Plastids – Types, Structure, Functions

What are Plastids?

• Plastids, double membrane-bound organelles, are crucial in the synthesis and storage of food in photosynthetic plant cells. Initially discovered and named by Ernst Haeckel, A. F. W. Schimper provided a clear definition of these organelles. Essential for life processes like photosynthesis and food storage, plastids vary in type: chloroplasts contain green pigment (chlorophyll), chromoplasts hold pigments other than green, and leucoplasts, lacking pigments, mainly involve in food storage. These organelles, approximately spherical or disc-shaped and varying in size from 1 µm to 1 mm, can also assume elongated, lobed, or amoeboid shapes. Present in all living plant cells and some protozoans like Euglena, plastids are key in food preparation and storage.
• Equipped with plastoglobuli, an internal membrane network of vesicles, and multiple copies of a small genome with 70s ribosomes, plastids are distinct from bacteria, fungi, and animal cells, which lack these organelles. Besides their role in manufacturing and storing food, they often contain pigments crucial for photosynthesis and can alter the color of the cell.
• Plastids, identified as intracellular endosymbiotic cyanobacteria, are found in eukaryotic organisms like plants and algae. Examples include chloroplasts for photosynthesis, chromoplasts for pigment synthesis and storage, and leucoplasts, non-pigmented plastids that can differentiate. The origin of plastids in the Archaeplastida clade, encompassing land plants, red and green algae, dates back approximately 1.5 billion years to an endosymbiotic event with a cyanobiont related to Gloeomargarita. Further endosymbiosis events have occurred in various organisms, including photosynthetic Paulinella amoeboids about 90–140 million years ago, involving plastids from the “PS-clade” (Prochlorococcus and Synechococcus genera of cyanobacteria).
• Plastids contain DNA that is circular, akin to prokaryotic cells’ chromosomes. Their importance is underscored by their retention even in organisms where photosynthetic properties are lost, due to their crucial role in producing molecules like isoprenoids. Therefore, understanding plastids’ structure, function, and evolution is vital for insights into plant biology and beyond.

Definition of Plastids

Plastids are double membrane-bound organelles found in the cells of plants and some algae, essential for processes like photosynthesis, pigment synthesis, and storage of food and nutrients. They vary in type, including chloroplasts, chromoplasts, and leucoplasts, each playing a specific role in the cell’s metabolism and functionality.

Types of Plastids

Plastids are specialized organelles in plant cells, integral for various cellular functions. Each type of plastid serves a distinct role, and their differentiation is a dynamic process dependent on the cell’s developmental stage and environmental conditions. This section will explore the major types of plastids, emphasizing their structure, function, and significance.

1. Chloroplasts Chloroplasts are the most well-known plastids, primarily responsible for photosynthesis. They contain thylakoids, structures where photosynthesis occurs, and chlorophyll, the pigment essential for capturing light energy. These organelles are vital for converting solar energy into chemical energy, thereby sustaining the plant’s metabolic needs.
2. Chromoplasts Chromoplasts specialize in storing and synthesizing pigments in plants. Found in flowering plants, fruits, and aging leaves, chromoplasts originate from the conversion of chloroplasts. They contain carotenoid pigments, which impart various colors to fruits and leaves. These colors play a crucial role in attracting pollinators and aiding in plant reproduction.
3. Leucoplasts Leucoplasts are non-pigmented plastids located in non-photosynthetic parts of the plant, like roots. They primarily serve as storage sites for starches, lipids, and proteins. Leucoplasts can differentiate into specific types based on the stored substance: amyloplasts for starch, elaioplasts for fat, and proteinoplasts for proteins. They also contribute to the synthesis of amino acids and fatty acids.
4. Gerontoplasts Gerontoplasts represent the aging phase of chloroplasts, typically found in senescing leaves. During the aging process, these plastids undergo structural and functional changes, shifting from photosynthetic activity to different roles as the leaf prepares for senescence.

