Chloroplasts: the origin, structure and function

Kloroplasti su organeli prisutni u biljnim stanicama i stanicama algi. Njihovo porijeklo najvjernije objašnjava teorija endosimbioze koja kloroplastima pripisuje prokariotsko porijeklo tvrdeći da su se kloroplasti razvili iz endosimbionata, odnosno cijanobakterija. Pri tom procesu su određeni geni prokariotskog simbionta izgubljeni, dok su neki geni integrirani u jezgru domaćina. Kloroplasti su veliki organeli obavijeni ovojnicom od dvije membrane. Vanjska membrana predstavlja granicu prema citoplazmi stanice, a unutrašnja obavija stromu koja sadrži sustav tilakoidnih membrana, ribosome, škrobna zrnca i plastoglobule. Na tilakoidima su smješteni specifični fotosintetski i pomoćni pigmenti: klorofili i karotenoidi, kao i ostali dijelovi fotosintetskog aparata. Kloroplasti su mjesto odvijanja fotosinteze, procesa u kojemu se Sunčeva energija pretvara u kemijsku energiju organskih spojeva, ponajprije ugljikohidrata. Zbog važnosti kloroplasta za biljnu stanicu, ali i za eukariote općenito, cilj ovog rada je opisati strukturu kloroplasta, njihov postanak, porijeklo i diferencijaciju te objasniti fotosintezu kao jedan od temeljnih prirodnih procesa kojim se osigurava metabolička energija za sve biološke sustave, a time i opstanak života na Zemlji.Chloroplasts are organelles found in plant cells and in algae cells.Their origin is best described by the endosymbiotic theory by which chloroplasts have prokaryotic origin and are developed from endosymbionts, most likely cyanobacteria. Through that process some of the genes of prokaryotic symbiont were lost, and some of them were integrated into the nucleus of the host. Chloroplasts are large organelles wrapped in tunica made out of two membranes. Outer membrane represents the boundary towards cytoplasm, while inner membrane envelops stroma, which contains thylakoid membranes, ribosomes, plastoglobules and starch granules. Thylakoids contain specific photosynthetic and assistant pigments: chlorophylls and carotenoids, as well as the rest of the photosynthetic apparatus. Chloroplasts take part in photosynthesis, process in which solar energy turns into chemical energy contained in organic compounds, mainly carbohydrates. Related to the importance of chloroplasts for plant cell, as well as for the eucaryots in general, the goal of this paper is to describe the structure of chloroplasts, their origin, evolution and differentiation, explain photosynthesis as one of the fundamental natural processes that provides metabolic energy for all biological systems, and, consequently, supports the survival of life on Earth