Plastids in a cell – function, types, structure and meaning

Plastids function largely in the production and storage of food in plant cells. As a result, they participate in activities like photosynthesis, amino acid and lipid synthesis, and material storage, among other things.

Biologists thought that the mitochondria formed before plastids and this is because endosymbiosis (a symbiotic connection in which one creature lives within another) is thought to have given rise to mitochondria before plastids, in part because mitochondria are found in all eukaryotes whereas plastids are absent in many.

Plastids meaning

Plastids are double membrane-bound organelles found in photosynthetic plant cells that are involved in food synthesis and storage.

The various types of plastids contribute to plant metabolism, thereby promoting plant growth and development. The fact that these organelles have a double membrane is one of their distinguishing features.

Plastids are primarily involved in the production and storage of food in cells and as a result, they participate in processes such as photosynthesis, amino acid and lipid synthesis, and material storage, among other things.

Plastids are present in a variety of organisms other than plants and algae, including ferns, mosses, parasitic worms, and marine mollusks (some sea slugs)

Plastids origin

Endosymbiotic cyanobacteria were earlier assumed to be plastids. But the Archaeplastida’s fundamental endosymbiotic event is thought to have occurred around 1.5 billion years ago, allowing eukaryotes to perform oxygenic photosynthesis. Since then, three Archaeplastida evolutionary lineages have evolved, each with its own set of plastid names: chloroplasts in green algae and/or plants, rhodoplasts in red algae, and muroplasts in glaucophytes.

The coloration and ultrastructure of the plastids are different with plant and green algae chloroplasts, for example, lack of phycobilisomes, the light-harvesting complexes seen in cyanobacteria, red algae, and glaucophytes instead it is comprised of stroma and grana thylakoids. In contrast to chloroplasts and rhodoplasts, the glaucocystophycean plastid is still enclosed by the cyanobacterial cell wall and two membranes surround all of these main plastids.

The plastid of photosynthetic Paulinella species, known as the ‘cyanelle‘ or chromatophore, had a much more recent endosymbiotic event about 90–140 million years ago; it is the only known primary endosymbiotic event of cyanobacteria outside of the Archaeplastida. The plastid belongs to the “PS-clade” (of the cyanobacterial genera Prochlorococcus and Synechococcus), which is not related to the Archaeplastida plastids.

In contrast to primary plastids, which are the result of a prokaryotic cyanobacteria’s primary endosymbiosis, there are complex plastids which are the result of a eukaryotic organism engulfing another eukaryotic organism that contains a primary plastid. When a eukaryote engulfs a red or green alga and keeps the algal plastid, the plastid is usually surrounded by more than two membranes.

These plastids’ metabolic and/or photosynthetic capacity may be reduced in some cases. For example, the heterokonts, haptophytes, cryptomonads, and most dinoflagellates (rhodoplasts) are algae with complex plastids derived from secondary endosymbiosis with a red alga. While euglenids and chlorarachniophytes (chloroplasts) are among those that endosymbiosed a green alga.

Plastids developmental cycle

J.M. Whatley postulated a plastid development cycle, which said that plastid development is a cyclic process that occurs numerous times. As indicated in the diagram above, proplastids are the precursors of more distinct kinds of plastids.

Plastids functions

1. Plastids are the sites of the production and storage of important chemical compounds used by autotrophic eukaryotic cells.
2. All of the enzymatic components required for photosyn­thesis are found in the thylakoid membrane of plastids. Within the thylakoid membrane, chlorophyll, electron carriers, coupling factors, and other components interact. Thus, the thylakoid membrane is a specialized structure that is essential for light capture and electron transport.
3. Chloroplasts which is an example of plastids are the sites of carbohydrate synthesis and metabolism.
4. Plastids play an important role not only in photosynthesis but also in the storage of primary foodstuffs, particularly starch.
5. plastids function is heavily reliant on the presence of pigments which are also responsible for the color of a plant structure, are typically found in plastids involved in food synthesis (e.g. green leaf, red flower, yellow fruit, etc.).
6. Plastids, like mitochondria, have their own DNA and ribosomes. As a result, they could be used in phylogenetic studies.

Types of plastids

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

Plastids, like all plant cells, are derived from meristem cells within the plant. Meristems, which are found at the shoot and root tips, are the source of undifferentiated cells in plants.

Proplastids, also known as progenitor plastids, are undifferentiated plastids derived from meristems, and further development of this progenitor results in the production of various types of plastids, which in turn serve various functions in the overall metabolism.

Chloroplasts

Chloroplasts are plastids found in mesophyll cells on plant leaves. When the vacuole presses the chloroplasts against the cell wall, it forms a monolayer on the plant leaves. Some chloroplasts can also be found in plant epidermal cells, but they are less developed than those found in mesophyll cells.

The size of chloroplasts varies between plant species and even within the same plant. For example, chloroplasts in epidermal cells are smaller and less developed, whereas chloroplasts in mesophyll cells are larger and more developed.

In terms of structure, chloroplasts have a thylakoid membrane, which is a large internal membrane that aids photosynthesis. The thylakoid membrane contains protein complexes containing chlorophyll molecules, which are directly involved in photosynthesis (capturing light and energy pathways).

