Structure and Function of Plastids in Plant Cell

In this structure and function of plastids in plant cell post we have briefly explained about plastids, definition, history, types of plastids, plastids structure, and functions of plastids in plant cell.

Structure and Function of Plastids in Plant Cell

Plant and algal cells contain plastids, which are double-membrane organelles. Plastids in plant cell are in charge of food production and storage. These frequently contain photosynthetic pigments as well as other pigments that can modify the colour of the cell.

Definition

Plastids are a type of cell organelle found in all plant cells, algae, and some protists such as Euglena. There are various varieties of it. Plastids are rarely found in eukaryotic animal cells. Plastids in plant cell, like mitochondria, are pigment-containing double membrane bound organelles with their own DNA and ribosome. Plastids in plant cell are mostly used in the production and storage of food.

Aside from that, they are plant cells’ largest cytoplasmic organelle. Proplastids, which are found in embryonic tissue, are used to make them. Proplastids are stroma- and DNA-containing double membraned structures.

Types of Plastids

There are different types of plastids in plant cell with their specialized functions. Among them, a few are mainly classified based on the presence or absence of the Biological pigments and their stages of development.

Chloroplasts

Chromoplasts

Gerontoplasts

Leucoplasts

1. Chloroplasts

Plant leaves contain chloroplasts, which are plastids in plant cell found in the mesophyll cells. The vacuole presses the chloroplasts against the cell wall, forming a monolayer. Some chloroplasts can be found in the plant’s epidermal cells, although they are less developed than those found in the mesophyll cells.

Chloroplasts vary in size between plant species and even within the same plant. Chloroplasts in epidermal cells, for example, are smaller and less developed, whereas those in mesophyll cells are larger and more developed.

Chloroplast

Structure and Function of Plastids in Plant Cell

Chloroplasts have a spheroid (oval) form in general, which could be due to the huge vacuole pressing against the cell wall. This, however, may vary depending on the plastid’s position.

The morphology has also been demonstrated to be dynamic, meaning that it can change shape over time. According to studies, the plastid is polarised and varies in width from 5 to 10 micrometres depending on the plant.

Chloroplasts, like other plastids in plant cell, contain a double membrane envelope with an outer and inner membrane (phospholipid layers). The stroma is an aqueous matrix that covers the space between the two membranes.

2. Chromoplasts

They are colourful plastids that contain coloured pigments such as yellow, orange, red carotenoids, and other coloured pigments other than green. As a result, they lack chlorophyll.

Except for green pigment, we may claim that this sort of plastid comprises coloured pigments. A chromoplast is a plastid that contains coloured pigments other than green.

Plastids

They are found in photosynthetic eukaryotes and are responsible for the many colours of flowers, fruits, leaves, and carrot roots.

Chromoplasts are formed from proplastids and chloroplasts, and chloroplasts can convert into chromoplasts, as when a green tomato ripens into a red tomato and green leaves change colour in the autumn.

The primary function of chromoplasts is to attract insects for pollination by giving flowers and fruits attractive colours.

3. Leucoplast

This type of plastid is an unpigmented organelle. It does not contain any pigment and grana. It is usually present in the non-photosynthetic parts of a plant such as underground parts of the plants like roots and tubers.

However, they are also present in some stems. Leucoplasts perform specialized functions such as the storage of starch, lipids, and proteins.

So, the major function of the leucoplast is the storage of food. Besides this, their shape is variable such as oval, rod-like, filamentous or spherical.

Types of leucoplast

Amyloplast: This type of leucoplast store carbohydrate molecules. They are oval in shape and can be found in potato tubers.

Elaioplast: They are oval-shaped plastids in plant cell. They store oil droplets or fatty acids or fat. They are found in oilseeds, such as caster seeds, groundnut, sunflower seeds, mustard seeds, etc.

Proteinoplast: They are also known as Aleuroplast. They store protein grains. They are found in pulses such as maize.

