s that relied on photosynthesis millions of years ago [3]. Therefore, almost all life on earth, directly or indirectly, depends on photosynthesis as a source of food, energy, and oxygen, making it one of the most important biochemical processes known. Plant photosynthesis occurs in leaves and green stems within cell structures called chloroplasts (Figure 4 & 5). One plant leaf is composed of tens of thousands of cells, and each cell contains 40 to 50 chloroplasts. The chloroplast, an oval-shaped structure, is divided by membranes into numerous disk-shaped compartments. These disklike compartments, called thylakoids, are arranged vertically in the chloroplast like a stack of plates or pancakes [4]. A stack of thylakoids is called a granum. The grana lie suspended in a fluid known as stroma [3] (Figure 6). Embedded in the membranes of the thylakoids are hundreds of molecules of chlorophyll (Figure 7), a light-trapping pigment required for photosynthesis. Additional light-trapping pigments, enzymes, and other molecules needed for photosynthesis are also located within the thylakoid membranes [4]. The pigments and enzymes are arranged in two types of units, Photosystem I and Photosystem II (Figure 8). Because a chloroplast may have dozens of thylakoids, and each thylakoid may contain thousands of photosystems, each chloroplast will contain millions of pigment molecules. Photosynthesis is a very complex process, and is divided into two stages. In the first stage, the light-dependent reaction, the chloroplast traps light energy and converts it into chemical energy contained in nicotinamide adenine dinucleotide phosphate (NADPH) and adenosine triphosphate (ATP) (Figure 9), two molecules used in the second stage of photosynthesis. In the second stage, called the light-independent reaction (formerly called the dark reaction), NADPH provides the hydrogen atoms that help form glucose, and ATP provides the energy for this and o...
