What is a chloroplast? Definition:
at chloroplasts (Greek chloros = green) are those organelles that give the plants and algae their typical green color. In the chloroplasts, the synthesis of the green dye chlorophyll takes place, which absorbs the sunlight and transmits it to the reaction center of the plant. In the process of photosynthesis, the plant also synthesizes its required organic substances from the inorganic substances water and carbon dioxide (in combination with sunlight). Thus, plants belong to the autotrophic (Greek autos = self, tropho = nourish) living creatures, because they feed themselves by the production of glucose so to speak themselves. Animals, however, are heterotrophic organisms and therefore dependent on the photosynthesis of the plants. In order to be able to produce the organic building blocks that are important for their survival, food must be supplied to the organism "from outside".
Structure of the chloroplasts
Individual chloroplasts are about 5-6 μm (microns) long. Like the mitochondria and the nucleus, chloroplasts are also enclosed by two semi-permeable cell membranes, allowing delivery and uptake of substances (e.g., water). This double membrane can be explained by the so-called Endosymbiontenhypothese, which represents the current state of research on the formation of eukaryotes. According to this, chloroplasts are formed by the migration of cyanobacteria into the precursors of plant cells. Means that chloroplasts were once autonomous, photosynthetic bacteria and, in the course of phylogenesis, are derived from the progenitor cells of plants by e.g. Phagocytosis were recorded.
Between the two membranes is the intermembrane space. The inner membrane of the chloroplasts is struck inwards and forms the thylakoids. The thylakoids form stacks called grana; Here, the decisive for the photosynthesis light reaction takes place. Inside the chloroplasts is the stroma, the cytosol of chloroplasts. The stroma also contains the DNA and ribosomes of the chloroplasts. Like the mitochondria, the chloroplasts have their own (ring-shaped) DNA. Chloroplasts replicate independently of the cell cycle of the rest of the cell and thus represent an autonomous organelle. All these facts are further arguments in favor of endosymbiont theory.
Function of the chloroplasts
The main function of chloroplasts is the operation of photosynthesis. For this reason, these organelles are not found in all plant cells; they are mainly found in green, upper parts of a plant. A plant cell may contain a single chloroplast or more. Photosynthesis takes place on the thylakoid membranes of the granules. Here are both the chlorophyll and the photo systems. Using chloroplasts, sunlight can be used as a form of energy to make sugar (glucose) and oxygen from carbon dioxide and water. Thus, organic substances are synthesized from inorganic substances. The resulting during photosynthesis sugar (glucose) is first stored in the stroma as starch grain and then transported on.