Plastids: Definition, Types, Examples, Diagram, Function

Edited By Irshad Anwar | Updated on Jul 02, 2025 05:53 PM IST

Plastids are unique cell organelles present in plant cells and algae. These are well known for their crucial role in the process of photosynthesis, pigment synthesis, and storage. Plastids is a topic of the chapter Cell: The Unit of Life in Biology.

What are Plastids?

Plastids are important cell organelles in plants and algae. These carry a range of functions, such as photosynthesis, pigment synthesis, and the storage of starches, oils, and proteins. Plastids contain their DNA and the system for synthesising proteins.

Plastids are essential for normal plant cell functioning. The process of energy production and metabolism is important for developing plants, and plastids play a consequential role in it. Plastids were first observed during the 19th century when scientists found the so-called photosynthetic organelle, the chloroplast.

This Story also Contains

What are Plastids?
Types of Plastids
Structure of Plastids
Functions of Plastids
Plastid biogenesis
Plastid Genome
Applications of Plastids

Plastids: Definition, Types, Examples, Diagram, Function

Types of Plastids

There are different types of plastids, each of which performs different functions:

Types of Plastids

Chloroplasts

The green pigment chlorophyll in plants is due to the presence of chloroplasts.
These are double membrane cell organelles containing thylakoid and stroma.
Chloroplasts are significant in the photosynthesis process.

Chromoplasts

A pigment named carotenoid is present in the chloroplasts.
The colour of different fruits and vegetables like tomatoes, bell peppers, and carrots is due to the presence of chromoplasts.
These do not have the thylakoid structure like in chloroplasts.
The chromoplasts function as the significant location to store and synthesise pigments.

Leucoplasts

These do not have any pigment and are colourless organelles.
Leucoplasts have many subtypes, namely amyloplast (stores and synthesises starch), elaioplast (stores fats and oils ), and proteinoplast (stores proteins).
Their main function is to store essential substances used by the plants.

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Gerontoplasts

This type of plastid plays a significant role in the ageing and senescence of Leaves.
These are formed from chloroplasts during the breakdown process of photosynthesis.

Etioplasts

The formation of this type of plastid takes place in the dark.
As the plant is exposed to light, etioplasts get converted into chromoplasts, and the process of photosynthesis starts.

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Structure of Plastids

The plastid structure can be differentiated into three major components: the outer membrane, the inner membrane, and the thylakoid membrane.
The outer membrane is smooth and permeable by small molecules as well as ions, but the inner membrane is less permeable and has transport proteins regulating the passage of materials.
The inside of a chloroplast consists of interconnected thylakoid membranes that are arranged in stacks called grana, where light-dependent reactions take place during photosynthesis.
The stroma is a dense fluid inside plastids surrounding the thylakoid membranes containing enzymes and the plastid DNA and ribosomes.

Different plastids have distinct structural differences related to specialisation:

For example, chloroplasts contain a highly structured internal membrane.
Chromoplasts, specialised for pigment synthesis, have few internal membranes.
Leucoplasts contain no pigments and serve as storage organelles for starch, oils, and proteins with little internal membrane.
Gerontoplasts arise as chloroplasts age. They contain reduced thylakoid membranous structures and enzymes to recycle nutrients.
Etioplasts are those that are developed in the absence of light. Contains prolamellar bodies that are a precursor to the thylakoid membranous system.

Functions of Plastids

Chloroplasts are the site of photosynthesis; they convert light energy to chemical energy, which is subsequently stored as glucose.
The photosynthetic machinery is set up in two ways: light-dependent reactions primarily occur in the thylakoid membranes.
Chlorophyll, in the presence of light, catalyses the generation of ATP and NADPH, and Calvin's cycle, which is a CO₂ fixation reaction, happens in the stroma.
The plant can then utilise the glucose both for its daily activities requiring energy and as raw material for the synthesis of other organic molecules.
Chromoplasts are the plastid type needed for synthesising and storing pigments like carotenoids and xanthophylls.
These pigments result in the conspicuous colours of fruits, flowers, and other plant organs, and all of these are necessary to attract the vectors that aid in pollination and seed dispersal.
The location of chromoplasts in petals and fruits demonstrates their function in enhancing the efficiency of pollination and aiding in the dissemination of seeds by animals attracted to colourful displays.
Leucoplasts are storage organelles mainly for starch, oils, and proteins.
They play a leading role in the growth and development of plants, acting as reservoirs for important nutrients.

