ABSTRACT
Although photosynthesis can happen in dierent ways in dierent species, some features are always the same. For example, the process always begins when energy from light is absorbed by proteins called photosynthetic reaction centers that contain chlorophylls. In plants, these proteins are held inside organelles called chloroplasts embedded within the cell membranes, while in bacteria they are embedded in the plasma membrane. is membrane may be tightly folded into cylindrical sheets called thylakoids, or bunched up into round vesicles called intra cytoplasmic membranes. ese structures can ll most of the interior of a cell, giving the membrane a very large surface area and therefore increasing the amount of light that the bacteria can absorb. Meanwhile, a typical plant cell contains about 10 to 100 chloroplasts. e chloroplast is enclosed by a membrane composed of a phospholipid inner membrane, a phospholipid outer membrane, and an intermembrane space between them. Within the membrane is an aqueous uid called the stroma. e stroma contains stacks (grana) of thylakoids, which are the site of photosynthesis. e thylakoids are attened disks, bounded by a membrane with a lumen or thylakoid space within it. e site of photosynthesis is the thylakoid membrane, which contains integral and peripheral membrane protein complexes, including the pigments that absorb light energy, which form the photosystems.
