Photosynthesis requires sunlight, carbon dioxide, and water as starting reactants (Figure 5.5 ). After the process is complete, photosynthesis releases oxygen and produces carbohydrate molecules, most commonly glucose. These sugar molecules contain the energy that living things need to survive.
The complex reactions of photosynthesis can be summarized by the chemical equation shown in Figure 5.6
Although the equation looks simple, the many steps that take place during photosynthesis are actually quite complex, as in the way that the reaction summarizing cellular respiration represented many individual reactions. Before learning the details of how photoautotrophs turn sunlight into food, it is important to become familiar with the physical structures involved.
In plants, photosynthesis takes place primarily in leaves, which consist of many layers of cells and have differentiated top and bottom sides. The process of photosynthesis occurs not on the surface layers of the leaf, but rather in a middle layer called the mesophyll (Figure 5.7). The gas exchange of carbon dioxide and oxygen occurs through small, regulated openings called stomata.
In all autotrophic eukaryotes, photosynthesis takes place inside an organelle called a chloroplast. In plants, chloroplast-containing cells exist in the mesophyll. Chloroplasts have a double (inner and outer) membrane. Within the chloroplast is a third membrane that forms stacked, disc-shaped structures called thylakoids. Embedded in the thylakoid membrane are molecules of chlorophyll, a pigment(a molecule that absorbs light) through which the entire process of photosynthesis begins. Chlorophyll is responsible for the green color of plants. The thylakoid membrane encloses an internal space called the thylakoid space. Other types of pigments are also involved in photosynthesis, but chlorophyll is by far the most important. As shown in Figure 5.7, a stack of thylakoids is called a granum, and the space surrounding the granum is called stroma(not to be confused with stomata, the openings on the leaves).