Chloroplasts

Chloroplasts are the most recognized type of plastids, essential for the process of photosynthesis. These organelles are primarily responsible for converting light energy into chemical energy, facilitating the synthesis of carbohydrates needed for the plant’s energy requirements.

• Distribution and Mobility in Plant Cells Chloroplasts are evenly distributed within the cytoplasm of plant cells, though their positioning can vary, sometimes concentrating around the nucleus or beneath the plasma membrane. Exhibiting both passive and active movements, chloroplasts are dynamic within the cell environment.
• Morphology and Size Variations Chloroplasts display diverse shapes, including filamentous, saucer-shaped, spheroid, ovoid, discoid, or slug-shaped forms, depending on the plant species. Typically, they measure 2-3 µm in thickness and 5-10 µm in diameter, with size variations observed between polyploid and diploid plant cells, as well as between plants grown in shade versus sunlight.
• Numerical Variability in Cells The number of chloroplasts per cell varies depending on the species and the cell’s physiological state. While algae cells might contain a single large chloroplast, higher plants generally house 20-40 chloroplasts per cell.
• Chemical Composition of Chloroplasts Chloroplasts consist of proteins (35–55% of dry weight), lipids (20-30%), carbohydrates, chlorophyll, carotenoids, RNA, and a small amount of DNA. The composition of these components varies, contributing to the chloroplast’s functionality.
• Ultrastructure of Chloroplasts The structure of chloroplasts comprises three main components: the envelope, stroma, and thylakoids.
  • Envelope: A double membrane that encloses the chloroplast, facilitating the exchange of molecules with the cytosol. It lacks chlorophyll but contains carotenoids.
  • Stroma: The internal matrix of the chloroplast, a gel-like fluid surrounding the thylakoids, containing 50% of the chloroplast’s soluble proteins, along with ribosomes and DNA molecules. It is the site of CO₂ fixation and synthesis of sugars, starch, fatty acids, and some proteins.
  • Thylakoids: Membranous sac-like structures, containing the photosynthetic pigments. Arranged in stacks called grana, they are crucial for the light-dependent reactions of photosynthesis. The number of thylakoids per granum varies, and they are connected by intergranal or stromal thylakoids.

Chromoplasts

Chromoplasts are specialized plastids responsible for storing and synthesizing pigments in plants. Found in various plant parts like flowers, fruits, and aging leaves, these organelles play a crucial role in coloring different plant sections, significantly influencing pollination and seed dispersal.

• Distribution in Plants Chromoplasts are present in flowering plants, fruits, and aging leaves. Notably, they are responsible for the vivid colors seen in these plant parts. For instance, the color transformation in fruits and the changing hues of leaves in the fall are attributed to the activity of chromoplasts.
• Transformation from Chloroplasts Interestingly, chromoplasts often originate from the transformation of chloroplasts. During this process, chloroplasts, initially involved in photosynthesis, convert into chromoplasts, changing their function from energy production to pigment storage and synthesis.
• Carotenoid Pigments and Coloration The primary function of chromoplasts is to store carotenoid pigments, which are responsible for the diverse colors observed in plants. These pigments vary in color, contributing to the reds, oranges, and yellows commonly seen in flowers and fruits. The purpose of these colors is primarily to attract pollinators and aid in the reproductive process.
• Structural Variability of Chromoplasts Chromoplasts exhibit a wide range of structures, varying from round and ellipsoidal to needle-shaped. The arrangement and type of carotenoids within these plastids can differ, influencing the intensity and type of color they impart to plant parts.
• Origin and Conversion Besides converting from chloroplasts, chromoplasts can also develop from leucoplasts. This transformation is evident in certain roots like carrots, which change from colorless to bright orange due to the development of chromoplasts from leucoplasts.
• Types of Chromoplasts Chromoplasts are categorized into two types based on their pigment content:
  • Phaeoplasts: Containing the pigment fucoxanthin, which absorbs light, phaeoplasts are found in organisms like diatoms, dinoflagellates, and brown algae.
  • Rhodoplasts: Characterized by the presence of phycoerythrin, rhodoplasts are predominant in red algae and play a role in light absorption.

