Origins of mitochondria and chloroplast
Both mitochondria and chloroplast have an inner and outer membrane, each a phospholipid bilayer with a unique collection of embedded proteins. The inner membrane of the mitochondria is convoluted, with infoldings called cristae which gives it a greater surface are to enhance productivity of cellular respiration, it also encloses the mitochondrial matrix, and contains built in enzymes that make ATP. The inner membrane of the chloroplast encloses the granum structures and stroma from cytosol. The majority of metabolic steps for both organelles occur in the mitochondrial matrix and stroma. Mitochondria are about 1 to 10 m long, and the chloroplast are about 2 m by 5 m in size.
Chloroplasts and Mitochondria generate ATP by the same basic mechanism: chemiosmosis. An electron transport chain assembled in a membrane translocates protons across the membrane as electrons are passed through a series of carriers that are progressively more electronegative. Built into the same membrane is an ATP synthase complex that couples the diffusion of hydrogen ions down their gradient to the phosphorylation of ADP. Some of the electron carriers, including quinones and cytochromes, are very similar in chloroplasts and mitochondria, and the ATP synthase complexes of the two organelles are also very much alike. But there are noteworthy differences between oxidative phosphorylation in mitochondria and photophosphorylation in chloroplast. In mitochondria, the high energy electrons dropped down the transport chain are extracted by the oxidation of food molecules. Chloroplast do not need food to make ATP; their photosystems capture light energy and use it to drive electrons to the top of the transport chain. In other words, mitochondrial transfer chemical energy from food molecules to ATP, while chloroplast transform light energy into chemical energy. It is an important difference.
The special organization of chemiosmosis also differs in chloroplast and mitochondria. The inner membrane of the mitochondrion pumps protons from the matrix out to the intermembrane space, which then serves as a reservoir of hydrogen ions that power the ATP synthase. The thylakoid membrane of the chloroplast pumps protons from the stroma into the thylakoid compartment, which functions as the H+ reservoir. The membrane makes ATP as the hydrogen ions diffuse from the thylakoid compartment back to the stroma through ATP synthase complexes, whose catalytic heads are on the stroma side of the membrane. Thus, ATP forms in the stroma.
According to the endosymbiotic model, a hypothetical model of the origin of the eukaryotic cell, the forerunners of the eukaryotic cells were symbiotic consortiums of prokaryotic cells, with certain species, termed endosymbionts, living with larger prokaryotes. Developed most extensively by Lynn Margulis, the endosymbiotic model focuses on the origin of chloroplasts and mitochondria. Chloroplasts are postulated to be descendants of photosynthetic prokaryotes that became endosymbionts within larger cells. The proposed ancestors of mitochondria were endosymbiotic bacteria that were aerobic heterotrophs. Perhaps they first gained entry to the larger cell as undigested prey or internal parasites. By whatever means the relationship began, it is not hard to imagine the symbiosis eventually becoming mutually beneficial. A heterotrophic host could derive nourishment from photosynthetic endosymbionts. And in a world that was becoming increasingly aerobic, a cell that was itself an anaerobe would have benefited from aerobic endosymbionts that turned the oxygen to advantage. As host and endosymbionts became more interdependent, the conglomerate of prokaryotes would gradually be integrated into a single organism, its parts inseparable.
The feasibility of an endosymbiotic origin of chloroplast and mitochondria rests partially on the existence of endosymbiotic relationships in the modern world. As another line of evidence, proponents of the endosymbiotic hypothesis cite various similarities between eubacteria and the chloroplast and mitochondria of eukaryotes. Comparisons of structure and function reveal that chloroplasts and mitochondria are appropriate size to be descendants of eubacteria, the overall dimensions of mitochondria and bacteria are similar, and the rod shaped bacteria is alike in shape to many types of mitochondria. The inner membranes of chloroplasts and mitochondria, perhaps derived from the membranes of endosymbiotic prokaryotes, have several enzymes and transport systems that resemble those found on the plasma membranes of modern prokaryotes. Due to advancements made in microbiology, it is now clear that there are respiratory assemblies organized in a similar fashion in both the bacterial and the inner mitochondrial membranes. Mitochondria and chloroplasts reproduce by a splitting process reminiscent of binary fission in bacteria. Chloroplasts and mitochondria contain DNA in the form of circular molecules not associated with histones or other proteins, as in prokaryotes. The organelles contain the transfer RNAs, ribosomes, and other equipment needed to transcribe and translate their DNA into proteins. These findings support the presence of a reasonably self replicative apparatus which is similar in its essentials to that of a free living microorganism. In fact, some of the subunits of the cytochromes and ATPases that function in chloroplast and mitochondria are known to be made in the organelles themselves. The ribosomes of chloroplast are more similar in size and biochemical characteristics to prokaryotic ribosomes than to the ribosomes outside the chloroplast in the cytoplasm of the eukaryotic cell. Mitochondrial ribosomes vary extensively from one group of eukaryotes to another, but they are generally more similar to prokaryotic ribosomes than their counterparts in the eukaryotic cytoplasm. There are also resemblance’s between the lipid composition of mitochondrial membranes and bacterial membrane. Indications from recent work show that there is a “premease” system in the mitochondrial membrane, similar to that in bacteria. Also figures in yeast mitochondria that greatly resemble the bacterial “mesosome” has been observed.
The limited evidence available so far from molecular systematics also suggest eubacterial origins for chloroplast and mitochondria. Comparisons of base sequence show that the ribosomal RNA of chloroplast, which is transcribed from genes within the organelles, is more similar to the RNA of certain photosynthetic eubacteria than it is to ribosomal RNA in eukaryotic cytoplasm, which is transcribed from nuclear DNA. Base-sequence comparisons also suggest a eubacterial origin for ribosomal RNA of mitochondria, although the similarity is not as close as the one between chloroplast and eubacteria.
Those skeptical about the endosymbiotic model point out that chloroplast and mitochondria are not even close to being genetically autonomous. The great majority of proteins in the organelles are made by cytoplasmic ribosomes translating messenger RNA transcribed from nuclear genes. Advocates of the endosymbiotic hypothesis answer that a billion years of co- evolution has been sufficient time for the host cell to develop extensive nuclear control over its symbionts, either by the accumulations of mutations or, more likely, by the direct transfer of DNA from the symbionts. In fact, the discovery of transposons has revealed that DNA is surprisingly mobile within the nuclear genome, and there is recent evidence that genes have also jumped between the genomes of organelles and the nucleus. When comparing bacteria and the mitochondria, the most similar characteristics between the two is the circular nature of MtDNA. Hitherto circular DNA molecules are usually only found in microorganisms, and there is no evidence of circular DNA in mammals. Nass and Nass (1963) observed that DNA tended to lie in the central region of the mitochondria which was analogous to the nucleoid region of bacteria.
In considering the origin of eukaryotes, we must understand that the endosymbiotic model is not mutually exclusive. Possibly, the chloroplast and mitochondria may have originated as endosymbionts. In addition, the endosymbiotic model doesn’t require an event leading to greater cellular complexity to happen just once in the course of evolution. Comparison of algal pigments and chloroplast construction suggests that, by whatever mechanism, photosynthetic protists likely evolved at least three times from separate prokaryotic ancestors.