It is called chloroplast a organelle present in the cells of plants, where develops photosynthesis (the process of metabolism allows certain organisms synthesize organic substances using light of the sun as an energy source).
Delimited by two membranes, chloroplasts have vesicles called thylakoids that house molecules capable of transforming light energy from the sun’s rays into chemical energy. One of these molecules is chlorophyll.
The outer membrane of the chloroplast has a good amount of proteins called porins. In the inner membrane, on the other hand, the porins are smaller. Inside the chloroplast you can see the stroma, a cavity where carbon dioxide is fixed. Thylakoids, with chlorophyll, carotenoids, and other photosynthetic pigments, are also found inside.
According to DigoPaul, in the chloroplast it is also possible to recognize the plastoglobules, protected by a membrane that is similar to the one that covers the thylakoids, from which they are detached. Different organic molecules are found in the plastoglobules, among which some lipids are especially common. It should be noted that all the functions of the plasma globule molecules have not yet been determined.
The photosynthesis takes place in two phases, each developed in different sectors of the chloroplast. The so-called light phase occurs in the membrane that surrounds the thylakoids, where the elements that convert light energy into chemical energy are found. The dark phase, for its part, is generated in the stroma. There an enzyme is responsible for fixing the carbon dioxide.
The cell content is divided into various compartments and this phenomenon represents a challenge at an organizational level due to the concept known as protein trafficking, which, in a eukaryotic cell, regulates:
* the classification signals, a reduced number of amino acids that form a compound known by the name of peptide, which in this case indicates with lysosomal enzymes of the mannose-phosphate group the proteins that are secreted;
* the receptors that notice these signals and move the proteins in which they are contained to the relevant compartments.
Importation of proteins through one or more outer membranes that act as boundaries occurs in four cell organelles: the chloroplast, the nucleus, the peroxisomes, and the mitochondria. An example can be seen in the proteins that import these organelles in the rough endoplasmic reticulum, which have amino acid sequences that act as a “home” for the receptors located on the outer membrane to recognize them.
While in the rough endoplasmic reticulum the import of proteins takes place almost contemporaneously with the translation, in the other organelles it occurs after the synthesis is completed in the free ribosomes of the cytoplasmic matrix (the so-called cytosol). In the specific case of chloroplasts, proteins can reach a series of well-defined subcompartments: inner and outer envelope membranes; intermembrane space; stroma; thylakoid membrane; thylakoid light.
The mechanisms used by the chloroplast to import proteins are very similar to those of the mitochondria, although the evolution of their translocations has been different.
It is important to mention that chloroplasts are present, beyond plants, in certain animals that acquire them through different processes, such as kleptoplasty, an endosymbiosis that consists of the plastids being assimilated by organisms that do not have them.
The kleptoplasty has the objective that the organisms that acquire the plastos can make use of their autotrophic capacity. This behavior has been observed in almost all sacoglossal mollusks and in certain dinoflagellates.
