Cell Biology

Plastids Definition, Types, Structure, Functions

Plastid is a two-membrane-bound organelle that is involved in the synthesis and preservation of nutrients. typically found in the photosynthesis cells of...

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This article writter by MN Editors on January 02, 2022

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Plastids Definition, Types, Structure, Functions
Plastids Definition, Types, Structure, Functions

Definition of Plastids 

Plastid is a two-membrane-bound organelle that is involved in the synthesis and preservation of nutrients. typically found in the photosynthesis cells of plants. Plastids were first discovered and identified in the work of Ernst Haeckel, but A. F. W. Schimper was the first to offer an accurate definition. They are vital for living processes such as photosynthesis and storage of food. A plastid that contains the green color (chlorophyll) is known as a chloroplast while a plastid with different pigments, apart from green, is referred to as the Chromoplast. A plastid with no pigments is known as an leucoplast. It is utilized mostly in the storage of food.

Types of Plastids

As with all plant cells, they are derived from meristem cells in the plant. They are located at the shoot and roots, meristems are the main source of undifferentiated cell within plants.

Proplastids are the progenitors of plastids are plastids undifferentiated which are derived from the meristems. The further growth of this progenitor result in the generation of various kinds of plastids, which perform different roles that contribute to the general metabolism.

Plastid and its various types with their respective organelle function. | Image source: https://bmcbioinformatics.biomedcentral.com/articles/10.1186/1471-2105-14-S14-S7

A. Chloroplasts

Chloroplasts are plastids found in mesophyll cell on leaves of plants. They create a monolayer when they are squeezed against the cell’s wall by the vacuole. Certain chloroplasts are located in epidermal cells in the plant however, they are not as developed as compared to the ones found within mesophyll cell.

For various plant species and even within plants the size of chloroplasts varies. For instance, while epidermal cells’ chloroplasts have smaller sizes and are less developed, those that are found in mesophyll cells are larger and more developed.

Regarding structure, chloroplasts have a thylakoid membrane that is a large inner membrane that helps in photosynthesis. The thylakoid membrane contains complexes of proteins that are containing chlorophyll molecules which are directly associated with the process of photosynthesis (capturing sunlight and energy channels).

The larger the surface area of the thylakoid thylakoid the higher the amount of chlorophyll found within the cell. The thylakoid membrane covers around 500 micrometers square in the chloroplast.

General Structure

In general, chloroplasts exhibit an spheroid-like shape (oval shape) and this may happen due to being pressurized on the cells’ wall with the huge vacuole. But, this can change in relation to the position of the chloroplastid.

The morphology is also demonstrated to be dynamic, meaning that the shape of the plastid can alter over the course of. Research has also revealed that the plastid is divided into two groups, ranging between 5 and 10 micrometers in size based what plant.

Similar to other plastids chloroplasts also have a double membrane envelope that consists of the inner and outer membrane (phospholipid layers). The space inside the double membranes is covered by an aqueous layer known as the stroma. This aqueous layer contains a variety of proteins and enzymes that are necessary for the cellular processes.

Other elements of a chloroplast comprise:

  • Grana Thylakoids, arranged in stacks (one over the other)
  • Peripheral reticulum Membranous tubules emerging from the membrane’s inner layer
  • Chloroplast DNA
  • Ribosome

B. Chromoplasts

“Chromo” comes from Greek word that means color. Chromoplasts are plastids with bright colors that serve as the place for pigmentation. They are usually found in fleshy fruits, flowers and other parts that are pigmented of the plant , like leaves.

Carotenoids are a pigment that is that accumulate in chromoplasts organelles play a crucial part in pollination since they function as visually appealing to animals that are involved in pollination.

In terms of structure, chromoplasts differ significantly according to the kind of carotenoids they have. Based on their structure they are classified in the following manner:

  • Reticulo-tubular chromoplasts
  • Simple chromoplasts that contain globules of pigments in their stroma
  • Chromoplasts with such crystals
  • Chromoplasts that have significant fibrillar/ tubular structures
  • Membranous Chromoplasts

Although chromoplasts can grow directly from their progenitors, they also have been proven to grow from chloroplasts in the the ripening process of fleshy fruit. In some instances it is possible for chromoplasts to revert to chloroplasts, which are sites for photosynthesis.

There are two kinds of chromoplasts that include:

  • Phaeoplast – Brownish , naturally occurring in brown algae
  • Rhodoplast – Plastids are discovered in red algae.

They are pigment-producing sites Chromoplasts play a crucial part in pollination as they draw a variety of species of birds and animals into the flower. When the animal comes in contact with the pollen of plants, this is able to ensure pollination when the animal travels between plants another.

C. Leucoplasts

Leucoplasts are generally colored plastids, which are usually located in leaves that are colorless and in rapidly growing tissues (tubers or stems, root systems, etc.). These plastids function as the source of starch formation and storage.

