Photosynthesis

Abstract

Photosynthesis is the process by which the energy of sunlight is captured for use in sustaining life. The organisms which are capable of performing this process are plants and algae (multi, as well as unicellular), and certain kinds of bacteria. In eukaryotic photosynthetic organisms (all plants and algae) the process occurs within an organelle called the chloroplast (see Chapter 2). The photosynthetic apparatus is embedded in lamellae which are densely packed in some regions (grana) and in other areas extend into the stroma (intergranal lamellae). The lamellae are closed double membranes forming the flattened closed vesicles called thylakoids (Fig. 6.1). The photosynthetic apparatus of the photosynthetic prokaryotes (photosynthetic bacteria and cyanobacteria) is also embedded in membranous structures which in these organisms extend throughout the cell. When photosynthetic bacterial cells are ruptured one often finds membrane fragments which are formed into closed vesicles from 300 to 500 Å in diameter. These vesicles are called chromatophores or chromatophore fractions (Fig. 6.2). They retain most of the photosynthetic activity. Functional membrane fractions from cyanobacteria are more difficult to obtain. The photosynthetic apparatus in these organisms is in membranes that are folded in a more or less complicated way inside the cell. Usually these membranes are also called thylakoids.