PLASTOS: CHARACTERISTICS, STRUCTURE AND TYPES - BIOLOGY - 2025

The plastid or plastidiosson group orgánulas semiautonomous cell with varied functions. They are found in cells of algae, mosses, ferns, gymnosperms, and angiosperms. The most notable plastid is the chloroplast, responsible for photosynthesis in plant cells.

According to their morphology and function, there is a great variety of plastids: chromoplasts, leukoplasts, amyloplasts, etioplasts, oleoplasts, among others. Chromoplasts specialize in storing carotenoid pigments, amyloplasts store starch, and plastids that grow in the dark are called etioplasts.

Surprisingly, plastids have been reported in some parasitic worms and in certain marine mollusks.

General characteristics

Plastids are organelles present in plant cells covered by a double lipid membrane. They have their own genome, a consequence of their endosymbiotic origin.

It is suggested that about 1.5 billion years ago a protoeukaryotic cell engulfed a photosynthetic bacterium, giving rise to the eukaryotic lineage.

Evolutionarily, three lines of plastids can be distinguished: the glaucophytes, the lineage of red algae (rhodoplasts) and the lineage of green algae (chloroplasts). The green lineage gave rise to plastids from both algae and plants.

The genetic material has 120 to 160 kb –in higher plants– and is organized in a closed and circular molecule of double-band DNA.

One of the most striking characteristics of these organelles is their ability to interconvert. This change occurs thanks to the presence of molecular and environmental stimuli. For example, when an ethioplast receives sunlight, it synthesizes chlorophyll and becomes a chloroplast.

In addition to photosynthesis, plastids fulfill various functions: synthesis of lipids and amino acids, storage of lipids and starch, functioning of stomata, coloring of plant structures such as flowers and fruits, and perception of gravity.

Structure

All plastids are surrounded by a double lipid membrane and inside they have small membranous structures called thylakoids, which can extend considerably in certain types of plastids.

The structure depends on the type of plastid, and each variant will be described in detail in the next section.

Types

There are a series of plastids that fulfill different functions in plant cells. However, the boundary between each type of plastid is not very clear, as there is a significant interaction between the structures and there is the possibility of interconversion.

Similarly, when comparing different cell types, it is found that the plastid population is not homogeneous. Among the basic types of plastids found in higher plants are the following:

Proplastids

They are plastids that have not yet been differentiated and are responsible for originating all types of plastids. They are found in the meristems of plants, both in the roots and in the stems. They are also in embryos and other young tissues.

They are small structures, one or two micrometers long and do not contain any pigment. They have the thylakoid membrane and their own ribosomes. In the seeds, proplastidia contain grains of starch, being an important reserve source for the embryo.

The number of proplastidia per cell is variable, and between 10 and 20 of these structures can be found.

The distribution of proplastidia in the process of cell division is essential for the correct functioning of the meristems or of a specific organ. When uneven segregation occurs and a cell does not receive the plastids, it is destined for rapid death.

Therefore, the strategy to ensure the equitable division of the plastids to the daughter cells is to be homogeneously distributed in the cell cytoplasm.

Likewise, proplastidia must be inherited by the descendants and are present in the formation of gametes.

Chloroplasts

Chloroplasts are the most prominent and conspicuous plastids of plant cells. Its shape is oval or spheroidal and the number normally varies between 10 and 100 chloroplasts per cell, although it can reach 200.

They are 5 to 10 µm in length and 2 to 5 µm in width. They are located mainly in the leaves of plants, although they can be present in stems, petioles, immature petals, among others.

Chloroplasts develop in plant structures that are not underground, from proplastidia. The most noticeable change is the production of pigments, to take on the characteristic green color of this organelle.

Like the other plastids, they are surrounded by a double membrane and inside they have a third membranous system, the thylakoids, embedded in the stroma.

Thylakoids are disc-shaped structures that are stacked into grains. In this way, the chloroplast can be structurally divided into three compartments: the space between the membranes, the stroma, and the lumen of the thylakoid.