Leucoplasts

Leucoplasts are non-pigmented, colorless plastids primarily found in non-photosynthetic parts of plants, such as roots. These organelles play a critical role in storing and synthesizing essential biomolecules like starches, lipids, and proteins, based on the plant’s needs.

• Location and Development in Plants Leucoplasts are predominantly located in embryonic and germ cells, meristematic cells, and areas not exposed to light, such as cotyledons and the primordium of the stem. In these regions, they initially develop from protoplasts and eventually differentiate into specific types of leucoplasts based on cellular requirements.
• Characteristics of True Leucoplasts True leucoplasts are found in fully differentiated cells like epidermal and internal plant tissues. These plastids never become green or photosynthetic and are characterized by the absence of thylakoids and ribosomes.
• Types of Leucoplasts Leucoplasts differentiate into various types, each specialized in storing different types of nutrients:
  • Amyloplasts: Amyloplasts are responsible for synthesizing and storing starch. Found in plant tissues like potato tubers, they enclose one to eight starch granules within their outer membrane. The starch granules exhibit concentric layers of starch.
  • Elaioplasts: Elaioplasts store lipids and are commonly found in the seeds of both monocotyledons and dicotyledons. They also contain sterol-rich sterinochloroplasts, which are involved in lipid storage.
  • Proteinoplasts: Proteinoplasts are specialized in storing proteins, mainly found in seeds. Although primarily non-photosynthetic, they may contain some thylakoids.

Gerontoplasts

Gerontoplasts represent a specific stage in the life cycle of chloroplasts, associated with the aging process of plant leaves. These specialized plastids are essentially aged chloroplasts that undergo structural and functional changes as the leaf ceases its photosynthetic activities, typically observed during the fall months or senescence period of the plant.

• Origin and Transformation from Chloroplasts Gerontoplasts develop from chloroplasts during the natural aging process of plant foliage. This transformation is marked by significant alterations in the chloroplast structure, indicating the shift from a photosynthetically active state to a senescent state.
• Structural Changes in Gerontoplasts The formation of gerontoplasts is characterized by distinct structural changes within the chloroplast, particularly in the thylakoid membranes. These membranes, once integral to the process of photosynthesis, undergo modifications that render them less efficient or inactive in this capacity.
• Functional Shift in Aging Leaves As leaves age, especially in the fall, their requirement for photosynthesis diminishes. This decrease in photosynthetic demand leads to the repurposing of chloroplasts into gerontoplasts. During this period, the focus shifts from energy production to other cellular functions, adapting to the changing physiological needs of the plant.

Structure of Plastids

• General Structure of Plastids
  Plastids are a diverse group of organelles found in plant cells, each type exhibiting unique structural characteristics tailored to their specific functions. While their overall structure shares some common features, variations exist among different types of plastids, such as chloroplasts, chromoplasts, and leucoplasts.
• Shape and Size Variations in Plastids
  In higher plants, plastids like chloroplasts can assume various shapes including spherical, ovoid, or discoid. In algae, they may exhibit more diverse forms such as stellate, cup-shaped, or spiral. The size of these organelles is typically around 4-6 µm in diameter, and their number can range from 20 to 40 per cell, evenly distributed throughout the cytoplasm.
• Membrane Structure of Chloroplasts
  Chloroplasts, a type of plastid, are enclosed by two lipoprotein membranes: an outer and an inner membrane. This structure creates an intermembrane space, which plays a role in the exchange of materials between the chloroplast and the cytoplasm.
• Internal Components of Chloroplasts
  Inside the inner membrane lies the stroma, a matrix that houses various structures and molecules crucial for chloroplast function. Within the stroma, small cylindrical structures called grana are present. Most chloroplasts contain between 10 to 100 grana, which are stacks of thylakoid membranes involved in photosynthesis.

The Grana and Thylakoids

In chloroplasts, grana and thylakoids are vital structures integral to the process of photosynthesis. Grana are stacks of disc-shaped membranous sacs known as thylakoids, which are essential for light absorption and energy conversion in plants.

