4.6: Photosynthesis takes place in the chloroplasts.

If a plant part is green, then you know it is photosynthetic. Leaves are green because the cells near the surface are packed full of chloroplasts, light-harvesting organelles, which make it possible for the plant to use the energy from sunlight to make sugars (their food) and other plant tissue (much of which animals use for food) (FIGURE 4-12). Other plant parts, such as stems, may also contain chloroplasts (in which case they, too, are capable of photosynthesis), but most chloroplasts are located within the cells in a plant’s leaves.

Let’s take a closer look at chloroplasts (FIGURE 4-13). The sac-shaped organelle is filled with a fluid called the stroma. Floating in the stroma is an elaborate system of interconnected membranous structures called thylakoids, which often look like stacks of pancakes. Once inside the chloroplast, you can be in one of two places: in the stroma or inside the thylakoids. The conversion of light energy to chemical energy—the “photo” part of photosynthesis—occurs inside the thylakoids. The production of sugars—taking place in the “synthesis” part of photosynthesis—occurs within the stroma.

We examine both the “photo” and the “synthesis” processes in greater detail later in this chapter. First, however, we examine the nature of light energy and of chlorophyll, the special molecule found in chloroplasts that makes the capture of light energy possible.