillion (10¹⁴) protons on a bathroom scale, the proton pile would weigh one gram, the weight of a packet of artificial sweetener.

Posted on September 5, 2003