Biochemistry - The Chemical Reactions of Living Cells, Volume 3 - D. Metzler 1980

Light in Biology
Photosynthesis
Structure of Chloroplasts

Like other energy-generating Organelles (Cell/35.html">Mitochondria), METABOLISM/14.html">Chloroplasts contain a highly organized system of internal membranes. Embedded in the colorless stroma are stacks of flattened disks known as grana. These disks, or thylakoids, are in turn composed of a pair of closely appressed membranes, 9 nm thick, separated by a narrow cleft called the loculus (Fig. 13-23). It is hypothesized that the outer layer contains protein subunits 4 nm in diameter, whereas the inner side consists of a lipid layer containing chlorophyll, carotenoids, and certain specific Lipids. However, chlorophyll is located predominantly in a protein complex (Sec. D, 3). The high content of galactosyl diglycerides, digalactosyl diglycerides, and sulfolipids in chloroplasts (Fig. 2-32) has already been mentioned. A possible function of these lipid components is interaction with chlorophyll molecules, thereby creating the microenvironmental heterogeneity mentioned above, though this question requires further investigation. Judging from CD spectra (Gregory [77], p. 111), the chlorophyll molecules within the membranes are specifically oriented relative to the plane of the thylakoids.

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FIG. 13-23. Electron micrograph of a chloroplast from an alfalfa leaf (courtesy of Harry T. Horner Jr., Iowa State University).

The question of the possible Organization of thylakoid membranes into photosynthetic units, or quantasomes, remains unresolved [86, 87]. As demonstrated by freeze-etching, quantasomes are ~20 nm in diameter and ~10 nm thick. However, some researchers have detected only smaller cubic particles, which may be ribulose diphosphate carboxylase molecules (cube edge ~12 nm; Ch. 7, Sec. K, 3, g) or coupling factor molecules involved in ATP synthesis (cube edge ~10 nm, Sec. D, 6).

Chemically, photosynthetic units have been characterized based on the number of various types of molecules present in the chloroplast membrane per pair of manganese atoms (Table 13-2). Each such unit is thought to contain one Photosystem I reaction center and one Photosystem II reaction center.