Structure of Plastids

Plastid is a double membrane-bound organelle present in photosynthetic plant cells that is involved in food synthesis and storage. Ernst Haeckel discovered and named plastids, but A. F. W. Schimper was the first to give them a precise definition. They’re required for life’s most basic functions, such as photosynthesis and food storage. A chloroplast is a plastid that contains green pigment (chlorophyll), whereas a chromoplast is a plastid that contains pigments other than green. A leucoplast is a plastid that lacks pigments and is used mostly for food storage. 

Types of plastids:

Plastids come in a variety of shapes and sizes, each with its own set of capabilities. The presence or absence of biological pigments, as well as their developmental stages, are used to classify a few of them: 

1. Chloroplasts
2. Chromoplasts
3. Gerontoplasts
4. Leucoplasts

Chloroplasts:

Chloroplasts are biconvex-shaped, semi-porous, double-membraned cell organelles located within the plant cell’s mesophyll. They are the places where food is synthesised through the photosynthesis process. 

Chromoplasts:

Chromoplasts is the name given to a compartment in the plant where all of the pigments are stored and manufactured. These are commonly seen in blossoming plants, wilting leaves, and ripening fruits. Chloroplasts become chromoplasts when they undergo a transformation. Carotenoid pigments in chromosomes provide for the variety of colours seen in plants and fruits. Its distinctive colour is primarily intended to attract pollinators. 

Gerontoplasts:

These are essentially aged chloroplasts. Gerontoplasts are the chloroplasts in leaves that assist them transform into other organelles when they are no longer used for photosynthesis, which happens in the autumn. 

Leucoplasts:

These organelles are colourless and non-pigmented. Most non-photosynthetic sections of the plant, such as the roots, include leucoplasts. Depending on the needs of the plants, they function as storage sheds for carbohydrates, lipids, and proteins. Amino acid and fatty acid conversion is the most common use for them. 

There are 3 types of leucoplasts:

1. 1. Amyloplasts: Amyloplasts, which store and synthesise starch, are the most numerous of the three.

• Proteinoplasts: Proteinoplasts are found in seeds and aid in the storage of proteins required by the plant. 

• Elaioplasts: Elaioplast aids the plant’s ability to store lipids and oils. 

Structure of plastids:

1. Higher plants have stellate, cup-shaped, or spiral chloroplasts, while algae have stellate, cup-shaped, or spiral chloroplasts.
2. They are usually 4-6 m in diameter and 20 to 40 in number, equally scattered throughout the cytoplasm of higher plants’ cells. 

1. The outer and inner membranes of the chloroplast are separated by an intermembrane gap.
2. The stroma, which contains small cylindri­cal structures known as grana, is enclosed by the inner membrane. The majority of chloroplasts have 10 to 100 grana. 

The Grana and Thylakoids:

1. A number of disc-shaped membranous sacs termed grana lamellae or thylakoids (80-120 across) are stacked one on top of the other in each granum.
2. Inter-grana, also known as stroma lamellae, is a network of anastomosing tubules that connects the grana.
3. Chloroplasts also include single thylakoids called stroma thylakoids.
4. In addition, the stroma matrix contains electron dense bodies, osmophilic granules, ribosomes (70S), circular DNA, RNA, and soluble Calvin cycle enzymes. 

1. The outer, inner, and thylakoid membranes are the three membranes that make up chloroplasts.
2. The thylakoid membrane is made up of lipoprotein and lipids such as galactolipids, sulpholipids, and phospholipids.
3. Due to small spheroidal quantasomes, the inner surface of the thylakoid membrane is gra­nular in organisation.
4. The photosynthetic units, known as quantasomes, are made up of two fundamentally separate photosystems, PS I and PS II, which each contain roughly 250 chlorophyll molecules. Each photosystem consists of antenna chlorophyll complexes and a reaction centre where energy is converted. Chlorophyll-a, chlorophyll-b, carotene, and xanthophyll are the pigments found in higher plants.

Functions of plastids:

1. Plastids are where autotrophic eukaryotes’ cells make and store essential chemical substances.
2. The enzyme components essential for photosynthesis are found in the thylakoid membrane. Within the thylakoid membrane, there is interaction between chlorophyll, electron carriers, coupling factors, and other components. As a result, the thylakoid membrane has a specialised structure that is important for light collection and electron transport.
3. Chloroplasts are thus the sites where carbohydrates are synthesised and metabolised. 

Conclusion:

Plants and algae have double membrane organelles called plastids. In some eukaryotic creatures, they are also found. They’re derived from an endosymbiotic relationship. Plastids are responsible for the production and storage of food. Many plastids include photosynthetic pigments, and some plastids have different types of pigments that can vary the cell’s colour.