In contrast to plastids such as the chromoplast and chloroplast they lack pigments such like chlorophyll. They are also located within the deep tissue of plant seeds , and thus are and are not exposed directly to sunlight.

Although the primary function is storage, a few of the leucoplasts also are involved in the production of lipids and fats.

Three principal types of leucoplasts:

(a) Amyloplasts

The term “Amylo” means starch. Amyloplasts are a kind of plastid that is involved in the long-term preservation of starch. As with other plastids amyloplasts are derived from proplastids. The biosynthetic pathway for starch is limited to the plastids. Amyloplasts play a significant role as storage for starch. As compared to other plastids an extremely thin membrane inside and are able to contain one or a few larger grains.

As chloroplasts amyloplasts have the double-membrane, which has the stroma. It is within the stroma of amyloplasts that starch granules can be produced and stored. Amyloplasts are also believed to have a significant role in gravimetric sensors. In this way, they help in directing the growth of roots to the soil. Apart from storing gravisensing and starch, the amyloplasts from certain species have been proven to create proteins (in in the GSGOGAT cycle) which promote nitrogen assimilation.

(b) Elaioplast (Lipoplasts)

The term “Elaiov” is a Greek word that means olive. As opposed to amyloplasts the elaioplasts belong to a different type of leucoplasts which contain oil. They are used to hold lipids and oils, that are the reason for the tiny drops of fat inside the plastids.

In terms of structure, elaioplasts don’t possess specific internal structure. Therefore, only oil droplets and lipids (plastoglobuli) exist. While other types of plastids might contain some plastoglobuli, it’s the large quantities of plastoglobule, as well as its composition that separates it from other plastids.

Elaioplasts also have their small size and spherical shape. They are however extremely rare compared to other plastids. They are typically found in the cells of the tapetal of some plants . They help in the maturation of the wall of pollen.

(c)Proteinoplasts

Proteinoplasts are richer in proteins compared to other plastids. The proteins are also huge enough to be observed under the microscope. The proteins can either form crystallized or amorphous inclusions or attached to membranes. Other elements (enzymes) that make up the organelle are:

  • Peroxidases
  • Polyphenol oxidases

D. Gerontoplasts 

Gerontoplasts are essentially chloroplasts that undergo the process of aging. They are the chloroplasts in leaves that are starting to transform into various organelles or are being reused as the leaf isn’t making use of photosynthesis (such as during the fall months). According to their morphology, as well as function, the plastids possess the capability to distinguish, or redifferentiate as well as other forms.

Structure of Plastids

  • Chloroplasts can be spherical discoid or ovoid in plants with higher levels and can be cups, stellate or spiral in certain algae.
  • They typically measure about 4-6 millimeters in diameter, and can range from 20 to 40 numbers in every cell of higher plants, distributed evenly across the cytoplasm.
  • The chloroplast is bound by two membranes of lipoproteins, one outer and the other an inner membrane. There is an intermembrane space in between.
  • The membrane’s interior is enclosed by an stroma-like matrix that is composed of small, circular structures, called the grana. Most chloroplasts contain 10-100 grana.
Structure of Plastids
Structure of Plastids

The Grana and Thylakoids

  • Each granum is made up of a variety of disc-shaped membranous sacs known as the thylakoids, or grana lamellae (80-120A across) that are stacked one on top of one.
  • The grana are linked through a network of anastomosing tubules known as inter-grana, or lamellae stroma.
  • Single thylakoids, also known as stroma thylakoidsare present in chloroplasts.
  • Osmophilic and granules together with Ribosomes (70S) round DNA, and other soluble Calvin cycle enzymes are also found within the stroma’s matrix.
  • Chloroplasts therefore have three distinct membranes: the outer membrane, the inner one and the Thylakoid membrane.
  • The thylakoid membrane is composed of lipoproteins with higher amounts of lipids that include galactolipids, sulfurolipids, and phospholipids.
  • The outer surface of the thylakoid cellular membrane is extremely granular due to the small quantosomes that are spheroidal.
  • Quantosomes are photosynthetic units. They consist of two distinct photosystems, PS I and PS II which contain around 250 molecules of chlorophyll. Each photosystem is equipped with complexes of chlorophyll antennas as well as a reaction center where energy conversion occurs. The pigments in higher plant species that are present are chlorophyll-a, chlorophyll b carotene, xanthophyll and.
  • The two photosystems and components of the electron transportation chain are distributed symmetrically throughout the thylakoid membrane. Electron acceptors for each PS I and PS II are on the outside (stroma) area of Thylakoid membrane. Electron donors from PS I are on the inner (thylakoid space) surface of the thylakoid space.

