Plastids

Plastids , (Plastids or Plastids). They are eukaryotic cell organelles, typical of plants and algae . They are self-replicating structures surrounded by a double membrane.

Summary

[ hide ]

• 1 Function
• 2 Origin
• 3 Features
• 4 Types of plastids
• 5 Development and Reproduction
• 6 Sources

Function

Its main function is the production and storage of important chemical compounds used by the cell . They usually contain pigments used in photosynthesis , although the type of pigment present can vary, determining the color of the cell.

Origin

The evolutionary origin of plastids is believed to be associated with an endosymbiosis between a cyanobacterium close to current genera such as Synechococcus or Nostoc , with a flagellated heterotrophic protist from which green algae, glaucocytophytes and red algae are derived . We distinguish two types of plastids by their origin:

• Primary plastids. They derive directly from the symbiosis of a cyanobacterium within a single-celled flagellate . From the latter derives the groups that carry primary plastids: red algae, green algae, and land plants, which evolved from freshwater green algae. The most “primitive” version of this organelle can still be seen in the glaucocytophyte cianellae, single-celled algae in whose plasto remains of the bacterial cell wall are preserved.
• Secondary plates. It is that unicellular eukaryotic algae have been assimilated, in a process of secondary endosymbiosis, by another eukaryote. This case has been repeated many times in evolution and the result is the immense diversity of algae plastids. The symbiote has sometimes been a green algae (eg in Chlorarachniophyta ), but derivatives of a red algae are more frequent. In some cases, the eukaryotic nucleus of the symbiote, which is called the nucleomorph , remains recognizable . These secondary plastids usually have complex envelopes with three or more membranes, some derived from the plasma membrane of the red algae, and sometimes with an endoplasmic reticulum. Brown algae, diatoms and other related groups (Chromophyta) plasties originated in this way.

There are tertiary and even more complex symbiosis tests, in which an algae carrying a secondary plasto has in turn become a symbiote for another protist. The maximum diversity and complexity of cases is observed in the dinoflagellate edge . They are characteristic organelles of plant eukaryotic cells. Their sizes can vary, they are surrounded by a double membrane that at the same time delimits it and they have ribosomes similar to those of prokaryotes. They are formed from proplasts, which are the young cell plastids. They are characterized by having lipid microdroplets and by possessing their own genetic material .

characteristics

The primary plastids are characteristic of an evolutionary branch that includes red algae , the green algae and plants. There are secondary plastids that have been acquired by endosymbiosis by other evolutionary lines and that are modified forms of plastid eukaryotic cells. The plastids of the plants appear as relatively large organelles, ellipsoidal in shape, and generally numerous. In a square millimeter section of a leaf, there can be more than 500,000 chloroplasts .

In protists they are often singular structures, which extend more or less extensively through the cytoplasm . They are limited from the rest of the cytoplasm by two structurally different membranes. They are often colored by fat-soluble pigments. Like mitochondria , they have naked, circular DNA . The plastids of the various eukaryotic groups are remarkably disparate. Those that appear on plants offer a suitable reference.

They appear delimited by the plastid envelope, formed by two membranes, the external plastid membrane and the internal plastid membrane. The space between the two, called the periplastidial, has a different composition and is homologous to the periplasmic space of bacteria.

Types of plastids

There are many types of plastids. According to the colors:

Green: chloroplasts .

No color: amyloplasts , oleoplasts , proteinoplasts …

With other colors: cromoplastos.

With very faint color: etioplasts and proplastids .

• Chloroplasts(only in plant and algae cells). They carry out photosynthesis. Chloroplasts are the cell organelles that in photosynthesizing eukaryotic organisms deal with photosynthesis. They are limited by an envelope formed by two concentric membranes and contain vesicles, the thylakoids, where the pigments and other molecules that convert light energy into chemical energy are organized.
• Chromoplasts(only in plant and algae cells). They synthesize and store pigments. Its presence in plants determines the red, orange or yellow color of some fruits, vegetables and flowers. The color of the chromoplasts is due to the presence of certain pigments; like carotenes , red and xanthophylls , yellow. For example, tomato and carrot contain many carotenoid pigments.
• Leukoplasts: These plastids are colorless and are located in the plant cells of organs not exposed to light, such as roots, tubers, seeds and organs that store starch.
• Proplastids: They are structurally very simple and small plastids. They are between 0.5 and 1 μm in size. They have all the common characteristics of plastids. They also have their own characteristics:

– They have poor vesicular development.

– They have small sacs in the stroma and sometimes are in continuity with the internal membrane.

– They have few ribosomes.

– They have starch granules (very small).

– They have or can be of various shapes, but it is normal for them to be spherical.

The rest of the plastids are obtained from proplastidium, that is, chloroplasts, etioplasts are going to be differentiated from proplastids …

• Ethioplasts: When a proplastidium takes place in the dark, it produces an ethioplast. If the proplastids develop in the light they give rise to the chloroplasts. The ethioplast formation process lasts about 15 days in which there is a progressive increase in the size of the plasto. Ethioplasts measure between 3 and 5 μm. Inside the plasto a prolamellar body is formed, made up of a highly organized tubular framework of about 18 nm in size.
• Amyloplast: They are plastids that accumulate the most important and abundant polysaccharide in plants, starch . Starch is made up of two fractions: amylose and amylopectin . Amylose is made up of α 1-4 glucose and amylopectin is made up of α 1-4 glucose with α 1-6 branches. The proportion of amylose and amylopectin depends on the species. The organization of amylose is looser and that of amylopectin is more compact.

The starch in the plastids accumulates in an inactive osmotic way, that is, in an insoluble way, forming granules. It is always synthesized and stored in a plasto. It is synthesized during the day, it is temporarily stored in the chloroplast and its degradation occurs at night and the products of the degradation are transported to the different tissues for use as an energy source. Starch accumulates in amyloplasts, which are abundant in underground organs. The size and shape of the amyloplasts ranges from 1 to 175 μm and the shape can be globular, lenticular, pear-shaped, conical etc. The shape and size usually depend on the number and shape of the starch granules.

Development and Reproduction

The plastids are multiplied by division . Protoplasts grow along with meristematic cells. In their development, through invaginations of the internal membrane, the plastids acquire a large surface. On this inner surface, the photosynthesizing pigments are arranged in an orderly fashion. In the dark, the protoplasts of plants can be transformed into crystalline structures called etioplasts , which by the effect of light, can in turn be transformed into photosynthetically active plastids. Plasters that are damaged or that are senile often have lipid droplets inside them, known by the name of plasma globules .

In the sexual reproduction of organisms, plastids are transmitted through gametes , in many cases through the female gamete. The DNA of the plastids is specific and is called plastid DNA , and it differs from nuclear DNA by the relationship between bases and thickness. plastid DNA is a circular double filament that is replicated by a specific DNA-plastid polymerase. and there is also a specific RNA-plastid polymerase for transcription.

A part of the proteins of the plasto is synthesized from the DNA-plastidial of 40 nm in length, and another part of the nuclear DNA. The genes of the plastids form the plastoma, while the set of plastids in a cell is called a plastidoma . The ribosomes of the plastids are smaller than those of the cytoplasm, with a sedimentation rate of 70 s. These plastid ribosomes are similar to those of prokaryotes. In photosynthetic bacteria and cyanophytes, photosynthetic pigments are located not in special organelles, but in chromatoplastoplasts, whose structure is similar to the thylakoids of eukaryotic cell plasts.