Plastids are double membrane bound organelles present in all plant cells, algae and in photosynthetic protists (euglenoids). Plastids are a diverse group of organelles found in the cells of plants and algae. They are unique to these organisms and are absent in animals and most other eukaryotic organisms.
PLASTIDS-
The outer membrane of plastids is smooth and acts as a barrier between the plastid and the cytoplasm. It helps maintain the integrity of the plastid and regulates the passage of molecules into and out of the organelle.The inner membrane of plastids is located adjacent to the outer membrane and is folded into structures called thylakoids in chloroplasts. Thylakoids are arranged in stacks called grana, and they contain the pigment chlorophyll and other components involved in photosynthesis. The inner membrane of other types of plastids may not have the same level of folding as in chloroplasts.
Plastids are of 3 types:-
1) Leucoplasts
2) chromoplasts
3) amyloplasts
Leucoplasts:- Leucoplasts are non-pigmented plastids that primarily function in the synthesis and storage of various substances, such as starch, oils, and proteins. They are commonly found in non-photosynthetic tissues, including roots, seeds, and storage organs like tubers and bulbs.
Futher leucoplast are also classified into 3 types-
Plastids are of 3 types:-
1) Leucoplasts
2) chromoplasts
3) amyloplasts
Leucoplasts:- Leucoplasts are non-pigmented plastids that primarily function in the synthesis and storage of various substances, such as starch, oils, and proteins. They are commonly found in non-photosynthetic tissues, including roots, seeds, and storage organs like tubers and bulbs.
Futher leucoplast are also classified into 3 types-
The main function of amyloplasts is to convert excess glucose into starch for long-term storage. Glucose molecules are polymerized into long chains of glucose units called amylose and amylopectin, which make up the starch granules stored within the amyloplasts.
B) elaioplast:- it is a non-pigmented plastid found in plant cells. Elaioplasts are specifically involved in the synthesis, storage, and metabolism of lipids or oils.
They are commonly found in tissues that require significant oil storage, such as seeds, fruits, and certain specialized cells in plants.
C) aleuroplasts:- Aleuroplasts, also known as proteinoplasts or protein bodies, are a specialized type of leucoplast found in certain plant cells. They are responsible for the synthesis, storage, and accumulation of proteins.Aleuroplasts are typically found in protein-rich tissues, such as seeds, storage organs, and endosperm cells.
The main function of aleuroplasts is to store proteins, primarily in the form of globulins and prolamins, which are important for seed development, germination, and early seedling growth.
Chromoplasts:- it is a type of non- photosynthetic plastids which is responsible for the synthesis, storage, and accumulation of pigments, giving various tissues and organs their characteristic colors due to the presence of carotenoid pigments.
Chromoplast are formed either from leucoplast or chloroplast.
Carotenoids are the most common pigments found in chromoplasts and are responsible for yellow, orange, and red colors. Other pigments, such as anthocyanins, can also be present in chromoplasts, contributing to purple, blue, and red hues.
The differentiation of chromoplasts from other plastids, such as chloroplasts or leucoplasts, is regulated by genetic factors and environmental cues. Under specific conditions, such as ripening of fruits or petal development, the existing plastids can transform into chromoplasts, leading to the synthesis and accumulation of pigments.
Chloroplasts:- it is a type of plastid which is responsible for the process of photosynthesis, which converts light energy into chemical energy in the form of glucose. Chloroplasts contain the green pigment chlorophyll, which gives them their characteristic color.The main function of chloroplasts is to capture sunlight and use it to produce energy-rich molecules through photosynthesis.
Chloroplasts are double membrane bound organelle. The outer membrane is a semi-porous membrane that serves as a barrier between the cytoplasm of the cell and the interior of the chloroplast and contains various transport proteins, which regulate the movement of molecules, ions, and metabolites into and out of the chloroplast.
The inner membrane is a selectively permeable membrane that controls the passage of molecules and ions into and out of the chloroplast. The inner membrane contains transport proteins, channels, and carriers that facilitate the transport of metabolites across the membrane.
Between the outer and inner membranes, there is a narrow intermembrane space that separates the two membranes.
