The Calvin-Benson Cycle: Overview

<h1>Text Preview</h1> <p>Both animals and plants synthesize carbohydrates. However, the synthesis of carbohydrates in animals always begins with precursors having at least three carbons. In contrast, photosynthetic organisms can synthesize carbohydrates from carbon dioxide and water, using the energy furnished by NADPH and ATP.</p> <p>The addition of carbon dioxide to organic molecules is called carbon assimilation. When carbon dioxide is assimilated via the Calvin-Benson cycle, the process is often referred to as carbon fixation. There are three stages in the Calvin-Benson cycle: fixation, reduction, and regeneration of the starting acceptor of carbon dioxide. </p> <p>The fixation of carbon consumes large amounts of ATP and NADPH. The energy-consuming process of carbon fixation is coupled to the energy-generating light reactions of photosynthesis. </p> <p>Most plants fix carbon dioxide into a three-carbon compound and are called C3 plants. The enzyme that catalyzes this reaction is rubisco, the most abundant enzyme in the world. However, the reaction with carbon dioxide is not the only reaction rubisco catalyzes. Rubisco catalyzes a competing reaction with oxygen. Oxygen and carbon dioxide compete with each other for rubisco’s active site.</p> <p>When rubisco catalyzes the reaction with oxygen, a useless product is made that wastes energy. This reaction is called photorespiration. Photorespiration is nothing like cellular respiration, the energy-generating reactions that take place in the mitochondria.</p> <p>Some plants have an adaptive mechanism that minimizes photorespiration. These plants use a different pathway that incorporates carbon dioxide into a four-carbon compound before it enters the Calvin-Benson cycle. Plants with this pathway are known as C4 plants.</p> <p>In this activity, we’ll learn how carbon dioxide is incorporated into organic compounds. We’ll also learn how photorespiration results in inefficiency and waste. Finally, we’ll examine the alternative pathway that has evolved in C4 plants to minimize this waste.</p> <p id="TextCopyright">Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education</p>