Etioplasts: They are colourless plastids and found in those parts of plants that are not exposed to sunlight. Due to the absence of light, their chlorophyll pigment is destroyed. In the presence of sunlight, they can convert into the chloroplast.

Besides this, the plastids in plant cell are interconvertible, which means one type of plastid can be converted into another type of plastid. For example, leucoplast can develop pigment and change into Chromoplasts and vice versa.

4. Gerontoplasts

Gerontoplasts are generated during senescence, unlike certain other plastids in plant cell. Senescence is defined as the breakdown of different organelles in a plant cell.

The thylakoid membrane of the chloroplast undergoes considerable structural alteration, followed by the creation of larger numbers of plastoglobuli, during this phase. As senescence progresses, the grana are gradually unstacked, but the gerontoplast membrane stays intact.

As a result, it’s been claimed that this plastid is involved in chloroplast degradation control. This permits the plant to keep the majority of the protein in the chloroplasts (75 percent of total leaf protein) while successfully removing the potentially hazardous chlorophyll and its metabolites.

Plastids Structure

The number of plastids in plant cell per cell in terrestrial plants has been shown to be relatively high, ranging from 30 to 40 in diploid cells to 100 to 150 in haploid cells. Plant plastids are also less complex than those seen in other species such as algae.

Plastids in plant cell can take on a number of shapes depending on the species (plant, algae, etc.). They can be discoid, spherical, dumbbell-shaped, or lens-shaped, to name a few.

Plastids

Stromules

The stromule is another key component linked with plastids in plant cell. The stromule ensures communication between the plastids and other cell organelles such as the mitochondria and the cell nucleus by connecting the plastids into a network (plasidome).

Stromules are also quite active, and they have been observed extending from the surface of many plastids in plant cell.

Double membrane

The double membrane has been found to be the only membrane that remains intact in all types of plastids in plant cell (permanent). It contains galactolipids like MGDG, as well as other lipids and proteins. Plastids in plant cell can only encode for a small number of proteins due to genome shrinkage in plastids, particularly in cells.

Plastid Stoma

The internal space encompassed by the plastid’s double membrane is referred to as the stroma. It’s filled with a whitish fluid/matrix that surrounds the plastid’s thylakoid as well as a number of other organelles.

Ribosome: It is a major characteristic of plastid stroma. In some cells, they may be present as polyribosome, which is a complex of the mRNA molecule (a group of ribosome that are linked by the messenger RNA). In a plastid, the presence of ribosome indicates protein synthesis activities.

Nucleoids: These include copies of the plastid DNA and RNA. Like the cell nucleus, these nucleoids are the functional unit of the plastid’s genome. Within the plastid, the nucleoids are attached to the thylakoids in chloroplasts or may be randomly spread in the stroma.

Internal Membrane

The internal membrane of plastids in plant cell is mostly found in land plants. It gradually develops from the inner membrane envelope (of the double membrane) as well as given lipid components.

Functions

1. All plant cells contain plastids in plant cell in some shape or form. This roll-call indicates their functional diversity and demonstrates that plastids in plant cell lie at the very core of plant cellular function. 

2. Plastids in plant cell are the sites where key chemical compounds utilised by autotrophic eukaryotes’ cells are manufactured and stored.

3. All of the enzyme components required for photosynthesis are found in the thylakoid membrane. Within the thylakoid membrane, chlorophyll interacts with electron carriers, coupling factors, and other components. As a result, the thylakoid membrane has a specialised structure that is essential for light collection and electron transport.

4. Chloroplasts are thus the centres of carbohydrate synthesis and metabolism; they are important not only in photosynthesis but also in the storage of primary nutrients, particularly starch; and their activity is primarily dependent on the presence of pigments. Pigments, which are also responsible for the colour of a plant structure, are commonly found in plastids involved in food synthesis (e.g. green leaf, red flower, yellow fruit, etc.).

5. Like mitochondria, plastids have their own DNA and ribosomes. Hence, they may be used in phylogenetic studies.

Further Readings

Reference