As in the mitochondria, the inheritance of chloroplasts from parents to children occurs by one of the parents (uniparental) and they have their own genetic material.

Features

In chloroplasts, the photosynthetic process occurs, which allows plants to capture the light from the sun and convert it into organic molecules. In fact, chloroplasts are the only plastids with photosynthetic capabilities.

This process begins in the thylakoid membranes with the light phase, in which the enzymatic complexes and the proteins necessary for the process are anchored. The final stage of photosynthesis, or dark phase, occurs in the stroma.

Amyloplasts

Amyloplasts are specialized in the storage of starch grains. They are found mostly in the reserve tissues of plants, such as the endosperm in seeds and tubers.

Most amyloplasts are formed directly from a protoplast during the development of the organism. Experimentally, the formation of amyloplasts has been achieved by replacing the phytohormone auxin with cytokinins, causing the reduction of cell division and inducing the accumulation of starch.

These plastids are reservoirs for a wide variety of enzymes, similar to chloroplasts, although they lack chlorophyll and photosynthetic machinery.

Perception of gravity

Amyloplasts are related to the response to the sensation of gravity. In the roots, the sensation of gravity is perceived by the cells of the columella.

In this structure are the statoliths, which are specialized amyloplasts. These organelles are located at the bottom of the cells of the columella, indicating the sense of gravity.

The position of the statoliths triggers a series of signals that lead to the redistribution of the hormone auxin, causing the growth of the structure in favor of gravity.

Starch granules

Starch is an insoluble semi-crystalline polymer made up of repeating glucose units, producing two types of molecules, amylopeptin and amylose.

Amylopeptin has a branched structure, while amylose is a linear polymer and they accumulate in most cases in a proportion of 70% amylopeptin and 30% amylose.

Starch granules have a fairly organized structure, related to amylopeptin chains.

In the amyloplasts studied from the endosperm of cereals, the granules vary in diameter from 1 to 100 µm, and can be distinguished between large and small granules that are generally synthesized in different amyloplasts.

Chromoplasts

Chromoplasts are highly heterogeneous plastids that store different pigments in flowers, fruits, and other pigmented structures. Also, there are certain vacuoles in cells that can store pigments.

In angiosperms it is necessary to have some mechanism to attract the animals responsible for pollination; for this reason, natural selection favors the accumulation of bright and attractive pigments in some plant structures.

Generally, chromoplasts develop from chloroplasts during the fruit ripening process, where the green fruit takes on a characteristic color over time. For example, unripe tomatoes are green and when ripe they are bright red.

The main pigments that accumulate in chromoplasts are carotenoids, which are variable and can present different colors. Carotenes are orange, lycopene is red, and zeaxanthin and violaxanthin are yellow.

The final coloring of the structures is defined by the combinations of said pigments.

Oleoplasts

Plastids are also capable of storing molecules of a lipid or protein nature. Oleoplasts are able to store lipids in special bodies called plastoglobules.

The floral antennae are found and their content is released on the wall of the pollen grain. They are also very common in certain species of cacti.

In addition, oleoplasts have different proteins such as fibrillin and enzymes related to the metabolism of isoprenoids.

Leukoplasts

Leukoplasts are plastids devoid of pigments. Following this definition, amyloplasts, oleoplasts and proteinoplasts could be classified as variants of leukoplasts.

Leukoplasts are found in most plant tissues. They do not have a conspicuous thylakoid membrane and have few plasma globules.

They have metabolic functions in the roots, where they accumulate significant amounts of starch.

Gerontoplasts

When the plant ages, a conversion of chloroplasts into gerontoplasts occurs. During the senescence process, the thylakoid membrane ruptures, plasma globules accumulate, and chlorophyll breaks down.

Ethioplasts

When plants grow in low light conditions, the chloroplasts do not develop properly and the plastid formed is called an ethioplast.

Ethioplasts contain starch grains and do not possess the extensively developed thylakoid membrane as in mature chloroplasts. If conditions change and there is enough light, etioplasts can develop into chloroplasts.

References

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