Plastids

Plastids are a group of phylogenetically and physiologically related organelles, the cell organelles found in all plant and algae cells. Apart from plants and algae, these cell organelles are also found in cells of fern, moss, some parasitic worms, and marine musks. Plastids play a crucial role in plant metabolism. Hence, they promote the development and growth of plants. But, what are plastids, precisely? Plastids are double-membraned cell organelles present in all plant cells. Primarily, these organelles are involved in food production and storage. Some of the processes where plastids are involved are- photosynthesis, amino acids and lipids synthesis, storage of numerous substances, and various other functions. 

Different types of plastids

Like all plant cells, plastids are obtained from meristem cells inside the plant. Situated at the shoot and root tips, meristems are the wellsprings of undifferentiated cells in plants. Proplastids, the progenitor plastids, are undifferentiated plastids that are obtained from meristems. Further improvement of this brings about the development of various kinds of plastids that thus serve various functions adding to general metabolism.

• Chloroplasts

These are the kinds of plastids located in the mesophyll cells of plant cells. The epidermal cells of the plant also have some chloroplasts. However, the chloroplasts located here are much less developed. These plastids vary in size for different species of plants and even within a plant.The process of photosynthesis occurs in these plastids. The green pigment, chlorophyll present in chloroplasts, is crucial for photosynthesis. 

These organelles are spheroid in shape. However, this shape can vary based on the plastid’s location. The double-membrane of chloroplasts is composed of an inner and outer membrane. These membranes are phospholipid layers. Stroma is an aqueous matrix in the space between the two membranes, and this matrix consists of numerous enzymes and proteins significant for some cellular processes. 

Structure of chloroplasts- Chloroplasts are regularly found in guard cells situated in plant leaves, about 1-2 μm thick and 5-7 μm in diameter. Chloroplasts are oval in shape and have two layers: an external and an internal layer. The intermembrane space is between the external and internal layers, roughly 10-20 nm wide. The space within the inner layer is the stroma, the thick liquid inside the chloroplast. This is the site where the transformation from carbon dioxide to carbs happens.

• Chromoplasts

Acting as the site of pigment accumulation, these plastids are brightly colored and are typically found in the flowers, fleshy fruits, and other pigmented parts of a plant like the leaves. In addition, Chromoplasts have an accumulation of carotenoids. Hence, they play a significant role in pollination. Furthermore, it is because they act as visual attractors for animals acting as pollination agents. Chromoplasts are of two types: rhodoplasts and phaeoplasts. 

• Gerontoplasts

Contrasted with other plastids, gerontoplasts are framed during senescence. Senescence includes the degradation of different organelles of a plant cell. During this interaction, the chloroplast goes through broad primary adjustment of the thylakoid membrane, followed by the development of expanded quantities of plastoglobuli. The grana are likewise steadily unstacked; however, the gerontoplast layer stays in one piece as senescence proceeds.

This plastid assumes a significant part in chloroplasts degradation. This permits the plant to hold the majority of the protein contained in the chloroplasts (75% of the leaf protein) and eliminates chlorophyll and byproducts that have been demonstrated to be possibly toxic.

• Leucoplasts

Leucoplasts are colorless plastids. These plastids are commonly found in colorless leaves and tissues that grow rapidly. The function of plastids- leucoplasts is to serve as the starch formation and storage sites. Leucoplasts do not contain chlorophyll. Also, they are not exposed to direct sunlight as they are located in deep tissues. Although the function of plastids- leucoplasts is storage, they are also involved in fats and lipids synthesis. Below are the three main types of leucoplasts:

• Amyloplasts- What are plastids involved in the long-term storage of starch? Amyloplasts are these plastids. In some species, amyloplasts are also responsible for enzyme production that promotes nitrogen assimilation. 
• Elaioplasts- Also known as lipoblasts, these leucoplasts contain oil. These plastids store oil and lipids. 
• Proteinoplasts- When compared to other plastids, proteinoplasts contain more protein. These are typically found in seeds. 

Conclusion

A. F. W. Schimper was the first to name and define plastids. They contain pigments utilized in photosynthesis. The type of pigment present in a plastid determines its color. They are additionally the site of production and storage of significant chemical compounds utilized by the cells of autotrophic eukaryotes. They have a double-stranded DNA molecule. Plastids are semi-autonomous bodies that contain their genetic material. This makes them capable of protein synthesis. The different types of plastids are- Chloroplasts, Chromoplasts and Leucoplasts.