The inner membrane encloses the stroma, which is the fluid-filled region where the light-independent reactions of photosynthesis occur. The stroma contains enzymes, DNA, ribosomes, and other components necessary for various metabolic processes within the chloroplast. Stroma contains large complex network of stacked sacs.
Each stack is called granules and each of the flattened sacs which makes up the granules is called thylakoid. The thylakoids of different grana are connected by flat membrane tubules called stroma lamellae. The membrane of thylakoids enclosed a space called a lumen.
During the light-dependent reactions, chlorophyll and other pigments in the thylakoid membranes of chloroplasts capture light energy. This energy is used to generate ATP and NADPH which are energy rich melocules.
In the light-independent reactions (Calvin cycle), which take place in the stroma of the chloroplasts, ATP and NADPH produced in the light-dependent reactions are used to convert carbon dioxide (CO2) into glucose and other organic molecules.
READ MORE: MITOCHONDRIA
Chloroplasts as semiautonomous organelle:-
Chloroplasts are also called semi autonomous organelle as they have their own genetic material for duplication and able to synthesize some of their own proteins with the help of 70s ribosomes, however most of the chloroplastal proteins are encoded by nuclear genetic material (dsDNA).Endosymbiotic theory- origin of chloroplast:-
According to the endosymbiotic theory, chloroplasts are believed to have evolved from a free-living cyanobacterium that was engulfed by a host eukaryotic cell.Endosymbiosis involved a eukaryotic host cell engulfing a photosynthetic prokaryote, specifically a cyanobacterium. Rather than digesting the cyanobacterium, the host cell established a symbiotic relationship with it. Over time,through natural selection and genetic changes, the cyanobacterium became integrated into the host cell, eventually evolving into what we now know as a chloroplast.
Evidences of Endosymbiotic theory:-
1) chloroplast is a self replicating organelle like bacteria and divide by binary fission as bacteria do.2) chloroplast has proteins called porins in the outer membrane which resembles with that of bacteria.
3) chloroplast have their own DNA and RNA and 70s ribosome as that of bacteria.
Conclusion:-
1) plastids are double membrane bound structure and contains three type of plastids leucoplast chromoplast and chloroplast.2) Leucoplasts are further classified into three types of plastids known as amyloplast elaioplast and aleuroplast.
3) amyloplast are the type of leucoplast that contains starch for example potato Tuber, rice wheat, etc.
4) elaioplast are the fats and oil storage Leucoplasts.for example Castor
5) aleuroplast are the protein storing leucoplast, for example aleurone grains of maize.
6) chloroplast is also called semi autonomous organelle.
7) Endosymbiosis involved a eukaryotic host cell engulfing a photosynthetic prokaryote, specifically a cyanobacterium. Rather than digesting the cyanobacterium, the host cell established a symbiotic relationship with it.
FAQs:-
Ques-1 what is the chloroplast?Ans- chloroplast is a type of plastid which is responsible for the process of photosynthesis, which converts light energy into chemical energy in the form of glucose.
Ques-2 where is chloroplast found?
Ans- chloroplast found in all green plants including green stem and in unripened fruits but most of the chloroplasts are found in the mesophyll cells of the leaves.
Ques-3 what is chromoplast and its function?
Ans- chromoplast is a type of non- photosynthetic plastids which is responsible for the synthesis, storage, and accumulation of pigments, giving various tissues and organs their characteristic colors due to the presence of carotenoid pigments.
Chromoplast are formed either from leucoplast or chloroplast.
Ques-4 what is leucoplast and its function?
Ans- Leucoplasts are non-pigmented plastids that primarily function in the synthesis and storage of various substances, such as starch, oils, and proteins. They are commonly found in non-photosynthetic tissues, including roots, seeds, and storage organs like tubers and bulbs.
Ques-5 why chloroplast is also called as semi autonomous organelle?
Ans- Chloroplasts are also called semi autonomous organelle as they have their own genetic material for duplication and able to synthesize some of their own proteins with the help of 70s ribosomes, however most of the chloroplastal proteins are encoded by nuclear genetic material (dsDNA).