• Structure of Grana Each granum is composed of numerous thylakoids piled atop one another. These thylakoids are about 80-120Å across and form the core structure of the grana. The grana themselves are interconnected by a network of tubules called inter-grana or stroma lamellae, facilitating communication and material transfer within the chloroplast.
• Thylakoid Membrane Composition The thylakoid membrane is primarily a lipoprotein structure with a high concentration of various lipids, including galactolipids, sulpholipids, and phospholipids. This membrane composition is crucial for the thylakoid’s function in photosynthesis.
• Organization of the Thylakoid Membrane The inner surface of the thylakoid membrane exhibits a granular organization due to the presence of small spheroidal structures called quantosomes. These quantosomes are the functional units of photosynthesis, containing two distinct photosystems, PS I and PS II, with approximately 250 chlorophyll molecules each.
• Photosystems and Pigment Composition Each photosystem within the quantosomes has antenna chlorophyll complexes and a reaction center where energy conversion occurs. The pigments present in higher plants’ chloroplasts include chlorophyll-a, chlorophyll-b, carotene, and xanthophyll, all of which play roles in capturing light energy.
• Distribution of Photosystems across the Thylakoid Membrane The two photosystems, along with the components of the electron transport chain, are asymmetrically distributed across the thylakoid membrane. Electron acceptors for both PS I and PS II are located on the outer (stroma) surface of the thylakoid membrane, while electron donors for PS I are situated on the inner (thylakoid space) surface.

Double-Membrane (Envelope Membrane)

Plastids, essential organelles in plant cells, are enclosed by a characteristic double membrane, also known as the envelope membrane. This membrane plays a crucial role in various cellular processes, including protein transport, signaling, and metabolic functions.

• Composition of the Double-Membrane The double membrane of plastids is primarily composed of galactolipids, such as MGDG (Monogalactosyldiacylglycerol), along with a variety of other lipids and proteins. This composition is critical for maintaining the structural integrity and functionality of the membrane.
• Protein Transport and Genome Reduction Due to genome reduction in plastids, these organelles can encode only a minimal number of proteins. Consequently, they rely heavily on proteins synthesized in the cell’s nucleus. The double membrane is vital for the transport of these nuclear-encoded proteins into the plastid, facilitating various plastid functions.
• Significance in Cellular Signaling The double membrane is also integral to cell signaling, particularly in mediating communication between the plastids and the cell nucleus. This communication is crucial for regulating gene expression, highlighting the membrane’s role in controlling cellular activities.
• Other Functional Roles of the Plastid Envelope Besides protein transport and signaling, the plastid double membrane is involved in several other important functions:
  • Transport of Other Materials: It facilitates the transport of essential metals and metabolites across the plastid boundary.
  • Metabolic Functions: The membrane is involved in the metabolism of fatty acids, carotenoids, and lipids, among other substances.
  • Synthesis of Plant Growth Regulators: The membrane plays a role in the production of plant growth regulators, influencing plant development and growth.
  • Interaction with Cellular Systems: It interacts with the cell’s endomembrane and cytokine systems, contributing to the overall cellular function and homeostasis.

Internal Membrane

The internal membrane of plastids is a key structural component found primarily in terrestrial plants. This membrane, distinct from the outer envelope membrane, plays a crucial role in various cellular processes, including the transport of substances.

• Development of the Internal Membrane The internal membrane of plastids develops from the inner envelope of the plastid’s double membrane. It also incorporates components from the stromal fluid, which is the liquid matrix within the plastid. This developmental process is essential for the formation of a functional internal membrane system in the plastid.
• Formation of the Peripheral Reticulum In certain instances, the internal membrane can extend and connect with the outer membranes of the plastid, forming a structure known as the peripheral reticulum. This structure is significant as it extends the functional capacity of the internal membrane system.
• Function in Substance Transport One of the primary functions of the internal membrane, and particularly the peripheral reticulum when present, is to facilitate the movement of various substances. This movement occurs between the cell’s cytoplasm and the plastid, indicating a crucial role in material exchange and transport within the cell.

