Plastids are the largest semi autonomous organelles found in all plant cells and euglenoids, absent in fungi and most of the bacteria. They develop from colorless precursors called proplastids. The term plastid was first used by A.F.W Schimper(1885). On the basis of pigments, the plastids are classified into three types.
i. Chloroplasts: They are self duplicating cellular organelles, where complete photosynthesis occurs. Chloroplasts occurs in the cytoplasm of all the green cells of the plants except autotrophic prokaryotes. They are usually found in the cells of leaves in angiosperms, gymnosperms, pteridophytes and vegetative cells of lower plants.
Structure of Chloroplast: Chloroplast in angiosperms and other vascular plants are usually discoid, ellipsoidal or biconvex lens shaped and measures about 4-10 micron in diameter and 1-3 micron in thickness. But in lower plants like algae they vary in size and shape. They are spiral, girdle-shaped, star shaped, bell shaped or reticulate. Their number varies from one per cell to 20-30 per cell in different organisms.
It shows that a chloroplast is bound by two membranes. The space between the two membranes is called periplastidial space. The inner content of the chloroplast is divisible into two, namely
i) Stroma or matrix: It is a hyaline, slightly electron dense proteinaceous watery substance. It contains all the necessary enzymes of photosynthesis. It also contains ribosomes, strands of chloroplast DNA and RNA. The DNA present in chloroplast is circular in shape and lacks histones. The nonliving inclusions like starch grains and osmiophilic droplets are also present in the stroma of chloroplasts. The stroma is the site of dark reaction of photosynthesis.
Grana: It is composed of a few to hundred membrane bounded flattened sacs or lamellae called thylakoids. They are arranged one above the other like a pile of coins to form grana. The grana of the chloroplast are interconnected by a membrane system called stroma lamellae. These lamellae may function as channels for transportation of various photosynthetic substrates. Chlorophyll, the chief coloring pigment and carotene xanthophyll etc. are present in the membranes of thylakoids. The grana are the regions of light reaction of photosynthesis.
ii) Chromoplasts: They are colored plastids found in the ripe fruits, flowers, leaves or croton plant, tubers of carrot, beetroot etc. The colored is due to the presence of pigments like carotene, xanthophyll, etc. The chromoplast can be formed from chloroplast or leucoplast or vice versa. The green color of many fruits changes into red, yellow, etc. due to this conversion.
iii) Leucoplasts: They are colorless plastids. Their shape is irregular and their main function is the storage of starch, oil or fats, protein, etc. If starch is stored, they are known as amyloplasts. Oil storing plastids are called lipoplasts, oleosomes or elaioplasts, and protein storing plastids are called aleuroplasts.
Leucoplasts may be converted into chloroplasts or chromoplasts. The young fruits of tomato and chilies are white. At maturity they turn green and when ripe color changes to yellow and finally turn red. The development of green color in potato tuber rhizome on exposure to sunlight is an example of the conversion of leaucoplast into chloroplast. Similarly, white color in carrot is changed into yellow orange due to the conversion of leucoplast into chromoplast.
