Chloroplast Anatomy

In all higher plants and algae, photosynthetic processes are localized in organelles called chloroplasts (Figure 12.11). In plants, most of the chloroplasts are found in cells just under the leaf surface (mesophyll cells). Each cell may contain 20 to 50 of these organelles (Figure 17.4c). The eukaryotic algae also have chloroplasts, but often only one very large one is found in each cell.

The internal structure of a chloroplast, as shown in Figure 17.4c, bears some resemblance to that of a mitochondrion. There is an outer, freely permeable membrane and an inner membrane that is selectively permeable. The inner membrane encloses a material called the stroma that is analogous to the mitochondrial matrix. Immersed in the stroma are many flat, saclike membrane structures called thylakoids, which are often stacked like coins to form units called grana (see Figure 17.4c). Individual grana are irregularly interconnected by thylakoid extensions called stroma lamellae. The thylakoid membrane encloses an interior space, the lumen of the thylakoid.

The division of labor within a chloroplast is simple. Absorption of light and all of the light reactions occur within or on the thylakoid membranes. The ATP and NADPH produced by these reactions are released into the surrounding stroma, where all of the synthetic dark reactions occur. Obviously, there are analogies in structure and role between mitochondrial matrix and chloroplast stroma and between the inner membrane of the mitochondrion and the thylakoid membrane of the chloroplast. Indeed, we shall find that a very similar kind of chemiosmotic ATP generation is carried out across these membranes in both mitochondria and chloroplasts.

Like mitochondria, chloroplasts are semiautonomous, carrying their own DNA to code for some of their proteins, as well as the ribosomes necessary for translation of the appropriate messenger RNAs. There is now much evidence that chloroplasts evolved from unicellular organisms similar to cyanobacteria (blue-green algae). Such prokaryotic photosynthesizers do not contain chloroplasts but have membrane structures that play the same roles as chloroplast membranes. To a certain extent, the cyanobacteria resemble free-living chloroplasts. It is believed that, early in evolution, primitive unicellular organisms took up cyanobacteria-like prokaryotes and that eventually the relationship became symbiotic: The photosynthetic organelles were no longer capable of independent life, and the algae depended upon them as energy sources. Today, some chloroplast genes are coded in the organelle genome, and some are in the cell nucleus.

See also: Stroma, Grana, Lumen of the Thylakoid Thylakoid Membranes, Chlorophyll, CF 0-CF 1 Complex

INTERNET LINKS:

> 1. Chloroplast Genome Page

> 2. Complete Chloroplast Genome Sequences