Plastid Stoma

The stroma refers to the internal space within a plastid, enclosed by its double membrane. This colorless, fluid-filled matrix plays a critical role in various cellular processes by housing multiple organelles and structures, including the thylakoid system and other elements essential for plastid function.

• Presence of Ribosomes in the Stroma One of the most prominent features of the plastid stroma is the presence of ribosomes. In some cells, these ribosomes can form polyribosomes, linked together by messenger RNA. The presence of ribosomes in the plastid indicates active protein synthesis, crucial for various cellular functions, including chemical processes and repair mechanisms.
• Nucleoids in Plastid Stroma Nucleoids, which consist of copies of the plastid’s DNA, are another key component of the stroma. Similar to a cell’s nucleus, they are central to the genomic function of the plastid. Nucleoids can be associated with chloroplast thylakoids or dispersed throughout the stroma. The number of nucleoids varies significantly among organisms, with chloroplasts generally having more nucleoids compared to non-green plastids.
• Semi-Autonomy of Plastids Plastids are semi-autonomous organelles, possessing their genetic material and capable of synthesizing essential proteins independently. However, they require close coordination with the cell for certain substances, reflecting a symbiotic relationship between plastids and the host cell.
• Other Components in the Stroma Additional elements found in the plastid stroma include:
  • Inclusion Bodies: Structures that may serve storage or metabolic functions.
  • Microtubules: Present in specific types of plastids like etioplasts, contributing to their structure and function.
  • Stromacenters: These may play a role in the organization and functioning of the stroma.
  • Starch Granules: Indicating the role of plastids in storage, especially in amyloplasts.
  • Plastoglobuli: Small lipid-containing bodies that are involved in lipid metabolism.

Inheritance of Plastids

• Uniparental Inheritance in Plants
  In most plant species, plastids are inherited from only one parent. This uniparental inheritance is common among both angiosperms and gymnosperms, although the source of the plastids differs. In angiosperms, the plastids are typically inherited from the female gamete, while in many gymnosperms, plastids are inherited from the male pollen. Consequently, the plastid DNA from the other parent is not passed on to the offspring.
• Inheritance Patterns in Algae
  Similar to higher plants, algae also exhibit uniparental inheritance of plastids. This pattern ensures that the offspring receives plastids from only one of the parents, maintaining consistency in the plastid lineage.
• Intraspecific Crossings and Plastid DNA Inheritance
  In intraspecific crossings, where hybrids of the same species are produced, the inheritance of plastid DNA is observed to be strictly uniparental. This means that in these normal hybridization events, the offspring inherit plastids exclusively from one parent, without any mixing of plastid DNA from both parents.
• Interspecific Hybridization and Plastid Inheritance
  The inheritance of plastids in interspecific hybridizations, involving the crossing of different species, is more variable. Although the maternal inheritance of plastids is predominant in these crosses, there have been reports of flowering plant hybrids containing plastids from the paternal lineage. This indicates a less strict pattern of plastid inheritance in interspecific hybrids compared to intraspecific crossings.
• Biparental Inheritance in Certain Angiosperms
  Approximately 20% of angiosperm species, including alfalfa (Medicago sativa), exhibit biparental inheritance of plastids. In these cases, plastids are inherited from both the male and female parents, leading to a mixture of plastid DNA in the offspring.

