Plastids

Introduction

Plastids are double membrane-bound cell organelle. Plastids are found in plants, algae and some eukaryotic cells. Ernest Haeckel discovered it and A. F. W. Schimper later provided a precise definition. These organelles are responsible for the synthesis and storage of food in them. They consist of chloroplast, which contains chlorophyll. Chlorophyll is responsible for trapping light energy from the sun for photosynthesis to occur. Photosynthesis is the process of food production in green plants and algae. It is also the primary food source for every living creature on earth. Thus, plastids contain one of the essential features responsible for life on earth. It also consists of its own DNA, RNA and ribosome as they are the descendants of a bacterial cell.

Structure of Plastids

Terrestrial plants contain more amounts of plastids than aquatic plants. It is interesting to know that its structure in plants is simpler than in algae. Their shape varies from species to species; some can be discoid, dumbbell-shaped and spherical-shaped. Under unfavorable or stressful conditions, the mitochondria are often seen inside it. The process of its entry is the intrusion.

Stromule acts as the communicating agent between the plastids and other cell-organelles like mitochondria and nucleus. It consists of a double-membrane structure. This structure is made up of galactolipids. Plastids can only code for a small number of proteins due to their genome reduction and so they depend upon proteins of the nucleus. The cell wall becomes the route for protein transfer from the cytoplasm to the inside. The cell wall also helps in communication.

The organelle’s internal space (stroma) is filled with a matrix. This matrix surrounds the parts lying within the organelle, such as thylakoids. It also consists of DNA and RNA as the plastid’s DNA/RNA. They are attached to the thylakoids or float freely in the matrix. These are the functional units of the plastid’s genome. The stroma also consists of ribosomes. Thus, due to their own genetic material, they can synthesize proteins. Proteins are the building blocks necessary for every metabolic and catabolic process. Therefore, their production is necessary for plastids too.

Types of Plastids

There are four categories of plastids, depending on the pigment present in them.

Chloroplast

Chloroplast is biconvex in shape, present in the mesophyll cells of the plant. Chloroplasts are double-membrane in structure. They contain thylakoids in which chlorophyll is present. Chlorophyll is the green pigment that imparts green colour to the leaves and is responsible for photosynthesis. The chlorophyll traps photons (light energy) from the sun for photosynthesis. The energy production in photosynthesis is in the form of ATP (adenosine triphosphate).

Chromoplasts

Chromoplasts are heterogenous organelles and are the types of plastids. They contain carotenoid-accumulating pigments. They impart colour to flowers and fruits. They are differentiated from chloroplasts; chloroplasts are the ancestors of chromoplasts. They are mostly present in fruits, flowers and ageing leaves. These bright colours of fruits and flowers attract the insects that cause pollination.

Leucoplasts

Colourless plastids are known as leucoplasts. They do not contain any specific pigment that can impart colour to them. They also lack photosynthesis and are present in the parts of plants that do not perform this function, such as roots. They may act as the storage parts of plants in the form of starch, lipids and proteins. They may synthesise amino acids and fatty acids.

Leucoplasts are sub-divided into three categories:

• Amyloplasts – Amyloplasts is the sub-category of colourless plastids known as leucoplasts. Amyloplasts store starch granules in the plants. Starch acts as the reserve food in plants. Amyloplast is present in both photosynthetic and non-photosynthetic plants
• Proteinoplasts – Proteinoplasts are the types of leucoplast that stores proteins. Protein storage is necessary as they are responsible for almost every metabolic activity. Proteinoplasts are also referred to as protoplasts and spheroplasts. They store protein in the form of crystalline bodies. It is mostly present in the seeds such as pulses, brazil nuts and peanuts
• Elaioplasts – Another category of leucoplast is elaioplasts. These are the fatty acids and lipid storage organelles. They not only store but also metabolise lipid synthesis. They look like small rounded organelles filled up with fat droplets. They are mostly present in the embryonic seeds of citrus fruits, oilseeds and anthers of flowering plants

Gerontoplasts

Gerontoplasts are the product of senescence. Senescence refers to the state of cells when they lack the division capability. However, they are not dead; they just stop dividing into daughter cells. The chloroplast undergoes several structural modifications of the thylakoids membrane. It leads to the formation of increased numbers of plastoglobuli and unstacked grana. The protoplast membrane is not affected by senescence. This indicates that plastids contribute an essential role in the controlled degradation of chloroplasts. It causes the plant to retain the protein in the chloroplast and remove chlorophyll and its toxic byproducts.

Plastid Inheritance

Most of the inheritance of plastids is from only one parent. In most plants, they are inherited from the mother cell. The angiosperms inherit them from their female gamete. On the other hand, the gymnosperms inherit them from the male gamete. The algal plastid is also a derivative from a single parent.

Conclusion

Plastids are the site of photosynthesis and food storage in the form of starch, protein and lipids. It is a double-membrane structure and can be discoid, dumbbell-shaped and spherical. They also consist of DNA, RNA and ribosomes as nucleoids, thus involving protein synthesis. Plastids are of four types – chloroplast, chromoplast, leucoplast and gerontoplasts. Chloroplasts contain the green pigment chlorophyll, and chromoplasts impart bright colours to the flowers and fruits. Leucoplasts are colourless and gerontoplasts are the result of senescence. The inheritance of plastids takes place from the single parent – mainly from the female gamete.