General Structure

Chloroplasts have a spheroid (oval) shape in general, which may be due to the large vacuole pressing against the cell wall, but this may vary depending on where the plastid is located. The morphology of chloroplasts has also been demonstrated to be dynamic, implying that the overall shape can change over time. The plastid is polarized and ranges in width from 5 to 10 micrometers depending on the plant.

Chloroplasts, like other plastids, have a double membrane envelope composed of an outer and an inner membrane (phospholipid layers). It also has a stroma which is an aqueous matrix that covers the space between the double membranes. This aqueous matrix contains a variety of enzymes and proteins that are required for cellular processes.

Chromoplasts

The term “chromo” is derived from the Greek word for color. Therefore, chromoplasts are brightly colored plastids that serve as pigment accumulation sites. They are commonly found in the fleshy fruits, flowers, and other pigmented parts of the plant such as the leaves.

With pigments like carotenoids accumulating in chromoplasts, plastids play an important role in pollination because they act as visual attractors for pollinating animals.

While chromoplasts can form directly from their progenitor, they have also been shown to form from chloroplasts during fleshy fruit ripening. In some cases, chromoplasts can revert to chloroplasts, which are photosynthesis sites.

General structure

The structure of chromoplasts varies greatly depending on the type of carotenoids they contain. Chromoplasts are classified according to their structures as follows:

• Reticulo-tubular chromoplasts
• Simple chromoplasts with pigment globules in their stroma
• Chromoplasts containing specific crystals
• Membranous chromoplasts
• Chromoplasts with prominent tubular/fibrillar structures

Types of chromoplasts

Chromoplasts are classified into two types namely phaeoplast (A brownish substance found naturally in brown algae) and rhodoplasts (plastids found in red algae).

Function of chromoplasts

Chromoplasts, as pigment storage sites, play an important role in pollination because they attract various animals and birds to the plant. When an animal comes into contact with plant pollen, it ensures pollination as the animal moves from one plant to another.

Gerontoplasts

Gerontoplasts, unlike some other plastids, are formed during senescence
(senescence is essentially the degradation of various organelles in a plant cell).

The chloroplast undergoes extensive structural modification of the thylakoid membrane during this process, which is followed by the formation of an increased number of plastoglobuli (lipid/oil droplets). As senescence progresses, the grana are gradually unstacked, but the gerontoplast membrane remains intact.

Function of gerontoplast

This plastid plays an important role in the controlled degradation of chloroplasts. This allows the plant to retain the majority of the protein contained in the chloroplasts (75 percent of total leaf protein) while effectively removing potentially toxic chlorophyll and its byproducts.

Leucoplasts

Leucoplasts, in general, are colorless plastids found in colorless leaves and rapidly growing tissues (tubers, stems, roots, etc). Leucoplasts are the site of starch formation and storage in this case.

Leucoplasts lack pigments such as chlorophyll when compared to plastids such as chloroplasts and chromoplasts. Furthermore, they are found in deep tissue, such as plant seeds, and thus are not directly exposed to light. While their primary function is storage, some leucoplasts are also involved in fat and lipid synthesis.

Types of leucoplasts

1. Amyloplasts
2. Elaioplast (Lipoplasts)
3. Proteinoplasts

Amyloplasts

The term “Amylo” refers to starch, hence, Amyloplasts are plastids that are involved in the long-term storage of starch. Amyloplasts, like other plastids, form from proplastids.

Starch’s biosynthetic pathway is restricted to plastids and amyloplasts play an important role in starch storage here. Amyloplasts have a thin internal membrane and one or more larger grains when compared to other plastids and like chloroplasts, they are surrounded by a double membrane that contains stroma.

Amyloplasts may also play an important role as gravimetric sensors, meaning, they play a role in directing root growth to the ground. Aside from starch storage and gravisensing, amyloplasts have been shown in some species to produce enzymes that boost nitrogen assimilation.

Elaioplast (Lipoplasts)

The word “Elaiov” is derived from the Greek word for olive and unlike amyloplasts, elaioplasts are leucoplasts that contain oil. They are used to store oils and lipids, which explains why there are small drops of fat inside the plastids.

The internal structures of elaioplasts are not well defined and as such there are only lipid/oil droplets (plastoglobuli) present. Although other types of plastids may contain some plastoglobuli, it is the high concentration of plastoglobules and their composition that distinguishes it from the others.

Elaioplasts are also distinguished by their small, spherical size and they are, however, uncommon in comparison to other plastids. Elaioplasts are typically found in the tapetal cells of some plants, where they aid in the maturation of the pollen wall.

Proteinoplasts

Proteinoplasts have more protein than other plastids and these proteins are also visible under a light microscope. Proteins can accumulate as amorphous or crystalline inclusions that are bound by a membrane. Proteinplasts have two additional organelle components (enzymes) in their structure and are peroxidases and polyphenol oxidases.