Origin of Plastids

• Origin from Endosymbiotic Cyanobacteria Plastids are believed to have originated from endosymbiotic cyanobacteria. This primary endosymbiotic event is hypothesized to have occurred around 1.5 billion years ago, allowing eukaryotes to perform oxygenic photosynthesis. This event marked a significant evolutionary step in the development of life on Earth.
• Diversification in the Archaeplastida Within the Archaeplastida group, three distinct evolutionary lineages emerged, each characterized by different types of plastids: chloroplasts in green algae and plants, rhodoplasts in red algae, and muroplasts in glaucophytes. These plastids vary in their pigmentation and ultrastructure. For instance, chloroplasts in plants and green algae contain stroma and grana thylakoids but lack phycobilisomes, which are present in cyanobacteria, red algae, and glaucophytes.
• Presence of Cyanobacterial Cell Wall in Glaucocystophycean Plastids The plastids in glaucocystophytes are unique as they retain remnants of the cyanobacterial cell wall, distinguishing them from chloroplasts and rhodoplasts. All these primary plastids are enclosed by two membranes, indicative of their endosymbiotic origin.
• Paulinella Chromatophore: A Recent Endosymbiotic Event The plastid in photosynthetic Paulinella species, often termed ‘cyanelle’ or chromatophore, resulted from a more recent endosymbiotic event about 90–140 million years ago. This event is unique and is the only known primary endosymbiosis of cyanobacteria outside of the Archaeplastida. The Paulinella plastid belongs to a different cyanobacteria clade compared to the plastids in the Archaeplastida.
• Complex Plastids from Secondary Endosymbiosis Complex plastids originated from secondary endosymbiosis, where a eukaryotic organism engulfed another eukaryote containing a primary plastid. These complex plastids, typically surrounded by more than two membranes, are found in various algae and can be reduced in metabolic or photosynthetic capacity. Examples include heterokonts, haptophytes, cryptomonads, dinoflagellates (derived from red algae), euglenids, and chlorarachniophytes (derived from green algae).
• Apicoplasts in Apicomplexa Apicomplexa, a phylum of parasitic alveolates, contains complex plastids known as ‘apicoplasts’. Although these organelles no longer perform photosynthesis, they are essential for the parasite’s survival and are targets for antiparasitic drug development.
• Kleptoplasty in Dinoflagellates and Sea Slugs Certain dinoflagellates and sea slugs, particularly in the genus Elysia, exhibit kleptoplasty, where they ingest algae and retain their plastids to benefit from photosynthesis. Eventually, these plastids are digested.

Functions of Plastids

• Photosynthesis and Carbohydrate Synthesis Chloroplasts, a type of plastid, are critical centers for photosynthesis and carbohydrate metabolism in plants. The thylakoid membrane within chloroplasts contains all the necessary enzymatic components for photosynthesis. This process involves the interaction of chlorophyll, electron carriers, coupling factors, and other components within the thylakoid membrane, leading to the synthesis of carbohydrates, a primary energy source for the plant.
• Pigment Storage and Coloration Plastids are also responsible for storing pigments, contributing to the coloration of various plant structures like green leaves, red flowers, and yellow fruits. The presence of specific pigments in plastids, such as chlorophyll in chloroplasts and carotenoids in chromoplasts, determines the color of the plant part.
• Attraction for Pollination Chromoplasts, found in flowers, contain pigments that help attract insects and birds for pollination. This function is crucial for the reproductive success of flowering plants.
• Starch Synthesis and Storage Amyloplasts, a type of leucoplast, are involved in the synthesis and storage of starch. This function is vital for providing energy reserves for the plant, particularly during periods of low photosynthetic activity or germination.
• Lipid Storage Elaioplasts, another type of leucoplast, specialize in storing lipids in seeds. These lipids are essential for seedling development and provide energy reserves for the growing plant.
• Protein Storage Proteinoplasts, also a type of leucoplast, are responsible for storing proteins in seeds and, in some cases, in thylakoids. These stored proteins are crucial for seed development and germination.
• Autonomous Genetic Material Like mitochondria, plastids contain their DNA and ribosomes, making them semi-autonomous organelles. This feature allows plastids to be used in phylogenetic studies, aiding in understanding plant evolution and relationships.
• Organic Compound Storage Plastids play a significant role in storing various important organic chemical compounds. This storage function is crucial for maintaining cellular homeostasis and providing resources for various metabolic processes.

FAQ

References

1. The Structure and Function of Plastids. (2006). https://doi.org/10.1007/978-1-4020-4061-0
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3. https://www.geeksforgeeks.org/plastids-definition-classification-structure-functions/
4. https://en.wikipedia.org/wiki/Plastid