Plastid biogenesis

The endosymbiotic theory accounts for the origin of plastids. This theory postulates that plastids arose from free-living cyanobacteria engulfed by ancestral eukaryotic cells. The genetic evidence is based on the presence of several genes, including plastid genes, indicating that plastid genomes share significant similarities with the genomes of cyanobacteria, indicating a common evolutionary ancestry.

Plastid development is marked by the differentiation of proplastids, the less developed and relatively undifferentiated plastids found in meristematic tissues. Various environmental factors, such as light, play an important role in their development into mature forms. For example, in the absence of light, proplastids can develop into etioplasts, a precursor to chloroplasts. These etioplasts finally form the chloroplasts.

Plastid Genome

The plastid DNA, also termed the plastome, is circular and compact, between 100 and 200 kb in length.
Unlike nuclear DNA, plastid DNA is less gene-dense and lacks a helical chromosome structure.
It most notably encodes proteins that mainly function in the photosynthesis process and gene expressions that include ribosomal proteins, RNA polymerases, and ATP synthase subunits.
Plastid and nuclear genomes are coordinated closely to ensure the proper functioning of plastids.
Indeed, the activity of these plastids has to be coordinated to maintain the efficiency and functioning of cellular activities.

Applications of Plastids

In the realm of biotechnology, chloroplast genetic engineering is emerging as a potent tool. Scientists modify the chloroplast genome to express biopharmaceuticals like therapeutic proteins and vaccines, using the fact that chloroplasts express protein at higher levels. Since the yield is higher than in other systems and contamination by human pathogens is low, it reduces the cost and time needed for pharmaceutical production.

In agriculture, plastids are instrumental in increasing the yield of nutrients in crops. These same chloroplasts can be engineered to develop plants with an improved ability to resist many environmental stresses, like drought, salinity, and pests. More importantly, such improved crop varieties help to reduce the application of chemical pesticides and fertilisers, thus promoting sustainable agriculture.

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Frequently Asked Questions (FAQs)

1. What are plastids and their functions?

Plastids are the organelles present in the cells of plants and algae, responsible for photosynthesis, pigment synthesis, and nutrient storage.

2. How do chloroplasts differ from other plastids?

Chloroplasts are the plastids responsible for photosynthesis, a feature that sets them apart from other plastids, including chromoplasts and leucoplasts, by utilising their green pigmentation, hence their capacity to convert light energy into chemical energy.

3. What is the significance of chromoplasts in plants?

Chromoplasts of plants play an important role in synthesising and storing pigments that give colour to the fruits and flowers, hence helping in the attraction of pollinators for reproduction.

4. How do leucoplasts contribute to plant growth?

Leucoplasts play the role of starch, oils, and protein storage for the biological processes of growth and development of a plant, more particularly in storage organs like roots and seeds, enabling seed germination while storing the said energy for use by the plant.

5. Can plastids be genetically engineered for better crop production?

Plastids can be genetically engineered, helping improve crops through genes responsible for high-yielding abilities, better nutritional qualities, or tolerance to biotic and abiotic stresses, which could contribute to solutions to challenges being faced in agriculture.

6. What is plastid transformation and why is it important in biotechnology?

Plastid transformation is a technique used to introduce foreign genes into the plastid genome. It's important in biotechnology because it allows for high levels of protein expression, maternal inheritance of introduced traits, and containment of transgenes. This technique is used to develop plants with improved traits or to produce pharmaceuticals and industrial products.

7. How do plastids contribute to plant stress responses?

Plastids play a crucial role in plant stress responses. They can alter their structure and function to help plants cope with environmental stresses like drought, high light, or temperature extremes. For example, chloroplasts may produce protective pigments or antioxidants to combat oxidative stress, while other plastids may accumulate or break down certain compounds to help the plant adapt.

8. How do plastids contribute to the nutritional value of fruits and vegetables?

Plastids, particularly chromoplasts, are responsible for producing and storing many of the beneficial compounds found in fruits and vegetables. These include carotenoids (like beta-carotene), which give orange and red colors and have antioxidant properties, as well as various vitamins and other phytonutrients that contribute to the nutritional value of plant-based foods.

