Plastids are membrane-bound semi-autonomous organelles found in eukaryotic cells of plants, algae, and other autotrophic organisms. They play a role in the synthesis and storage of various important compounds, such as carbohydrates and fatty acids. Similar to mitochondria, plastids are enveloped by a double membrane. They proliferate by amitosis like bacteria. They contain circular double-stranded DNA molecules and can autonomously synthesize some of the proteins they require.
Based on pigment and function, plastids are classified into three types: leucoplasts, chloroplasts, and chromoplasts. They all originate from a precursor organelle called a proplastid.
Leucoplasts: Amyloplasts, Proteinoplasts, Elaioplasts
The colorless leucoplasts, can be further classified into three types based on their function: amyloplasts, proteinoplasts, and elaioplasts.
Amyloplasts: These leucoplasts are responsible for the synthesis and storage of starch, a complex carbohydrate used as a long-term energy reserve in plants. Amyloplasts are usually present in storage organs such as roots, tubers, and seeds.
Proteinoplasts: They are involved in the synthesis and storage of proteins. Proteinoplasts are found in protein-rich tissues such as seeds, nuts, and grains.
Elaioplasts: They are plastids involved in the synthesis and storage of lipids, including oils and fats. They are present in oil-rich tissues such as seeds and fruits.
Chromoplasts: pigment, carotenoids
Chromoplasts are another type of plastid found in plant cells. Some proplastids differentiate into chromoplasts which are responsible for the synthesis, accumulation, and storage of pigments. Chromoplasts contain special pigments known as carotenoids which give rise to the vibrant colors in fruits, flowers and roots. Chromoplasts can be developed directly from proplastids or transformed from chloroplasts. Fruits require large amounts of chloroplasts to synthesize nutrients as they grow, which give them a green color. When the fruit ripens, the excess chloroplasts are transformed into chromoplasts giving the fruit a yellow, orange or red color.
Chromoplasts are mainly found in flowers, fruits, leaves, and certain plant roots, such as carrots, giving them yellow, orange, red, or purple hues. These carotenoids play a significant role in attracting pollinators and protecting plants from excessive light and oxidative damage. Additionally, carotenoids are associated with various health benefits due to their antioxidant properties and potential as dietary nutrients.