Both animal and plant cells have mitochondria to provide them with life-sustaining ATP. In addition, plant cells and green algae have organelles called chloroplasts that capture the energy of sunlight to synthesize simple sugars (Fig. 5.28). This process, called photosynthesis, results in the release of oxygen as a waste product. Like the nucleus and mitochondria, chloroplasts are surrounded by a double membrane. They also have a third, internal membrane. This membrane defines a separate internal compartment called the thylakoid. The thylakoid membrane contains specialized light-collecting molecules called pigments, of which chlorophyll is the most important. The green color of chlorophyll explains why so many plants have green leaves.
Chlorophyll plays a key role in the chloroplast’s ability to capture energy from sunlight. Using the light energy collected by this pigment, enzymes present in the chloroplast use carbon dioxide as a carbon source to produce carbohydrates. Photosynthesis is discussed in greater detail in Chapter 8.