9. What is the role of plastids in nitrogen fixation in legumes?

In legumes, specialized plastids called symbiosomes play a crucial role in nitrogen fixation. These plastids are found in root nodule cells and house nitrogen-fixing bacteria (rhizobia). The plastids provide a protected environment for the bacteria and facilitate the exchange of nutrients between the bacteria and the plant cell.

10. What is the role of plastids in non-photosynthetic plants?

Even in non-photosynthetic plants, such as parasitic plants, plastids still play important roles. They may be involved in the synthesis of essential compounds like fatty acids and amino acids. In some cases, these plastids may retain vestigial photosynthetic capabilities or evolve new functions specific to the plant's lifestyle.

11. What are plastids and why are they important in plant cells?

Plastids are specialized organelles found in plant and algal cells. They are important because they perform various crucial functions like photosynthesis, storage of starch, proteins, and lipids, and synthesis of certain amino acids and fatty acids. Plastids are essential for plant survival and growth, as they are responsible for energy production and storage of nutrients.

12. How do chloroplasts carry out photosynthesis?

Chloroplasts contain chlorophyll and other pigments that capture light energy. This energy is used to convert carbon dioxide and water into glucose and oxygen through a series of chemical reactions. The process occurs in the thylakoid membranes and stroma of the chloroplast, involving light-dependent and light-independent reactions.

13. How do chromoplasts contribute to plant appearance and function?

Chromoplasts store and display various pigments, giving fruits and flowers their vibrant colors. These colors attract pollinators and seed dispersers, aiding in plant reproduction. Additionally, some pigments in chromoplasts have antioxidant properties, protecting the plant from damage caused by excess light or environmental stress.

14. What are the different types of leucoplasts and their functions?

There are three main types of leucoplasts:

15. How do plastids move within a cell?

Plastids can move within a cell using the cell's cytoskeleton, particularly actin filaments. This movement allows plastids to position themselves optimally for their functions, such as chloroplasts moving towards light for efficient photosynthesis or amyloplasts repositioning in response to gravity.

16. What is plastid DNA and how does it differ from nuclear DNA?

Plastid DNA (also called plastome) is the genetic material found within plastids. It is typically circular, like bacterial DNA, and much smaller than nuclear DNA. Plastid DNA primarily encodes proteins essential for plastid function, such as those involved in photosynthesis. Unlike nuclear DNA, plastid DNA is usually inherited only from the maternal parent in most plants.

17. How do plastids replicate?

Plastids replicate through a process called binary fission, similar to bacterial cell division. This process involves the duplication of the plastid's DNA, division of the internal structures, and finally, the separation into two daughter plastids. The ability to replicate independently is a key feature that distinguishes plastids from other organelles.

18. What is the evolutionary origin of plastids?

Plastids are believed to have originated through endosymbiosis, where an ancient photosynthetic bacterium (similar to modern cyanobacteria) was engulfed by a eukaryotic cell. Over time, this symbiotic relationship evolved, with the bacterium becoming an integral part of the cell as a plastid. This theory explains why plastids have their own DNA and can replicate independently.

19. How do plastids differ from other cell organelles?

Plastids are unique because they have their own DNA and can replicate independently. Unlike other organelles, plastids are found only in plant and algal cells, not in animal cells. They also have a double membrane structure and can change their form and function based on the cell's needs.

20. How do plastids communicate with the cell nucleus?

Plastids and the nucleus communicate through a process called retrograde signaling. Plastids send chemical signals to the nucleus to regulate gene expression based on their functional state or environmental conditions. This two-way communication ensures that the cell can respond appropriately to changes in plastid function or external stimuli.

21. What is the relationship between proplastids and other plastid types?

Proplastids are undifferentiated plastids found in meristematic cells. They can develop into any of the specialized plastid types (chloroplasts, chromoplasts, or leucoplasts) depending on the cell's needs and environmental conditions. This ability to differentiate allows plants to adapt to changing conditions.

22. How do environmental factors influence plastid development?

Environmental factors like light, temperature, and nutrient availability can significantly influence plastid development. For example, light exposure can trigger the conversion of proplastids into chloroplasts, while darkness may lead to the formation of leucoplasts. Stress conditions can also cause changes in plastid structure and function as the plant adapts to its environment.

23. What is the difference between etioplasts and chloroplasts?

Etioplasts are precursors to chloroplasts found in plants grown in darkness. They contain a specialized internal membrane structure called the prolamellar body but lack chlorophyll. When exposed to light, etioplasts rapidly transform into chloroplasts, developing thylakoid membranes and producing chlorophyll for photosynthesis.