Structure of plastids

• Double-Membrane (Envelope membrane)
• Plastid stroma
• Internal membrane
• Mitochondria
• Stromule
• The grana and thylakoids

The number of plastids per cell in terrestrial plants is relatively high, ranging from 30 to 40 in diploid cells and 100 to 150 in haploid cells. Plant plastids are also simpler than those found in other organisms such as algae.

Depending on the species (plant, algae, etc.), plastids can take on a variety of shapes such as discoid, spherical, dumbbell-shaped, or lens-shaped, among others.

Double membrane (Envelope membrane)

The double membrane has been shown to be the only membrane that remains intact for all types of plastids (permanent). It’s made up of galactolipids like MGDG, as well as other lipids and proteins. Plastids can only encode for a limited number of proteins due to genome reduction, particularly in cells. As a result, they rely heavily on proteins encoded by the cell nucleus.

Functions of plastids double membrane

• The plastids’ double-membrane envelope, then, plays an important role in the transport of protein from the cell’s cytoplasm into the plastid.
• Aside from protein transport, the membrane is critical in the signaling process. Communication between plastids and the nucleus is critical, especially during gene expression. As a result, the membrane plays an important role in cell signaling and in the regulation of gene expression.
• Play a role in the transportation of other materials, such as vital metals and metabolites.
• Aid in fatty acid, lipid, and carotenoid metabolism, among other things
• Help in the manufacturing of plant growth regulators.
• Aid in the interaction with the endomembrane systems of the cell

Plastid Stroma

This is the internal region of the plastid that is enclosed by a double membrane. The stroma is made up of colorless fluid/matrix that surrounds a number of organelles including the thylakoid membrane within the plastid.

Components of plastid stroma

• Ribosomes
• Nucloides
• Inclusion bodies
• Microtubules ( E.g. etioplasts)
• Stromacenters
• Starch
• plastoglobuli

Ribosomes

The ribosome is a significant feature of the plastid stroma, for instance, they are found in polyribosomes, which are a complex of the mRNA molecule. The presence of ribosomes in a plastid implies protein synthesis activity. Proteins are essential for a variety of tasks, including chemical reactions and damage repair. As a result, the presence of a ribosome is required for a variety of plastid activities to occur within a cell.

Nucloides

These include plastid DNA and RNA copies and these nucleoids, like the cell nucleus, are the functional unit of the plastid genome. The nucleoids are connected to the thylakoids in chloroplasts or may be randomly distributed in the stroma within the plastid.

The number of nucleoids differs greatly between organisms. Chloroplasts, for example, have a higher number of nucleoids than non-green plastids. The nucleoids in plastids can be grouped into a ring and grown into a continuous ring of DNA. In plastids, however, linear genomes have also been discovered.

Plastids, like mitochondria, are semi-autonomous bodies and as a result, they have their own genetic material and are able to synthesize proteins necessary for normal operation. During plastid growth, however, precise coordination between the plastids and the cell is critical since they may rely on the cell for specific materials required during operations.

Internal membrane

Plastids have an interior membrane that is typically seen in terrestrial plants. It grows out of the inner membrane envelope (of the double membrane) and specific lipid components over time.

This membrane may form a membrane system known as the peripheral reticulum when it attaches to the plastid’s inner membrane. This mechanism is essential for the movement of numerous materials from the cell’s cytoplasm to the plastid and vice versa.

Mitochondria

Mitochondria have also been found in plastids under stressful conditions (by intrusion). This has been demonstrated in the presence of plastids such as chloroplasts surrounding mitochondria.

Stromule

The stromule is another important structure associated with plastids. The stromule plays a key role in ensuring communication between the plastids and other cell organelles like the mitochondria and the cell nucleus by connecting the plastids into a network (plasidome). Stromules are also highly dynamic, extending from the surface of all types of plastids.

The grana and thylakoid

These are a number of disc-shaped membranous sacs termed grana lamellae or thylakoids are layered one on top of the other in each granum. The inter-grana or stroma lamellae are a network of anastomosing tubules that connect the grana and single thylakoids, known as stroma thylakoids, can also be found in chloroplasts with electron-dense bodies, as well as osmophilic granules, ribosomes (the 70S), circular DNA, RNA, and soluble Calvin cycle enzymes in the stroma matrix.

The thylakoid membrane is made up of lipoprotein and lipids such as galactolipids, sulpholipids, and phospholipids. However, due to small spheroidal quantosomes, the inner surface of the thylakoid membrane is gra­nular in their organization.

The photosynthetic units, known as quantosomes, 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 center where energy is converted. Chlorophyll-a, chlorophyll-b, carotene, and xanthophyll are the pigments found in higher plants.

The two photosystems, as well as the electron transport chain’s components, are spread asymmetrically over the thylakoid membrane. PS I and PS II electron acceptors are found on the thylakoid membrane’s outer (stroma) surface. PS I has electron donors on the inner (thylakoid space) surface.

Plastids inheritance

Many plants descended from plastids from a single parent as many gymnosperms inherit plastids from male pollen, whereas angiosperms inherit plastids from the female gamete. Algae only receive plastids from one parent and the plastids’ DNA appears to be 100% uniparental in inheritance. The inheritance of plastid appears to be more irregular during hybridization.

FAQ about plastids