Plastids

Like mitochondria, plastids are double-membraned organelles that contain their own DNA, RNA and 70S ribosomes, but they are larger than mitochondria and can be seen under a light microscope. Plastids are classified according to the pigments they contain, into leucoplasts, chromoplasts and chloroplasts.

Leucoplasts and chromoplasts

Leucoplasts contain no pigments and come in a variety of shapes and sizes; they are used mainly for storing nutrients. Different types of leucoplast are named for what they store: amyloplasts store starch, elaioplasts store oils, and aleuroplasts store proteins. Chromoplasts, on the other hand, carry pigments such as carotene and xanthophyll, and it is these pigments that give many flowers and fruits their red, yellow or orange colours -- explaining, for instance, why a potato tuber (full of starch-storing leucoplasts/amyloplasts) looks white, while shoe-flower petals (full of chromoplasts) look red. Neither leucoplasts nor chromoplasts contain the photosynthetic apparatus found in chloroplasts.

Chloroplasts

Chloroplasts are found in plant cells, in algal cells, and in a few protists such as Euglena. Their size, number and shape vary between different cells -- Spirogyra, for example, has a distinctive ribbon-shaped chloroplast, Chlamydomonas has a single large chloroplast per cell, while mesophyll cells in a leaf may have anywhere from 20 to 40 chloroplasts. In plants, chloroplasts are found in the green parts of the plant, mainly in the mesophyll of the leaf, and while the typical leaf chloroplast is lens-shaped, chloroplasts elsewhere can be oval, spherical, discoid or ribbon-like. Chloroplasts contain the green pigment chlorophyll, along with the enzymes needed to produce sugar through photosynthesis. …