24. What is the significance of plastid number and size in different plant tissues?

The number and size of plastids can vary greatly between different plant tissues and cell types. This variation reflects the specific metabolic needs of each cell type. For example, leaf cells typically have numerous large chloroplasts for efficient photosynthesis, while root cells may have fewer, smaller leucoplasts for storage. The plastid population can also change in response to environmental conditions or developmental stages.

25. How do plastids contribute to fruit ripening?

During fruit ripening, chloroplasts often transform into chromoplasts. This transformation involves the breakdown of chlorophyll and the synthesis and accumulation of carotenoids and other pigments. These changes not only affect the fruit's color but also contribute to changes in texture, flavor, and nutritional content. The process is often triggered by ethylene and involves complex signaling between plastids and other cellular components.

26. What is the significance of the plastid envelope?

The plastid envelope consists of two membranes that surround the organelle. It controls the exchange of materials between the plastid and the cytoplasm, regulates plastid division, and plays a role in signaling between the plastid and the nucleus. The envelope is crucial for maintaining the plastid's unique internal environment.

27. How do plastids contribute to plant adaptation to different light environments?

Plastids, particularly chloroplasts, can adapt to different light environments. In high light, they may reduce their light-harvesting capacity and increase photoprotective mechanisms. In low light, they may increase their light-harvesting pigments and adjust their internal structure for more efficient light capture. These adaptations allow plants to optimize photosynthesis across a range of light conditions.

28. What are the main types of plastids?

The main types of plastids are:

29. How do plastids contribute to plant cell differentiation?

Plastid differentiation is often closely linked to cell differentiation in plants. As cells specialize, their plastids may transform into specific types to support the cell's function. For example, as a leaf cell matures, its proplastids develop into chloroplasts. This coordinated differentiation is crucial for proper tissue and organ development in plants.

30. What is the role of plastids in gravitropism?

Plastids, specifically amyloplasts, play a crucial role in gravitropism - the growth response of plants to gravity. In specialized cells called statocytes, amyloplasts act as statoliths. They settle to the bottom of the cell in response to gravity, triggering a signaling cascade that ultimately leads to differential growth and bending of the plant organ towards or away from gravity.

31. How do plastids contribute to plant senescence?

During plant senescence (aging), plastids undergo significant changes. In leaves, chloroplasts often degrade, breaking down chlorophyll and other components. This process allows the plant to recycle nutrients. Some plastids may transform into gerontoplasts, which are involved in the mobilization and export of nutrients from senescing tissues to other parts of the plant.

32. What is the role of plastids in CAM (Crassulacean Acid Metabolism) photosynthesis?

In CAM plants, which are adapted to arid conditions, chloroplasts play a unique role. During the night, when stomata are open, carbon dioxide is fixed into organic acids and stored in vacuoles. During the day, these acids are transported to chloroplasts where they are decarboxylated, releasing CO2 for use in the Calvin cycle. This adaptation allows CAM plants to conserve water while still performing photosynthesis.

33. How do plastids contribute to photorespiration?

While photorespiration primarily occurs in peroxisomes and mitochondria, chloroplasts play a crucial role in initiating the process. In chloroplasts, the enzyme RuBisCO sometimes fixes oxygen instead of carbon dioxide, producing a compound that must be processed through photorespiration. This process involves coordination between chloroplasts, peroxisomes, and mitochondria.

34. How do plastids contribute to plant defense mechanisms?

Plastids contribute to plant defense in several ways. They can produce and store defensive compounds like alkaloids and terpenes. In response to pathogen attack, chloroplasts can generate reactive oxygen species as part of the plant's immune response. Some plastids also produce salicylic acid, an important signaling molecule in plant defense.

35. What is plastid division and how is it regulated?

Plastid division is the process by which plastids increase their numbers within a cell. It involves the formation of a division ring, constriction, and separation into two daughter plastids. This process is regulated by both plastid and nuclear genes. Factors like light, plant hormones, and the cell cycle can influence plastid division rates, ensuring that cells maintain appropriate numbers of plastids.

36. What is the role of plastids in symbiotic relationships?

Plastids play crucial roles in various symbiotic relationships. In addition to their role in nitrogen-fixing root nodules, plastids are essential in symbioses between plants and fungi (mycorrhizae) or algae and fungi (lichens). In these relationships, plastids often function in nutrient exchange and energy production, supporting the symbiotic association.

