Plants provide us with food to eat and oxygen to breathe. They perform this amazing feat by the process of photosynthesis. Let's take a closer look. Photosynthesis requires carbon dioxide, which diffuses into the leaf through small pores, and then enters the cells. Inside the cell, carbon dioxide diffuses into the chloroplasts, where photosynthesis takes place. Chloroplasts use energy from light to transform carbon dioxide and water into sugar and oxygen. Zooming into a chloroplast, we see these flattened membranous sacs called thylakoids. Here, light energy is converted to chemical energy in the first phase of photosynthesis, the light reactions. The two green structures you see here are photosystems - large complexes of proteins and chlorophyll that capture light energy. An electron transport chain connects the two photosystems. Notice the small mobile electron carriers that shuttle electrons from one large complex to another. Now, let's take a closer look at the steps of the light reactions. The photosystem on the left absorbs light energy, exciting electrons that enter the electron transport chain. Electrons are replaced with electrons stripped from water, creating oxygen as a by-product. The energized electrons flow down the electron transport chain, releasing energy that is used to pump hydrogen ions (the blue balls) into the thylakoid. In the photosystem on the right, light energy excites electrons, and this time the electrons are captured by an electron carrier molecule - NADPH. The high concentration of hydrogen ions inside the thylakoid powers ATP synthase, producing ATPs. The light reactions in the thylakoid have produced two energy products-- ATP and NADPH-- that will now power the production of sugar in the Calvin cycle. The Calvin cycle takes place outside the thylakoids in the stroma - the thick fluid of the chloroplast. At the beginning of the cycle, carbon dioxide molecules combine with molecules called RuBP. The resulting molecules go through a series of reactions powered by ATP and NADPH from the light reactions. Sugar molecules known as G3Ps are produced. Most of the G3Ps are rearranged back into RuBPs that will begin the Calvin cycle again. But the important product of photosynthesis is the remaining G3P sugar. Some G3Ps are used to build glucose, which can combine into starch or cellulose. Still other G3Ps form sucrose. And some of the sugar is broken down by cellular respiration using oxygen in the plant's own mitochondria, generating ATPs that can power other work of the plant. Excess oxygen diffuses out of the leaf through the pores, while more carbon dioxide enters. With three simple ingredients-- carbon dioxide, water, and light-- plants produce sugar and oxygen by photosynthesis, powering plant metabolism and ultimately providing your fuel as well. Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings