Specialized structures found in the cytoplasm of plant cells, diverse in distribution, size, shape, composition, structure, function, and mode of development. A number of different types are recognized. Chloroplasts occur in the green parts of plants and are responsible for the green coloration, for they contain the chlorophyll pigments. These pigments, along with certain others, absorb the light energy that drives the processes of photosynthesis, by which sugars, starch, and other organic materials are synthesized. Amyloplasts, nearly or entirely colorless, are packed with starch grains and occur in cells of storage tissue. Proteoplasts are less common and contain crystalline, fibrillar, or amorphous masses of protein, sometimes along with starch grains. In chromoplasts the green pigment is masked or replaced by others, notably carotenoids, as in the cells of carrot roots and many flowers and fruits. See Carotenoid, Chlorophyll
All types of plastids have one structural feature in common, a double envelope consisting of two concentric sheets of membrane. The outer of these is in contact with the cytoplasmic ground substance; the inner with the plastid matrix, or stroma. They are separated by a narrow space of about 10 nanometers.
Another system of membranes generally occupies the main body of the plastid. This internal membrane system is especially well developed in chloroplasts, where the unit of construction is known as a thylakoid. In its simplest form this is a sac such as would be obtained if a balloon-shaped, membrane-limited sphere were to be flattened until the internal space was not much thicker than the membrane itself. It is usual, however, for thylakoids to be lobed, branched, or fenestrated.
The surface area of thylakoids is very large in relation to the volume of the chloroplast. This is functionally significant, for chlorophyll molecules and other components of the light-reaction systems of photosynthesis are associated with these membranes. A chloroplast, however, is much more than a device for carrying out photosynthesis. It can use light energy for uptake and exchange of ions and to drive conformational changes. The stroma contains the elements of a protein-synthesizing system—as much deoxyribonucleic acid (DNA) as a small bacterium, various types of ribonucleic acid (RNA), distinctive ribosomes, and polyribosomes. There is evidence to indicate that much of the protein synthesis of a leaf takes place within the chloroplasts. See Photosynthesis, Protein, Ribosomes
One of the most challenging problems in cell biology concerns the autonomy of organelles, such as the plastids. Chloroplasts, for instance, have their own DNA, DNA-polymerase, and RNA-polymerase; can make proteins; and, significantly, can mutate. All this suggests a measure of independence. It is known, however, that some nuclear genes can influence the production of molecules that are normally found only in chloroplasts, so their autonomy cannot be complete. It remains to be seen whether they control and regulate their own morphogenetic processes.