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During senescence, gerontoplasts are plastids that arise from chloroplasts. Chloroplasts are plastids with a lot of green pigments in them (chlorophyll). They have three membrane systems. These are the thylakoid system, the outer membrane, and the inner membrane. The photosynthetic processes that are light-dependent occur in the thylakoid system. It consists of a stack of disks (granum) linked by stromal thylakoids to another granum.
The gerontoplast is formed in senescent plant tissue that was formerly green. The gerontoplast grows from chloroplast during leaf senescence under genetic control. The gerontoplast has a decreased thylakoid system and numerous plastoglobuli, similar to the chromoplast. It, on the other hand, is unable to subdivide. However, the gerontoplast can only revert to a chloroplast for a limited amount of time. When a cell reaches the end of its life cycle, it dies, and the change from chloroplast to gerontoplast is irreversible.
PLASTIDS
Plastids are a phylogenetic and physiologically associated collection of organelles present in all plants and algae. The many kinds of plastids have different roles in plant metabolism, supporting plant growth and development. The fact that these organelles have a double membrane is one of their most distinctive features. Plastids are largely involved in the production and storage of food in cells. They are therefore engaged in activities like photosynthesis, amino acid and lipid synthesis, as well as the storage of numerous resources.
STRUCTURE
Higher plants have spherical, ovoid, or discoid chloroplasts, while algae have stellate, cup-shaped, or spiral chloroplasts.
Two lipoprotein membranes, an outer and an inner membrane, surround the chloroplast, with an intermembrane region between them.
The stroma, a matrix containing small cylindrical structures known as grana, is enclosed by the inner membrane. Most chloroplasts have between 10 and 100 grana.
FUNCTIONS
Plastids are found in all plant cells in some form or the other. Some of the functional variety of plastids and how they are at the heart of plant cellular activity are stated below –
Plastids are the places where key chemical compounds used by autotrophic eukaryotic cells are created and stored.
All of the enzyme components necessary for photosynthesis are present in the thylakoid membrane. Within the thylakoid membrane, chlorophyll reacts with electron carriers, coupling factors, and other components. As a result, the thylakoid membrane has a specialised structure that helps in light collection and electron transport.
Chloroplasts are thus the centres of carbohydrate synthesis and metabolism.
They are vital not only for photosynthesis but also for the storage of fundamental nutrients, notably starch.
The presence of pigments is critical to its function. Pigments, which are also responsible for the colour of a plant structure, are commonly found in plastids engaged in food production (e.g., green leaf, red flower, yellow fruit, etc.).
Plastids, like mitochondria, have their DNA and ribosomes. As a consequence, they can be used in phylogenetic analyses.
THE GRANA AND THALAKOIDS
Several disc-shaped membranous sacs named grana lamellae or thylakoids are layered one on top of the other in each granum.
Inter-grana or stroma lamellae are a system of anastomosing tubules that connect the grana.
Chloroplasts also contain single thylakoids known as stroma thylakoids.
The stroma matrix also contains electron-dense bodies, osmophilic granules, ribosomes (the 70S), circular DNA, RNA, and soluble Calvin cycle enzymes.
The outer, inner, and thylakoid membranes are the three membranes that constitute chloroplasts.
The thylakoid membrane is composed of lipoprotein and lipids like galactolipids, sulpholipids, and phospholipids.
Owing to small spheroidal quantosomes, the internal surface of the thylakoid membrane is granular in structure.
The photosynthetic units, called quantosomes, are made up of two structurally separate photosystems, PS I and PS II, which each contain about 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 present in higher plants.
The two photosystems, as well as the electron transport chain’s components, are dispersed asymmetrically over the thylakoid membrane. PS I and PS II electron acceptors are located on the thylakoid membrane’s outer (stroma) surface. PS I contains electron donors on the inner (thylakoid space) surface.
CONCLUSION
Plastids, which are descended from prokaryotic endosymbionts, are a special set of interconvertible organelles found in plant cells. The chloroplast, which conducts the primordial plastid function of photosynthesis, is the most recognized plastid type. Plastid activities were increasingly intertwined with cellular metabolism and functions, as well as plant developmental processes, over time, resulting in the emergence of newer forms of non-photosynthetic plastids. The chromoplast, a carotenoid-rich organelle found in flowers and fruits, is one of them. Plastids play an important role in cell food creation and storage. As a result, they assist in processes such as photosynthesis, amino acid and lipid synthesis, and material storage, among others. Endosymbiosis (a symbiotic relationship in which one organism lives within another) is believed to have given rise to mitochondria before plastids, in part since mitochondria are found in all eukaryotes but plastids are absent in many.
