In electron micrographs of sections, chloroplasts are seen to be bounded by an envelope consisting of two (green algae, red algae, higher plants), three (Euglenophyta, dinoflagellates) or four (brown algae, diatoms, prymnesiophytes, haptophytes) membranes, and to be filled with a granular matrix. This matrix is referred to as the stroma and is composed of a concentrated solution or gel of proteins, consisting mainly of the enzymes used in carbon dioxide fixation. Embedded within the stroma there is an array of flattened, membrane-bounded sacs known as thylakoids. These lie approximately parallel to the main plane of the chloroplast and when seen in electron microscope sections at right angles to that plane, they present the appearance of membranes in pairs. There are large numbers of thylakoids in each chloroplast. In the algae most of the thylakoids extend from one end of the chloroplast to the other.
The different classes of algae differ from one another in the way in which the thylakoids are grouped together. The simplest arrangement is that found in the red algae (Fig. 8.2b). These have single thylakoids, about 20 nm thick, lying separately in the stroma. Attached to the outer surface of each thylakoid is an array of particles 30 to 40 nm in diameter, known as phycobilisomes and consisting mainly of biliprotein molecules.
The next simplest type of thylakoid arrangement is that found in the Cryptophyta, which have thylakoids loosely associated in pairs (Fig. 8.3a). They are thicker than the thylakoids of other algae, being 19 to 36 nm across, and their interior space is filled with a finely granular, electron-dense material, which is thought to consist of the biliprotein pigments.
In all other classes of algae, with the exception of some of the Chlorophyta, the thylakoids are grouped, or stacked, in threes (Fig. 8.3b). These stacks of thylakoids are referred to as compound lamellae. In aquatic higher plants and in some green algae a more complex arrangement of the thylakoids is found. They occur in much bigger stacks consisting typically of 5 to 20 thylakoids, but the individual thylakoids in the stacks are of much smaller diameter than a typical algal thylakoid. Each such stack is referred to as a granum. There may be 40 to 60 grana in a typical chloroplast. There are numerous interconnections between the different grana and between the different thylakoids within a stack (Fig. 8.4). In this type of chloroplast the thylakoids can best be regarded as individual compartments in a complex, ramifying system of interconnected compartments that constitutes the photosynthetic membrane system of the chloroplast. The thylakoid system in the chloroplast of a submerged aquatic higher plant may be seen in Fig. 8.4b.
Blue-green algae (Cyanophyta) are prokaryotes and so the various cellular functions are not compartmented within separate membrane-bounded organelles. These algae also have thylakoids but these lie free within the cytoplasm (Fig. 8.2a). They are similar in type to those of the red algal chloroplasts, occurring as separate, single thylakoids with phycobilisomes on the surface. A type of prokaryotic alga, Prochloron, which occurs in symbiotic association with certain marine ascidians, does
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