An important difference between the organization of mitochondria and chloroplasts is highlighted in Figure 14–38. The inner membrane of the chloroplast is not folded into cristae and does not contain electron-transport chains. Instead, the electron-transport chains, photosynthetic light-capturing systems, and ATP synthase are all contained in the thylakoid membrane, a separate, distinct mem-brane that forms a set of flattened, disc-like sacs, the thylakoids. The thylakoid membrane is highly folded into numerous local stacks of flattened vesicles called grana, interconnected by nonstacked thylakoids. The lumen of each thylakoid is connected with the lumen of other thylakoids, thereby defining a third internal compartment called the thylakoid space. This space represents a separate com-partment in each chloroplast that is not connected to either the intermembrane space or the stroma. Chloroplasts Capture Energy from Sunlight and Use It to Fix Carbon We can group the reactions that occur during photosynthesis in chloroplasts into two broad categories: 1. The photosynthetic electron-transfer reactions (also called the “light reactions”) occur in two large protein complexes, called reaction centers, embedded in the thylakoid membrane. A photon (a quantum of light) knocks an electron out of the green pigment molecule chlorophyll in the first reaction center, creating a positively charged chlorophyll ion. This electron then moves along an electron-transport chain and through a sec-ond reaction center in much the same way that an electron moves along the respiratory chain in mitochondria. During this electron-transport process, H+ is pumped across the thylakoid membrane, and the resulting LEAF upper epidermis lower epidermis nucleus cytosol vacuole air space cell wall mitochondrion (B) (C)(A) 10 mm chloroplast chloroplast (D) 5 µm 0.5 µm
Figure 14–37 Chloroplasts in the cell.
Schematic cross section through the leaf of a green plant. (B) Light microscopy of a plant leaf cell—here, a mesophyll cell from Zinnia elegans—shows chloroplasts as bright green bodies, measuring several micrometers across, in the transparent cell interior. (C) the electron micrograph of a thin, stained section through a wheat leaf cell shows a thin rim of cytoplasm— containing chloroplasts, the nucleus, and mitochondria—surrounding a large, water-filled vacuole. (D) at higher magnification, electron microscopy reveals the chloroplast envelope membrane and the thylakoid membrane within the chloroplast that  is highly folded into grana stacks (Movie 14.9). (B, courtesy of John Innes Foundation; C and D, courtesy of
K.
plaskitt.)
Figure 14–38 A mitochondrion and chloroplast compared. Chloroplasts are generally larger than mitochondria. In addition to an outer and inner envelope membrane, they contain the thylakoid membrane with its internal thylakoid space. the chloroplast thylakoid membrane, which is the site of solar energy conversion in plants and algae, corresponds to the mitochondrial cristae, which are the sites of energy conversion by cellular respiration. Unlike the crista membrane, which is continuous with the inner mitochondrial membrane at cristae junctions, the thylakoid membrane is not connected to the inner chloroplast membrane at any point.
electrochemical proton gradient drives the synthesis of ATP in the stroma. As the final step in this series of reactions, electrons are loaded (together with H+) onto NADP+, converting it to the energy-rich NADPH molecule. Because the positively charged chlorophyll in the first reaction center quickly regains its electrons from water (H2O), O2 gas is produced as a by-product. All of these reactions are confined to the chloroplast.