What is Plastids?
The most obvious difference between plant cells and other eukaryotic cells is that cells of most plants contain unique organelles called plastids, which include chloroplasts, chromoplasts, and leukoplasts.
Chloroplasts contain a special pigment known as chlorophyll, which absorbs sunlight energy. This sunlight energy is then converted to another form of energy utilizable by other living organisms. This energy conversion process is referred to as photosynthesis, which combines carbon dioxide from the air and hydrogen from water to form sugars and other organic compounds. Oxygen is returned to the atmosphere as a byproduct.
Chloroplasts, like mitochondria, also contain DNA and are postulated to have arisen as prokaryotes that entered the eukaryotic cells and evolved to become part of the cell's structure. Chloroplasts are bounded by a double membrane. The interior membrane is explanded into pancake-shaped membranes, or thylakoids, which are assembled in stacks called grana. A dense solution called the stroma surrounds the grana.
Chromoplasts synthesize and store pigments such as yellow xanthophylls, orange carotenes, and various red pigments.
Leukoplasts are an important component of food plants, and these colorless or white organelles are where plant cells store starches, proteins, and lipids.