37. What is the relationship between plastids and plant hormones?

Plastids both respond to and produce plant hormones. For example, chloroplasts are involved in the biosynthesis of auxins, gibberellins, and abscisic acid. Conversely, hormones can influence plastid development and function. This interplay is crucial for coordinating plant growth and development with environmental conditions and metabolic needs.

38. How do plastids contribute to the production of secondary metabolites in plants?

Plastids are key sites for the production of many secondary metabolites, including terpenes, alkaloids, and phenolic compounds. These compounds often play roles in plant defense, pollinator attraction, or stress responses. The ability of plastids to produce these diverse compounds contributes significantly to the chemical diversity found in plants.

39. How do plastids contribute to the plant's response to temperature stress?

Plastids play a crucial role in plant responses to both heat and cold stress. Under heat stress, chloroplasts may produce heat shock proteins and adjust their membrane composition to maintain function. In cold stress, plastids may increase production of cryoprotectants and alter their membrane lipids to maintain fluidity. These responses help protect the photosynthetic apparatus and other plastid functions under temperature extremes.

40. What is the significance of plastid-to-nucleus signaling?

Plastid-to-nucleus signaling, also known as retrograde signaling, is crucial for coordinating plastid function with nuclear gene expression. This signaling allows plastids to communicate their functional state, developmental stage, or stress conditions to the nucleus. The nucleus can then adjust gene expression to support plastid function or respond to environmental changes, ensuring cellular homeostasis and appropriate responses to external stimuli.

41. How do plastids contribute to the synthesis of plant cell walls?

While the bulk of cell wall synthesis occurs in the Golgi apparatus, plastids play an important role by producing precursors for cell wall components. For example, plastids are involved in the synthesis of fatty acids used in cutin and suberin production, which are important cell wall components. Additionally, some plastid-derived compounds contribute to cell wall lignification.

42. What is the role of plastids in photoperiodism and flowering?

Plastids, particularly chloroplasts, play a role in photoperiodism - the response of plants to day length. They are involved in the production of signaling molecules that influence flowering time. Additionally, the redox state of the plastid electron transport chain can act as a signal in photoperiodic responses, linking photosynthetic activity to developmental decisions like flowering.

43. How do plastids contribute to the plant's antioxidant system?

Plastids are major sites of reactive oxygen species (ROS) production in plant cells, particularly during photosynthesis. To counter this, they have developed robust antioxidant systems. These include enzymatic antioxidants like superoxide dismutase and ascorbate peroxidase, as well as non-enzymatic antioxidants like carotenoids and tocopherols. This antioxidant system is crucial for protecting the plant cell from oxidative damage.

44. What is the role of plastids in plant mineral nutrition?

Plastids play important roles in plant mineral nutrition. They are involved in the assimilation of nitrogen and sulfur, converting these elements into organic compounds. Chloroplasts are key sites for iron utilization in the photosynthetic apparatus. Some plastids also store minerals, such as iron in ferritin complexes, helping to maintain proper mineral balance in the cell.

45. How do plastids contribute to plant circadian rhythms?

Plastids, especially chloroplasts, are integral to plant circadian rhythms. The redox state of the plastid and the production of sugars through photosynthesis provide important timing cues to the cell. Additionally, some clock-regulated genes are involved in plastid function, and disruptions to plastid signaling can affect the plant's circadian clock. This interplay helps plants coordinate their metabolic activities with daily light-dark cycles.

46. What is the role of plastids in programmed cell death in plants?

Plastids play a significant role in programmed cell death (PCD) in plants. They can initiate PCD by releasing pro-death signals such as reactive oxygen species or specific proteins. During PCD, plastids undergo characteristic changes, including membrane disruption and release of their contents. This process is important in plant development, senescence, and responses to pathogens.

47. How do plastids contribute to plant salt tolerance?

Plastids contribute to salt tolerance in several ways. They are involved in the synthesis of compatible solutes like glycine betaine, which help maintain cellular osmotic balance under salt stress. Chloroplasts also adjust their photosynthetic activity to cope with the reduced water availability caused by salt stress. Additionally, some plastids may sequester excess ions to protect the cytoplasm from salt toxicity.

48. What is the role of plastids in C4 photosynthesis?

In C4 plants, two types of chloroplasts work together to improve phot