Double-Membrane (Envelope Membrane)

For all kinds of plastids The double membrane is found to be the sole membrane that is in good shape (permanent). It is composed of galactolipids like MGDG in addition to various lipids and proteins. Because of the genome reduction of plastids in particular the cells, plastids can be only able to encode the smallest number of proteins.

In turn, they depend heavily on proteins produced by the cell’s nucleus. This means that the double-membrane membrane of plastids is a key element in the movement of proteins from the cell’s cell cytoplasm to the plastid.

In addition to transporting proteins The membrane also is a key component in the process of signaling. The communication between plastids and cell nucleus is essential, especially when it comes to gene expression. The membrane is a key element in cell signaling, and consequently in the control in gene expression.

The other functions of plastid envelopes are:

  • Other materials are transported, including important metals and the metabolites
  • Metabolism of fatty acid carotenoid, and lipids among other substances
  • The production of plant growth regulators
  • Interaction with cells’ endomembrane and cytokine systems.

Internal Membrane

The plastid’s internal membrane is typically found in terrestrial plants. It is gradually developed from the membrane’s inner envelope (of that double membrane) and also from the liquid components.

In certain instances this membrane could be able to connect to the membranes of the plastid and create a membrane known by the name of the peripheral reticulum. This system plays a crucial function in the movement of different substances from the cell’s cytoplasm to the plastid and the reverse.

Plastid Stoma

Stroma is the name given to the internal space, which is covered with the membrane that is doubled in the plastid. It’s surrounded by a colorless matrix or fluid that surrounds the thylakoid and several other organelles in the plastid.

Other elements of the stroma are:

Ribosome 

Ribosome is the most prominent characteristic of the plastid stroma. In certain cells, it is possible to find them in the form of polyribosome which is a part of the messenger RNA protein (a group of ribosomes which are connected by messenger RNA). In a Plastid, the presence of the ribosome suggests protein synthesis.

Proteins are essential for a variety of tasks, such as diverse chemical processes, as well as damage repair. So the existence of a ribosome is vital for the various plastid processes inside cells.

Nucleoids 

Nucleoids, which are duplicates of the plastid’s DNA and DNA. Much like the nucleus in the cell they are the main functional component of the genome of the plastid. Within the plastid nucleoids are linked to chloroplast thylakoids or could be distributed randomly within the stroma.

The amount of nucleoids is different drastically from one organism to the next. In particular, in comparison to non-green plastids that are green, chloroplasts have a greater amount of nucleoids. In plastids, nucleoids could be organized in an elongated ring before developing into the continuous DNA ring. However linear genomes also have been found in the plastids.

As mitochondria, plastids are semi-autonomous entities. They also contain the genetic materials of their own, and therefore are capable of synthesizing the proteins needed to function normally. But, close coordination between the plastids and cell is crucial in the development of plastids since they might depend on cells for certain substances required in the process.

Other elements of the plastid which could also be located in the stroma comprise:

  • Inclusion bodies
  • Microtubules – E.g. Etioplasts
  • Stromacenters
  • Starch
  • Platoglobuli

Inheritance of Plastids

The majority of plants inherit plastids only from one parent. Angiosperms generally get plastids from female gamete, while the majority of gymnosperms inherit plastids from male pollen. Algae also inherit plastids from one parent. The plastid DNA from one parent has been consequently, gone. In normal intraspecific crosses (resulting as normal hybrids from one species) the transmission of plastid DNA is believed to be essentially 100 100% uniparental. However, in interspecific hybridisations this inheritance is reported to be more variable. While plastids are inherited mostly maternally during interspecific hybridisations. However, there are numerous reports of hybrids between flowering plants that carry plastids that belong to the father. About 20 percent of the angiosperms such as Alfalfa (Medicago sativa) typically exhibit biparental inheritance of plastids.

Functions of Plastids

Every plant cell has plastids that are in some shape or shape or. This is a roll-call of their many functions and shows that plastids are at the core of the plant cell’s function.

  • Plastids are the place of production and storage of key chemical compounds that are utilized by cells of the autotrophic eukaryotes.
  • The thylakoid membrane is home to all the enzymatic elements needed to produce photosynthesis. Interaction between chlorophyll, electron transporters as well as coupling factors and other components occurs within the thylakoid layer. This is why the thylakoid is a special structure that plays an important function for the capture and storage of electrons and light.
  • So, chloroplasts are organelles of synthesis and metabolism of carbohydrates.
  • They’re not just important in photosynthesis, but also in the storage of food items especially starch.
  • Its purpose is mostly dependent in the amount of pigments. A plastid in food synthesis generally has pigments that are also that determine the colour of the plant structure (e.g. the red flower, green leaf or yellow fruit, etc. ).
  • As mitochondria, plastids also have an individual DNA, as well as the ribosomes. Therefore, they can be utilized in phylogenetic research.
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